Sensory Perception: Lessons from Synesthesia
Identifieur interne : 002823 ( Pmc/Curation ); précédent : 002822; suivant : 002824Sensory Perception: Lessons from Synesthesia
Auteurs : Joshua Paul HarveySource :
- The Yale Journal of Biology and Medicine [ 0044-0086 ] ; 2013.
Abstract
Synesthesia, the conscious, idiosyncratic, repeatable, and involuntary sensation of one sensory modality in response to another, is a condition that has puzzled both researchers and philosophers for centuries. Much time has been spent proving the condition’s existence as well as investigating its etiology, but what can be learned from synesthesia remains a poorly discussed topic. Here, synaesthesia is presented as a possible answer rather than a question to the current gaps in our understanding of sensory perception. By first appreciating the similarities between normal sensory perception and synesthesia, one can use what is known about synaesthesia, from behavioral and imaging studies, to inform our understanding of “normal” sensory perception. In particular, in considering synesthesia, one can better understand how and where the different sensory modalities interact in the brain, how different sensory modalities can interact without confusion ― the binding problem ― as well as how sensory perception develops.
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PubMed: 23766741
PubMed Central: 3670440
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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Sensory Perception: Lessons from Synesthesia</title><author><name sortKey="Harvey, Joshua Paul" sort="Harvey, Joshua Paul" uniqKey="Harvey J" first="Joshua Paul" last="Harvey">Joshua Paul Harvey</name></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">23766741</idno><idno type="pmc">3670440</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3670440</idno><idno type="RBID">PMC:3670440</idno><date when="2013">2013</date><idno type="wicri:Area/Pmc/Corpus">002823</idno><idno type="wicri:Area/Pmc/Curation">002823</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Sensory Perception: Lessons from Synesthesia</title><author><name sortKey="Harvey, Joshua Paul" sort="Harvey, Joshua Paul" uniqKey="Harvey J" first="Joshua Paul" last="Harvey">Joshua Paul Harvey</name></author></analytic><series><title level="j">The Yale Journal of Biology and Medicine</title><idno type="ISSN">0044-0086</idno><idno type="eISSN">1551-4056</idno><imprint><date when="2013">2013</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>Synesthesia, the conscious, idiosyncratic, repeatable, and involuntary sensation
of one sensory modality in response to another, is a condition that has puzzled
both researchers and philosophers for centuries. Much time has been spent
proving the condition’s existence as well as investigating its etiology, but
what can be learned from synesthesia remains a poorly discussed topic. Here,
synaesthesia is presented as a possible answer rather than a question to the
current gaps in our understanding of sensory perception. By first appreciating
the similarities between normal sensory perception and synesthesia, one can use
what is known about synaesthesia, from behavioral and imaging studies, to inform
our understanding of “normal” sensory perception. In particular, in considering
synesthesia, one can better understand how and where the different sensory
modalities interact in the brain, how different sensory modalities can interact
without confusion ― the binding problem ― as well as how sensory perception
develops.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Cytowic, Re" uniqKey="Cytowic R">RE Cytowic</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Simner, J" uniqKey="Simner J">J Simner</name></author><author><name sortKey="Mulvenna, C" uniqKey="Mulvenna C">C Mulvenna</name></author><author><name sortKey="Sagiv, M" uniqKey="Sagiv M">M Sagiv</name></author><author><name sortKey="Tsakanikos, E" uniqKey="Tsakanikos E">E Tsakanikos</name></author><author><name sortKey="Witherby, Sa" uniqKey="Witherby S">SA Witherby</name></author><author><name sortKey="Fraser, C" uniqKey="Fraser C">C Fraser</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Grossenbacher, Pg" uniqKey="Grossenbacher P">PG Grossenbacher</name></author><author><name sortKey="Lovelace, Ct" uniqKey="Lovelace C">CT Lovelace</name></author></analytic></biblStruct><biblStruct><analytic><author><name 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Yale J Biol Med</journal-id><journal-id journal-id-type="iso-abbrev">Yale J Biol Med</journal-id><journal-id journal-id-type="pmc">yjbm</journal-id><journal-id journal-id-type="publisher-id">YJBM</journal-id><journal-title-group><journal-title>The Yale Journal of Biology and Medicine</journal-title></journal-title-group><issn pub-type="ppub">0044-0086</issn><issn pub-type="epub">1551-4056</issn><publisher><publisher-name>YJBM</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">23766741</article-id><article-id pub-id-type="pmc">3670440</article-id><article-id pub-id-type="publisher-id">yjbm862203</article-id><article-categories><subj-group subj-group-type="heading"><subject>Focus: Psychiatry and Psychology</subject></subj-group><series-title>Focus: Psychiatry and Psychology</series-title></article-categories><title-group><article-title>Sensory Perception: Lessons from Synesthesia</article-title><subtitle>Using Synesthesia to Inform the Understanding of Sensory
Perception</subtitle></title-group><contrib-group><contrib contrib-type="author"><name><surname>Harvey</surname><given-names>Joshua Paul</given-names></name></contrib><aff>Balliol College, Oxford, England</aff></contrib-group><author-notes><corresp>To whom all correspondence should be addressed: Joshua Paul Harvey, Balliol
College, Broad Street, Oxford, England OX13BJ; Tele: +447716908105; Email:
<email>joshua.harvey@balliol.ox.ac.uk</email>.</corresp></author-notes><pub-date pub-type="epub"><day>13</day><month>6</month><year>2013</year></pub-date><pub-date pub-type="collection"><month>6</month><year>2013</year></pub-date><volume>86</volume><issue>2</issue><fpage>203</fpage><lpage>216</lpage><permissions><copyright-statement>Copyright ©2013, Yale Journal of Biology and
Medicine</copyright-statement><copyright-year>2013</copyright-year><copyright-holder>Yale Journal of Biology and Medicine</copyright-holder><license license-type="open access"><license-p>This is an Open Access article distributed under the terms of the
Creative Commons Attribution Non-Commercial No Derivatives License, which
permits for noncommercial use, distribution, and reproduction in any digital
medium, provided the original work is properly cited and is not altered in
any way.</license-p></license></permissions><abstract><p>Synesthesia, the conscious, idiosyncratic, repeatable, and involuntary sensation
of one sensory modality in response to another, is a condition that has puzzled
both researchers and philosophers for centuries. Much time has been spent
proving the condition’s existence as well as investigating its etiology, but
