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The common response properties to simple visual stimuli (light impulses, light steps, and movement of simple patterns at different speeds) has been investigated by intracellular recording from Giant Vertical Cells (VS) in the lobula plate of the blowflyCalliphora erythrocephala.
The impulse response begins < 10ms after onset of the photoreceptor signal (Fig. 6), and shows several phases which gradually subside within about 0.5 s. Very late events, which would hint at recurrent or far-reaching sidepaths, were not observed.
The step response is highly non-linear in that both, the increase and decrease of brightness elicit transient depolarization. The excitatory transients are followed by inhibitory waves (Figs. 7, 8), similar to those observed in impulse responses. The possible significance of this succession of excitation and inhibition is discussed.
Vertical movement of arbitrary patterns (dot, edges, bar, and gratings) elicit, invariably and irrespective of contrast polarity, depolarizing responses with downward movement, and hyperpolarizing responses with upward movement (Fig. 10). Both responses increase nonlinearly with contour length (Fig. 11). Possible mechanisms, and the functional significance of such nonlinear summation are discussed.
The velocity dependence of movement responses to periodic gratings was investigated at both high and low pattern luminance and contrast. Under these conditions VS-cells respond best at a contrast frequency of ≈ 2 Hz, which corresponds with that of velocity dependent optomotor reactions.
These results confirm earlier findings that giant vertical cells have many response properties in common. They are best suited to perceive widefield motion, which occurs when a fly performs rotatory and translatory movements in a resting environment. VS-cells are therefore most likely involved in the visual control of such movements.
The present results are not sufficient to indicate which of the VS-cells contribute to which of the optomotor reactions. A subsequent publication will be addressed to these questions.
