Velocity constancy and models for wide-field visual motion detection in insects

Abstract

The tangential neurons in the lobula plate region of the flies are known to respond to visual motion across broad receptive fields in visual space.When intracellular recordings are made from tangential neurons while the intact animal is stimulated visually with moving natural imagery,we find that neural response depends upon speed of motion but is nearly invariant with respect to variations in natural scenery. We refer to this invariance as velocity constancy. It is remarkable because natural scenes, in spite of similarities in spatial structure, vary considerably in contrast, and contrast dependence is a feature of neurons in the early visual pathway as well as of most models for the elementary operations of visual motion detection. Thus, we expect that operations must be present in the processing pathway that reduce contrast dependence in order to approximate velocity constancy.We consider models for such operations, including spatial filtering, motion adaptation, saturating nonlinearities, and nonlinear spatial integration by the tangential neurons themselves, and evaluate their effects in simulations of a tangential neuron and precursor processing in response to animated natural imagery. We conclude that all such features reduce interscene variance in response, but that the model system does not approach velocity constancy as closely as the biological tangential cell.

Appendix

To quantify the degree of saturation of a signal subjected to a limiting nonlinearity, we measured the fraction of time during the course of a simulation that the signal resides in the upper 10% or lower 10% of the range of the nonlinear function. The time spent in transition between these states is therefore consistent with the engineering concept of rise or fall time. We define moderate saturation as corresponding to 60–65% of simulated time spent in those parts of the range near the extrema, by the above criterion. This criterion was applied to both saturating nonlinearities, at early vision output and at EMD correlator output.

When a limiting nonlinearity was included in the signal path in simulations of the motion processing pathway, we measured and averaged the time spent near the extrema for the limited signal at five different latitudinal locations in the array, and adjusted the scaling of the signal at the input to the nonlinear block with the goal of obtaining moderate saturation. When motion adaptation was included in the model, a single scaling constant could be found that achieved moderate saturation for all five animated images in the test set, in all cases. However, when motion adaptation was not included, saturation necessarily fell outside the moderate range for some of the images in the set, due to the differing global contrasts. In these instances, the time spent near the extrema ranged from 48.5% to 52.3% for the lowest-contrast image gardens, and 82.3% to 89.0% for the image hamlin. The other three images were in or near the moderate saturation range.