Some quantitative aspects of vibrissa-driven neuronal responses in rat neocortex

Abstract

Single-unit responses to whisker deflections were studied in the rat somatosensory cortex, primarily with cortical layer V neurons, under urethane anesthesia. Manual stimulation tests showed that 31% of the total sample responded to deflections of 1 whisker only, while the remainder responded to 2 or more whiskers (up to 14 whiskers). The average of the receptive-field (RF) size was 3.9 in terms of the number of effective whiskers. The latency of unit responses to supramaximal deflections of the center whisker in the most preferred direction averaged 10.6 ms. When tested with deflections of 5 ms duration, the velocity threshold (measured at 10 mm from the whisker base) was 23.5 mm/s (or 134.4.degree./s) on the average, the amplitude threshold was 117 .mu.m (0.68.degree.). The longer the latency of a neuron was, the higher was its threshold. Neurons with larger RF tended to show lower thresholds and lesser degree of directional selectivity. Threshold tuning curves were negatively sloped, and most of the slopes were between 0 (ideal amplitude detector) and -1 (ideal velocity detector). The highly significant linearity of individual slopes suggested a constant contribution of the amplitude and velocity components of whisker deflection within the range of frequencies tested (0.31-17.8 Hz) in a given neuron. Although the ratio of contribution of the 2 components was different for different neurons, these neurons, as a population, were more sensitive to the velocity than to the amplitude, as indicated by a mean slope value of -0.78.