Department of Brain and Cognitive Sciences, Center for Biolology and Computer Learning, Massachusetts Institute of Technology
Responses of neurons during target selection in macaque area v4
Visual scenes are usually cluttered with many objects and rarely only one object is present on a homogeneous background. As one ascends the ventral stream (from V1 to IT), thought to be involved in object recognition, the classical receptive fields (RFs) of the cells increase in size. As a result, during viewing of natural scenes usually more than one object falls within the RFs of cells necessitating mechanisms by which the objects can be disambiguated. This is particularly important during visual search where a target has to be selected among distractors. Previous studies have shown that responses in the ventral stream communicate information about the targets of interest (Chelazzi et al. 1993). The question about the nature of the processes utilized by cells to selectively respond to their inputs, however, remains open. One possibility is that the responses of the neurons are captured by the object of interest only, and as a result all other objects are ignored irrespective of their location. Another possible mechanism involves spatial modulation of the cells' RF profiles to focus on the object of interest.
We recorded the extracellular activity from neurons in V4 to study the spatial mechanisms involved in target selection. After the monkey acquired fixation, two objects appeared simultaneously on a monitor, a target and a distractor-probe. Both the locations of the target and distractor-probe were chosen randomly from a predetermined set of positions, across different trials. We could therefore map the RF profile of the neurons with the distractor-probe for different positions of the target. The target would appear both inside and outside the receptive field on different trials. The monkey was rewarded for making a saccadic eye movement to the target as soon as the stimuli appeared. We find a shift of the cells' spatial sensitivity profile towards the saccade target suggesting the involvement of spatial mechanisms in target selection.
Supported by NIH EY00676 and NIH EY02621.