what can be learned from synesthesia remains a poorly discussed topic. Here,
synaesthesia is presented as a possible answer rather than a question to the
current gaps in our understanding of sensory perception. By first appreciating
the similarities between normal sensory perception and synesthesia, one can use
what is known about synaesthesia, from behavioral and imaging studies, to inform
our understanding of “normal” sensory perception. In particular, in considering
synesthesia, one can better understand how and where the different sensory
modalities interact in the brain, how different sensory modalities can interact
without confusion ― the binding problem ― as well as how sensory perception
develops.</p></abstract><kwd-group><kwd>synesthesia</kwd><kwd>synaesthesia</kwd><kwd>cross-modal perception</kwd><kwd>sensory perception</kwd><kwd>binding problem</kwd><kwd>Maurer</kwd></kwd-group></article-meta></front><body><sec id="s1"><title>Introduction</title><p>The word synesthesia has an ancient Greek origin: <italic>syn</italic>, meaning
together, and <italic>aesthesis</italic>, meaning sensation [<xref ref-type="bibr" rid="R1">1</xref>]. This is an apt etymology for a
condition whereby stimulation of one sensory pathway of the brain leads to the
autonomatic and involuntary sensation of a second pathway. For example, the
perception of a vivid red (the inducer) could cause the synesthete to hear a middle
C (the concurrent) (<xref ref-type="fig" rid="F1">Figure 1</xref>). The prevalence
of synesthesia is debated, but is estimated to be between 1 percent and 5 percent
[<xref ref-type="bibr" rid="R2">2</xref>].</p><p>The etiology of synesthesia is a contentious subject. Today, a debate exists between
Ramachandran and Hubbard’s hyperconnectivity hypothesis and Grossenbacher and
Lovelace’s disinhibition-unmasking hypothesis. The former describes direct
connections between sensory cortical regions, while the latter implicates a loss of
inhibitory feedback between the cortical regions [<xref ref-type="bibr" rid="R3">3</xref>,<xref ref-type="bibr" rid="R4">4</xref>]. Other
theories include Calkin’s learned association theory, Cytowic’s awareness theory,
and Maurer’s neonatal synaesthesia theory [<xref ref-type="bibr" rid="R5">5</xref>,<xref ref-type="bibr" rid="R6">6</xref>,<xref ref-type="bibr" rid="R7">7</xref>] (<xref ref-type="table" rid="T1">Table 1</xref>). This essay
does not attempt to resolve the parsimonious questions of synesthesia etiology but
rather addresses a pressing issue: what can one learn of sensory perception from
synesthesia.</p><p>What can be learned from synesthesia depends on its precise definition; for instance,
Grossenbacher’s defined synesthesia as unusual, and this precludes Maurer’s theory
that everyone is born with synesthesia [<xref ref-type="bibr" rid="R1">1</xref>,<xref ref-type="bibr" rid="R3">3</xref>]. For this review,
synesthesia is considered a condition that is defined as an inducer causing a
conscious, involuntary, idiosyncratic, and stable (repeatable) experience of an
atypical concurrent (picturing grass when hearing the word green is not
atypical).</p><p>Those with synesthesia rarely consider it a disability; as such, the study of
synesthesia does not seek a cure but a greater understanding of the brain. With this
in mind, one can view synesthesia not as a complex problem but a complex answer to
some of the most difficult questions neuroscience posits.</p></sec><sec id="s2"><title>Cross-Modal Perception</title><p>Aristotle originally divided senses into five separate modalities according to their
individual sense organs. However, he could not have appreciated the extent to which
the five senses interact in the brain [<xref ref-type="bibr" rid="R9">9</xref>]. As our knowledge of sensory perception has advanced, there
is an increased understanding of the integration of the senses, namely via the
process of cross-modal perception (CMP).</p><p>CMP is perception involving the interaction of two or more sensory modalities and is
vital for gaining the most accurate estimate of the surrounding world. The first
example of this was given by Köhler (1929), who provided evidence of CMP through the
bouba/kiki effect, in which the word “bouba” is associated with curved shapes and
“kiki” with angular shapes [<xref ref-type="bibr" rid="R8">8</xref>]. Another example is the McGurk effect, an interaction between
hearing and vision in speech perception [<xref ref-type="bibr" rid="R9">9</xref>]. The McGurk effect describes the phenomenon in which audition
is altered by vision, such that seeing someone mouth the sound “Faa” while hearing
“Baa” makes it impossible to hear anything but “Faa.” Evidence for the interaction
of vision with touch and sound has also been shown [<xref ref-type="bibr" rid="R10">10</xref>,<xref ref-type="bibr" rid="R11">11</xref>]. It is
likely that with further study, CMP will be shown to have an increasing role in
everyday sensory perception.</p><p>Why is CMP important clinically? Answering the questions posed by CMP could lead to
better understanding and treatment of disorder of CMP such as Balint syndrome,
characterized by optic ataxia, oculomotor apraxia, and simultanagnosia
[<xref ref-type="bibr" rid="R12">12</xref>].</p><p>This essay will focus on what can be learned about CMP from synesthesia, in
particular its location, underlying processes, the binding problem, and its
development.</p></sec><sec id="s3"><title>Can Synesthesia Really Inform an Understanding of CMP?</title><p>In order for what is known about synesthesia to inform our understanding of CMP, one
must assume that there are similarities between the two phenomena and therefore the
characteristics of one can inform the other. It has been shown experimentally that
synesthesia and CMP show many similarities, and so conclusions regarding synesthesia
may therefore inform current thinking regarding CMP.</p><p>Ward et al. (2006) showed their similarities by playing 70 tones of varying timbres
to 10 synesthetes and 10 controls. Irrespective of timbre, both groups showed an
identical trend to associate low pitch with dark colors and high pitch with light
colors [<xref ref-type="bibr" rid="R13">13</xref>]. This is to be
expected, as there is cultural association between pitch and color. Both color and
pitch can be thought of as linear scales, and it is logical to assume that high
pitch and light colors, presumably both high on a linear scale, should be
associated. It is likely that non-synesthetes would make a similar association.</p><p>Ward went further to provide evidence of common percepts between synesthetes and
non-synesthetes that are not simply based on magnitude. Ward et al. showed
similarities that cannot be explained by this linear theory. The group showed that
synesthetes with colored hearing, and non-synesthetes reported that timbre affects
the saturation of color, with middle C often eliciting the most saturation
[<xref ref-type="bibr" rid="R14">14</xref>]. This is surprising,
as it is not based on a culture association. This finding suggests that there are
