Whisking profiles along cortical columns in the rat primary somatosensory cortex

Rats use their whiskers to explore the spatial and textural features of their environment. Due to their poorly developed visual system, rats have an excellent ability to discriminate between textures and to explore objects and gaps they encounter as they navigate their dim world. They do so successfully by constantly altering their whisking patterns. The whisking patterns are represented as neuronal response profiles in the cerebral cortex of the brain. Response profiles of cortical neurones in response to whisking response patterns of neurones to the three parameters - velocity, amplitude and frequency of whisker movements, are poorly characterised in the primary somatosensory cortex of the rat. These parameters change in exploratory and discriminative whisking. It was hypothesised that (1) tuning response profiles for velocity and frequency varied down a cortical column as cell type changed and (2) that for both types of whisking, cortical neurons were more responsive to changes in velocity than frequency. To test this, trapezoidal and sinusoidal stimulus waveforms were delivered to the whiskers of anaesthetised rats while recording neuronal responses from whisker barrel cortex. These waveforms were used to approximate whisker movements during exploratory (trapezoidal) and discriminative (sinusoidal) whisking. Extracellular recordings were taken from layers 2, 3, 4 and 5a of the primary somatosensory cortex in response to five amplitudes (0.05, 0.1, 0.2, 1.0 and 2.0 mm) and eight velocity combinations (3, 7, 15, 30, 45, 90, 150 and 220 mm/s). For frequency tuning, neuronal responses to the same amplitudes combined with ten frequencies (1, 3, 5, 9, 15, 30, 60, 100, 300 and 600 Hz) were also characterised. It was found that cortical neurons were sensitive to changes in velocity and frequency, and to a lesser extent to amplitude of whisker movement. Neuronal firing rates increased with velocity, amplitude (to a small degree) and with cortical depth for exploratory whisking. For discriminative whisking, firing rates increased with frequency and amplitude. For high amplitude movements, responses were meaningful only at low frequencies before firing rates saturated, and for small amplitude movements responses were only meaningful at high frequencies. When frequency was plotted in the velocity domain, the effects of amplitude on discriminative whisking were abolished. This supported the hypothesis that velocity tuning underlies frequency tuning. The response profiles varied between cortical layers. The supragranular layer contained neurones with less robust responses and lower firing rates compared with the granular and infragranular layers. Layer specific differences were attributed to differences in cell type and cell density. It was concluded that whisking velocity was the main parameter signalling neurones in the cortical columns of the rat’s somatosensory cortex. Therefore, it is the change in velocity that is the most important information bearing parameter used in exploratory and discriminative whisking.