TAMMERO, Lance; DICKINSON, Michael: A characterization of the response to visually simulated object approach in Drosophila

As a fly approaches an object, image expansion occurring across the fly’s retina and a fly is faced with the choice of either avoiding the object or landing on it. To quantify the dynamics of the fly’s response to looming visual stimuli, we measured both the collision avoidance and landing responses in flies tethered in a “virtual reality” flight arena. During these experiments, flies control the position of a 15� x 15� square in closed-loop by varying the amplitude of their left and right wings. At 6 second increments, the square expanded in both the horizontal and vertical directions. We measured changes in a fly’s wing stroke amplitude and frequency in response to this square expanding with velocities between 100 and 10,000�/sec, while optically tracking the fly’s legs to monitor stereotyped landing responses. The nature of the responses elicited by the expanding square depends upon the position of the square at the onset of expansion. If the square is in the lateral portion of the fly’s field of view at the onset of expansion, the fly increased stroke amplitude in one wing while decreasing amplitude in the other, an alteration causing the square to be rotated to the rear of the arena. This response represents a tethered flight analog of the saccades elicited by image expansion during free-flight. Frontal expansion of the stimulus results in wing responses causing no rotation of the square, but elicits an increase in wing beat frequency and leg extension indicative of a landing response. To determine how these separate responses elicited by similar visual stimuli interact, the dependence of each on of the position and rate of expansion was characterized. A model in which separate thresholds gate the neural estimates of lateral and frontal expansion is sufficient to account for the tethered flight data.