In the primate visual system, information about color is known to be carried in separate divisions of the retino-geniculo-cortical pathway. From the retina, responses of photoreceptors to short (S), medium (M), and long (L) wavelengths of light are processed in two different opponent pathways. Signals in the S-opponent pathway, or blue/yellow channel, have been found to lag behind signals in the L/M-opponent pathway, or red/green channel in primary visual area V1, and psychophysical studies have suggested similar perceptual delays. However, more recent psychophysical studies have found that perceptual differences are negligible with the proper controls, suggesting that information between the two channels is integrated at some stage of processing beyond V1. To study the timing of color signals further downstream in visual cortex, we examined the responses of neurons in area V4 to colored stimuli varying along the two cardinal axes of the equiluminant opponent color space. We used information theory to measure the mutual information between the stimuli presented and the neural responses in short time windows in order to estimate the latency of color information in area V4. We found that on average, despite the latency difference in V1, information about S-opponent signals arrives in V4 at the same time as information about L/M-opponent signals. This work indicates a convergence of signal timing among chromatic channels within extrastriate cortex.

We consider a fast-slow excitable system subject to a stochastic excitatory input train, and show that under general conditions its long term behavior is captured by an irreducible Markov chain. In particular, the ring probability to each input, expected number of response failures between rings, and distribution of slow variable values between rings can be obtained analytically from the distribution of interexcitation intervals. The approach we present immediately generalizes to any pair of input trains, excitatory or inhibitory and synaptic or not, with distinct switching frequencies. We also discuss how the method can be extended to other models, such as integrate-and- re, that feature a single variable that builds up to a threshold where an instantaneous spike and reset occur. The Markov chain analysis guarantees the existence of a limiting distribution and allows for the identication of dieren t bifurcation events, and thus has clear advantages over direct Monte Carlo simulations. We illustrate this analysis on a model thalamocortical (TC) cell subject to two example distributions of excitatory synaptic inputs, in the cases of constant and rhythmic inhibition. The analysis shows that there is a drastic drop in the likelihood of ring just after inhibitory onset in the case of rhythmic inhibition, relative even to the case of elevated but constant inhibition. This observation provides support for a possible mechanism for the induction of motor symptoms in Parkinson's disease, analyzed in (Rubin and Terman, 2004).