Single-unit responses to kinetic stimuli in New World monkey area V2: physiological characteristics of cue-invariant neurones

Abstract

In order to investigate the neural processes underlying figure-ground segregation on the basis of motion, we studied the responses of neurones in the second visual area (V2) of marmoset monkeys to stimuli that moved against dynamic textured backgrounds. The stimuli were either "solid" bars, which were uniformly darker or lighter than the background's average, or kinetic ("camouflaged") bars, formed by textural elements that matched the spatial and temporal modulation of the background. Camouflaged bars were rendered visible only by the coherent motion of their textural elements. Using solid bars, we subdivided the population of marmoset V2 neurones into motion-selective (uni- and bi-directional units, 73.3% of the sample) and weakly-biased (26.7%) subpopulations. The motion selective subpopulation was further subdivided into cue-invariant neurones (units which demonstrated a similar selectivity for the direction of motion of the solid and camouflaged bars) and non-cue-invariant neurones (units which showed selectivity to the direction of motion of solid bars, but had weak or pandirectional responses to camouflaged bars). Cells with cue-invariant responses to these stimuli were as common in V2 as in the primary visual area (V1; approximately 40% of the population). In V2, neurones with cue-invariant and non-cue-invariant motion selectivity formed distinct populations in terms of classical response properties: cue-invariant neurones were characterized by a sharp axis of motion selectivity and extensive length summation, while the majority of non-cue-invariant neurones had broader motion selectivity and were end-stopped. In the light of previous studies, these different constellations of classical response properties suggest a correlation with more traditionally recognized categories of V2 units and modular compartments. The responses of V2 cells to kinetic stimuli were slightly delayed relative to their responses to luminance-defined stimuli.