The cortical representation of foveal stimuli: evidence from quadrantanopia and TMS-induced suppression.
To address the extent to which the visual foveal representation is split, we examined a 29-year-old patient with a lower right quadrantanopia following surgical removal of the left occipital cortex above the calcarine sulcus and compared her performance with subjects receiving transcranial magnetic stimulation (TMS) over the occipital lobes. In a letter/digit classification task, the patient responded accurately to targets presented in the upper visual field, for all horizontal eccentricities. In the lower visual field, she failed to discriminate letters from digits when targets were presented in the right, but not the left visual field (RVF and LVF, respectively). This pattern was also true for the foveal targets, with poor performance to foveal-RVF (0.5 degrees to the right of fixation) but not foveal-LVF (0.5 degrees to the left of fixation) targets. Similar patterns of normal performance to LVF but not RVF or foveal-RVF targets were observed in a group of nine normal observers when TMS was applied over their left occipital cortex. Complementary impairments to LVF and foveal-LVF target classification were induced with TMS over the right occipital cortex. Thus, we have induced an hemianopic pattern in normal observers contralateral to the magnetically stimulated hemisphere. This correspondence between real and TMS-induced visual field defects is further evidence, in neurologically intact subjects, that the cortical representation of the fovea is split between the two hemispheres along the vertical meridian.
Chiang
Tzu-Ching
TC
Institute of Cognitive Neuroscience and Department of Psychology, University College London, London, UK.
Walsh
Vincent
V
Lavidor
Michal
M
D000328:Adult; D016009:Chi-Square Distribution; D004558:Electric Stimulation; D005260:Female; D005584:Fovea Centralis / Q000503:physiopathology / Q000528:radiation effects; D007839:Functional Laterality / Q000502:physiology / Q000528:radiation effects; D006423:Hemianopsia / Q000503:physiopathology* / Q000601:surgery; D006801:Humans; D007266:Inhibition, Psychological; D008280:Magnetics; D016571:Neural Networks, Computer; D009778:Occipital Lobe / Q000503:physiopathology / Q000528:radiation effects* / Q000601:surgery; D010775:Photic Stimulation / Q000379:methods; D011930:Reaction Time / Q000187:drug effects; D013997:Time Factors; D014794:Visual Fields / Q000502:physiology / Q000528:radiation effects*; D014796:Visual Perception / Q000502:physiology / Q000528:radiation effects*