TRAVELING WAVES AND THE PROCESSING OF WEAKLY TUNED INPUTS IN A CORTICAL NETWORK MODULE

Recent studies have shown that local cortical feedback can have an imp ortant effect on the response of neurons in primary visual cortex to t he orientation of visual stimuli. In this work, we study the role of t he cortical feedback in shaping the spatiotemporal patterns of activit y in cortex. Two questions are addressed: one, what are the limitation s on the ability of cortical neurons to lock their activity to rotatin g oriented stimuli within a single receptive field? Two, can the local architecture of visual cortex lead to the generation of spontaneous t raveling pulses of activity? We study these issues analytically by a p opulation-dynamic model of a hypercolumn in visual cortex. The order p arameter that describes the macroscopic behavior of the network is the time-dependent population vector of the network. We first study the n etwork dynamics under the influence of a weakly tuned input that slowl y rotates within the receptive field. We show that if the cortical int eractions have strong spatial modulation, the network generates a shar ply tuned activity profile that propagates across the hypercolumn in a path that is completely locked to the stimulus rotation. The resultan t rotating population vector maintains a constant angular lag relative to the stimulus, the magnitude of which grows with the stimulus rotat ion frequency. Beyond a critical frequency the population Vector does not lock to the stimulus but executes a quasi-periodic motion with an average frequency that is smaller than that of the stimulus. In the se cond part we consider the stable intrinsic state of the cortex under t he influence of isotropic stimulation. We show that if the local inhib itory feedback is sufficiently strong, the network does not settle int o a stationary state but develops spontaneous traveling pulses of acti vity. Unlike recent models of wave propagation in cortical networks, t he connectivity pattern in our model is spatially symmetric, hence the direction of propagation of these waves is arbitrary. The interaction of these waves with an external-oriented stimulus is studied. It is s hown that the system can lock to a weakly tuned rotating stimulus if t he stimulus frequency is close to the frequency of the intrinsic wave.