RECURRENT EXCITATION IN NEOCORTICAL CIRCUITS

The majority of synapses in the mammalian cortex originate from cortic al neurons. Indeed, the largest input to cortical cells comes from nei ghboring excitatory cells. However, most models of cortical developmen t and processing do not reflect the anatomy and physiology of feedback excitation and are restricted to serial feedforward excitation. This report describes how populations of neurons in cat visual cortex can u se excitatory feedback, characterized as an effective ''network conduc tance,'' to amplify their feedforward input signals and demonstrates h ow neuronal discharge can be kept proportional to stimulus strength de spite strong, recurrent connections that threaten to cause runaway exc itation. These principles are incorporated into models of cortical dir ection and orientation selectivity that emphasize the basic design pri nciples of cortical architectures.