Single unit studies in the visual cortex of rodents.

This paper deals with the visual topography in the rat and hamster visual cortex, and with receptive field maps and trigger features of visual units in rat visual cortex. The visuotopic organization of the primary visual cortex (area 17) and the extrastriate visual regions surrounding it (areas 18a and 18) was studied in gray rats and hamsters using standard microelectrode techniques. The results confirm and extend previous observations in the rat and reveal in hamster an organization more complex than previously reported. In the rat, apart from the representation of the contralateral visual field (VF) in area 17, in which the upper VF is represented caudally and the nasal VF laterally, there are additional representations of the VF in the extrastriate cortex. In lateral extrastriate cortex (area 18a) there are at least four such representations, namely latero-medial (LM), antero lateral (AL), latero intermediate (LI) and latero-lateral (LL). In LM (second visual area) the upper VF is represented caudally at the nasal FV, medially, being thus a mirror image of V1. In AL (third visual area) the upper VF is represented rostrally and the nasal VF, medially, being thus a mirror image of LM. In LI the upper VF is medial, and the nasal VF, lateral, being thus a mirror image of LM, or a reduced copy of V1. In LL, the upper VF is caudal and the nasal VF, medial, being thus a mirror image of LI, or a reduced copy of LM. In medial extrastriate cortex (area 18) there are two representations of the temporal VF, labelled antero-medial (AM) and postero medial (PM). In AM, the upper temporal VF is medial and the lower temporal VF, lateral, the extreme temporal field being rostral. The 30 degree azimuth provides the boundary between AM and PM. Thus, AM is organized as a counter clock wise rotation by 90 degrees of the V1 representation. In PM, the upper lower VF topography is like in AM, but the extreme temporal VF is caudal, being thus a mirror image of AM. Our preliminary study of visual topography is hamster shows an organization at least as elaborate as that of rat. Finally, we have compared RFs of striate vs extrastriate units. Our extrastriate units showed similar trigger features and field maps as in V1, but with larger RF sizes and a proportion of rapidly habituating cells. The percent distribution of field types, at least in LM and AM was similar to that in V1.