Identification of directionally selective motion-detecting neurones in the locust lobula and their synaptic connections with an identified descending neurone

Abstract

The anatomy and physiology of two directionally selective motion-detecting neurones in the locust are described. Both neurones had dendrites in the lobula, and projected to the ipsilateral protocerebrum. Their cell bodies were located on the posterio-dorsal junction of the optic lobe with the protocerebrum. The neurones were sensitive to horizontal motion of a visual stimulus. One neurone, LDSMD(F), had a preferred direction forwards over the ipsilateral eye, and a null direction backwards. The other neurone, LDSMD(B), had a preferred direction backwards over the ipsilateral eye.Motion in the preferred direction caused EPSPs and spikes in the LDSMD neurones. Motion in the null direction resulted in IPSPs. Both excitatory and inhibitory inputs were derived from the ipsilateral eye. The DSMD neurones responded to velocities of movement up to and beyond 270°s−1. The response of both LDSMD neurones showed no evidence of adaptation during maintained apparent or real movement. There was a delay of 60–80 ms between a single step of apparent movement, in either the preferred or the null direction, and the start of the response. There was a monosynaptic, excitatory connection between the LDSMD(B) neurone and the protocerebral, descending DSMD neurone (PDDSMD) identified in the preceding paper (Rind, 1990). At resting membrane potential, a single presynaptic spike did not give rise to a spike in the postsynaptic neurone.