some underlying similarities between the perception of synesthetes and
non-synesthetes. Evidence such as this suggests that synesthesia can truly inform
our understanding of CMP.</p></sec><sec id="s4"><title>Where Does CMP Take Place?</title><p>The first question considered in this essay is: Where does CMP occur? To date, a
range of cortical regions, including the superior temporal sulcus, intraparietal
sulcus, and fronto-insula have all been implicated; however, evidence is conflicting
and often depends on the experimental paradigms [<xref ref-type="bibr" rid="R14">14</xref>]. The precise location of these cortical regions is
unimportant, as the focus of this review considers the evidence that high order
cortical such as those listed above are more likely to be important in CMP and
synesthesia.</p><p>The notion of high and low order cortices is not necessarily intuitive. It is
believed that sensory processing occurs at a multistage level in the cortex. Low
order cortical regions, cortical regions that are early in the processing pathway,
such as the primary visual cortical regions, are thought to be involved in simple
processing. Higher order cortical regions are thought to be involved in more complex
processing [<xref ref-type="bibr" rid="R15">15</xref>].</p><p>The concept of high and lower order cortical regions was demonstrated by the work of
Hubel and Wiesel. They showed that the receptive fields of cat V1 neurons consisted
of simple receptive fields. They then showed that neurons from higher order cortical
regions responded to not only more complex signals, but that these signals were
constituted from an integration of input from lower order V1 cells. Thus, Hubel and
Wiesel proposed that visual processing occurs in a hierarchical configuration.</p><p>Clavagnier, Falchier, and Kennedy (2004) reviewed this work more than 40 years later
[<xref ref-type="bibr" rid="R16">16</xref>]. It is now known
that sensory processing is not as simple as this, and there is a large amount of
both feed-forward and feedback interaction between the high and low order cortical
regions. In particular, it is thought that high order cortical regions are able to
influence low order cortical regions, which is thought to be one of the
neurophysiological processes behind attention [<xref ref-type="bibr" rid="R17">17</xref>].</p><p>The idea of lower order cortical regions feeding into and being refined by higher
order cortical regions was outlined in Damasio’s theory of convergence zones. He
suggested that sequential convergence zones correspond to higher and higher order
cortical regions that identify objects as well as link with other convergence zones
that process the other senses of that object. For instance, the convergence zones
involved in visual recognition of an apple will link with the convergence zones
responding to the taste of that apple [<xref ref-type="bibr" rid="R18">18</xref>].</p><p>What evidence is there that synesthetic perception involves high rather than low
order cortices? Paulesu et al. (1995) used positron emission tomography (PET) to
measure brain activation in six sound-vision synasthetes and six controls
[<xref ref-type="bibr" rid="R19">19</xref>]. PET involves the
subject consuming a radionucleotide tracer and then using a gamma detector to detect
where in the body the tracer has accumulated. The tracer uses in an analog of
glucose and therefore localizes in areas of high metabolic activity. Areas of the
brain that are correlated with high tracer emissions are corresponded to areas of
high metabolic activity and therefore neural activity.</p><p>When listening to inducing words compared with pure tone controls, the synesthetes
had an increased activation in the occipital and parietal cortical regions, the
bilateral inferior temporal gyrus, and the left lingual gyrus. This increase in
activity in these high order cortical regions suggests that they have more of a role
in synesthetic perception than they do in normal auditory perception. Interestingly,
no change was found in V1 or V4, considered low order cortical regions. This
suggests that their activity and thus function differs little between synesthetes
and normal individuals, indicating that V1 and V4 are not as important in the
synesthetic precept. This also suggests that higher order cortical regions are more
important, although this experiment could never hope to exclude low order cortical
regions from having a role in synesthesia. Additional evidence regarding this
conclusion comes from Esterman et al., who showed that transcranial magnetic
stimulation (TMS) disruption over the right posterior parietal lobe but not V1 or
the left posterior parietal cortex reduced synesthetic binding [<xref ref-type="bibr" rid="R20">20</xref>]. TMS uses electromagnets to create
electric currents within the subject’s brain. TMS has been used in a variety of
neuroscience experiments for its ability to selectively deactivate areas of the
brain, thus providing a quick and reversible alternative to lesion experiments,
i.e., electrical ablation of areas of the cortex.</p><p>Another experiment by Aleman and colleagues did show lower order cortical region
activation [<xref ref-type="bibr" rid="R21">21</xref>]. The
experiment involved taking functional magnetic resonance imaging (fMRI) readings
from a synesthete who, on hearing a word, visualized the word in a particular
colour. fMRI also aims to detect metabolic activity by measuring the ratio between
oxygen rich and oxygen poor blood.</p><p>When there was no visual stimulus, the group showed activation of the primary visual
cortex. The patient showed significantly more activation during her synesthetic
perceptions compared with controls, hearing and responding to non-inducer tones. The
group suggests that this is evidence of feedback links between the visual cortical
regions and higher order cortical regions (<xref ref-type="fig" rid="F2">Figure
2</xref>).</p><p>This experiment both supports and contributes to the theory advanced above about the
importance of higher order cortical areas. The evidence does not, however, show a
definitive mechanism for synesthesia and thus cross-modal perception; it presents a
working hypothesis for future experiments to build on, which unfortunately may be
the extent of our knowledge regarding the question of where CMP takes place.</p><p>One of the difficulties in drawing firm conclusions in this area of neuroscience is
due to the excessive reliance on functional imaging, due in part to its ease of use
in humans and limited number of synesthetic test subjects. More definitive
conclusions may rely on different types of experiments such as lesion studies.</p><p>Lesion studies, both through mechanical ablation and TMS, affirm the importance of
high order cortical areas. Damage to the angular gyrus was shown to interfere with
cross-modal stimuli-matching as shown with the aforementioned bouba/kiki test
[<xref ref-type="bibr" rid="R22">22</xref>]. Additionally, a TMS
