WAVELENGTH AUTOMATED PERIMETRY ( SWAP ) Chris

The pioneering work of Stiles provided a means of psychophysically isolating and measuring the sensitivity of individual color vision mechanisms through the two-color increment threshold procedure. Basically, this approach involved decreasing the sensitivity of some color vision mechanisms (termed π mechanisms by Stiles) by using a chromatic adapting background light, and then measuring the sensitivity of another color vision mechanism by means of a narrow band chromatic stimulus. According to Stiles’ terminology, π0 refers to the sensitivity of the rod system, π1, 2 and 3 are short wavelength (“blue”) sensitive mechanisms, π4 is a middle wavelength (“green”) sensitive mechanism, and π5 is a long wavelength (“red”) sensitive mechanism. Isolation of π1, the principal short wavelength (“blue”) sensitive mechanism, was best achieved with a high luminance (greater than 50 cd/m) white or broad spectrum yellow background (530 nm short wavelength “cutoff” filter), and a large (greater than 2 degrees in diameter) narrow band (440 nm peak wavelength with a 10-20 nm bandwidth) short wavelength stimulus. The figure below and to the right shows the spectral sensitivity of three color vision mechanisms (π1, π4, and π5) under normal viewing conditions on the left graph. The vertical blue line indicates the peak of the short wavelength mechanism and the vertical yellow line indicates the peak wavelength of the background. The graph is plotted in a threshold versus wavelength format, in which the background chromaticity and luminance are constant, the stimulus wavelength is varied and the stimulus increment threshold is determined. Note that under these conditions, sensitivity to a short wavelength stimulus is higher for the middle and long wavelength systems than for the short wavelength system. The graph to the right shows the same spectral sensitivity profile in the presence of a bright broadband yellow background. Here one can observe that there is substantially decreased sensitivity for the middle and long wavelength mechanisms, thereby permitting the short wavelength mechanism’s sensitivity to be isolated and measured.