Mechanisms underlying the representation of angles embedded within contour stimuli in area V2 of macaque monkeys

We previously found that surprisingly many V2 neurons showed selective responses to particular angles embedded within continuous contours [M. Ito & H. Komatsu (2004)Journal of Neuroscience, 24, 3313-3324]. Here, we addressed whether the selectivity is dependent on the presence of individual constituent components or on the unique combination of these components. To reveal roles of constituent half-lines in response to whole angles, we conducted a quantitative model study after the framework of cascade models. Our linear-non-linear summation model implemented a few subunits selective to particular half-lines and was fitted to neuronal responses for each neuron. The study indicates that the best-fitting models well replicate the selectivity in the majority of V2 neurons and that the angle selectivity is dependent on a linear combination of responses to individual half-line components of the angles. The implication is that optimal angles are given by a combination of two preferred half-line components and the selectivity is sharpened by introducing suppression to non-preferred half-line components, rather than a specific facilitatory interaction between two preferred half-line components. The study indicates the participation of the gain control of responsiveness according to the number of half-line components. We also showed that the selectivity to acute angles depends on a combination of responses to one preferred component and weak responses to another component. Therefore, we concluded that the angle selectivity is dependent on selective responses to individual half-line components of the angles rather than a unique combination between them, whereas neurons could be selective to various angle widths at area V2.