study showed that disruption of the PPC attenuated visual-tactile CMP in normal
individuals [<xref ref-type="bibr" rid="R23">23</xref>]. What is not
clear but would be an interesting experiment is whether disruption of lower order
cortical regions such as V1 in the Aleman study interferes with synesthesia.</p><p>Despite these conclusions, it is important not to place too much weight on the
findings of fMRI and other imaging techniques. fMRI studies by definition can only
show association and not causality. They demonstrate an increase in activity in
certain brain regions, but this may be due to a variety of causes. As shown in the
Aleman study, there is increased activity in V1, but the cause of this can only be
hypothesized.</p><p>fMRI studies are often limited to low sample groups, both due to the heterogeneity of
synesthetes and the rare nature of the disease. This means one must be prudent in
extrapolating conclusions based on fMRI studies beyond the test subjects in
question.</p><p>Another criticism of fMRI studies is that V1 activation may also be a sign of mental
imagery. For example, if one were to hear the words green grass, one instinctively
imagines a visual scene that may be the cause of low order cortical activation. Such
activation of V1 in these circumstances has been well demonstrated in
non-synesthetes [<xref ref-type="bibr" rid="R24">24</xref>]. An
experiment by Klein and colleagues showed that when subjects imagined a mental
image, there was a reproducible activation of the primary visual cortex according to
fMRI.</p></sec><sec id="s5"><title>Cross-Modal Perception Processes</title><p>Synesthesia can also inform our understanding of the processes of CMP. Synesthesia is
a complex condition characterized by heterogeneity. Synesthetes can be associators,
see concurrents in the mind’s eye, or projectors, perceive concurrents in the
environment transposed on top of the inducer. Another difference is the varied
modalities of the inducers and concurrents. This hints at not one but many
underlying processes that likely interact through the network of cortical regions
mentioned in the previous section.</p><p>Evidence for connections between cortical regions comes from injecting retrograde
tracers in the primate striate cortex. Falchier et al. (2002) showed lifelong
connections between V1 and A1 and connections between the multimodal temporal region
and unimodal V1 [<xref ref-type="bibr" rid="R25">25</xref>]. This
study also provides an anatomical substrate for the aforementioned integration of
processes. However, it is worth noting at this point that CMP functioning may not
rely on direct connections via cortical regions but may occur via thalamocortical
loops, connections from the cortex to the thalamus [<xref ref-type="bibr" rid="R26">26</xref>].</p><p>What do anatomical connections between cortical regions show us? The
hyperconnectivity hypothesis suggests that interaction between sensory cortical
regions causes the mixing of the senses. For this to occur, there must be
connections between the sensory cortical regions to facilitate their interaction.
Therefore, evidence of unimodal to unimodal connections supports the
hyperconnectivity hypothesis [<xref ref-type="bibr" rid="R27">27</xref>]. However, the disinhibition-unmasking hypothesis would require
connections between high order and low order cortical regions to facilitate the
proposed inhibition exhibited by areas of the brain such as the temporal lobe
[<xref ref-type="bibr" rid="R28">28</xref>].</p><p>The experiment by Falchier shows evidence for both the connections between lower
order cortical regions, V1 to A1, as well as between high and lower order cortical
regions, V1 to the temporal region. Falchier’s experiment, therefore, neither
supports one theory nor the other but shows evidence for both and further how these
processes may co-exist in the brain both to produce synesthetic phenomena and to
underlie CMP. It is therefore possible that both direct and indirect connections
exist between V1 and A1 and neither connection may function without the input of the
other (<xref ref-type="fig" rid="F3">Figure 3</xref>).</p><p>Thus far, only the cortex has been considered. Are there sub-cortical neurons that
could also contribute? Multi-sensory integration (MSI) cells have been found to have
maps of sensory space for more than one sense and are possible candidates
[<xref ref-type="bibr" rid="R28">28</xref>]. These cells have
been found in the superior colliculus of guinea pigs, primates, and cats
[<xref ref-type="bibr" rid="R29">29</xref>,<xref ref-type="bibr" rid="R30">30</xref>,<xref ref-type="bibr" rid="R31">31</xref>]. More
recently, MSI cells were found in the primate cortex [<xref ref-type="bibr" rid="R32">32</xref>]. Burnet et al. (2007) showed when MSI cells in the
superior colliculus were lesioned, there was a preferential loss of multisensory
behavior over unisensory behavior [<xref ref-type="bibr" rid="R33">33</xref>]. Additionally, there was a reduction in the receptive field of
the remaining neurons.</p><p>A possible improvement to Burnet’s experiment could involve a more selective lesion,
involving only the bottom layers of the superior colliculus that are more dominated
by multimodal input compared with the superficial layers [<xref ref-type="bibr" rid="R34">34</xref>]. MSI cells are likely to have a
role in synesthesia and CMP. One explanation of MSI cell function could be that the
cells in the superior colliculus are characterized by simple reflexive behavior and
those in the cortex with complex CMP. A similar experiment to Burnet’s with varying
complexities of multimodal behavior could provide evidence for this.</p><p>In this essay, the similarities between synesthesia and CMP have been used to inform
the latter. However, there are also important differences [<xref ref-type="bibr" rid="R35">35</xref>,<xref ref-type="bibr" rid="R36">36</xref>]. Two EEG studies have shown important differences between CMP
and synesthesia.</p><p>The first experiment used EEG recordings to investigate the early sensory processing
of synesthetes compared with those or normal individuals while viewing visual
stimuli that do not activate a synesthetic perception. If the sensory processing
between synesthetes and normal individuals was the same, one would expect that the
EEG recordings should show no difference. However, this experiment showed extra
brain potentials in the synesthetes that indicate hyperactivation of sensory
cortical areas compared with controls [<xref ref-type="bibr" rid="R37">37</xref>].</p><p>The second experiment also used EEG recording to investigate the difference between
synesthetes and controls that were played tones which in the synesthetes were
associated with synesthetic concurrents. This group showed EEG differences
corresponding to activity in the auditory cortex early on (within 100msecs of tone
onset) between the controls and synesthetes. The group therefore predicted
differences in the way that synesthetic and normal brains perceived sound
