Synaesthesia refers to the crossing ofsenses that affects an estimated 1 in 2,000 people, including thevisually impaired and even the blind (Kher, 2001; Nold, 1997). Insynaesthesia, the stimulation of one sensory system induces aperception in a different, physically unstimulated sensory system(Wager, 1999). For example, as cited by Wager (1999), Cytowicreported on a synaesthete who perceived the touch of a glass columnwhen he tasted mint. The variations and combinations of senses arenumerous in synaesthesia, with no two synaesthetes reporting the sameassociations (Kher, 2001). A music piece may induce a visual scene ofcolor explosions for one person while inducing a scene of coloredwaves for another. In color-hearing synaesthesia, an auditorystimulus creates a visual perception. In other words, whencolor-hearing synaesthetes hear different sounds, they perceptuallysee different colors and shapes that are not physically presented tothem.

Scientists have not yet discovered whatcauses synaesthetic perception. One theory is that synaesthesia isthe norm in lower vertebrate animals; it is a quality that humanshave left behind in the process of evolution (Freeman, 1998). Mostresearchers, however, seem to agree that the answer to the questionlies in the concept of modularity. The human brain is efficientlyorganized so that specific mental tasks are processed in specificareas of the brain. Each of the sensory systems has a correspondingarea, or module, of the brain. While there are normal interactionsbetween the brain activities of different modules, as when usingvision to detect movement, the current understanding of humanperception cannot account for the stimulation of one module inducingbrain activity in a different module.

There are conflicting theories of modularityas the source of synaesthesia. Lloyd (1996) describes the brainactivity of a synaesthete as "confused," but not as the interminglingof different modules. In contrast, Gray (2001) proposes two possiblemodels of modularity that may explain the occurrence of synaesthesia:(1) a breakdown in normal modularity, or (2) an extra module designedto process more than one sensory modality that occurs in only someindividuals. Both of these possibilities would, theoretically, befeasible because neither circumstance would violate the currentunderstandings of the rules governing the modularity of human brains.Anderson (1998) proposed the idea that some neural modules are notinnate&emdash;they can be learned. If supported, this theory wouldsuggest the possibility that synaesthesia develops as the result ofan extra, learned module. There is no theory regarding the period oflife in which this extra module would develop since all synaesthetesreport having experienced their synaesthetic perceptions as far backin time as they can remember (Cytowic,1995).

The experience of color-hearing synaesthesiaand imagery can be compared in that both are a visual representationof an unreal stimulus. The difference is that, in synaesthesia, theimages are actually seen outside of the individual, whereas inimagery they are merely imagined within the individual's mind(Cytowic,1995). Brandt and Stark (1997) show intheir study that patterns of eye-movements are similar when an imageis imagined, and thus not physically presented, as when the image isactually viewed. Researchers already know that the eyes follow asystematic pattern of movement when viewing an image. It is alsounderstood that most people follow a systematically similar patternof eye movement when scanning the same image. These relativelystandardized patterns of eye movement are the brain's way oforganizing an image (Brandt & Stark, 1997). Since eye patternsare indicative of image content during imagery tasks, it isreasonable to believe that synaesthetes will also exhibit eyemovements representing the content of their visual scene, even thoughthe images they see are not physically present.

Several methods of tracking eye-movementshave been used in research (Mulligan, 1997). Choosing a single methodof eye-tracking involves the consideration of many advantages anddisadvantages associated with each available system. Most commercialsystems are expensive and invasive because they involve the use ofhead gear, chin rests, and bite-down armatures as well as requiringthat the head be stationary during testing (; Krugman & Fox, 1994).Currently, no system is appropriate for use in every researchsituation. Of these disadvantages, the largest obstacle facing thepresently available commercial systems is their inability to separatehead movements from eye movements (Mulligan& Beutter, 2002). This limitation meansthat these systems can only be used in studies where theparticipant's head is immobile, prohibiting research of eye movementsin realistic situations. Research on the eye-movements in infants oryoung children is also limited because of the difficulties involvedin keeping maintaining head immobility in very young participantswhile awake. A few of the recently developed techniques ofeye-tracking have overcome these obstacles, including the AppliedScience Laboratories Model 425OR eye tracker and the use ofcompressed video images (Krugman & Fox, 1994; Mulligan& Buetter, 2002).

