OCULOCENTRIC SPATIAL REPRESENTATION IN PARIETAL CORTEX

Parietal cortex comprises several distinct areas. Neurons in each area are selective for particular stimulus dimensions and particular regions of space. The representation of space in a given area reflects a particular motor output by which a stimulus can be acquired. Neurons in the lateral intraparietal area (LIP) are active in relation to both visual and motor events. LIP neurons do not transmit an unambiguous saccadic command. Rather, they signal the location at which an event has occurred. These spatial locations are encoded in oculocentric coordinates, that is, with respect to the current or anticipated position of the center of gaze. When an eye movement brings the spatial location of a recently flashed stimulus into the receptive field of an LIP neuron, the neuron responds to the memory trace of that stimulus. This result indicates that, for nearly all LIP neurons, stored visual information is remapped in conjunction with saccades. Remapping of the memory trace maintains the alignment between the current image on the retina and the stored representation in cortex. Further, when an eye movement is about to occur, more than a third of LIP neurons transiently shift the location of their receptive fields. This anticipatory remapping allows the neuron to begin to respond to a visual stimulus even before the saccade is initiated that will bring the stimulus into the fixation-defined receptive field. Both kinds of remapping serve to create a constantly updated representation of stimulus location that is always in terms of distance and direction from the fovea. This oculocentric representation has the advantage that it already matches that known to exist in the frontal eye fields and the superior colliculus, the output targets of LIP, and it does not require further coordinate transformation in order to contribute to spatially accurate behavior. These results indicate that LIP can analyze visual space without ever forming a representation of absolute target position.