Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness.
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.
Department of Experimental Psychology, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Devin B
DB
Terhune
Department of Experimental Psychology, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Elizabeth
E
Murray
Douglas Mental Health University Institute and Department of Psychiatry, McGill University, Montreal, Canada.
Jamie
J
Near
Centre for Functional MRI of the Brain (FMRIB), Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Charlotte J
CJ
Stagg
Department of Experimental Psychology, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Alan
A
Cowey
Department of Experimental Psychology, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Roi
R
Cohen Kadosh
D000328:Adult; D000704:Analysis of Variance; D001364:Awareness / Q000502:physiology*; D002540:Cerebral Cortex / Q000502:physiology*; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism*; D006801:Humans; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D010468:Perceptual Disorders; D010719:Phosphenes / Q000502:physiology*; D010775:Photic Stimulation; D012684:Sensory Thresholds / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D000080311:Synesthesia; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid
GABA; TMS; awareness; glutamate; phosphene; synesthesia; visual perception