Saturation in a wide-field, directionally selective movement detection system in fly vision

In the third optic lobe of the fly large-field spiking neurons are found which detect movement in a directionally selective way. For a wide variety of stimuli the responses of one of these, the H1 neuron, can very well be described and predicted by Reichardt's correlation model which is based on behavioural optomoter responses. However, when the spike rate is driven to large values with intense stimuli, the correlation model needs to be extended in order to account for the saturation phenomena that occur. Saturation can take place in all the elements of the large and extensive movement detecting system which contains interneurons in the optic lobes which process and guide the signals from the peripheral receptors to the central H1 neuron. To separate saturation at the peripheral site from that at the central level of the H1 neuron, a special stimulation technique was chosen. In measuring the saturation at the central level the stimulus parameter was the size of the stimulus field, while the modulation depth of the moving grating in this field stayed constant. Saturation at the peripheral site of the system was studied with a stimulus in which the modulation depth of the moving grating was the parameter and the size of the stimulus field was small and constant. When a simple feedback loop is incorporated in the final stage of the correlation model the saturation phenomena at the central level for steady-state stimulus conditions can very well be described. Saturation phenomena at the peripheral site of the system can also be explained by the same kind of feedback mechanisms in the input channels.