Properties of shape tuning of macaque inferior temporal neurons examined using Rapid Serial Visual Presentation

De Baene W, Premereur E, Vogels R.
J Neurophysiol. 2007 Jan 24;

We used Rapid Serial Visual Presentation (RSVP) to examine the tuning of macaque inferior temporal cortical (IT) neurons to 5 sets of 25 shapes each that varied systematically along pre-defined shape dimensions. A comparison of the RSVP technique using 100 ms presentations with that using a longer duration showed that shape preference can be determined with RSVP. Using relatively complex shapes that vary along relatively simple shape dimensions, we found that the large majority of neurons preferred extremes of the shape configuration, extending the results of a previous study using simpler shapes and a standard testing paradigm (Kayaert et al. 2005). A population analysis of the neuronal responses demonstrated that, in general, IT neurons can represent the similarities among the shapes at an ordinal level, extending a previous study that used a smaller number of shapes and a categorization task (Op de Beeck et al. 2001). However, the same analysis showed that IT neurons do not faithfully represent the physical similarities among the shapes. The responses to the two-part shapes could be predicted, virtually perfectly, from the average of the responses to the respective two parts presented in isolation. We also showed that IT neurons adapt to the stimulus distribution statistics. The neural shape discrimination improved when a shape set with a narrower stimulus range was presented, suggesting that the tuning of IT neurons is not static but adapts to the stimulus distribution statistics, at least when stimulated at a high rate with a restricted set of stimuli.

Fulltext: http://jn.physiology.org/cgi/reprint/00741.2006v1

Speed dependence of tuning to one-dimensional features in V1

Mechler F, Ohiorhenuan IE, Victor JD.
J Neurophysiol. 2007 Jan 24;

Using drifting compound grating stimuli matched in energy and frequency spectrum, we previously showed (Mechler et al. 2002) that neurons in the primary visual cortex (V1) were tuned to line-like, edge-like, and intermediate one-dimensional features. Because these compound grating stimuli were drifting, allowing for potential interaction between shape and motion, we examine here the dependence of V1 feature tuning on drift speed. We find that the feature selectivity and specificity of individual V1 neurons strongly depend on speed. A simple model explains these observations in terms of an interaction between linear filtering by the receptive field and the static nonlinearity of spike threshold, embedded in a recurrent network. While the speed-dependent behaviors in single V1 neurons preclude their acting as extractors of one-dimensional features, the population as a whole retains a representation of a full suite of features.

Fulltext: http://jn.physiology.org/cgi/reprint/00713.2006v1