Monday, May 22, 2017

To bees, edges are green

In color science, we like to start from spectral data. To obtain the relative response in a photoreceptor, we multiply the reflectance function of a stimulus with the illuminant spectrum and then integrate over the visual spectral range using the receptor's spectral sensitivity function as the integration measure, up to a normalization factor.

Most often, what changes are the stimuli. Sometimes, we change the illuminant to predict the response under a different light source. When we study the response of people with color vision deficiencies, we swap the spectral sensitivity functions, for example, we shift the peak frequencies of the M or L catch probabilities to simulate deuteranomaly respectively protanomaly. For people with normal color vision, the standard values for the peak sensitivities are approximately 430 nm (S-cones), 540 nm (M-cones), and 570 nm (L-cones).

The approach is not limited to humans. For example, bees also have three receptors, with peak sensitivities at 344 nm (S), 436 nm (M), and 544 nm (L): their visual spectrum is shifted towards the ultraviolet. If we taught color naming to bees, their red would correspond to our green. Actually, looking at the honeybee (Apis mellifera) sensitivity functions, their color vision is different from ours because they have a secondary peak in the UV region. With only 10,000 ommatidia, their vision also has a much lower spatial resolution.

spectral sensitivity functions of the honeybee (Apis mellifera) receptors

In their paper Multispectral images of flowers reveal the adaptive significance of using long-wavelength-sensitive receptors for edge detection in bees, Vera Vasas et al. use a collection of multispectral photographs of flowers preferred by bees.

Assuming bees and flowers coevolved to maximize pollination, the authors perform an interesting statistical analysis of what bees would see, to determine under what condition they can best recognize the flower's center areas where the nectar and the stamens/carpels are located. An important boundary condition is that the process has to work in the presence of movement, as flowers are swayed by Zephyr.

The statistical analysis suggests that bees use only the L-receptors to identify edges and segment the visual field and detect movement. This process is different from us humans who use the M- and L-receptors to analyze an essentially monochromatic image.

Citation: Vasas, V., Hanley, D., Kevan, P.G. et al. J Comp Physiol A (2017) 203: 301. doi:10.1007/s00359-017-1156-x