No significant reduction in fibre number in the anterior optic tracts.

The lobula plate is strongly reduced in size. A prominent bundle of giant fibers is seen between the lobula and lobula plate in supraoesohageal sections. This bundle traverses the posterior border of the lobula, extending from the midbrain to the medulla where the fibres ramify. The pattern of visually induced nervous activity (measured by 3H-deoxyglucose autoradiography) in the lamina and medulla does not differ substantially from wild-type, but no stimulus-specific activity is seen in the lobula plate.

Mushroom bodies have normal size and shape. Normal olfactory learning. Structural defects are restricted to visual system.

Lobula plate (third-order optic ganglion) missing most of its usual neuropil (Fischbach and Heisenberg, 1984), including an absence of most "small-field" visual neurons; but the giant fibers of this lobe remain; certain of these (i.e. "VS"-cells) enter the neuropil of a more peripheral lobe (the medulla), as opposed to their orthodox (more central) targets; optomotor roll and pitch responses to movements in the visual field are weak or absent. Visually mediated stimulus-specific labelling (in 2-deoxyglucose experiments) does not occur in lobula plate, though it is normal in the mutant's lamina (first order optic lobe) and medulla (Bulthoff and Buchner, 1985). Circadian rhythms of locomotor activity are quite robust and have normal ca. 24 hours free-running periodicities (Helfrich, 1986).