There are two recognized modes of firing activity in thalamic cells, burst and tonic. A low threshold (LT) burst (referred to from now on as "burst") comprises a small number of high frequency action potentials riding the peak of a LT calcium spike which is preceded by a silent hyperpolarized state >50 msec. This is traditionally viewed as a sleep-like phenomenon, with a shift to tonic mode at wake-up. However, bursts have also been seen in the wake state and may be a significant feature for full activation of recipient cortical cells. Here we show that for visual stimulation of anaesthetized cats burst firing is restricted to a reduced area within the receptive field center of LGN cells. Consistently, the receptive field size of all the recorded neurons decreased in size proportionally to the percentage of spikes in burst versus tonic spikes, an effect that is further demonstrated with pharmacological manipulation. The role of this shrinkage may be distinct from that also seen in sleep-like states and we suggest that this is a mechanism which trades spatial resolution for security of information transfer.