[<xref ref-type="bibr" rid="R38">38</xref>].</p><p>Can synsthesia still inform the understanding of CMP? Yes, but one must proceed
cautiously. Many of the studies described in this section have been functional
imaging studies and thus are limited in showing associative links between cortical
regions rather than causality. There appear to be strong similarities in the
underlying process of synesthesia and normal sensory perception. The true extent of
the similarities and what they demonstrate is still to be seen.</p><p>Studying CMP raises another closely related question: With constant cross-modal
interaction, how does the human brain ensure the auditory percept of an object is
bound to its visual percept? Synesthesia can again enlighten the situation, this
time with regards to the binding problem.</p></sec><sec id="s6"><title>Binding Problem</title><p>The computer in front of me consists of features such as shape, sound, and smell. How
does this information, transduced in the eyes, ears, and nose, respectively, and
then processed in different cortical regions combine to form a uniform perception of
the object? The binding problem asks: How can CMP occur without confusing the
senses?</p><p>What is feature binding? Imagine a child playing with numerous colored blocks. At
first, the blocks represent a jumbled selection of different features ― some are
square, some are round, and some are triangle. In addition, the blocks are blue,
red, yellow, green, and so on. It is only when the child starts grouping the blocks,
e.g., all of the red blocks together, associations between the different blocks
become evident. Grouping objects by color would seem illogical, as a blue pencil and
a blue book share no other characteristics other than their color. However, if
features share the same geographical place in our visual scene, it is much more
likely that they are in fact the same object. This is how the human brain is thought
to bind features [<xref ref-type="bibr" rid="R39">39</xref>].</p><p>A range of theories have proposed solutions to the binding problem. A seminal work
was forwarded in Treisman and Schmidt’s 1980 and 1982 papers regarding feature
integration theory (FIT) [<xref ref-type="bibr" rid="R39">39</xref>]. They proposed that features are represented in a feature map in
the brain that corresponds to the location in the visual field and identity.
Therefore, the appearance and the sound of the computer are bound by virtue of their
shared geographical location in the visual field. The pair also state that our
visual field consists of a variety of objects, and as such, there is activation in
our brains caused by all the features of all the objects in our visual field.
However, they state that binding only occurs when attention is directed toward a
particular location [<xref ref-type="bibr" rid="R40">40</xref>,<xref ref-type="bibr" rid="R41">41</xref>]. It is this assumption, that
binding requires attention, that will be the focus of the following discussion for
the following reasons.</p><p>The binding problem is a real problem in that it has not been adequately solved, and
a solution could help better understand some of the perception deficits seen in
patients with common conditions such as stroke [<xref ref-type="bibr" rid="R39">39</xref>]. Understanding the binding problem can help better
understand how attention influences perception and with that consequences of stroke
such as neglect, a rare but highly debilitating condition in which patients can in
some cases fail to perceive large parts of their environment [<xref ref-type="bibr" rid="R42">42</xref>,<xref ref-type="bibr" rid="R43">43</xref>].</p><p>Although binding is often very accurate, this is not always the case. It is possible
for the brain to make mistakes. How many times have you thought one person said
something when in fact it was someone else? These errors in binding (illusory
conjunctions) have been used to inform FIT, and researchers have used them to
suggest evidence both for and against the requirement of attention for binding
[<xref ref-type="bibr" rid="R44">44</xref>]. Treisman maintained
that binding requires attention, and therefore, like the child and their blocks,
before attention the visual scene consists of a collection of jumbled unbound
features.</p><p>Treisman investigated illusory conjunctions by showing subjects a line of colored
shapes or letters, on either side of which were two black digits [<xref ref-type="bibr" rid="R41">41</xref>]. The investigators told the
subjects to report on the two black digits, thereby diverting their attention from
the color shapes. The results showed that the subjects could accurately report on
the digits, but not on the shapes; therefore, attention was needed for binding.
Treisman suggested that primitive features such as color, orientation, and intensity
are available pre-attentively, but such features are unbound prior to attention.</p><p>However, there is no consensus as to whether binding can occur prior to attention.
Tsal (1989), among others, has criticized FIT, saying binding can occur prior to
attention [<xref ref-type="bibr" rid="R45">45</xref>]. The evidence
demonstrating that binding can occur prior to attention is presented in the
remainder of this section.</p><p>The study of projector synesthetes can help resolve this issue by showing attention
is not required for synesthetic binding and therefore would not necessarily be
required for normal binding (<xref ref-type="fig" rid="F3">Figure 3</xref>).</p><p>One way to consider projector synesthetes is that they are demonstrating a form of
incorrect binding. For instance, the binding of the color red to the number 7.
Several pieces of evidence suggest that this type of synesthesia can occur without
the awareness of the inducer, the number 7, and thus is pre-attentive.</p><p>Smilek et al. (2001) showed that when objectively black shapes were shown on a
colored background, their detection was more accurate when the induced color was
different from the background [<xref ref-type="bibr" rid="R46">46</xref>] (<xref ref-type="fig" rid="F4">Figure 4</xref>).</p><p>This suggests that the shape was bound to the synesthetic color prior to perception,
and thus binding was pre-attentive. This is, therefore, a contradiction to FIT that
suggests that binding requires attention. If binding required attention, as stated
by Treisman, it would follow that it occurred after attention and therefore
perception, which would mean that this form of synesthetic binding should not
interfere with perception.</p><p>However, Mattingley et al. (2001) suggested that attention is necessary for the
binding in synesthesia. His study tested 15 synesthetes by visually presenting them
with inducing letters or digits, which were either above or below the detection
threshold [<xref ref-type="bibr" rid="R47">47</xref>]. If, for
example, the letter T was bound to green, then grayscale T would be flashed and
either green or red presented. Synesthetes were slower to name the inconsistent
rather than the consistent color patches when the letters and digits were detected.
