IN layer 4 of cat visual cortex, the monocular, concentric receptive f
ields of thalamic neurons, which relay retinal input to the cortex, ar
e transformed into 'simple' cortical receptive fields that are binocul
ar and selective for the precise orientation, direction of motion, and
size of the visual stimulus(1). These properties are thought to arise
from the pattern of connections from thalamic neurons(1-6), although
anatomical studies show that most excitatory inputs to layer 4 simple
cells are from recurrently connected circuits of cortical neurons(7-9)
. We examined single fibre inputs to spiny stellate neurons in slices
of cat visual cortex, and conclude that thalamocortical synapses are p
owerful and the responses they evoke are unusually invariant for centr
al synapses. However, the responses to intracortical inputs, although
less invariant, are strong enough to provide most of the excitation to
simple tells in vivo. Our results suggest that the recurrent excitato
ry circuits of cortex may amplify the initial feedforward thalamic sig
nal, subserving dynamic modifications of the functional properties of
cortical neurons(10-12).