Theory and Experiments Concerning the Heterochromatic Threshold-Reduction Factor (HTRF)*

Selective chromatic adaptation occurs when a change in spectral distribution and/or luminance of an adapting field alters the sensitivity of the eye (as measured by the reciprocal threshold of superposed test flashes) differentially with respect to wavelength of the test stimuli. A convenient test of selective chromatic adaptation is to measure homochromatic (e.g., red on red) and heterochromatic (e.g., red on green) thresholds for two different relative spectral distributions, each being used in a conditioning field as well as in a test flash. An index $\overline{\eta }$ is defined by the sum of the two homochromatic thresholds minus the sum of the two heterochromatic thresholds, where all values are expressed in log units This index expresses the factor by which the heterochromatic thresholds are lower than the homochromatic ones (the heterochromatic threshold-reduction factor or HTRF). It is shown that $\overline{\eta }$ must equal zero for an eye having only one class of spectrally selective mechanism; data obtained under scotopic conditions are presented to verify this conclusion. It is also shown that if $\overline{\eta }$ is significantly different from zero, more than one class of mechanism must have contributed to the threshold response, and further that these two or more classes of mechanisms have been selectively adapted. Data are also presented to illustrate such selective adaptation where $\overline{\eta }$ has been measured at photopic levels as a function of field luminance and spectral separation between stimuli being compared.