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Hiroshi Horiguchi, Yaping Joyce Liao, Brian A Wandell, Jonathan Winawer; Intact extrastriate maps following V1 quarterfield lesion. Invest. Ophthalmol. Vis. Sci. 2014;55(13):349.
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© ARVO (1962-2015); The Authors (2016-present)
Primary visual cortex (V1) is a key cortical area in distributing signals across visual cortex. There are other pathways that bypass V1 which convey signals from the eyes to extrastriate visual areas. Here we report a case study of a 68 year old man with a lesion to one-quarter of V1 (right ventral). Our principal goal was to characterize the portions of the extrastriate maps that do not receive input from V1.
Visual function was assessed with Humphrey and Goldmann visual field testing. T1- and T2-weighted MRI scans (3T) were used to identify the lesion extent. Population Receptive Fields (pRFs) were measured using functional MRI with a moving bar stimulus paradigm (Dumoulin and Wandell 2008). The pRF method was used to identify retinotopic maps and measure the visual field coverage within the maps.
The lesion resulted in a complete loss of ventral visual field maps bilaterally (hV4 and VO-1/2) and the ventral (but not dorsal) portion of right V1/V2/V3, representing the left upper visual field. Consistent with the V1/V2/V3 lesion, visual field testing showed a complete homonymous quadrantanopia in the left upper visual field. Visual field coverage was assessed by superimposing the pRFs within a map (Winawer et al. 2010). Right V1 covered only the lower left quarterfield, as expected due to the lesion. In contrast, several extrastriate map clusters in the right hemisphere - V3-A/B, temporal-occipital (TO-1/2) , and lateral occipital (LO-1/2) - each covered the complete left hemifield. PRF size was quantified as a function of eccentricity separately for upper and lower visual field. Within each map cluster, in both upper and lower visual field, pRF size increased with eccentricity. PRF size also varied systematically between clusters, increasing from V1 to LO-1/2 to V3-A/B to TO-1/2. The sizes did not differ systematically between upper visual field and lower visual field within any of the maps.
One quarter of the visual field was not represented in V1/V2/V3, but nonetheless was intact in several extrastriate visual field map clusters. Surprisingly, pRF measures for the quarterfield in which the patient cannot see were quantitatively similar to measures for the regions in which the patient can see. Hence, pathways which bypass V1/V2/V3 do not enable the patient to see but do support intact extrastriate visual field maps.
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