Eye movements are an observable behaviorresulting from the subjective experience of perception. Richardson(1999) explains that subjective experiences will produce observablebehaviors. In the case of this study, the subjective experience ofextra-sensory, synaesthetic perceptions should produce observableeye-movement patterns different from the patterns of eye movements innon-synaesthetic individuals. Using this logic and the knowledge thatimagery will produce patterns of eye-movements similar to those ofactual vision, it is reasonable to assume that color-hearingsynaesthetes would exhibit a systematically different pattern of eyemovements than non-synaesthetes receiving the same visual andauditory stimulation. With this assumption in mind, it ishypothesized that color-hearing synaesthetes will have significantlymore visual fixations as well as a significantly higher number ofareas of visual interest, or areas with many visual fixations thannon-synaesthetes.

Participants

Twenty non-synaesthetic participants will berecruited from the local university and will receive class credit fortheir participation. Twenty color-hearing synaesthetic participantswill be recruited through the American Synaesthesia Association andwill be paid for their participation. The number of participants willbe relatively low due to the expensive and time-consuming proceduresavailable for the use of measuring eye movements (Krugman & Fox,1994). The participants are expected to range in age from 20 to 30years old. Approximately 20% of the participants will be male and 80%of the participants will be female. More females will be studiedbecause synaesthesia is more common in females than in males(Cytowic,1995). The sample will include a number ofdifferent ethnicities as well as a number of different socioeconomicclasses.

Materials

The materials used to stimulate the senseswill be a black and white photograph of a woman's face and taperecording of "Romance," the second movement from Wolfgang Mozart'sEine Kleine Nachtmusik. The auditory stimulus will be heard throughearphones connected to a tape player.

The eye movements will be monitored andrecorded using the Applied Science Laboratories Model 425OR eyetracker (Krugman & Fox, 1994). This system reports points ofvisual fixation without the use of head-worn equipment, meaningparticipants are free to move as they might move in the normalenvironment. Participants will sit in front of a screen on which thewoman's photograph will be projected from a projection unit locatedbeside the participants' seat.

Procedure

Participants will first be asked to view aphotograph of a woman's face for 90 seconds while receiving noauditory stimulus. This is to establish a baseline and to ensure thatthe any difference in the number of visual fixations or in the numberof areas of visual interest is due to the added auditory stimulus andnot due to innate differences between the two groups of participants.Participants will wear the headphones in the trials where no auditorystimulus is presented to ensure that neither group of participants isexposed to noises within the room. Extra noise exposure couldincrease the number of visual fixations and the number of areas ofvisual interest in the synaesthete group, giving a false baseline andpossibly implying a difference between the groups when no auditorystimulus is presented. Next, participants will be asked to view asecond, similar photograph of a woman for 90 seconds while listeningto "Romance" through headphones. Points of visual fixation will berecorded during both the baseline measurements and the experimentalmeasurements.

The data will be analyzed using a betweensubjects 2x2 ANOVA procedure. The two variables will be synaesthetecondition (whether the participant was synaesthetic ornon-synaesthetic) and presence of the auditory stimulus (whether theauditory stimulus was present or not present).

The difference in the number of visualfixations (see Figure 1) will be analyzed by comparing the meannumber of visual fixations in synaesthetes to the mean number ofvisual fixations in non-synaesthetes. There will be no significantdifference between synaesthetes and non-synaesthetes for the baselinemeasurement of the number of visual fixations when auditory stimulusis not presented to the participants. It will be found, however, thatcolor-hearing synaesthetes have a significantly higher number ofvisual fixations than non-synaesthetes when asked to view thephotograph while the auditory stimulus is presented toparticipants.

Figure 1: A schematic showingfixations.

