Experiments were carried out to learn about changes in sensory cortical processing associated with different levels of anaesthesia. Traditionally this question has been addressed by studying single neurons. Because state changes are likely to influence the relationships between neurons, the present experiments were undertaken to investigate the spatial and temporal firing patterns distributed across cortex. Using 5 × 5 or 10 × 10 microelectrode arrays, spontaneous and stimulus-evoked activity of multineuron clusters was recorded from rat somatosensory 'barrel' cortex (the whisker representation) during a light surgical stage of urethane anaesthesia, and after two supplemental doses of urethane which led to intermediate and deep levels of anaesthesia. At all depths of anaesthesia, spontaneously occurring action potentials at a single electrode tended to be clustered into 'bursts.' With increasing anaesthetic depth, bursts became more prominent and rhythmic, and increasingly synchronized between cortical barrel-columns. Burst frequency decreased and fewer spikes occurred outside bursts, leading to a decrease in the overall spontaneous firing rate. The cortical territory engaged by individual whiskers contracted with increasing depth of anaesthesia, leading to the spatial segregation of whisker representations. At all stages of anaesthesia, whisker stimulation produced the maximal cortical response when delivered close to burst onset. These observations show that ongoing spontaneous activity modulates sensory response properties and makes peripheral tactile information accessible to a cortical territory whose size is determined by the phase of burst cycle. The possible significance of the cyclic cortical responsiveness encountered during urethane anaesthesia to cortical processing in awake rats is considered.
