The transmission of information by first and second order neurons in the fly visual system

1. Random (white noise) fluctuations of both point and wide field light sources were presented to eyes of the fly, Calliphora stygia. At the same time, intracellular recordings were made from photoreceptor cells and from large monopolar cells (LMCs) in the lamina ganglionaris. 2. Fourier analysis was used to determine the linear frequency response functions and coherence functions between the input light fluctuations and the resultant fluctuations in membrane potentials of the two types of cells. 3. The frequency response function of the receptor-to-LMC pathway was estimated by dividing the overall light-to-LMC frequency response function by the light-to-receptor frequency response function. For small signal conditions this function is primarily a phase shift of 180° (inversion) and some increase in sensitivity with frequency. 4. Comparison of results obtained with point and wide field stimuli did not reveal any significant differences between the frequency response functions of receptors or LMCs to either stimulus. No evidence of lateral interaction between receptors or LMCs was found. 5. For a linear system the coherence function provides a measure of the signal-to-noise ratio as a function of frequency. Comparison of the coherence functions obtained from receptors and LMCs showed that there is no significant improvement or deterioration in the signal-to-noise ratio as the signal passes from receptors to LMCs.