Recurrent projections comprise a universal feature
of cerebral organization. Here, we show that the corticofugal
projections from the striate cortex (V1) to the lateral
geniculate nucleus (LGN) robustly and multiplicatively
enhance the responses of parvocellular neurons, stimulated
by gratings restricted to the classical receptive field and
modulated in luminance, by over two-fold in a contrast-independent
manner at all but the lowest contrasts. In the equiluminant
plane, wherein stimuli are modulated in chromaticity with
luminance held constant, such enhancement is strongly contrast
dependent. These projections also robustly enhance
the responses of magnocellular neurons but contrast
independently only at high contrasts. Thus, these
results have broad functional significance at both network
and neuronal levels by providing the experimental basis
and quantitative constraints for a wide range of models
on recurrent projections and the control of contrast gain.