The Neural Substrates of Synesthesia - Literature review Example

Synaesthesia differs from most other neuropsychological conditions in that it is a positive symptom, i.e., it is defined by the presence of a trait not found in other members of the population rather than by the absence of a function, as in neglect, amnesia, aphasia, and so on (Ward & Mattingly, 2006). Functional neuroimaging studies using positron emission tomography (PET) and functionalmagnetic resonance imaging (fMRI) have demonstrated significant differences between the brains of synaesthetes and non-synaesthetes. At the neurophysiological level, the literature suggests that synaesthetic experience arise from a failure of neural pruning or some form of disinhibition. According to research by Semir Zeki and colleagues, area V4 appears to be active when looking at colored images, but not when that same image is perceived in shades of gray (Zeki, 1993). Perhaps the same is true of synaesthetes who perceive 0 and 1 as white or black. The neural substrates of synesthesia have been thoroughly studied in grapheme-color synesthesia (in which numbers and letters evoke colors) using both psychophysical tests and functional imaging. At the neurophysiological level, the literature suggests that synaesthetic experience arise from a failure of neural pruning or some form of disinhibition, as proposed by Hubbard and Ramachandran in 2005. Neural pruning is one of the key mechanisms of synaptic plasticity, in which connections between brain regions are partially eliminated with development and specialization. Ramachandran and Hubbard proposed that synesthesia results from an excess of neural connections between associated modalities, possibly due to decreased neural pruning between (typically adjacent) regions that are interconnected in the fetus (Hubbard & Ramachandran, 2005). In the case of synaesthesia, since regions involved in the identification of letters and numbers lie adjacent to area V4, and a failure in the normal process of pruning in these brain regions (e.g. between area V4 and then number area in the fusiform gyrus) may evoke the experience of seeing colours; hence looking at graphemes may be causing �cross-activation� of area V4 (Brang & Ramachandran, 2011). Consistent with this suggestion, a number of studies have demonstrated anatomical differences in the inferior temporal lobe near regions related to grapheme and color processing in synaesthetes, including increased fractional anisotropy (reflecting increased white matter or coherence of white matter), and increased gray matter volume; increased connectivity has been found in other forms of synesthesia as well (Brang & Ramachandran, 2011). Functional neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have demonstrated significant differences between the brains of synaesthetes and non-synaesthetes. In a paper by Sperling and colleagues, using functional neuroimaging, the authors contrasted patterns of brain activity for achromatic graphemes that elicited either color or �colourless� (i.e. greys, whites, or blacks) synaesthetic experiences, and found bilateral activity in area V4 (Sperling et al., 2006). Structural imaging studies complement the functional studies found in literature, suggesting morphometric differences in similar brain regions: synaesthetes exhibited increased gray and white matter density in the fusiform gyrus (including V4), and the parietal and primary visual cortices (Rouw et al., 2011). In research conducted by Banissy et al. in 2012, a concomitant decrease was observed in anterior regions of left FG and left MT/V5 (Banissy et al., 2012). Additionally, similar to the results of Sperling, Hubbard and Ramachandran found a correlation between behavioural and fMRI results; subjects who performed better on behavioural experiments showed larger fMRI responses in early retinotopic visual areas, including V1, V2, V3, AND hV4 (Hubbard & Ramachandran, 2005). Early research demonstrated that synaesthesia is a familial trait, and suggested that it is more common in women than in men, perhaps through an X-linked dominant mode of inheritance. Recent data suggests that the genetic mechanisms underlying synaesthesia may be more complex than the straightforward X-linked dominant account (Hubbard & Ramachandran, 2005). Given that synaesthesia is known to run in families, it has been suggested that a genetic difference may be responsible for either decreased pruning or decreased inhibition in the synaesthete brain, leading to increased