Computational Neuroscience: Trends in Research, 1998, J. M. Bower, ed., Plenum Press, New York (1998). Extracellular Recording from Multiple Neighboring Cells: Response Properties in Parietal Cortex J. S. Pezaris1, M. Sahani1,2, R. A. Andersen1,2 1Computation and Neural Systems 2Sloan Center for Theoretical Neurobiology California Institute of Technology Pasadena, CA 91125, U.S.A. Abstract Multiple single unit extracellular recordings were made using tetrodes in macaque posterior parietal cortex while the animal was performing a visual memory saccade task. Recordings were made over a 2x2 mm area at both superficial and deep locations in one hemisphere. Signals were analyzed using an Expectation Maximization algorithm for spike separation based on spike peak height or the first two principle components of spike shape. 27 sites were selected for analysis based on task response and clarity of separation, yielding 85 total neurons with a mode of 3 cells per site. The response criteria and stereotaxic location used were consistent with identifying neurons within the lateral intraparietal area (LIP). For cells within the set of selected sites, responses to the task were further categorized based upon spatial characteristics (preferred direction) and temporal characteristics (time of maximal response). Neighboring cells were found to be very likely to have similar tuned direction (75% within one octant), but not comparatively likely to have similar temporal characteristics. We take this as evidence that area LIP is heterogeneous at the local level. John Pezaris, Caltech, Mail Code 216-76, Pasadena, CA 91125, john [at] pezaris [dot] com, 14 July 1997.