Neuronal mechanisms subserving directional selectivity in the locust optomotor system

Summary

• 1. The locust optomotor interneurons show velocity-response functions independent of pattern wavelength over a wide range. Thus the conditions for asymmetric direction detecting networks are fulfilled and this type of network can be used as the basis for a model.

• 2. When the pattern is moved during a short dark period the optomotor interneurons respond on reillumination of the pattern depending on the direction of displacement. These responses appear equivalent to behaviour previously described as optokinetic memory.

• 3. Experiments where the magnitude of pattern displacement was varied reveal that the type 1 (M1) optomotor interneuron receives inputs from small field cells (field dia. 1.5°). Interactions occur between every fifth cell along a horizontal row.

• 4. The type 2 (M2) neuron receives inputs from similar small field cells, with every fifth cell along a horizontal row forming an interacting pair. As well, the M2 neuron receives inputs from wider field cells (field dia. 2.5°–4°) and here the receptive fields of interacting pairs are separated by a distance equivalent to six ommatidia.

• 5. By varying the duration of the dark period it was seen that the lateral interactions of the M1 networks have only a short duration, thus mediating a response to fast movements. A selective facilitation explains the high discharge of these neurons to long slow movements.

• 6. The excitatory lateral interactions between the wide field cells connecting to the M2 neuron show a double peaked time course which corresponds well with the upper range of the M2 velocity-response function.

• 7. The experimental data can now be fitted to precise models for the inputs to the M1 and M2 neurons.