Fig 1. Example plot of conditioning index data.
Animals had been trained to discriminate a reference stimulus of 500 nm from stimuli of other wavelengths as indicated on the horizontal axis. The vertical axis shows for each pair how many animals, above chance, gave the correct response. The black solid line shows a linear interpolation of the data points. The dashed line indicates an (arbitrarily chosen) threshold of 20%. δλi are the ranges between the reference wavelength and the intersection between the threshold and the interpolation of the data, as indicated by the gray horizontal lines. The midpoints of these ranges, λ1 and λ2, are the virtual reference wavelengths as used to derive the wavelength discrimination function [24].