Neuronal representation of optic flow experienced by unilaterally blinded flies on their mean walking trajectories.

Asymmetries in the optic flow on both eyes may indicate an unintended turn of an animal and evoke compensatory optomotor responses. On a straight path in an evenly structured environment, the optic flow on both eyes is balanced corresponding to a state of optomotor equilibrium. When one eye is occluded an optomotor equilibrium is expected to be reached on a curved path provided that the translatory optic flow component is cancelled by a superimposed rotation. This hypothesis is tested by analysing how the HSE cell, a constituent element of the fly's optomotor system, represents optic flow in behavioural situations. The optic flow as seen on the average trajectory of freely walking monocular flies is reconstructed. This optic flow is used as stimulus of the HSE cell in electrophysiological experiments and as input of a model of the fly's optomotor system. The responses of the HSE cell and of the model fluctuate around the resting potential. On average, they are much smaller than the responses evoked by optic flow experienced on a straight path. These results corroborate the hypothesis that the mean trajectory of monocular flies corresponds to a path of optomotor equilibrium.