title: Transcallosal inhibition dampens neural responses to high contrast stimuli in human visual cortex.
abstract: Visual cortical areas in the two hemispheres interact via the corpus callosum, but the precise role of the callosal pathway in visual processing remains controversial. Here we have investigated the function of transcallosal projections in human primary visual cortex (V1). Visual evoked potentials (VEPs) triggered by grating stimuli of different contrasts were recorded before and after functional inactivation of the occipital cortex of one hemisphere via off-line low-frequency repetitive transcranial magnetic stimulation (rTMS; 0.5 Hz stimulation for 20 min). VEPs were recorded in V1 before (T0), immediately after (T1) and 45' following the completion of rTMS (T2). We found that low-frequency rTMS had an inhibitory effect on VEPs amplitudes at all contrasts in the treated side. Remarkably, reduction of VEP amplitudes in the inhibited hemisphere at T1 was accompanied by an increase in VEP amplitudes in the contralateral side only at mid-high contrasts (50-90%). This disinhibitory effect was observed with both central and hemifield stimulation. No changes in VEP amplitudes were observed when rTMS was applied to a cortical site more anterior with respect to V1. These data provide the first evidence that a mechanism of transcallosal inhibition dampens neural responses at high contrasts in human visual cortex.
authors.affiliation: Department of Neuroscience, Unit of Neurology, Pisa University Medical School, Via Roma 55, 56126 Pisa, Italy; Department of Neuroscience, Neurology and Clinical Neurophysiology Section, Azienda Ospedaliera Universitaria Senese, Viale Bracci 1, 53100 Siena, Italy.
authors.forename: T
authors.initials: T
authors.lastname: Bocci
authors.forename: M
authors.initials: M
authors.lastname: Caleo
authors.forename: E
authors.initials: E
authors.lastname: Giorli
authors.forename: D
authors.initials: D
authors.lastname: Barloscio
authors.forename: L
authors.initials: L
authors.lastname: Maffei
authors.forename: S
authors.initials: S
authors.lastname: Rossi
authors.forename: F
authors.initials: F
authors.lastname: Sartucci
mesh_terms: D000328:Adult; D003337:Corpus Callosum / Q000502:physiology*; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D009434:Neural Pathways / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D014793:Visual Cortex / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult