Efficiency of information transmission by retinal ganglion cells

Background: Different types of retinal ganglion cells convey different messages to the brain. Messages are in the form of spike patterns, and the number of possible patterns per second sets the coding capacity. We asked if different ganglion cell types make equally efficient use of their coding capacity or whether efficiency depends on the message conveyed. Results: We recorded spike trains from retinal ganglion cells in an in vitro preparation of the guinea pig retina. By calculating, for the observed spike rate, the number of possible spike patterns per second, we calculated coding capacity, and by counting the actual number of patterns, we estimated information rate. Cells with "brisk" responses, i.e., high firing rates, and a general message transmitted information at high rates (21 ± 9 bits s^-1). Cells with "sluggish" responses, i.e., lower firing rates, and specific messages (direction of motion, local-edge) transmitted information at lower rates (13 ± 7 bits s^-1). Yet, for every type of ganglion cell examined, the information rate was about one-third of coding capacity. For every ganglion cell, information rate was very close (within 4%) to that predicted from Poisson noise and the cell's actual time-modulated rate. Conclusions: Different messages are transmitted with similar efficiency. Efficiency is limited by temporal correlations, but correlations may be essential to improve decoding in the presence of irreducible noise.