Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness

Terhune, Devin Blair; Murray, Elizabeth; Near, Jamie; Stagg, Charlotte J.; Cowey, Alan and Cohen Kadosh, Roi. 2015. Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness. Cerebral Cortex, 25(11), pp. 4341-4350. ISSN 1047-3211 [Article]

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Abstract or Description

Phosphenes are illusory visual percepts produced by the application of transcranial magnetic stimulation to occipital cortex. Phosphene thresholds, the minimum stimulation intensity required to reliably produce phosphenes, are widely used as an index of cortical excitability. However, the neural basis of phosphene thresholds and their relationship to individual differences in visual cognition are poorly understood. Here, we investigated the neurochemical basis of phosphene perception by measuring basal GABA and glutamate levels in primary visual cortex using magnetic resonance spectroscopy. We further
examined whether phosphene thresholds would relate to the visuospatial phenomenology of grapheme-color synesthesia, a condition characterized by atypical binding and involuntary color photisms. Phosphene thresholds negatively correlated with glutamate concentrations in visual cortex, with lower thresholds associated with elevated glutamate. This relationship was robust, present in both controls and synesthetes, and exhibited neurochemical, topographic, and threshold specificity. Projector synesthetes, who experience color photisms as spatially colocalized with inducing graphemes, displayed lower phosphene thresholds than associator synesthetes, who experience photisms as internal images, with both exhibiting lower thresholds than controls. These results suggest that phosphene perception is driven by interindividual variation in glutamatergic activity
in primary visual cortex and relates to cortical processes underlying individual differences in visuospatial awareness.

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Additional Information:

This research was supported by the Cogito Foundation to D.B.T.
He is also supported by a Marie Skłodowska-Curie Intra-European
Fellowship within the 7th European Community Framework
Programme. R.C.K. is supported by the Wellcome Trust
(WT88378). Funding to pay the Open Access publication charges
for this article was provided by Wellcome Trust (WT88378)


awareness, GABA, glutamate, phosphene, synesthesia, TMS, visual perception

Departments, Centres and Research Units:

Psychology > Timing, Awareness, and Suggestion Lab


26 February 2015Published Online
November 2015Published

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Date Deposited:

31 Mar 2016 13:48

Last Modified:

12 Mar 2021 17:38

Peer Reviewed:

Yes, this version has been peer-reviewed.


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