W. Martin Usrey, John B. Reppas and R. Clay Reid
Department of Neurobiology, Harvard Medical School
In previous work (Alonso, Usrey and Reid. Nature 383, 815-819.), we have found that neurons in the lateral geniculate nucleus (LGN) are often highly correlated when their receptive field centers are overlapped. These correlations are tighter than any correlation seen in the retina; they peak near zero time and have a width of less than 1.0 msec. When the two neurons have nearly identical receptive fields, these correlation are always present and are quite strong (10-40% of the spikes). Pairs of neurons with the strongest correlations are also very likely to connect to the same simple cells in striate cortex (Reid and Alonso, Nature, 378, 281). Tight correlations are also found when there is only partial overlap of the centers, or even between on-center and off-center neurons, but these correlations are seen only in a fraction of cell pairs (10-20%) and weaker (1-5% of the spikes).
In order to examine the source and the potential implications of these synchronous events, we have made intra-ocular recordings from single retinal ganglion cells simultaneously with recordings from up to eight neurons in the LGN. The electrodes were spaced closely enough that occasionally the same neuron was recorded on two electrodes and often all receptive fields were partially overlapped.
We have recorded simultaneously from pairs of LGN neurons with nearly identical receptive fields, together with a retinal ganglion cell that provides strong input to these cells. In this case, the LGN cells were clearly being synchronized by the common input from the single ganglion cell. Further, it appeared that the retinal influence on each geniculate cell was itself correlated; if a retinal spike produced a spike in on LGN cell, it was more likely to do so in the other. One potential source of this ``concerted efficacy'' of a single input in driving two thalamic neurons is a form of pair-pulse facilitation: the second of two closely spaced retinal spikes is more likely to drive a thalamic neuron. Implications of this correlated transmission of information from retina to visual cortex are being explored.
(Supported by NIH EY10115, EY06604, HHMI, The Klingenstein Fund, and The Harvard Mahoney Neuroscience Institute)