  The latency of visual responses generally decreases as contrast increases.
Recording in the lateral geniculate nucleus (LGN), we find that response
latency increases with increasing contrast in ON cells for some visual stimuli.
We propose that this surprising latency trend can be explained if ON cells rest
further from threshold at higher contrasts. Indeed, while contrast changes
caused a combination of multiplicative gain change and additive shift in LGN
cells, the additive shift predominated in ON cells. Modeling results supported
this theory: the ON cell latency trend was found when the distance-to-threshold
shifted with contrast, but not when distance-to-threshold was fixed across
contrasts. In the model, latency also increases as surround-to-center ratios
increase, which has been shown to occur at higher contrasts. We propose that
higher-contrast full-field stimuli can evoke more surround inhibition, shifting
the potential further from spiking threshold and thereby increasing response
latency.
