Integration of one- and two-dimensional motion signals in infants: evidence from the barber-pole illusion

Several previous studies in adults have investigated how one- and two-dimensional moving features are integrated into a coherent global motion percept by studying the "barber-pole illusion"; when a one-dimensional moving grating is presented within a rectangular aperture, the two-dimensional line terminators at the edges of the aperture bias the perceived direction of motion toward the longer axis of the aperture. In the current study, we used barber-pole stimuli to investigate the development of motion mechanisms that integrate one- and two-dimensional motion signals. Using a directional eye movement technique, we measured responses to obliquely moving gratings presented within horizontally vs. vertically oriented apertures, in infants (ages 2-5 months) and adults. For all ages, we found that horizontal eye movements were significantly stronger when gratings were presented within horizontal than within vertical apertures, as predicted by the barber-pole illusion. Additionally, we devised a way to infer the "effective shift" in eye movement direction produced by the barber-pole illusion. Using a simple motion integration model, effective shift values were then used to calculate the relative weightings of one- and two-dimensional motion signals to direction coding. The results show that by 2 months of age, infants integrate one- and two-dimensional motion signals, and that the relative weighting of one- and two-dimensional signals remains roughly constant from 2 months of age into adulthood.