Striate cortex increases contrast gain of macaque LGN neurons

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 e nhance the responses of parvocellular neurons, stimulated by gratings restr icted 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 wi th luminance held constant, such enhancement is strongly contrast dependent . These projections also robustly enhance the responses of magnocellular ne urons but contrast independently only at high contrasts. Thus, these result s have broad functional significance at both network and neuronal levels by providing the experimental basis and quantitative constraints for a wide r ange of models on recurrent projections and the control of contrast gain.