EEG-dependent modulation of response dynamics of cat dLGN relay cells and the contribution of corticogeniculate feedback.

Single unit recordings were made extracellularly from the dorsal lateral geniculate nucleus (dLGN) in the anaesthetized and paralysed cat. The impulse rates of phasic (peak) and tonic components of visual responses to stimulation of the receptive field center by a flashing spot were determined during different states of the EEG, during local cortical cooling and during micro-iontophoretic application of the excitatory amino acid receptor agonists, quisqualate (QUIS) and N-methyl-D-aspartate (NMDA). Typically, visual responses were phasic during low frequency/high amplitude EEG patterns, resembling slow wave sleep (SWS). During high frequency EEG patterns (non-SWS) visual responses of X- and Y-cells exhibited a prominent tonic response component. This tonic component could be clearly reduced during ipsilateral cortical cooling in the non-SWS state. QUIS or NMDA, applied in order to mimic corticofugal activity, augmented the tonic response component, most efficiently during SWS EEG. The effects did not differ significantly for X- and Y-cells. During non-SWS EEG Y-cells exhibited a tonic response component similar to X-cells, but because of their higher peak rates the responses of Y-cells were on average more phasic than those of X-cells. Our results indicate that state-dependent changes in CNS activity modulate dL'GN responsiveness in part via the corticogeniculate feedback, and that predominantly the tonic response component is modulated.