title: Enhanced cortical excitability in grapheme-color synesthesia and its modulation.
abstract: Synesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions. Previous research suggests that synesthetes display enhanced modality-specific perceptual processing, but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. In three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia. Using transcranial magnetic stimulation, we show that synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. We next used transcranial direct current stimulation to discriminate between two competing hypotheses of the role of hyperexcitability in the expression of synesthesia. We demonstrate that synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. A control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia. These results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms.
authors.affiliation: Department of Experimental Psychology, University of Oxford, Oxford OX1 2JD, UK. devin.terhune@psy.ox.ac.uk
authors.forename: Devin Blair
authors.initials: DB
authors.lastname: Terhune
authors.forename: Sarah
authors.initials: S
authors.lastname: Tai
authors.forename: Alan
authors.initials: A
authors.lastname: Cowey
authors.forename: Tudor
authors.initials: T
authors.lastname: Popescu
authors.forename: Roi
authors.initials: R
authors.lastname: Cohen Kadosh
mesh_terms: D003118:Color Perception / Q000502:physiology*; D046690:Deep Brain Stimulation; D006801:Humans; D010468:Perceptual Disorders / Q000503:physiopathology* / Q000628:therapy*; D010719:Phosphenes / Q000502:physiology; D050781:Transcranial Magnetic Stimulation; D014793:Visual Cortex / Q000502:physiology*
references.citation: Cortex. 2006 Feb;42(2):264-80
references.pmid: 16683501
references.citation: Nat Neurosci. 2007 Jun;10(6):671-2
references.pmid: 17525757
references.citation: Neuron. 2005 Nov 3;48(3):509-20
references.pmid: 16269367
references.citation: Behav Brain Res. 2011 Sep 30;223(1):48-52
references.pmid: 21504763
references.citation: Brain Stimul. 2008 Jul;1(3):206-23
references.pmid: 20633386
references.citation: Nat Neurosci. 2006 Jun;9(6):740-2
references.pmid: 16699505
references.citation: Cortex. 2006 Feb;42(2):175-83
references.pmid: 16683491
references.citation: J Cogn Neurosci. 2004 May;16(4):521-7
references.pmid: 15165345
references.citation: Neuroimage. 2008 Nov 1;43(2):192-203
references.pmid: 18692578
references.citation: Cognition. 2005 Nov;98(1):53-84
references.pmid: 16297676
references.citation: J Neurosci. 2010 May 5;30(18):6205-13
references.pmid: 20445046
references.citation: Neuroscientist. 2011 Aug;17(4):368-81
references.pmid: 21311054
references.citation: Cereb Cortex. 2010 Sep;20(9):2166-71
references.pmid: 20051361
references.citation: Neuropsychologia. 2010 Aug;48(10):2789-810
references.pmid: 20542047
references.citation: Q J Exp Psychol (Hove). 2007 May;60(5):681-95
references.pmid: 17455076
references.citation: Neuron. 2005 Mar 24;45(6):975-85
references.pmid: 15797557
references.citation: Mem Cognit. 1982 Jul;10(4):389-95
references.pmid: 7132716
references.citation: Neuroscientist. 2012 Jun;18(3):208-15
references.pmid: 21571722
references.citation: Am J Hum Genet. 2009 Feb;84(2):279-85
references.pmid: 19200526
references.citation: J Neurosci. 2009 Jun 3;29(22):7271-7
references.pmid: 19494149
references.citation: Brain. 2009 Jan;132(Pt 1):65-70
references.pmid: 19028762
references.citation: Trends Cogn Sci. 2007 Apr;11(4):177-84
references.pmid: 17331789
references.citation: Exp Brain Res. 2009 Jul;196(4):565-71
references.pmid: 19533108
references.citation: Neuropsychologia. 2011 Apr;49(5):1355-1358
references.pmid: 21219918
references.citation: Neuroscientist. 2011 Feb;17(1):37-53
references.pmid: 21343407
references.citation: Neuroimage. 2010 Oct 15;53(1):268-74
references.pmid: 20547226
references.citation: Dev Psychol. 2009 Jan;45(1):151-9
references.pmid: 19209998
references.citation: Neuropsychologia. 2001;39(4):415-9
references.pmid: 11164880
references.citation: Cogn Neuropsychol. 2005;22(8):1069-85
references.pmid: 21038290
references.citation: Trends Cogn Sci. 2006 May;10(5):204-11
references.pmid: 16603406
references.citation: Nat Rev Neurosci. 2002 Jan;3(1):43-52
references.pmid: 11823804
references.citation: Eur J Neurosci. 2009 Apr;29(8):1703-10
references.pmid: 19419432
references.citation: Eur J Neurosci. 2009 Mar;29(6):1287-93
references.pmid: 19302164
references.citation: J Neuropsychol. 2011 Sep;5(2):214-42
references.pmid: 21923787
references.citation: Neuroimage. 2008 Nov 15;43(3):605-13
references.pmid: 18723094
references.citation: Nat Neurosci. 2007 Jun;10(6):792-7
references.pmid: 17515901
references.citation: Trends Neurosci. 2008 Jul;31(7):335-42
references.pmid: 18550184