ASCENDING PROJECTIONS OF SIMPLE AND COMPLEX CELLS IN LAYER-6 OF THE CAT STRIATE CORTEX

Receptive field properties vary systematically across the different la yers of the cat striate cortex. Understanding how these functional dif ferences emerge requires a precise description of the interlaminar con nections and the quality of information that they transmit. This study examines the contribution of the two physiological types of neuron in layer 6, simple and complex, to the cortical microcircuit, The approa ch was to make whole-cell recordings with dye-filled electrodes in viv o to correlate visual response property with intracortical projection pattern. The two simple cells we stained projected to layer 4, as prev iously reported (Gilbert and Wiesel, 1979; Martin and Whitteridge, 198 4). Six of the eight complex cells that we labeled projected to the su perficial layers, a pathway not previously described in the cat. The r emaining two cells targeted the infragranular layers. Layer 4 is domin ated by simple cells, whereas layers 5 and 2+3 are mainly composed of complex cells (Hubel and Wiesel, 1962; Gilbert, 1977). Hence, our resu lts indicate that the ascending projections of simple cells in layer 6 target other simple cells. In parallel, the ascending projections of a population of complex cells in layer 6 favor other complex cells. An atomical experiments in several species (Lund and Boothe, 1975; Burkha lter, 1989; Usrey and Fitzpatrick, 1996; Wiser and Callaway, 1996) had also demonstrated that layer 6 gives rise to two separate intracortic al pathways. Pooling the results of these anatomical studies with our own suggests a common feature of the laminar organization: cells that project to different intracortical targets have distinct functional ch aracteristics.