[Show abstract][Hide abstract] ABSTRACT: Although it is estimated that as many as 4% of people experience some form of enhanced cross talk between (or within) the senses, known as synaesthesia, very little is understood about the level of information processing required to induce a synaesthetic experience. In work presented here, we used a well-known multisensory illusion called the McGurk effect to show that synaesthesia is driven by late, perceptual processing, rather than early, unisensory processing. Specifically, we tested 9 linguistic-color synaesthetes and found that the colors induced by spoken words are related to what is perceived (i.e., the illusory combination of audio and visual inputs) and not to the auditory component alone. Our findings indicate that color-speech synaesthesia is triggered only when a significant amount of information processing has occurred and that early sensory activation is not directly linked to the synaesthetic experience.
[Show abstract][Hide abstract] ABSTRACT: The majority of studies support a role of the anterior cingulate cortex (ACC) in the attentional control necessary for conflict resolution in the Stroop task; however, the time course of activation and the neural substrates underlying the Stroop task remain contentious. We used high-density EEG to record visual-evoked potentials from 16 healthy subjects while performing a manual version of the traditional Stroop colour-word task. Difference waveforms for congruent-control and incongruent-control conditions were similar in amplitude and had a similar spatial distribution in the time window of 260-430 ms post stimulus onset. Source estimation indicated particularly middle cingulate involvement in congruent-control and incongruent-control difference waveforms. In contrast, the difference waveform for the incongruent-congruent contrast was observed later (in the time window of 370-480 ms), had a different spatial distribution, and source estimation indicated that the anterior cingulate underlies this difference waveform. As congruent-control and incongruent-control differences have a similar timeframe and cingulate source, we propose that this indicates early attentional allocation processes. That is, the identification of two sources of information (the word and the colour it is printed in) and the selective attention to one. The later peak in the incongruent-congruent difference wave, originating in anterior cingulate, likely reflects identification (and subsequent resolution) of conflict in the two sources of information.
Brain research 01/2009; 1253:139-48. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In one of the most common forms of synaesthesia, linguistic-colour synaesthesia, colour is induced by stimuli such as numbers, letters, days of the week, and months of the year. It is not clear, however, whether linguistic-colour synaesthesia is determined more by higher level semantic information--that is, word meaning--or by lower level grapheme or phoneme structure. To explore this issue, we tested whether colour is consistently induced by grapheme or phoneme form or word meaning in bilingual and trilingual linguistic-colour synaesthetes. We reasoned that if the induced colour was related to word meaning, rather than to the acoustic or visual properties of the words, then the induced colours would remain consistent across languages. We found that colours were not consistently related to word meaning across languages. Instead, induced colours were more related to form properties of the word across languages, particularly visual structure. However, the type of inducing stimulus influenced specific colour associations. For example, colours to months of the year were more consistent across languages than were colours to numbers or days of the week. Furthermore, the effect of inducing stimuli was also associated with the age of acquisition of additional languages. Our findings are discussed with reference to a critical period in language acquisition on synaesthesia.
[Show abstract][Hide abstract] ABSTRACT: Although the condition known as synaesthesia is currently undergoing a scientific resurgence, to date the literature has largely focused on the heterogeneous nature of synaesthesia across individuals. In order to provide a better understanding of synaesthesia, however, general characteristics need to be investigated. Synaesthetic experiences are often described as occurring 'internally' or in the 'mind's eye', which is remarkably similar to how we would describe our experience of visual mental imagery. We assessed the role of visual imagery in synaesthesia by sampling a large group of synaesthetes and found that they report experiencing more vivid mental images than controls. These findings have important implications for our general understanding of synaesthesia and, in particular, emphasize the need to control for visual imagery in behavioural and neuroimaging paradigms.
Consciousness and Cognition 10/2008; 17(3):1032-9. · 2.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synesthesia is a condition where stimulation of a single sensory modality or processing stream elicits an idiosyncratic, yet reliable perception in one or more other modalities or streams. Various models have been proposed to explain synesthesia, which have in common aberrant cross-activation of one cortical area by another. This has been observed directly in cases of linguistic-color synesthesia as cross-activation of the 'color area', V4, by stimulation of the grapheme area. The underlying neural substrates that mediate cross-activations in synesthesia are not well understood, however. In addition, the overall integrity of the visual system has never been assessed and it is not known whether wider differences in sensory-perceptual processing are associated with the condition. To assess whether fundamental differences in perceptual processing exist in synesthesia, we utilised high-density 128-channel electroencephalography (EEG) to measure sensory-perceptual processing using stimuli that differentially bias activation of the magnocellular and parvocellular pathways of the visual system. High and low spatial frequency gratings and luminance-contrast squares were presented to 15 synesthetes and 15 controls. We report, for the first time, early sensory-perceptual differences in synesthetes relative to non-synesthete controls in response to simple stimuli that do not elicit synesthetic color experiences. The differences are manifested in the early sensory components of the visual evoked potential (VEP) to stimuli that bias both magnocellular and parvocellular responses, but are opposite in direction, suggesting a differential effect on these two pathways. We discuss our results with reference to widespread connectivity differences as a broader phenotype of synesthesia.
