Abstract
We investigated the response properties of primary visual cortical neurons of the anesthetized cat when presented with natural movies. First, we computed the inter-spike interval (ISI) histograms of the 60 cells in our database. We found the majority of the cells to exhibit a predominance of short ISIs. The maximum value in the histograms was below 2 ms in 39 cells, and below 4 ms for 52 cells. Next, we compared the firing rate distributions of each neuron to exponential (y=a*exp(b*x)) and power (y=a*x^b) functions. We found 43 of the cells were significantly better characterized by power functions (p0.75), while 13 of the cells were best fit by exponential functions. Finally, we investigated the spike-count and spike-time variability across repetitions within 40 ms windows, equivalent to the duration of a single movie frame. We compared the variability of each cell to 1000 surrogates generated using a time-varying Poisson process with a relative refractory period computed from the ISI histograms. With the rate changing every 40 ms, we found that only 1.2% of the windows with an average of at least 1 spike per window exhibited Fano Factors that were lower than 95% of the surrogates. This percentage remained invariant with faster rate changes in the surrogates. Using an entropy measure to quantify spike-time variability, we found 49.3% of the aforementioned windows exhibited lower variability than 95% of the surrogates with rates changing every 40 ms. This percentage decreased to 37.28%, when the rates changed every 5 ms instead. Our results indicated that when stimulated with natural movies, striate cortical neurons exhibited responses: 1) with short intervals, 2) that were better described using power-law distributions instead of exponential distributions, and 3) with spike-time variability that was substantially lower than that predicted from time-varying Poisson processes with relative refractory periods.
This work was supported by grants from the National Eye Institute and the Singapore Ministry of Education AcRF Tier 1 Fund (R263000355112, R263000355133).