Spatial processing of visual information in the movement-detecting pathway of the fly

Abstract

1. Spatial processing of visual signals in the fly's movement-detecting pathway was studied by recording the responses of directionally-selective movement-detecting (DSMD) neurons in the lobula plate. The summarized results pertain to a type of neuron which preferentially responds to horizontal movement directed toward the animal's midline. Three kinds of visual stimuli were used: moving gratings, reversing-contrast gratings and reversing-contrast bars. 2. Contrast-sensitivity functions were measured for reversing-contrast gratings. With horizontally-oriented gratings, sensitivity is maximum at the lowspatial-frequency end and falls off toward high frequencies. With vertically-oriented gratings, sensitivity is maximum at an intermediate spatial frequency (Fig. 7). These results are consistent with a neural organization in which the DSMD neuron receives its input through an array of small-field (" sampling") units, each unit having a receptive field comprising an excitatory centre and horizontally-extending inhibitory flanks (Fig. 17). 3. Threshold contrast functions were measured for reversing-contrast bars (Figs. 11 and 12). The results for horizontally-oriented bars differ from those for vertically-oriented bars in a way that is consistent with the hypothesized neural organization. 4. Response to horizontally-moving, verticallyoriented gratings of various spatial frequencies were measured (Figs. 13 and 14) and the results used to infer the azimuthal angle A~ between the visual axes of sampling units participating in directionally-selectire movement detection (Fig. 18). At a mean luminance of 10 cd/m e, the inferred value of A'~ is approximately equal to the angle between the visual axes of

Neural correlates of illusory motion perception in Drosophila

“…When flicker was decoupled from motion in the reverse-phi out of phase stimulus, the neuron responded more strongly to a stimulus moving in its standard preferred direction (CCW). HSN also responded to a nondirectional full-field flicker stimulus in agreement with previous electrophysiological results from blowfly LPTCs (27,37). We attribute the weaker calcium responses to null direction reversephi and reverse-phi out of phase stimuli to the flicker components of these stimuli (see below).…”

A `bright zone' in male hoverfly (Eristalis tenax) eyes and associated faster motion detection and increased contrast sensitivity

“…Type 1 EMDs appear to dominate the response of both HSN and HSNE cells (Table·1). This agrees with previous experimental work on other flies in light-adapted conditions (Buchner, 1984;Srinivasan and Dvorak, 1980). The amount of saturation in the model can be inferred from the flatness of the curve, and the overall gain can be inferred by the height of the curve.…”

“…In control experiments (e.g. Figs·3, 4), the validity of this approach was confirmed by a contrast step technique, which has been used in earlier studies of contrast sensitivity in the fly Srinivasan and Dvorak, 1980). Contrast steps were performed for moving, Gaussianwindowed sinusoidal gratings at a given spatial and temporal frequency after exposure to a mean gray screen for at least 3·s.…”

“…This property has been previously exploited to provide confirmation of the validity of the correlator model in application to fly optomotor responses (Buchner, 1976;Buchner, 1984;van Hateren, 1990) and to directly compare the sampling distance of fly LPTCs with the spacing of ommatidia (Schuling et al, 1989;Srinivasan and Dvorak, 1980). We used this property combined with the known angular spacing of Eristalis ommatidia (Fig.·1A,C) to estimate the relative contributions of the EMD types 1 and 2 (with inputs from neighboring ommatidia and next-but-one neighboring ommatidia, respectively).…”

“…For this study, a fixed criterion response value of 2·mV above the resting potential was used as a threshold, analogous to the fixed criterion used by Srinivasan and Dvorak (Srinivasan and Dvorak, 1980). Other authors have used a 'detectability criterion', e.g.…”