[Show abstract][Hide abstract] ABSTRACT: The lateralisation of colour processing is not well understood, although there is a reasonable amount of evidence indicating a right hemisphere bias for colour processing. Tasks that require colour naming are associated with a left hemisphere bias and it is likely that asymmetry of colour processing is influenced by task demands. It is not known whether object colour knowledge is lateralised. In the current study colour and achromatic Mondrian-like objects were presented to either the left or right hemisphere to assess the lateralisation of colour processing. Participants were required to judge whether the objects were colour or achromatic. To assess colour knowledge, congruently and incongruently coloured familiar objects were presented to either the left or right hemisphere and participants were required to judge whether the objects were correctly or incorrectly coloured. The data show that both colour processing and colour knowledge are associated with a right hemisphere bias.
[Show abstract][Hide abstract] ABSTRACT: The term synaesthesia has been applied to a range of different sensory-perceptual and cognitive experiences, yet how these experiences are related to each other is not well understood. Not only are there disparate types of synaesthesia, but even within types there are vast individual differences in the way that stimuli induce synaesthesia and in the subjective synaesthetic experience. An investigation of the inheritance patterns of different types of synaesthesia is likely to elucidate whether a single underlying mechanism can explain all types. This study is the first to systematically survey all types of synaesthesia within a familial framework. We recruited 53 synaesthetes and 42% of these probands reported a first-degree relative with synaesthesia. We then directly contacted as many first-degree relatives as possible and collected complete data on synaesthetic status for all family members for 17 families. We found that different types of synaesthesia can occur within the same family and that the qualitative nature of the experience can differ between family members. Our findings strongly indicate that various types of synaesthesia are fundamentally related at the genetic level, but that the explicit associations and the individual differences between synaesthetes are influenced by other factors. Synaesthesia thus provides a good model to explore the interplay of all these factors in the development of cognitive traits in general.
[Show abstract][Hide abstract] ABSTRACT: Language anomalies and left-hemisphere dysfunction are commonly reported in schizophrenia. Additional evidence also suggests differences in the integration of information between the hemispheres. Bilateral gain is the increase in accuracy and decrease in latency that occurs when identical information is presented simultaneously to both hemispheres. This study measured bilateral gain in controls (n=20) and individuals with schizophrenia (n=10) using a lexical-decision task where word or non-word judgements were made to letter strings presented in the left visual field (LVF), right visual field (RVF) or bilaterally (BVF). Language was not abnormally lateralized in the schizophrenia group. Controls exhibited the expected decrease in latency when words were presented bilaterally. This effect was not observed in the schizophrenia group who were actually disadvantaged in this condition. The lack of bilateral gain in schizophrenia is discussed as arising from differences in the connections between areas in each hemisphere that mediate language.
Consciousness and Cognition 07/2007; 16(2):436-44. · 2.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study investigated line bisection in 10 males with schizophrenia and 15 controls. There was an overall leftward bias, consistent with slight right "pseudoneglect", but the schizophrenia group showed more variation with experimental conditions, suggesting impaired interhemispheric transfer. Specifically, the rightward bias was especially marked when the lines were positioned on the right side of the page, when the right hand was used, and when a right to left scan was adopted. The rightward bias was associated with the predominance of negative symptoms. Findings are discussed with reference to the role of the corpus callosum in the transfer of spatial attention.
[Show abstract][Hide abstract] ABSTRACT: To assess the mechanisms underlying lack of speeded information transfer asymmetry (faster right to left) for verbal information in schizophrenia.
Interhemispheric transfer times (IHTT) between the hemispheres were assessed using a lateralized lexical-decision task in males with schizophrenia (N = 12) and matched controls (N = 12). Words were presented to the left visual field (LVF), right visual field (RVF), or bilaterally (BVF) while 128-channel EEG was recorded continuously. A direct measure of IHTT in each direction was obtained by comparing the latencies of the N160 evoked potential (EP) component in the hemispheres contralateral and ipsilateral to stimulation.
Controls showed faster information transfer from the right to left hemisphere (R-to-L) for linguistic stimuli. The two groups did not differ for IHTTs L-to-R. Lack of IHTT asymmetry in the schizophrenia groups was associated with an overall concomitant decrease in the amplitude of the N160 in the right hemisphere.
Differences in IHTT asymmetry may be attributed to lack of right hemisphere activation and not callosal dysfunction as has been previously suggested.
It is suggested that a relative excess of myelinated axons in the right hemisphere speeds IHTT faster R-to-L, findings are discussed with reference to differences in right hemisphere white matter connectivity in schizophrenia.
