Physiological properties of macaque V1 neurons are correlated with extracellular spike amplitude, duration, and polarity

In the lateral geniculate nucleus (LGN) the large neurons of the magnocellu lar layers are functionally distinct and anatomically segregated from the s mall neurons of the parvocellular layers. This segregation of large and sma ll cells is not maintained in the primary visual cortex (V1); instead a het erogeneous mixture of cells occurs, particularly in the output layers. Neve rtheless, our results indicate that for the middle and upper layers of V1, cell size remains a predictor of physiological properties. We recorded extr acellularly from neurons in V1 of alert monkeys and analyzed the amplitude, duration, and polarity of the action potentials of 199 cells. Of 156 cells that could be assigned to specific cortical layers, 137 (88%) were localiz ed to the middle and upper cortical layers, layer 4 and above. We summarize evidence that the large-amplitude spikes are discharged by large cells, wh ereas small-amplitude spikes are the action potentials of smaller cells. La rge spikes were predominantly negative and of longer duration, whereas smal l spikes were predominantly positive and briefer. The putative large cells had lower ongoing activity, smaller receptive field activating regions and higher selectivity for stimulus geometry and stimulus motion than the small cells. The contrasting properties of the large and the small cells were il lustrated dramatically in simultaneous recordings made from adjacent cells. Our results imply that there may be an anatomic pairing or clustering of s mall and large cells that could be integral to the functional organization of the cortex. We suggest that the small and the large cells of area V1 hav e different roles, such that the small cells may shape the properties of th e large output cells. If some of the small cells are also output cells, the n cell size should be a predictor of the type of information being sent to other brain regions. Because of their high activity and relative ease of st imulation, the small cells also may contribute disproportionately to in viv o images based on metabolic responses such as changes in blood flow.