Synaesthesia Honours Project: Review of Functional connectivity study on auditory-visual synesthetes

Research to read for today is:

Disinhibited feedback as a cause of synesthesia: Evidence from a functional connectivity study on auditory-visual synesthetes (Neufeld et al., 2012)

J. Neufeld, C. Sinke, M. Zedler, W. Dillo, H.M. Emrich, S. Bleich, G.R. Szycik

In this research scientist tested auditory-visual synaesthetes and non-synaesthetes with magnetic resonance imaging (fMRI) and non-linguistic sound stimuli. For synaesthetes increased activation has been shown in the left inferior parietal cortex (IPC) – an area involved in multimodal integration, feature binding and attention guidance. No significant group-differences were detected in V4 area – related to colour vision and form processing. Conclusion of the test is that parietal cortex (IPC) act as sensory nexus area in auditory-visual synaesthesia and as a common neural correlate for different types of synaesthesia.

Experiment:

Stimuli: tones and chords. 6 different sound classes: major, minor and dissonant piano chords and pure piano, sine and bassoon tones. Duration: 2 seconds. Interval of 13 seconds
Concurrent: colour or (mostly three-dimensional) coloured shapes moving in space

Hypothesis: if synaesthesia was mediated by direct cross-activation, increased connectivity between the AC and at least one area of concurrent representation (e.g., the visual cortex) would be expected. If disinhibited feedback from IPC leads to synaesthesia, such increased connectivity should not be found (Neufeld et al., 2012, pp. 1472)

Subjects: Fourteen auditory-visual synaesthetes and control group of fourteen non-synaesthetes. Matched for age, IQ sex, musical expertise (determined by Ollen Musical Sophistication Index) (Ollen, 2006) and years of musical lessons.

Test: 36 different tones. Pseudo-random order. Presented via pneumatic headphones in an event-related design in three sessions. 48 stimuli per session.

Subjects had to indicate colour related to tones presented from different instruments and with different pitches. Non-synaesthes were asked to choose colour that they believed fit the tone best.

After three presentations of the stimuli (in random order), the geometric distance in the RGB colour space of the colour choices indicated by subject for each item, were calculated. Responses from synaesthetes were more consistent in comparison to non-synaesthetes. All synaesthetes were identified as ‘associators’.

Synesthetes reported strong synaesthesia (with open and closed eyes). Non-synaesthetes reported no synaesthetic sensations.

Synaesthetes also reported additional synaesthetic sensation caused by sound of fMRI scanner.

Functional connectivity analysis was based on hypothesis that if two regions interact within a network, their activity patterns should be strongly correlated and has to examine how different brain areas work together during the perception of stimuli in synaesthesia.

Analysis revealed one significant cluster in the inferior parietal cortex (IPC) for the main effect group (this area exhibited stronger activation in synaesthetes compared to control group). And two clusters in the left and right auditory cortex (AC) for the main effect ‘stimulation’.

Second step of analysis was focused on hypotheses regarding the connectivity differences between the controls and syneasthetes during the perception of auditory stimulation.

Results:

No significant differences between synaesthetes and control group in the number of correctly identified stimuli or reaction times in the tone-chord discrimination task conducted during fMRI.

No increased connectivity in the control group compared to synesthetes in any region or any of the seed areas.

Hypothesis about stronger connectivity between the AC and visual cortex in synaesthetes was not confirmed. Right AC merely exhibited increased activity while left AC showed no differences between the groups.

Main result of the study: stronger connectivity of the left IPC to both the left primary auditory cortex and the right primarily visual cortex in the synaesthetes. Processing of auditory stimuli in synaesthetes is accompanied by a stronger interaction between these areas.

Findings support the disinhibited feedback model of synaesthesia. No evidence to support the cross-activation model.

The lack of increased connectivity between the IPC and V4 may be explained by analyse of different form of synaesthesia.

Moreover, individual synaesthetes might perceive their synaesthetic sensations differently (like different spatial locations) within one form of synaesthesia.

Figure 1 Acoustically induced synesthetic photisms. The photisms induced by single tones (sine, violin and guitar) in A, which were painted by three different synesthetes, are shown exemplarily. The photisms were perceived in 3D, and the forms changed according to the mounting and fading of the tone. The arrows indicate the direction of movement. (Neufeld et al., 2012, pp. 1473)

Figure 2 Significant group differences in functional connectivity of the IPC. The connectivity analysis revealed two brain areas exhibiting significantly more connectivity to the inferior parietal cortex (IPC) in synesthetes compared to controls (p < 0.05, corrected for multiple comparisons): (A) a cluster in the left temporal cortex (center of mass at MNI coordinates xyz = −40, −26, 8), identified as the primary auditory cortex (BA 41) and (B) a cluster in the left occipital cortex (center of mass at MNI coordinates xyz = 12, −94, −6), identified as the primary visual cortex (BA 17). p = posterior, a = anterior, r = right, l = left, color bars indicate the strength of activation. (Neufeld et al., 2012, pp. 1473)

