Both pre-synaptic and post-synaptic suppression have been proposed as the cellular mechanism underlying reduced neuronal sensitivity in early visual cortex following adaptation (Finlayson & Cynader, 1995; Sanchez-Vives, et al., 2000). At the behavioral level, these two cellular mechanisms can be distinguished using an external noise method and an observer model (Dao, Lu, and Dosher, VSS, 2003). Based on an analysis of full psychometric functions in multiple external noise levels and at multiple stimulus contrasts, we predicted that adapting to a grating stimulus selectively reduces sensory gain to a narrow range of the stimulus domain. In this study, we empirically tested this prediction by directly measuring the orientation bandwidth of adaptation. Observers were adapted to a grating of +45 deg, and then detected the presence of a +45 deg grating in a 2IFC procedure in the presence of external noise with different orientation bandwidths. Nine different noise bandwidths were tested: +/- 0, 1, 5, 10, 15, 30, 45, 60, and 90 deg about the center orientation of +45 deg. Compared to performance in unadapted control conditions, adaptation led to a general elevation in threshold in all noise conditions, with greater threshold elevation in the no noise (0 bandwidth) condition. A contrast gain control perceptual template model (cgcPTM) accounts for all of the data with an estimated bandwidth of adaptation of 20 deg. In this and our previous study, adaptation translates the psychometric functions to higher contrasts without changing the form in all noise conditions. This pattern is consistent with a post-synaptic hyper-polarization mechanism of adaptation.

Supported by AFOSR, NSF and NIMH.