This is because when the red color was presented after the T, this interfered with
the green ― the synaesthetic concurrent, an example of Stroop interference.</p><p>Mattingley concludes that this demonstrates attention is necessary for binding using
the logic presented below (<xref ref-type="fig" rid="F5">Figure 5</xref>):</p><list list-type="simple"><list-item><p>1a) The synesthete shows stroop interference when presented with colors
inconsistent with their normal concurrent, e.g., seeing red when seeing the
letter T, which they normally associate with green.</p></list-item><list-item><p>1b) The synesthete does not show interference when the inducer is below
detection threshold, i.e., they are unable to recognize the letter or digit
(also shown by Mattingly in the same experiment).</p></list-item><list-item><p>2a) The synesthete only shows interference after letter and digit recognition
is complete.</p></list-item><list-item><p>2b) Interference requires binding, otherwise there would be no confusion if
green was not associated T when red is presented.</p></list-item><list-item><p>3. The autonomic binding of color and alphanumeric form therefore arises
after letter and digit recognition is complete.</p></list-item><list-item><p>4. Letter and digit recognition requires attention.</p></list-item><list-item><p>5. Thus binding requires attention.</p></list-item></list><p>There are three clear criticisms of Mattingley’s experiment.</p><p>Firstly, he does not say whether the synesthetes are associators or projectors. If
they are the former, then it is not likely that his subject synesthetes are a good
model of sensory binding. This is because only projectors bind the concurrent with
the inducer in the visual scene.</p><p>Secondly, his control was not effective. To show that the lack of Stroop interference
in synesthetes was not due to the lack of digit recognition by the visual system,
when the inducers were below detection threshold, Mattingley showed the digits
caused interference in naming the next digit. However, the control is not robust, as
it is likely numbers and colors are processed in different cortical areas, dorsal
and ventral stream respectively [<xref ref-type="bibr" rid="R48">48</xref>]. It is conceivable that there are two different thresholds for
these two types of perception.</p><p>Finally, it is known attention can modulate synesthesia and binding at a later stage.
This has already been well demonstrated by Navon et al., who used Navon-type
local-global stimuli to show the effects of selective attention on synesthetes
[<xref ref-type="bibr" rid="R49">49</xref>] (<xref ref-type="table" rid="T2">Table 2</xref>). The table demonstrates Navon type
stimuli and interference in reading the final image, the letter T, as it is made up
of an interfering character, in this case the letter S. There is no interference in
the first two images, but in the last image the letter S interferes with the overall
shape of the letter T.</p><p>Therefore, it is possible the masking did not reduce synesthetic Stroop effects
because binding required attention, but rather because of the attentional
modification of synesthesia.</p><p>One explanation that would reconcile conflicting literature is that there are
differences between projector and associator synesthetes. This was indeed shown in
Stroop-like tests on both types of synesthetes. In a study by Dixon, it was shown
the projectors have a greater level of Stroop interference than associators, which
may be accounted for in different forms of binding [<xref ref-type="bibr" rid="R50">50</xref>]. This could lead to confusion when reviewing
literature as some studies concentrate on projectors, some on associators, some on
both, and some, such as Mattingley’s work, don’t state one way or the other.</p><p>Another option is that the dichotomy created between pre-attentive and post-attentive
binding is artificial. This is a notion forwarded by Anderson who states that the
conflicting evidence presented above is not likely to represent two different types
of perception but rather different aspects of a single mechanism of synesthesia
[<xref ref-type="bibr" rid="R51">51</xref>]. What is learned
from studying synesthesia is that binding does not always require attention.</p><p>This shows that future theories of binding should accommodate the fact that normal
binding can, although not always, occur pre-attentively.</p><p>What else can synesthesia demonstrate regarding sensory perception? Another
controversial subject questions how cross-modal perception develops and the theory
of neonatal synesthesia.</p></sec><sec id="s7"><title>Development of Cross-Modal Perception</title><p>How does CMP develop? Synesthesia may again help to answer this question. Familial
studies have provided strong evidence that there is a genetic component to
synesthesia [<xref ref-type="bibr" rid="R52">52</xref>].
Additionally, a recent study has used linkage analysis to identify chromosomes 2, 5,
6, and 12 as important in synesthesia [<xref ref-type="bibr" rid="R52">52</xref>]. If synesthesia is caused by genes, is it possible that it
is present from birth? Maurer (2005) goes further and suggests that all neonates are
born with a form of synesthesia [<xref ref-type="bibr" rid="R7">7</xref>]. Her neonatal synesthesia (NS) hypothesis suggests that everyone
is born with synesthesia, and during development, the modularity of senses appears,
causing the synesthesia to disappear; when this development of modularity fails,
synesthesia persists. If this hypothesis were correct, it would drastically change
the way synesthesia and the development of CMP is considered.</p><p>Maurer divides evidence for the NS hypothesis into three areas [<xref ref-type="bibr" rid="R7">7</xref>] (<xref ref-type="table" rid="T3">Table 3</xref>).</p><p>The first area of evidence is anatomical. Studies have shown the immature cortex is
less specialized in the newborn than in the adult. An experiment by Sur showed that
when retinal axons are rewired to the auditory cortex in young ferrets, the auditory
cortex no longer develops with normal tonotopic stripes but with pinwheels seen in
the visual cortex. The neurons in the auditory cortex then become sensitive to
visual stimuli. This suggests that the specialization and organization of primary
sensory cortical areas are determined by the nature of its input, and thus the
modality of the sense is not fixed by virtue of the location of the cortex
[<xref ref-type="bibr" rid="R53">53</xref>]. Additionally, an
experiment in cats showed that enucleation of the eyes at birth, thus depriving the
cat of visual stimulus, allowed the auditory cortex to respond to visual stimuli
[<xref ref-type="bibr" rid="R54">54</xref>]. These experiments
are thought to demonstrate the unmasking of normally silent inputs, i.e., visual
stimuli to the auditory, which may be present in the newborn and underlie
synesthesia [<xref ref-type="bibr" rid="R7">7</xref>].</p><p>The second line of evidence comes from techniques for recording neural activity. It
was shown that stimulation of the wrist elicits a somatosensory evoked response
which in newborns is enhanced when accompanied by a white noise [<xref ref-type="bibr" rid="R55">55</xref>]. Tzourio-Mazoyer (2002) used a PET
scan to show that activation in response to faces compared to illuminated diodes
activated areas associated with speech [<xref ref-type="bibr" rid="R56">56</xref>]. Both these experiments are taken to show that is enhanced
cross-modal interaction in neonates.</p><p>The final line of evidence is behavioral. Normally, infants squeeze smooth objects
more often than granular objects. An experiment showed when a granular tactile
stimulus was coupled with a smooth visual stimulus, the frequency of squeezing
increased [<xref ref-type="bibr" rid="R57">57</xref>]. When a smooth
tactile stimulus was accompanied by granular visual stimulus, the squeeze frequency
fell, suggesting newborns can compare sensory information across modalities.</p><p>The evidence for the developmental theory of synesthesia remains unconvincing. The
aforementioned three branches of experiments do not go beyond that which can be
reconciled with the cross-modal transfer (CMT) hypothesis. The CMT hypothesis
proposes that objects can be recognized in more than one modality. This is supported
by a range of evidence [<xref ref-type="bibr" rid="R58">58</xref>].
