Contrast and luminance dependence of target choice and visual orientation in walking stick insects.

When presented with static images, animals show robust preferences for particular visual features, and reliably turn towards and approach selected visual landmarks. In target choice paradigms, stick insects tend to approach edges with high image contrast, but also show robust orientation based on luminance alone. To better understand which stimulus features actually govern turning towards static visual targets, this study tests the relative importance of two elementary cues of spatial vision - luminance and contrast. We do so in a large open-field arena, using luminance-modulated, static 360° patterns with and without high-contrast edges. We show that target choice strongly depends on image contrast, though with a bias towards areas of low luminance. Comparison of heading directions during approach with terminal locations at the arena wall suggests an early, coarse orientation based on luminance, with subsequent steering towards high-contrast regions. When walking towards a target with high-contrast edges, the likelihood to turn away towards a Gaussian distractor image increases with decreasing edge contrast of the original target. Subjective equality of the two images occurs for an approximate 2:1 weighing of contrast and luminance, indicating that a stronger contrast-dependent edge-orientation mechanism acts in parallel with a weaker luminance-dependent phototaxis mechanism. Given the significance of stick insects as study organisms for the control of legged locomotion, future research may now test whether the two visual orientation mechanisms lead to distinct turning responses at the level of step pattern or leg movement variables.