Synaesthesia explanation, theories etc. once again:

Stimulus (stimuli), inducer – source of synaesthesia like e.g. sound

Concurrent – internally generated sensation e.g. objects visualisation

Synaesthetes can be categorised as:

- Associators: experience synaesthesia ‘in their minds’ eye

- Projectors: experience concurrents externally

Three main classes of neuropsychological theories:

- Cross-activation model: direct linkage between areas of inducer-concurrent representation

- Re-entrant feedback model: crosstalk between inducer and concurrent brain areas with additional feedback from higher-level areas

- Disinhibited feedback model: unusual activation of the concurrent-related brain areas caused by the disinhibition of feedback to these areas from ‘multi-sensory nexus’ area e.g. parietal cortex.

Majority of knowledge comes from most common type: grapheme-colour synaesthesia. Recent researches in this area extended cross activation theory in the form of two-stage model and cross-tuning model of synaesthesia.

Also, there is evidence that grapheme-colour synaesthesia involves spatially adjacent brain areas responsible for colour processing and grapheme representation (V4) (Brang, Hubbard, Coulson, Huang, & Ramachadran, 2010; Hubbard, Arman, Ramachadran, & Boynton, 2005; Nunn et al., 2002). In recent study with fMRI colour processing centers were identified individually in each participant – which challenge previously mentioned findings. The spatial proximity of the two areas suggest direct cross-activation as the mechanism responsible for grapheme-colour synaesthesia.

Other studies with fMRI support additional involvement of the parietal cortex which speaks against cross-activation as the only mechanism in synaesthesia.

In auditory-visual synaesthesia (acoustic stimulation leads to a visual experience) additional mechanisms apart from cross-activation could be responsible.

Current adequate explanation suggests combined model of cross-activation together with parietal ‘hyperbinding’ mechanism (Esterman, Verstynen, Ivry, & Robertson, 2006):

Concurrent is evoked directly by the activation of areas that process the inducing stimuli

Inducer and concurrent are bound together to form a holistic experience by parietal modulating mechanisms in a second step (Hubbard, 2007)

Left parietal cortex was identified as a major hub region which was more functionally incorrect in synaesthetes has been seen in recent EEG study on synaesthetes with visual concurrent evoked by spoken words and letter (Jäncke & Langer, 2011). These studies suggest a well-distributed network as relevant for synesthetic colour perception.

References:

Neufeld, J., Sinke, C., Zedler, M., Dillo, W., Emrich, H., Bleich, S. and Szycik, G. (2012). Disinhibited feedback as a cause of synesthesia: Evidence from a functional connectivity study on auditory-visual synesthetes. Neuropsychologia, 50(7), p.1472.

Bibliography:

Neufeld, Sinke, Dillo, Emrich, Szycik, Dima, . . . Zedler. (2011). The neural correlates of coloured music: A functional MRI investigation of auditory–visual synaesthesia. Neuropsychologia, 50(1), 85-89.

Brang, D., Hubbard, E. M., Coulson, S., Huang, M., & Ramachandran, V. S. (2010). Magnetoencephalography reveals early activation of V4 in grapheme-color synesthesia. Neuroimage, 53, 268–274.

Esterman, M., Verstynen, T., Ivry, R. B., & Robertson, L. C. (2006). Coming unbound: Disrupting automatic integration of synesthetic color and graphemes by transcranial magnetic stimulation of the right parietal lobe. Journal of Cognitive Neuroscience, 18, 1570–1576.

Hubbard, E. M. (2007). Neurophysiology of synesthesia. Current Psychiatry Reports, 9, 193–199.

Hubbard, E. M.,Arman,A. C., Ramachandran,V. S., & Boynton, G. M.(2005). Individual differences among grapheme-color synesthetes: Brain-behavior correlations. Neuron, 45, 975–985

Jäncke, L., & Langer, N. (2011). A strong parietal hub in the small-world network of coloured-hearing synaesthetes during resting state EEG. Journal of Neuropsychology, 5, 178–202.

Nunn, J. A., Gregory, L. J., Brammer, M., Williams, S. C., Parslow, D. M., Morgan, M. J., et al. (2002). Functional magnetic resonance imaging of synesthesia: Activation of V4/V8 by spoken words. Nature Neuroscience, 5, 371–375.

Ollen, J. E. (2006). A criterion-related validity test of selected indicators of musical sophistication using expert ratings. Ohio: Ohio State University

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