ENCODING OF VISUAL-MOTION INFORMATION AND RELIABILITY IN SPIKING AND GRADED POTENTIAL NEURONS

We investigated the information about stimulus velocity inherent in th e membrane signals of two types of directionally selective, motion-sen sitive interneurons in the fly visual system. One of the cells, the H1 -cell, is a spiking neuron, whereas the other, the HS-cell, encodes se nsory information mainly by a graded shift of its membrane potential. Using a pseudo-random velocity waveform by which a visual grating is m oving along the horizontal axis of the eye, both cell types follow the stimulus velocity at higher precision than in response to a step-like velocity function. To measure how much information about the stimulus velocity is preserved in the cellular responses, we calculated the co herence between the stimulus and the neural signals as a function of s timulus frequency At frequencies up to similar to 10 Hz motion informa tion is well contained in the electrical signals of HS- and H1-cells: For HS-cells the coherence value amounts to similar to 70%, and for H1 -cells this value is similar to 60%. Comparing these values with the c oherence expected from a linear encoding reveals that the fidelity of the original stimulus is deteriorated in the neural signal partly by n eural noise and partly by the nonlinearity inherent in the process of visual motion detection The degree to which this nonlinearity contribu tes to the decrease in coherence depends on the maximum velocity used in the experiments; the smaller the stimulus amplitude, the higher the coherence and, thus, the smaller the nonlinearity in encoding of stim ulus motion. All these results are in agreement with model simulations in which visual motion is processed by an array of local motion detec tors, the spatially integrated output of which is considered the equiv alent of the neural signals of HS- and H1-cells.