Visual sensitivity to varying levels of brightness. While visual acuity concerns the discrimination of stimuli with maximum contrast, contrast sensitivity has to do with the detection of visual displays that are neither maximally dark nor light. If the visual display consists of a sine wave grating pattern of alternating dark and light bars (or stripes), then it can be analysed in terms of its spatial frequencies (i.e., the number of cycles of dark and light bars over a given region of the display). The contrast threshold is the smallest difference in spatial frequency between the dark and light bars that enables the observer to detect them. Accordingly, contrast sensitivity is 1/contrast threshold, which provides an indication of the visual system’s ability to analyse and synthesize variations in luminance. Developmental data, based on preferential looking and visual evoked potentials, point to marked improvements in both contrast sensitivity and visual acuity over the first three months after birth (see figure below). It develops at a low spatial frequency to an asymptote at about 9 weeks of age, and continues to develop at high spatial frequencies until grating acuity is fully developed.
