LATENCY VARIABILITY OF RESPONSES TO VISUAL-STIMULI IN CELLS OF THE CATS LATERAL GENICULATE-NUCLEUS

We constructed average histograms from responses evoked by flashing st imuli and noted previously described variations in the shape of the re sponse profile, particularly with respect to sharpness of the peak. To express this variable, we measured the half-rise latency, which is th e latency from stimulus onset required to reach half the maximum respo nse. A short half-rise latency, which is characteristic of nonlagged c ells, is associated with a brisk response and sharp peak; a long half- rise latency, characteristic of lagged cells, is associated with a slu ggish response and broad peak. Nonlagged cells were readily seen; we a ttempted to identify cells with long latencies as lagged, but we were unable to do so unambiguously due to failure to observe lagged propert ies other than latency. We thus refer to these latter cells as having ''lagged-like'' responses to indicate that we are not certain whether these are indeed lagged cells. In addition to the histograms, we analy zed the individual response trials that were summed to create each his togram, and we used spike density analysis to estimate the initial res ponse latency to the flashing spot for each trial. We found that lagge d-like responses were associated with more variability in initial resp onse latency than were nonlagged responses. We then employed an alignm ent procedure to eliminate latency variation from individual trials; t hat is, responses during individual trials were shifted in time as nee ded so that each had a latency equal to the average latency of all tri als. We used these ''aligned'' trials to create a second, ''aligned'' response histogram for each cell. The alignment procedure had little e ffect on nonlagged responses, because these were already well aligned due to consistent response latencies amongst trials. For lagged-like r esponses, however, the alignment made a dramatic difference. The align ed histograms looked very much like those for nonlagged responses: the responses appeared brisk, with a sharply rising peak that was fairly high in amplitude. We thus conclude that the slow build up to a relati vely low peak of firing of the lagged-like response histogram is not a n accurate reflection of responses on single trials. Instead, the slug gishness of lagged-like responses inferred from average response histo grams results from temporal smearing due to latency variability amongs t trials. We thus conclude that there is relatively little difference in briskness between nonlagged and lagged-like responses to single sti muli.