Processing of vibrissa sensory information within the rat neocortex.

Abstract

Neuronal response properties were compared among different layers of the urethan-anesthetized rat vibrissa cortex. Measurements were made of the receptive-field (RF) size, the degree of directional selectivity, the latency of driving, the velocity threshold, and the tuning-curve slope. The RF size was defined by the number of whiskers that, when deflected individually, activated a neurons. For the center whisker of the RF (usually whisker C3), the response to deflection in the most preferred direction was compared with that in the opposite direction to classify the neuron as either strongly directional, weakly directional, or nondirectional. For the most preferred direction of the center whisker, the minimum velocity of deflection required to drive the unit was defined as the velocity threshold, the latency of driven response to a standard supramaximal velocity was measured, and finally, using exponential ramp-and-hold deflection, the threshold amplitude was determined at different values of time constant to construct a tuning-curve slope. Cortical layer IV neurons, as a whole, have the lowest threshold velocity. Layer Vb neurons stand on the opposite extreme in having the highest mean velocity threshold value. Although this difference is consistent with the generally held view that the "barrels" in layer IV represent the input stage of cortical information processing, the lack of laminar differences in latency and RF size support the idea that neurons of other cortical layers also receive direct thalamocortical inputs. The population of cortical neurons thus appears quite homogeneous across different layers as far as the results of examination with short-pulsed stimulation are concerned. Correlation of pairs of parameters (RF, directionality, velocity threshold, and latency) was tested in the two layers (layer IV and layer Vb). The latency and velocity threshold are highly correlated within both layers. Also, most of correlation coefficients of the corresponding pairs of the two layers are similar. However, the use of exponential ramp-and-hold deflection of whiskers revealed a difference in tuning-curve slope between layer IV and layer Vb (also layers II-III); layer IV neurons show flatter tuning-curve slopes (more oriented for detection of the amplitude component of whisker deflection) than neurons of layer Vb and layers II-III, which are more oriented for velocity detection. During the hold phase of whisker deflection, layer IV neurons tend to show sustained discharges, whereas layer Vb (also layers II-III) neurons mainly exhibit transient responses.(ABSTRACT TRUNCATED AT 400 WORDS)