Also called "vector edge" (VE) units by Northrop and Guignon, these units fired at the same average rate in LSS and DSS. However, the firing in DSS was more regular, less random. They constituted 7% of the 181 OL units studied. They gave a sharp burst at OFF of general illumination, followed by a long period of reduced firing before the unit regained its DSS rate. There was no burst at ON. The receptive field of the VE units appeared to be the entire ipsilateral eye, with the posterior half generally more sensitive. VE units were not responsive to mechanical or acoustic stimuli.
VE units were caused to fire above their base rate by the motion of a contrasting, linear (black/white) boundary in the preferred direction. The preferred direction for Romalea VE units was generally distributed around the anterior + dorsal direction, that is, forward and up. Object velocity in the opposite, or null, direction would suppress the LSS firing rate under certain conditions. If a single, long, black/white boundary was used as the test object, there would be no firing rate suppression when the motion was in the null direction if the white field was anterior. Suppression occurred when the black field was anterior.
VE units gave little or no response to small contrasting spots moved in the preferred direction; the object must have a linear contrasting boundary subtending at least 30°, perpendicular to the velocity vector. VE units respond over a very wide range of velocities in the preferred direction. Response drops off as the cosine of the angle between the preferred direction and the velocity vector of the object. When a multiple striped object was used (square-wave pattern), VE unit response to continuous motion in the preferred direction adapts. The suppression effect for continuous motion in the null direction also adapts. VE units are extremely sensitive to small incremental movements of 30.33° in the preferred direction, giving a large burst for such incremental motion. VE unit responses fall off for square-wave striped patterns with periods less than ~4°. Clearly, there appears to be a trade-off between spatial resolution (object size) and incremental motion sensing.
Northrop (1974) reported having found another class of directionally sensitive units giving maximum response to small, contrasting spots rather than to long contrasting boundaries. These vector spot (VS) units responded similarly to the VE units except they showed a burst at ON followed by a relatively long period of suppressed firing. The VS units firing was suppressed by spot motion in the null direction. VS units showed little adaptation to continuous spot motion. They also responded to stripes moving in the preferred direction. Lesioning of VS units showed that recording sites were in the lobula or in the tracts of large fibers running from the lobula to the protocerebrum.
Romalea VE units are probably the same as the type BG units described by Horridge et al. (1965) in the OLs of Locusta. Many other workers have identified directionally sensitive units in insect eyes. Collett and Blest (1966) found VE units in the hawk moth OLs. Swihart (1968) reported directionally-sensitive (DS) units in the butterfly Heliconius. Eight subtypes of DS units were specified for flies by McCann and Dill (1968); more will be said on fly units below. DeVoe et al. (1982) has reported on DS units in the OLs of bees.
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