Visual Angle (deg)

Fig. 4.3. Percentage of correct choices as a function of the visual angle subtended by different color discs presented against an achromatic (gray) background. Each symbol corresponds to a different color stimulus. A 95% confidence interval is depicted for each data point. The dotted horizontal line represents the 60% choice level used to determine the minimum visual angle (amin) that the disc must subtend at the eye in order to be detected.

Variation in UV and blue contrast did not affect detection (Giurfa et al. 1996b).

These results suggest that achromatic and chromatic cues are used in succession during the approach flight: from afar, detection requires the presence of green contrast (an achromatic cue), whereas, as the bee comes closer, it uses chromatic (color) information.

Detection versus recognition: alternative use of achromatic and chromatic cues

The results above (Giurfa et al. 1996b) suggested the alternative use of achromatic and chromatic cues in the detection and recognition of colored targets. However, the stimuli that were detected at an angle of 5° (Fig. 4.3) contained both chromatic and achromatic (green) contrast, whereas those that were only detected at a visual angle of 15° contained chromatic contrast, but no green contrast. The data do not let us conclude that green contrast cannot be used when the stimulus subtends a large angle at the n xi r m

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