Development of infant reaching in the dark to luminous objects and 'invisible sounds'

Two studies were conducted to investigate the existence of an unusual U-shaped developmental function described by Wishart et al (1978) for human infants reaching towards invisible sounds. In study 1, 2-7 month olds were presented with four conditions: (i) an invisible auditory stimulus alone, (ii) a glowing visual stimulus alone, (iii) auditory and visual stimuli on the same side (ie combined), and (iv) auditory and visual stimuli on opposite sides (ie in conflict). Study 2 was designed to examine the effects of practice and possible associations made when using the 'combined conflict' paradigm. Infants of 5 and 7 months of age were given five trials with the auditory stimulus, with or without prior visual experience, and five trials with the visual stimulus, with the position of the stimulus varied on each trial. Stimuli were presented individually at the midline, and +/- 30 and +/- 60 degrees from the midline. In both studies testing was conducted in complete darkness. Results indicated that the auditory-alone condition was slower to elicit a reach from the infants, relative to the visual-alone one, and reaches were least frequent to the auditory target. No U-shaped function was obtained, and reaching for auditory targets occurred later in age than for visual targets, but even at 7 months of age did not occur as often and was achieved by fewer infants. In both studies the quality of the reach was significantly poorer to auditory than to visual targets, but there were some accurate reaches. This research adds to our understanding of the development of auditory-manual coordination in sighted infants and is relevant to theories of auditory localization, visually guided reaching, and programming for the blind.