Representing spatial relationships in posterior parietal cortex: single neurons code object-referenced position.

The brain computes spatial relationships as necessary to achieve behavioral goals. Loss of this spatial cognitive ability after damage to posterior parietal cortex may contribute to constructional apraxia, a syndrome in which a patient's ability to reproduce spatial relationships between the parts of an object is disrupted. To explore neural correlates of object-relative spatial representation, we recorded neural activity in parietal area 7a of monkeys performing an object construction task. We found that neurons were activated as a function of the spatial relationship between a task-critical coordinate and a reference object. Individual neurons exhibited an object-relative spatial preference, such that different neural populations were activated when the spatial coordinate was located to the left or right of the reference object. In each case, the representation was robust to translation of the reference object, and neurons maintained their object-relative preference when the position of the object varied relative to the angle of gaze and viewer-centered frames of reference. This provides evidence that the activity of a subpopulation of parietal neurons active in the construction task represented relative position as referenced to an object and not absolute position with respect to the viewer.