Neurons in the lateral geniculate nucleus (LGN) of the thalamus are the major source of visual input to the cerebral cortex. Much of our understanding of the physiology of LGN neurons comes from data collected in anaesthetized animals. This study examines the visual responses of LGN neurons in alert animals and compares these responses to those from anaesthetized animals. Compared to the anaesthetized animal, LGN neurons in the alert animal respond to visual stimuli with stronger responses and follow stimuli drifting at higher spatial and temporal frequencies. Stronger responses are likely to translate into an increased coupling between the LGN and visual cortex, as firing rate and interspike interval are known to influence the dynamics of synaptic communication between the two structures.
Despite the increasing use of alert animals for studies aimed at understanding visual processing in the cerebral cortex, relatively little attention has been focused on quantifying the response properties of neurons that provide input to the cortex. Here, we examine the response properties of neurons in the lateral geniculate nucleus (LGN) of the thalamus in the alert macaque monkey and compare these responses to those in the anaesthetized animal. Compared to the anaesthetized animal, we show that magnocellular and parvocellular neurons in the alert animal respond to visual stimuli with significantly higher firing rates. This increase in responsiveness is not accompanied by a change in the shape of neuronal contrast response functions or the strength of centre–surround antagonism; however, it is accompanied by an increased ability of neurons to follow stimuli drifting at higher spatial and temporal frequencies.