Wavelength sensitivity was measured in the guppyfish by means of optomotor responses to a special apparent-motion display. A set of red and green bars appeared to humans to move to the left if red was darker than green, but to the right if red was lighter than green. At equiluminance there was no apparent motion. By noting the direction in which the fish swam to follow the stripes we were able to record equiluminance points for red, green and blue. Store-bought guppies (Poecilia reticulata) were mildly protan compared with humans, and wild-strain guppies were strongly protan, being 50% more sensitive to short wavelengths and 67% more sensitive to medium wavelengths than human observers. We also measured optomotor responses to achromatic Michelson contrast: responses were maximum if the contrast exceeded 0.3. Finally, the optomotor threshold (signal/noise ratio) for motion coherence was 20% for fine dots and 40% for coarse dots. These stimuli should be easy to use on any non-verbal species.
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