activation in colour processing regions (Ramachandran & Hubbard, 2001). It is very interesting to note that the letters o and i and the numbers 0 and 1 are relatively frequently seen as black or white. Looking at the literature, it can be seen that research has not yet been conducted to determine why this is. "I havent yet considered why synaesthetes have the particular associations that they do. Why do most synaesthetes think that 0 is white?" (Ward, 2008). Circular and elongated rectangular shapes are fundamentally recognized by the receptive fields in the visual system. The graphemes 1 and 0 evoking black and white colours in many synaesthetes may be due to this fundamental orientation sensitivity in early visual areas. According to research by Semir Zeki and colleagues, area V4 appears to be active when looking at colored images, but not when that same image is perceived in shades of gray (Zeki, 1993). Perhaps the same is true of synaesthetes who perceive 0 and 1 as white or black. If synaesthetes perceiving graphemes 1 and 0 as black or white in colour is related to the receptive fields of the LGN and retinal ganglion cells being orientation selective, most processing would occur before area V4 and other brain regions responsible for color. Thus the level of activity may be lower in area V4 for those specific graphemes, which should be looked at when conducting future research. Much of what is known about cortical receptive fields comes from Hubel and Wiesel during their 25 years of collaboration. They recorded the activity of cortical neurons while displaying patterned stimuli, mainly line segments and spots, and varied the positions of the stimuli on a screen to find their receptive fields. They also first discovered two different types of V1 cells that are selective to stimuli of different spatial orientations, and termed these Simple and Complex cells, which may play key roles in synaesthetic experience of 0/Os and 1/Is appearing “colourless”. Since this phenomenon runs in families, it is tempting to speculate that a genetic mutation may cause excessive proliferation (or defective pruning) of neural connections between adjacent brain maps (Ramachandran & Hubbard, 2001). Synesthesia tends to be a unidirectional phenomenon such that numbers evoke colors but colors wont typically evoke the concept of numbers (Brang & Ramachandran, 2011). This prospect of unidirectionality should constrain theories on the types of anatomical and functional connections that mitigate synesthetic experiences, (perhaps simple cell activation in instances of black and white colored graphemes). Area V4 has been found to be active in both word-colour and grapheme-colour synaesthesia; however, neuroimaging technology does not have the spatial and temporal resolution to distinguish between pruning and disinhibition feedback theories. Future studies investigating the types of anatomical and/or functional connections that mitigate synesthesia should aim to clarify this matter. The study of synaesthesia is likely to yield important insights into intra-modal and cross-modal perception; perceptual awareness; brain development and plasticity; the way that perception interacts with language and memory; and individual differences of cognition more generally (Ward & Mattingley, 2006). Banissy, M. J., Stewart, L., Muggleton, N. G., Griffiths, T. D., Walsh, V. Y., Ward, J., & Kanai, R. (2012). Brang, D., Ramachandran, V.S. (2011). Survival of the Synesthesia Gene: Why Do People Hear Colors and Taste Words? PLoS Biology, 9(11): e1001205. doi:10.1371/journal.pbio.1001205 Brang, D., Ramachandran, V.S. (2011). Survival of the Synesthesia Gene: Why Do People Hear Colors and Taste Words? PLoS Biology, 9(11): e1001205. doi:10.1371/journal.pbio.1001205 Hubbard, E., & Ramachandran, V. S. (2005). Neurocognitive mechanisms of synesthesia. Neuron, 48, 509-520. doi: 10.1016/j.neuron.2005.10.012 Ramachandran, V. S., & Hubbard, E. M. (2001). Psychophysical investigations into the neural basis of synaesthesia. Neural basis of synaesthesia, 268, 979-983. doi: 10.1098/rspb.2000.1576 Sperling, J. M., Pruvulovic, D., Linden, D. E. J., Singer, W., & Stirn, A. (2006). Neuronal correlates of colour grapheme synaesthesia: A fmri study. Cortex, 42, 295-303. Ward, J., & Mattingley, J. B. (2006). Synaesthesia: An overview of contemporary findings and controversies. Cortex, 42, 129-136. Ward, J. (2008). The frog who croaked blue. (pp.58-86). New York: Routledge. Zeki, S. (1993). A vision of the brain. Oxford: Blackwell Read More