[Show abstract][Hide abstract] ABSTRACT: While there is much evidence to suggest left hemisphere dysfunction and interhemispheric transfer deficits in schizophrenia, the right hemisphere is rarely implicated. This study uses 128-channel EEG to assess whether asymmetry of interhemispheric transfer found in normal individuals is present in those with schizophrenia, and whether this might point to a right-hemisphere dysfunction.
Simple reaction time (RT) was recorded to stimuli presented to the left visual field (LVF), right visual field (RVF) or bilaterally (BVF) in 13 males with schizophrenia and 13 controls. 128-Channel EEG was simultaneously recorded. Interhemispheric transfer time (IHTT) in each direction was calculated by comparing the latencies of N160 EP components in the hemispheres contralateral and ipsilateral to stimulation.
While controls showed faster information transfer from the right-to-left hemisphere, this asymmetry was not present in the schizophrenia group who also exhibited a concomitant decrease in the amplitude of the N160 in the right hemisphere.
Results are interpreted with reference to a loss of rapidly conducting myelinated axons in the right hemisphere in schizophrenia.
Schizophrenia Research 06/2005; 74(2-3):171-8. · 4.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study uses the Poffenberger (1912) paradigm, which compares the difference between "crossed" (stimuli and motor response areas are contralateral) and "uncrossed" (stimuli and motor response areas are ipsilateral) conditions to estimate interhemispheric transfer time. Simple reaction time (RT) was recorded to stimuli presented to the left visual field (LVF), right visual field (RVF), or bilaterally (BVF) in individuals with schizophrenia (n = 10) and controls (n = 14), who responded using either the left or right hand. While the results provide no evidence for differences between the groups in information transfer between the hemispheres, the schizophrenia group were significantly slower to respond to LVF stimuli, suggesting right hemisphere dysfunction.
[Show abstract][Hide abstract] ABSTRACT: Both reaction time (RT) and evoked potential (EP) studies have shown that interhemispheric transfer is faster from the right to the left hemisphere than vice versa. This has been explained either in terms of an asymmetry of callosal fibres or as a result of hemispheric specialization. Here we suggest that it may be due to greater activity resulting from a greater number of fast-conducting, myelinated fibres in the right hemisphere than in the left. Interhemispheric transfer times (IHTTs) were measured in 13 males by comparing latencies and amplitudes of N160 EPs ipsilateral and contralateral to checkerboard stimuli presented to the left or right visual field. IHTT estimates were obtained from three homologous electrode pairs. The shorter IHTT from right-to-left was associated with a concomitant increase in N160 negativity in the right hemisphere. There was no evidence from RTs to stimuli in each visual field to suggest that the right hemisphere was dominant for this task, suggesting that the faster speed of transfer from the right-to-left hemisphere may depend on faster axonal conduction in the right hemisphere relative to the left.
[Show abstract][Hide abstract] ABSTRACT: Five people lacking the corpus callosum (two callosotomized, three with agenesis of the corpus callosum) and neurologically normal subjects were shown vertical lines that appeared instantaneously between pairs of rectangles in one or other visual field. When one of the rectangles flashed prior to the presentation of the line, and the line was in the same visual field, all subjects perceived the line as spreading from the flashed rectangle to the other. Normal subjects and one of the callosotomized subjects showed a slight but significant right visual-field advantage, perhaps reflecting a left-hemispheric superiority in processing rapid temporal events. The illusion was also induced when the line and the flash were in opposite visual fields in one of the callosotomized, one of the acallosal subjects, and about half of the normal subjects, implying interhemispheric integration even in the absence of the corpus callosum.
[Show abstract][Hide abstract] ABSTRACT: We measured simple reaction time (RT) to light flashes, presented either singly or in pairs, in two people who had undergone callosotomy, one person with agenesis of the corpus callosum, and 17 normal subjects. The three split-brained subjects' RTs were decreased to bilateral pairs beyond predictions based on a simple race between independent unilateral processes, while those of the normal subjects were actually longer than predicted by the race model. This effect was present whether the bilateral pairs were in mirror-image locations or not, but was not present when the pairs were presented unilaterally. Since summation does not depend on close spatial correspondence, and also occurs when inputs are staggered in time, we suggest that it is due to cortical projection to a subcortical arousal system, and is normally inhibited by the corpus callosum.
Journal of Cognitive Neuroscience 12/2002; 14(8):1151-7. · 4.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In a sample of 250 healthy undergraduate students, scores on a scale of magical ideation rose to a peak at the point of ambilaterality on a scale of hand preference, and fell away with increasing right- or left-handedness. This effect mirrors that reported by Crow, Crow, Done, and Leask (1998) who found a dip in academic abilities at the point of ambilaterality, or what they call ''the point of hemispheric indecision''. We relate these findings to genetic theories of laterality in which one allele (RS+) codes for left-cerebral dominance while the other (RS-) leaves laterality to chance. RS-- homozygotes may be susceptible to a lack of dominance, resulting in a disposition to magical ideation and an increased risk of schizophrenia, but also enhanced creativity and lateral thinking.