Encoding of visual information by LGN bursts

Encoding of visual information by LGN bursts. J. Neurophysiol. 81: 2558-256 9, 1999. Thalamic relay cells respond to visual stimuli either in burst mod e, as a result of activation of a low-threshold Ca2+ conductance, or in ton ic mode, when this conductance is inactive. We investigated the role of the se two response modes for the encoding of the time course of dynamic visual stimuli, based on extracellular recordings of 35 relay cells from the late ral geniculate nucleus of anesthetized cats. We presented a spatially optim ized visual stimulus whose contrast fluctuated randomly in time with freque ncies of up to 32 Hz. We estimated the visual information in the neural res ponses using a linear stimulus reconstruction method. Both burst and tonic spikes carried information about stimulus contrast, exceeding one bit per a ction potential for the highest variance stimuli. The "meaning" of an actio n potential, i.e., the optimal estimate of the stimulus at times preceding a spike, was similar for burst and tonic spikes. In within-trial comparison s, tonic spikes carried about twice as much information per action potentia l as bursts, but bursts as unitary events encoded about three times more in formation per event than tonic spikes. The coding efficiency of a neuron fo r a particular stimulus is defined as the fraction of the neural coding cap acity that carries stimulus information. Based on a lower bound estimate of coding efficiency; bursts had similar to 1.5-fold higher efficiency than t onic spikes, or 3-fold if bursts were considered unitary events. Our main c onclusion is that both bursts and tonic spikes encode stimulus information efficiently, which rules out the hypothesis that bursts are nonvisual respo nses.