Decoding Visual Information from a Population of Retinal Ganglion Cells.
David K. Warland, Pamela Reinagel, and Markus Meister.
Molecular and Cellular Biology Department, Harvard University.
APStracts 4:147N, 1997.
ABSTRACT
This work investigates how a time-dependent visual stimulus is encoded by the
collective activity of many retinal ganglion cells. Multiple ganglion cell
spike trains were recorded simultaneously from the isolated retina of the
tiger salamander using a multi-electrode array. The stimulus consisted of
photopic, spatially uniform, temporally broad-band flicker. From the recorded
spike trains, an estimate was obtained of the stimulus intensity as a function
of time. This was compared to the actual stimulus to assess the quality and
quantity of visual information conveyed by the ganglion cell population. Two
algorithms were used to decode the spike trains: an optimized linear filter in
which each action potential made an additive contribution to the stimulus
estimate; and an artificial neural network, trained by back-propagation to
match spike trains with stimuli. The two methods performed indistinguishably,
suggesting that most of the information about this stimulus can be extracted
by linear operations on the spike trains. Individual ganglion cells conveyed
information at a rate of 3.2ñ1.7 bits/s (mean ñ SD), with an average
information content per spike of 1.6 bits. The maximal possible rate of
information transmission compatible with the measured spiking statistics was
13.9ñ6.3 bits/s. On average, ganglion cells used 22% of this capacity to
encode visual information. When a decoder received two spike trains of the
same response type, the reconstruction improved only marginally over that
obtained from a single cell. However, a decoder using an ON and an OFF cell
extracted as much information as the sum of that obtained from each cell
alone. Thus cells of opposite response type encode different and non-
overlapping features of the stimulus. As more spike trains were provided to
the decoder, the total information rate rapidly saturated, with 79% of the
maximal value obtained from a local cluster of just 4 neurons of different
functional types. The decoding filter applied to a given neuron's spikes
within such a multi-unit decoder differed substantially from the filter
applied to that same neuron in a single-unit decoder. This shows that the
optimal interpretation of a ganglion cell's action potential depends strongly
on the simultaneous activity of other nearby cells. The quality of the
stimulus reconstruction varied greatly with frequency: flicker components
below 1?Hz and above 10Hz were reconstructed poorly, and the
performance was optimal near 2.5Hz. Further analysis suggests that
temporal encoding by ganglion cell spike trains is limited by slow
phototransduction in the cone photoreceptors and a corrupting noise source
proximal to the cones.
Received 11 February 1997; accepted in final form 2 July 1997.
APS Manuscript Number J123-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997