For instance, one study found that 12-month-olds look longer at an object they had
just explored orally; this is presumably because the baby can differentiate between
two objects by sight even if they have only felt the shape of the object via the
mouth due to an abstract representation of the shape [<xref ref-type="bibr" rid="R59">59</xref>,<xref ref-type="bibr" rid="R60">60</xref>]. It
appears that Maurer’s evidence reflects CMT rather than NS.</p><p>Baron-Cohen (1996) proposed an imaging experiment to resolve this issue
[<xref ref-type="bibr" rid="R58">58</xref>]. He suggested an
fMRI study over the course of development to investigate abnormal sense cortical
regions’ activation. He predicted that as the infant ages, this abnormal activation
will decrease. This experiment is impractical due to the ethical considerations
concerned with testing infants for a condition such as synesthesia. Also, the
experiment fails to demonstrate that the activation being measured is caused by
synesthesia rather than CMT. This study would also fail to prove a conscious
percept, which many believe to be necessary for synesthesia [<xref ref-type="bibr" rid="R5">5</xref>]. The future of this field may lie in
the refinement of animal models of CMP, allowing more invasive electrophysiological
and lesion studies in neonatal animals [<xref ref-type="bibr" rid="R61">61</xref>,<xref ref-type="bibr" rid="R62">62</xref>].</p><p>Discounting the NS hypothesis begs the question: What can one learn about CMP from
synesthesia? What is immediately apparent is there is not a simple answer to this
question. Both main theories of synesthesia, hyperconnectivity and
disinhibition-unmasking, implicate environmental influences on synaptic connections
in the development of the condition [<xref ref-type="bibr" rid="R63">63</xref>]. However, the hereditability of synesthesia suggests an
important genetic impact of synaptic wiring. Synesthesia, therefore, gives evidence
for CMP being a function of nature and nurture.</p></sec><sec id="s8"><title>Conclusion and Outlook</title><p>What can synesthesia demonstrate regarding CMP?</p><p>Synesthesia is a form of enhanced CMP. It has been shown from fMRI and TMS disruption
studies that the CMP involved in synesthesia is likely to be more localized to
higher rather than lower order cortical areas. The etiology of synesthesia is
unclear, but it is likely that CMP occurs both as a result of direct connections and
feedback connections from higher order cortical structures. Synesthesia also
demonstrates the role of attention in binding the features of the visual scene and
again suggests that the answer to the binding problem may be somewhere between the
two theories of pre-attentive and post attentive models. Finally, consideration of
synesthesia suggests CMP is a product of both biological and environmental
factors.</p><p>As our knowledge has expanded, new questions have arisen: How is crossmodal
information integrated in the brain? What role does memory and attention have on CMP
[<xref ref-type="bibr" rid="R64">64</xref>]? As more questions
arise, synesthesia will continue to be a guide to the unpredictable road ahead.</p></sec></body><back><glossary><title>Abbreviations</title><def-list><def-item><term>CMP</term><def><p>cross-modal perception</p></def></def-item><def-item><term>V1</term><def><p>primary visual cortex</p></def></def-item><def-item><term>PET</term><def><p>positron emission tomography</p></def></def-item><def-item><term>V4</term><def><p>V4 visual cortex</p></def></def-item><def-item><term>TMS</term><def><p>transcranial magnetic stimulation</p></def></def-item><def-item><term>fMRI</term><def><p>functional magnetic resonance imaging</p></def></def-item><def-item><term>PPC</term><def><p>posterior parietal cortex</p></def></def-item><def-item><term>A1</term><def><p>primary auditory cortex</p></def></def-item><def-item><term>MSI</term><def><p>multi-sensory integration</p></def></def-item><def-item><term>EEG</term><def><p>electroencephalography</p></def></def-item><def-item><term>FIT</term><def><p>feature integration theory</p></def></def-item><def-item><term>CMT</term><def><p>cross-modal transfer</p></def></def-item></def-list></glossary><ref-list><ref id="R1"><element-citation publication-type="book" publication-format="print"><name><surname>Cytowic</surname><given-names>RE</given-names></name><source>Synesthesia: A Union of the Senses</source><edition>2nd ed.</edition><publisher-loc>Cambridge</publisher-loc><publisher-name>MIT Press</publisher-name><year>2002</year></element-citation></ref><ref id="R2"><element-citation publication-type="journal" publication-format="print"><name><surname>Simner</surname><given-names>J</given-names></name><name><surname>Mulvenna</surname><given-names>C</given-names></name><name><surname>Sagiv</surname><given-names>M</given-names></name><name><surname>Tsakanikos</surname><given-names>E</given-names></name><name><surname>Witherby</surname><given-names>SA</given-names></name><name><surname>Fraser</surname><given-names>C</given-names></name><etal></etal><article-title>Synaesthesia: the prevalence of atypical cross-modal
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neurons.</td></tr><tr><td rowspan="1" colspan="1">Disinhibition-unmasking hypothesis</td><td rowspan="1" colspan="1">Grossenbacher and Lovelace</td><td rowspan="1" colspan="1">Caused by a decreased level of feedback from inhibitory cortical
areas.</td><td rowspan="1" colspan="1">This theory suggests constitutive inhibitory cortical feedback is
present in everyone.</td></tr><tr><td rowspan="1" colspan="1">Learned association theory</td><td rowspan="1" colspan="1">Calkin</td><td rowspan="1" colspan="1">Suggests that synesthetic links are caused by learned associations early
in life.</td><td rowspan="1" colspan="1">Discredited due to genetic component and increased incidence in
women.</td></tr><tr><td rowspan="1" colspan="1">Awareness theory</td><td rowspan="1" colspan="1">Cytowic</td><td rowspan="1" colspan="1">Suggests that synesthesia is part of a normal perceptual process, and
