Effects of field area and configuration on chromatic and border discriminations

Visual discrimination was tested with juxtaposed fields that differed only in luminance, S cone excitation, or the ratio of L to M cone excitation. Observers were asked to make discriminations based upon a criterion of color difference or of the perception of a border between fields. The area of the fields was varied either by increasing the length of their common junction or by increasing their widths at constant junction length. For fields larger than 0.2-0.4 deg2, area controls sensitivity. For smaller fields, discrimination may be worse for narrow fields with long common junctions than for wide ones with small common junctions, especially for discriminations that depend upon S cones, in which the field components tend to become spatially integrated. Decreasing viewing time reduces the magnitude of this effect. S cone and L-M channel acuities were estimated at about 7 and 32 c/deg, respectively.