Discrimination of edge orientation by bumblebees

Abstract

Simple feature detectors in the visual system, such as edge-detectors, are likely to underlie even the most complex visual processing, so understanding the limits of these systems is crucial for a fuller understanding of visual processing. We investigated the ability of bumblebees (Bombus terrestris) to discriminate between differently angled edges. In a multiple-choice, "meadow-like" scenario, bumblebees successfully discriminated between angled bars with 7° differences, significantly exceeding the previously reported performance of eastern honeybees (Apis cerana, limit: 15°). Neither the rate at which bees learned, nor their final discrimination performance were affected by the angular orientation of the training bars, indicating a uniform performance across the visual field. Previous work has found that, in dual-choice tests, eastern honeybees cannot reliably discriminate between angles with less than 25° difference, suggesting that performance in discrimination tasks is affected by the training regime, and doesn't simply reflect the perceptual limitations of the visual system. We used high resolution LCD monitors to investigate bumblebees' angular resolution in a dual-choice experiment. Bumblebees could still discriminate 7° angle differences under such conditions (exceeding the previously reported limit for Apis mellifera, of 10°, as well as that of A. cerana). Bees eventually reached similar levels of accuracy in the dual-choice experiment as they did under multiple-choice conditions but required longer learning periods. Bumblebees show impressive abilities to discriminate between angled edges, performing better than two previously tested species of honeybee. This high performance may, in turn, support complex visual processing in the bumblebee brain.

Fig 1. Experimental setup.

A: Multiple-choice arena. A bumblebee colony was housed in a wooden box, connected to the experimental arena by a transparent tunnel. Ten feeders were presented in a circle configuration so that all were visible from the centre of the arena. B: Feeder. Each feeder consisted of a white paper disc with a printed black bar, and a feeding tube in the centre. A heavy block weight at the rear of the feeder supported a rotatable 360° protractor, allowing the bar to be rotated to the required angle. Five feeders, allocated at random, had the bar oriented to angle A and contained a drop of sucrose solution; the others were oriented to angle B and contained water only. C: Dual-choice arena. Two computer monitors were arranged at a 60° angle at the rear of the arena. Each presented a magenta bar in a white circle. A feeding tube was placed at the centre of each bar. One bar, allocated at random was oriented to angle A and the feeding tube contained a drop of sucrose solution; the other was oriented to angle B and contained water. D: Stimuli used in dual-choice experiments. Top row, two stimuli used during pre-training, one on each screen. Bottom row, example bar stimuli used for training and testing: left, 0° bar; right, 7° bar. All stimuli were displayed in magenta (RGB: 255, 0, 255) on a white background (RGB: 255, 255, 255).

Fig 2. Learning curves for bees discriminating angles that differ by 7°.

Each marker shows the mean proportion of choices (±S.E.) for the trained angle, A, across a block of 10 consecutive choices. Red circles, experiment 1 (multiple-choice, N = 25); blue circles, experiment 2 (dual-choice, N = 6). Dashed grey horizontal lines indicate a chance level of performance (0.5) and the threshold criterion we used as a proxy for the end of the learning period (0.8, but note that bees had to reach this proportion of correct choices over 20 feeder visits).

Fig 3. Angular discrimination is unaffected by training bar angles, in multiple-choice tests.

A: Mean proportion of choices (±S.E.) for the trained angle, A, across each block of 10 consecutive choices for bees in experiment 1. Red triangles indicate bees that were trained to discriminate bars of -60° ± 7° (N = 9); blue circles, bees trained on 45° ± 7° bars (N = 9); green squares, bees trained on 90° ± 7° bars (N = 6). Dashed grey horizontal lines indicate a chance level of performance (0.5) and and the threshold criterion we used as a proxy for the end of the learning period (0.8 but note that bees had to reach this proportion of correct choices over 20 feeder visits). B: Proportion of choices for angle A (correct choices) across the last 20 feeder visits by bees trained to each group of angular orientations. Red lines indicate group median, boxes indicate the interquartile range and whiskers indicate the range. Filled circles show the proportion of choices for angle A, made by each individual. There are no significant differences between groups.