Cone Inputs to Simple and Complex Cells in V1 of Awake Macaque

Rules by which V1 neurons combine signals originating in the cone photoreceptors are poorly understood. We measured cone inputs to V1 neurons in awake, fixating monkeys with white-noise analysis techniques that reveal properties of light responses not revealed by purely linear models used in previous studies. Simple cells were studied by spike-triggered averaging that is robust to static nonlinearities in spike generation. This analysis revealed, among heterogeneously tuned neurons, two relatively discrete categories: one with opponent L- and M-cone weights and another with nonopponent cone weights. Complex cells were studied by spike-triggered covariance, which identifies features in the stimulus sequence that trigger spikes in neurons with receptive fields containing multiple linear subunits that combine nonlinearly. All complex cells responded to nonopponent stimulus modulations. Although some complex cells responded to cone-opponent stimulus modulations too, none exhibited the pure opponent sensitivity observed in many simple cells. These results extend the findings on distinctions between simple and complex cell chromatic tuning observed in previous studies in anesthetized monkeys.

Space-Time Maps and Two-Bar Interactions of Different Classes of Direction-Selective Cells in Macaque V-1

We used one-dimensional sparse noise stimuli to generate first-order spatiotemporal maps and second-order two-bar interaction maps for 65 simple and 124 complex direction-selective cells in alert macaque V1. Spatial and temporal phase differences between light and dark space-time maps clearly distinguished simple and complex cell populations. Complex cells usually showed similar direction preferences to light and dark bars, but many of the directional simple cells were much more direction selective to one sign of contrast than the reverse. We show that this is predicted by a simple energy model. Some of the direction-selective simple cells showed multiple space-time-slanted subregions, but others (previously described as S1 cells) had space-time maps that looked like just one subregion of an ordinary simple cell. Both simple and complex cells showed directional interactions (nonlinearities) to pairs of flashed bars (a 2-bar apparent-motion stimulus). The space-time slant of the simple cells correlated with the optimum d X/d T (velocity) of the paired-bar interactions. Some complex cells also showed a space-time slant; the direction of the slant usually correlated with the preferred direction of motion, but the degree of slant correlated with the inferred velocity tuning only when measured by a weighted-centroid calculation. Principal components analysis of the simple-cell space-time maps yielded one fast temporally biphasic component and a slower temporally monophasic component. We saw no consistent pattern for the spatial phase of the components, unlike previous reports; however, we show that principal components analysis may not distinguish between spatial offsets and phase offsets.

Suppression outside the classical cortical receptive field

The important visual stimulus parameters for a given cell are defined by the classical receptive field (CRF). However, cells are also influenced by visual stimuli presented in areas surrounding the CRF. The experiments described here were conducted to determine the incidence and nature of CRF surround influences in the primary visual cortex. From extracellular recordings in the cat's striate cortex, we find that for over half of the cells investigated (56%, 153/271), the effect of stimulation in the surround of the CRF is to suppress the neuron's activity by at least 10% compared to the response to a grating presented within the CRF alone. For the remainder of the cells, the interactions were minimal and a few were of a facilitatory nature. In this paper, we focus on the suppressive interactions. Simple and complex cell types exhibit equal incidences of surround suppression. Suppression is observed for cells in all layers, and its degree is strongly correlated between the two eyes for binocular neurons. These results show that surround suppression is a prevalent form of inhibition and may play an important role in visual processing.