Bursting in the LGN

BACKGROUND: The information encoded by the retina is relayed to visual cortex by lateral geniculate nucleus (LGN) relay cells. Individual LGN cells can operate in either of two regimes: tonic mode (left in figure) and burst mode (right). In tonic mode, each retinal spike is relayed by a single LGN spike. In burst mode, a single input spike is relayed as a stereotyped burst of spikes. Bursts have long been known to have a role in non-visual activity such as sleep waves. More recently it has been suggested that the LGN cell also transfers information about visual stimuli via bursts, but it performs a nonlinear amplification of the visual signal, sacrificing signal discrimination for signal detectability. LGN cells fire in both modes in awake animals during visual stimulation, though bursts are relatively infrequent when animals are alert.

EXPERIMENTS: When I was a postdoc with Christof Koch, we collaborated with experimentalists Dwayne Godwin and Murray Sherman, then at SUNY Stony Brook, to explore the information transfer properties of LGN cells in burst mode vs. in relay mode in more detail. Our approach was to present white noise flicker visual stimuli and use linear filters to reconstruct the stimuli from the times of the two types of LGN cell action potentials. We found that bursts carry visual information, and do so with high efficiency.

This earlier work left open the possiblity that the bursts were simply serving as equivalent to single spikes: competent to carry visual signals, but not carrying out any distinct function. Since then, work in several labs has addressed this second question. For our more recent contribution see here.