ORIENTATION TUNING OF INTRACELLULAR POTENTIALS AND SPIKE RESPONSES AT PINWHEEL CENTERS AND ISO-ORIENTATION DOMAINS IN PRIMARY VISUAL CORTEX J.M. Schummers1*; J. Marino2; M. Sur1 1. Brain & Cognitive Sci, Massachusetts Inst Tech, Cambridge, MA, USA 2. Dept. Medicina, Univ. of A Coruna, A Coruna, Spain The representation of orientation across the primary visual cortex is inhomogeneous. That is, the orientation map contains foci in which neurons preferring many different orientation are situated in close proximity (pinwheel centers) as well as wider regions where neurons share similar orientation preference (iso-orientation domains). Suprisingly, neurons located in pinwheel centers are sharply tuned for orientation despite the broad orientation representation surrounding them. However, there is no evidence that the dense network of local connections respects the inhomogeneities in the orientation representation. We have investigated whether the membrane potential responses to oriented stimuli, which should reveal synaptic inputs from the local network, differ between neurons in pinwheel centers and iso-orientation domains. We have found substantial variability in the orientation selectivity of the synaptic response, regardless of map location. Many neurons show significant depolarization in response to only a narrow range of stimulus orientations. However, many neurons respond with robust subthreshold activity to all orientations, and their spiking tuning curve is dramatically sharpened by an iceberg effect. We have detected a tendency for the latter class of neurons to be found near pinwheel centers, and for the former to be found in iso-orientation domains. We conclude that the orientation representation in the local region of the map may profoundly influence the tuning properties of synaptic inputs to a neuron. Supported by: NIH grants EY07023 & NS39022;JS is an HHMI predoctoral fellow