The effect of stimulus intensity on the size of perceptive fields

Perceptive field sizes for the four elemental hues were measured at 10° temporal retinal eccentricity for a series of stimuli ranging in intensity -0.7 to 3.3 log td. Following 30 min dark adaptation, the three observers used the "4+1" color-naming paradigm to describe the color appearance of 23 monochromatic stimuli (440-660nm) for five different stimulus sizes. Mean hue responses as a function of stimulus size were determined for each wavelength at each intensity level. The Michaelis-Menton function was then fitted to each of the mean hue responses to derive perceptive field sizes. Results indicate that stimulus intensity affects the size of perceptive fields and that, in general, perceptive field sizes decrease as stimulus intensity is increased. Furthermore, whereas green is always the largest perceptive field and red is always the smallest, the differences in the sizes among the four perceptive fields diminish at the higher intensity levels. The results from this study will be discussed in terms of the contribution of rod signals to color appearance.