A possible human homologue of the macaque V6A

Abstract

We have recently mapped the retinotopic organization of the area V6 in the human parieto-occipital sulcus thanks to a wide-field retinotopic stimulation, up to 168 degrees (Pitzalis et al., 2004). In monkey, dorsal and anterior to V6 there is the visuomotor area V6A. Monkey V6A is retinotopically less organized than V6, showing only a rough tendency for the eccentricity to increase dorso-ventrally in both the anterior bank of POS and the precuneate cortex (Galletti et al. 1996, 1999a). Visuomotor neurons in V6A have been found to respond more during reaches than during saccades to visual targets. Here we used phase-encoded fMRI, cortical flattening and wide-filed retinotopic mapping to detect the presence in the human POS of a visual area having retinotopic properties similar to those of the macaque area V6A. We also combined event-related fMRI with cortical surface reconstruction, in order to isolate movement-related activity for reaching and saccades in relation to the individual sulcal anatomy. We found an area in the POS located just anteriorly to V6 and medial to V3A which has an inconsistent polar angle representation but a clear trend in the eccentricity maps. In agreement with functional data on monkey V6A, this region of the human brain shows a marginal effect of cue location (contralateral side slightly preferred) and a transient activity during reaching trials stronger than during saccade trials. We propose that this region may be the putative homologous of area V6A.