We model feedback from primary visual cortex to the dorsal
lateral geniculate nucleus (dLGN). This feedback makes dLGN
neurons sensitive to orientation discontinuity (Sillito et al.,
1993; Cudeiro & Sillito, 1996). In the model, each dLGN
neuron receives retinotopic input driven by layer 6 cortical
neurons in a full set of orientation columns. Excitation is
monosynaptic, while inhibition is through perigeniculate neurons
and dLGN interneurons. The stimulus consists of drifting gratings,
one within and the other outside a circular region centered
over the receptive field of the model dLGN relay neuron we study.
They appear as a single grating when they are aligned with equal
contrast. The model reproduces experimental results showing
an increasing inhibitory effect of feedback on the firing rate
of dLGN neurons as the two gratings move towards the aligned
position. Moreover, enhancement of dLGN cell center-surround
antagonism by feedback is revealed by measuring the responses
to drifting gratings inside a circular window, as a function
of window radius. This effect is related to the observed length
tuning of dLGN cells. Sensitivity to orientation discontinuity
could be mediated in the model by feedback from either simple
or complex cells. The model puts constraints on the feedback
synaptic footprint and shows that its elongated shape does not
play a crucial role in sensitivity to orientation discontinuity.
The inhibitory component of feedback must predominate overall,
but the feedback signal from a cortical neuron to a dLGN neuron
with the same or nearby receptive-field center can be dominated
by excitation. Predictions of the model include (1) robust stimuli
for layer 6 cortical neurons give pronounced nonlinearities
in the responses of dLGN neurons; (2) the sensitivity to
orientation discontinuity at low contrast is twice that at high
contrast.