Oxyopia Abstract

October 23, 2009
Friday, 4:00 PM
481 Minor Hall

Alessandra Angelucci, MD, PhD
Associate Professor of Ophthalmology & Visual Science, Adjunct Associate Professor of Bioengineering, Dept. of Ophthalmology and Visual Science, Moran Eye Center, University of Utah, Salt Lake City
Host: Michael Silver

Title

Contextual effects in primary visual cortex: pathways and mechanisms

Abstract

The response of neurons in primary visual cortex to oriented stimuli inside their receptive field (RF) is modulated by oriented stimuli in the RF surround. Our interest is to identify the circuitry and mechanisms generating orientation-specific surround modulation in V1. We propose that there are two components to the surround operating at different spatio-temporal scales: a near surround, subserved by feedforward and slow intra-areal horizontal (HZ) connections, and a far surround, subserved by divergent and fast inter-areal feedback (FB) connections. We provide anatomical evidence for two FB systems: one diffuse and unspecific terminating in upper layer 1, the other patchy and orientation-specific terminating in deeper V1 layers. We suggest that the latter FB system may serve surround modulation in V1.

To investigate the circuitry and mechanisms for surround modulation, we have generated an anatomically and physiologically-constrained recurrent network model of macaque V1. In the model, HZ and FB connections generate contrast-dependent size tuning and near surround modulation, while FB connections mediate far surround modulation by targeting horizontally-projecting V1 cells. I will present experiments designed to test several model's predictions. One such prediction, supported by our recent data, is that the far surround can be facilitatory when the RF is weakly stimulated, e.g., by a stimulus of low contrast or of sub-optimal orientation.

We have also examined the orientation tuning of surround facilitation and suppression and found them both to be tuned to the orientation "seen," rather than "preferred," by the RF, with suppression being strongest and facilitation weakest for iso-oriented center and surround stimuli. In a 2D version of our model this property emerges from the interaction of orientation-specific HZ (and or FB) connections and strong but balanced recurrent connections. Iso-orientation surround suppression independent of the input selectivity of V1 neurons may serve the perceptual purpose of enhancing V1 responses to local orientation contrast. We are exploring this hypothesis in our model.

[Back to Angelucci - Oxyopia Page]