The same statistical analysis will beapplied to analyze the difference in the number of areas of visualinterest. For our purposes, areas of visual interest are the areaswhere the most densely packed points of visual fixation are grouped(see Figure 2). When no auditory stimulus is presented toparticipants, there will be no significant difference betweensynaesthetes and non-synaesthetes for baseline measurement of thenumber of areas of visual interest. In contrast, synaesthetes willhave a significantly higher number of areas of visual interest thannon-synaesthetes when auditory stimulus is presented.

Figure 2: A schematic showing a areas ofvisual interest.

Discussion

When viewing the photograph alone duringbaseline measurements, both synaesthetes and non-synaesthetes willshow a similar number of visual fixations. This indicates that thereare no inherent differences between synaesthetes and non-synaesthetesthat should cause them to have different patterns of eye movementswithout the extra sensory stimulation. When the musical stimulus isalso presented, however, synaesthetes will exhibit significantly morevisual points than non-synaesthetes as well as a significantly highernumber of areas of visual interest than non-synaesthetes.

Although the relationship implied by thisstudy is correlational and not a cause-and-effect relationship, it isassumed that synaesthetes show extra visual points and extra areas ofvisual interest because they experience the visual stimulus, commonto both groups, as well as visual sensations induced by the auditorystimulus. This supports Richardson's theory that all subjectiveexperiences will produce observable behaviors (Richardson, 1999). Thesubjective synaesthetic experience will in fact induce an observablebehavior, described in this study as patterns of eyemovements.

These results will add to the empiricalevidence that synaesthesia is a real phenomenon. Previously, the onlyobjective means of measuring sensory experience was through brainimaging. Eye tracking is a less expensive and more accessible methodof objectively identifying synaesthesia than brain imagingtechniques.

Tracking eye movements can also make theidentification of color-hearing synaesthesia in individuals moresystematic. Currently, identification of synaesthesia is based solelyon subjective reports. I dentification may be possible through theuse of brain imaging, but these techniques are expensive and, in manycircumstances, inaccessible and unrealistic. The compressed videoimages technique of tracking eye movements is a relativelyinexpensive alternative with the necessary PC software available forless than $1,000 (Mulligan& Beutter, 2002). This technique wouldalso make it easier to identify synaesthesia in people who cannotgive subjective reports, as in the cases of young children or adultswithout speech.

Accurate and inexpensive identification ofsynaesthesia can benefit scientists who research the phenomenon.Because synaesthetic research participants are often paid, it ispossible that non-synaesthetes can claim to have synaesthesia andparticipate in research for the money. Using eye-movement tracking toidentify color-hearing synaesthesia can prevent this relativelyeasily and inexpensively, which would in turn help researchers to besure that they are actually dealing with color-hearing synaestheticparticipants.

Because the study of synaesthesia is stillrelatively new, several areas of research are still open forscientists to pursue. It would be interesting to see research donecomparing the eye movements of different synaesthetes. Synaesthesiais currently described as being unique in experience from onesynaesthete to another, but this is based on the subjective reportsof synaesthetes (Kher, 2001). Objective observations through the useof eye-tracking could provide empirical data that would eithersupport the current theory that synaesthesia is unique to eachindividual or show that there are in fact systematic similaritiesbetween individual color-hearing synaesthetes that could not havebeen previously recognized.

The use of eye-tracking as a means ofidentifying synaesthesia could also help researchers to determinewhen synaesthesia develops in synaesthetic individuals. Sincesynaesthesia is proposed to be inherited from synaesthetic parents,further research could include longitudinal studies of synaesthetes'offspring to determine when in life their children developsynaesthesia (Cytowic,1995).

Also of interest for future research wouldbe the adaptiveness of synaesthesia. How does synaesthesia givesynaesthetic individuals an advantage over non-synaesthetes? Or, ifCytowic's theory that synaesthesia is a trait that humans haveevolved away from, what advantage do non-synaesthetes have oversynaesthetes? This is a topic that has received little attentionbecause there is no empirical data on the subject. Research comparingsynaesthetes' and non-synaesthetes' abilities to perform differenttasks may show minor differences.

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