Visual afferences to flight steering muscles controlling optomotor responses of the fly.

In tethered flying house-flies (Musca domestica) visually induced turning reactions were monitored under open-loop conditions simultaneously with the spike activity of four types of steering muscles (M.b1, M.b2, M.I1, M.III1). Specific behavioral response components are attributed to the activity of particular muscles. Compensatory optomotor turning reactions to large-field image displacements mainly occur when the stimulus pattern oscillates at low frequencies. In contrast, turning responses towards objects are preferentially induced by motion of relatively small stimuli at high oscillation frequencies. The different steering muscles seem to be functionally specialized in that they contribute to the control of these behavioral responses in different ways. The muscles I1, III1 and b2 are preferentially active during small-field motion at high oscillation frequencies. They are much less active during small-field motion at low oscillation frequencies and large-field motion at all oscillation frequencies which were tested. M.b2 is most extreme in this respect. These steering muscles thus mediate mainly turns towards objects. In contrast, M.b1 responds best during large-field motion at low oscillation frequencies and, thus, is appropriate to control compensatory optomotor responses. However, the activity of this muscle is also strongly modulated during small-field motion at high oscillation frequencies and, therefore, may be involved also in the control of turns towards objects. These functional specializations of the different steering muscles in mediating different behavioral response components are related to the properties of two parallel visual pathways that are selectively tuned to large-field and small-field motion, respectively.