Thalamo-cortical processing of vibrissal information in the rat. II. spatiotemporal convergence in the thalamic ventroposterior medial nucleus (VPm) and its relevance to generation of receptive fields of S1 cortical "barrel" neurones.

One hundred and twenty-six cells, sampled in the vicinity of the D1 barreloid in the ventroposterior medial nucleus of the thalamus, were tested for magnitude and latency of response to brief deflections (3 ms; 1.14 degrees) of vibrissae in adult rats under controlled conditions of light urethane anaesthesia. Similar results were achieved for D1 and non-D1-dominant cells. D1-dominant cells (N = 76) responded to the centre-receptive field (D1) vibrissa with a mean of 1.08 spikes per stimulus at modal latencies of 3-12 ms (inter-quartile range 4-5 ms) and to surrounding vibrissae with a mean of 0.26 spikes per stimulus at latencies of 3-41 ms (interquartile range 5-8 ms). Surround-receptive fields showed extensive overlap but were reduced and finally eliminated by deepening anaesthesia. A cell-by-cell analysis showed no correlation between latency and response magnitude for responses to surround vibrissae. Response magnitudes to the surround- and centre-receptive field inputs for D1-dominant barrel cells were some 2.5- and 1.7-fold greater, respectively, than for thalamic cells under identical experimental conditions. The latencies to centre- and surround-receptive field inputs for D1-dominant barrel cells were 2.5 and 10-20 ms later than for thalamus, respectively. These data on a mismatch of latencies for surround- and centre-receptive fields in thalamus and cortex support the notion that surround-receptive fields of cortical barrel cells are almost entirely constructed intracortically during light anaesthesia (Armstrong-James et al., '91), although it is argued that surround-receptive fields of thalamic cells conceivably could be relayed in other cortical states or serve a role in plasticity.