the phenomenon is caused by a failure of our brain to suppress the
concurrent which he hypothesizes occurs in everyone.</td><td rowspan="1" colspan="1">Implicates the limbic system as important especially the
hippocampus.</td></tr><tr><td rowspan="1" colspan="1">Neonatal synaesthesia</td><td rowspan="1" colspan="1">Maurer</td><td rowspan="1" colspan="1">This theory suggests that humans are all born with synesthesia-like
tendencies, which in “normal” people are lost through age.</td><td rowspan="1" colspan="1">Widely refuted. Discussed at length later.</td></tr></tbody></table></table-wrap><table-wrap id="T2" orientation="portrait" position="float"><label>Table 2</label><caption><title>Navon stimuli.</title></caption><table frame="hsides" rules="groups"><thead align="left"><tr><td rowspan="1" colspan="1"><bold>Consistent</bold></td><td rowspan="1" colspan="1"><bold>Neutral</bold></td><td rowspan="1" colspan="1"><bold>Conflicting</bold></td></tr></thead><tbody align="center"><tr><td colspan="3" rowspan="1"></td></tr><tr><td rowspan="1" colspan="1">TTTTTTTTTTTTTTTTTTTTTTTTT</td><td rowspan="1" colspan="1">+++++++++++++++++++++++++</td><td rowspan="1" colspan="1">SSSSSSSSSSSSSSSSSSSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTTTTTTTTTTTTTTTTTTT</td><td rowspan="1" colspan="1">+++++++++++++++++++++++++</td><td rowspan="1" colspan="1">SSSSSSSSSSSSSSSSSSSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTTTTTTTTTTTTTTTTTTT</td><td rowspan="1" colspan="1">+++++++++++++++++++++++++</td><td rowspan="1" colspan="1">SSSSSSSSSSSSSSSSSSSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr><tr><td rowspan="1" colspan="1">TTTTTTTT</td><td rowspan="1" colspan="1">++++++++</td><td rowspan="1" colspan="1">SSSSSSSS</td></tr></tbody></table></table-wrap><table-wrap id="T3" orientation="portrait" position="float"><label>Table 3</label><caption><title>Mauer's three lines of evidence for neonatal synesthesia.</title></caption><table frame="hsides" rules="groups"><thead align="left"><tr><td rowspan="1" colspan="1"><bold>Line of reasoning</bold></td><td rowspan="1" colspan="1"><bold>Explanation</bold></td><td rowspan="1" colspan="1"><bold>Criticism</bold></td></tr></thead><tbody align="left"><tr><td colspan="3" rowspan="1"></td></tr><tr><td rowspan="1" colspan="1">Anatomical</td><td rowspan="1" colspan="1">Evidence of plasticity in the neonatal brain – may underlie
synesthesia.</td><td rowspan="1" colspan="1">Demonstrates that the neonate brain may be more susceptible to
crossmodal cortical connections, but not that this causes
synesthesia.</td></tr><tr><td rowspan="1" colspan="1">Recording</td><td rowspan="1" colspan="1">Sensory percepts of one modality are enhanced by those of other
modalities.</td><td rowspan="1" colspan="1">This is very difficult to demonstrate anything other than cross modal
perception rather than synesthesia.</td></tr><tr><td rowspan="1" colspan="1">Behavioral</td><td rowspan="1" colspan="1">Behavioral evidence suggesting neonates can compare sensory information
across modality.</td><td rowspan="1" colspan="1">Again the studies cited by Mauer cannot demonstrate neonatal
synesthesia.</td></tr></tbody></table></table-wrap><fig id="F1" orientation="portrait" position="float"><label>Figure 1</label><caption><p><bold>An example of synesthetic perception.</bold> Diagram demonstrating what a
synesthete might see when they look at the above characters.</p></caption><graphic xlink:href="yjbm_86_2_203_g01"></graphic></fig><fig id="F2" orientation="portrait" position="float"><label>Figure 2</label><caption><p><bold>Cortical processing schematic.</bold> This schematic shows the hypothesis
of Aleman and colleagues. They hypothesize a role for high order cortical areas
to activate cross-modal cortical areas. What is not clear is to what extent
activation of primary sensory cortical regions are able to act more directly,
via direct links (red arrow) and via other cortical areas, e.g., V4 (green
arrow).</p></caption><graphic xlink:href="yjbm_86_2_203_g02"></graphic></fig><fig id="F3" orientation="portrait" position="float"><label>Figure 3</label><caption><p><bold>Preattentive and postattentive binding.</bold> This diagram demonstrates
the two different forms of sensory binding. In example 1, the sensory percepts,
e.g., visual and auditory, are bound after perception and therefore, attention
is not required to bind them. In the second example, after perception, the
visual and auditory perceptions are unbound and remain so until attention is
directed at them. There are differing views as to which schematic better
represents our own perception. Synesthesia may help show that attention is not
required for binding, and thus example 1 is more accurate. </p></caption><graphic xlink:href="yjbm_86_2_203_g03"></graphic></fig><fig id="F4" orientation="portrait" position="float"><label>Figure 4</label><caption><p><bold>Diagram illustrating Smilek et al. method.</bold> In the experiment by
Smilek and colleagues, they showed black shapes (that appeared colored to the
synesthetes by virtue of their condition) on different colored backgrounds. The
detection of the shapes was more accurate when the color that the shapes
appeared was different from the background. This figure demonstrates this
effect. Here, a black 7 appears red because of the synesthetic perception. On a
red background, this 7 is obscured, but on a blue background, the 7 is more
visible.</p></caption><graphic xlink:href="yjbm_86_2_203_g04"></graphic></fig><fig id="F5" orientation="portrait" position="float"><label>Figure 5</label><caption><p><bold>An example of Stroop interference.</bold> Try naming the color of the font
of the first row of words and then the second. The color of the font interferes
with the naming of the font.</p></caption><graphic xlink:href="yjbm_86_2_203_g05"></graphic></fig></floats-group></pmc></record>Pour manipuler ce document sous Unix (Dilib)
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