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Abstract

Positional information is required in solving orientation tasks. In open loop experiments it was found that a resting stripe does not influence the time averaged (over 120 s) yaw torque signal of a tethered flying fly irrespective of the position at which it is presented. An open loop response was elicited when either the stripe was moved or the head of the fly was free to move (Reichardt 1973, Wehrhahn 1980). Thus light flux changes are necessary for the extraction of positional information. It is known that flies are able to perceive motion and a mechanism responsible for the computation of motion proposed earlier (Hassenstein and Reichardt 1956), was found to apply also for the fly visual system (Fermi and Reichardt 1963). This mechanism selectively extracts the phase relations between the signals of neighbouring photoreceptors and thus enables the fly to follow a moving object. The direction of the response is inverted when this phase relation is inverted. The existence of a mechanism selectively extracting the position of an object can be tested by stimulating a fly with a resting object, whose luminosity is modulated in time. The result of the first of such experiments where the time averaged torque response to a flickered stripe has been determined, was negative. Thus it was concluded that no separate mechanism exists for the computation of position (Reichardt 1973). However, the motion and flicker stimuli were different in these experiments.