Burst responses in thalamic relay cells of the awake behaving cat

Abstract

1. Relay cells of the dorsal lateral geniculate nucleus (LGN) respond in one of two modes: tonic or burst. The purpose of this study was to determine whether, and under what conditions, burst responses occur in LGN cells of an awake, behaving animal. 2. We recorded the extracellular responses of cells located in the A layers of LGN in two cats trained to perform a simple fixation task. In our paradigm, head position was fixed and gaze monitored with the use of the scleral search coil technique. They were trained to fixate on a small target light while we probed the receptive field of an isolated LGN cell with drifting gratings. 3. Many LGN cells (37/48; 77%) recorded in the awake cat showed some form of burst firing. However, the degree of burstiness a given cell displayed was relatively low. The probability of recording a burst response during any second within the fixation task was 0.09. This reflects the fact that during wakefulness, LGN cells are generally in a state of relative depolarization and the low-threshold Ca2+ conductance underlying burst firing is inactivated. 4. The majority of bursts occurred (262/377; 71%) during visual stimulation. These comprised the initial response to the grating and were confined to the early phase of fixation. As the cat continued to hold fixation and as subsequent cycles of the grating passed through the cell's receptive field, the response shifted from a burst to a tonic firing pattern. Some bursts (67/377, 18%) were related to eye movements. All of these were postsaccadic, and most occurred > 150 ms from the onset of the eye movement. Finally, some bursts were neither visually driven nor related to eye movements. However, these spontaneous bursts were infrequent (41/377; 11%) and never rhythmic. 5. Burst firing in LGN cells has a dual purpose. During quiescent states such as slow-wave sleep, they support the decoupling of retinal signals from LGN. During the waking state, bursts can facilitate signal transmission during target acquisition and early phases of fixation.