Infant contrast detectors are selective for direction of motion

In order to investigate the presence of directionally selective mechanisms in 3-month-old infants, we employed a summation-near-threshold paradigm previously developed for studies of adult vision (Levinson & Sekuler, 1975 Journal of Physiology (London), 250, 347-366); Watson, Thompson, Murphy & Nachmias, 1980 Vision Research, 20, 341-347). The degree of contrast summation occurring between two sinusoidal gratings moving in opposite direction was determined by comparing the contrast threshold for a compound stimulus (a counterphase-reversing grating) with the contrast threshold for one of its components (a single moving grating). Using the forced-choice preferential looking (FPL) technique, contrast thresholds were obtained for both counterphase and single moving gratings within individual infant subjects. Data were collected at several speeds, ranging from 2.8 to 66.8 degrees/sec (temporal frequency range: 0.7-16.7 Hz). At slow speeds, infants' thresholds were approximately equal for counterphase and moving gratings, indicating that non-directional mechanisms were responsible for detection. At an intermediate speed (22.3 degrees/sec), thresholds were nearly twice as high for counterphase gratings as for single moving gratings, indicating the existence of directionally selective mechanisms at detection threshold for this speed. For faster speeds, relative thresholds for the two types of stimuli fell between the two extremes; a model incorporating probability summation between directionally selective mechanisms was sufficient to account for the data. These results demonstrate that, at speeds greater than or equal to 22.3 degrees/sec (t.f. = 5.6 Hz), 3-month-old infants possess directionally selective mechanisms at threshold.