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J Wattam-Bell; Vertical Motion Asymmetries in Infants: Contrast Increment Thresholds and Contrast Matches . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3996.
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Purpose: 11-week-olds show a preference for downwards- over upwards-drifting gratings which is greatest for contrasts of 10 & 20%, weaker at 40%, and absent at threshold (Wattam-Bell 2001: Perception 30, 159). These results suggest that at intermediate contrasts downwards directional mechanisms have a higher gain than upwards mechanisms, while at high contrasts both mechanisms saturate, and are therefore equally responsive. To test this, the present experiments on 10-12-week-olds measured (a) contrast increment thresholds for the two directions; (b) contrast matches for gratings moving in opposite directions; and (c) preferences between high contrast gratings moving in opposite directions. Methods: All experiments used 0.25 c/deg horizontal gratings drifting vertically at 22.6 deg/sec, presented in pairs on a large video monitor. The infants were tested with forced-choice preferential looking. Increment thresholds for each direction were measured separately with 2-up/1-down staircases; trials for the 2 directions were interleaved. The contrast matching experiment used a 1-up/1-down staircase. Direction preferences were based on 20-30 trials per infant. Different groups of infants participated in each experiment. Results: (a) For base contrasts in the range 5-40%, increment thresholds were significantly smaller for downwards motion. (b) A high contrast (97%) upwards-drifting grating was matched by a significantly lower contrast (65%) downwards grating. (c) At both 50 & 97% contrast, infants showed a significant preference for downwards motion (40 and 32% respectively). Conclusion: The increment threshold data indicate that in 2-month-old infants, downwards directional mechanisms have a higher gain. However, neither the matching or preference data show evidence for a reduced downwards bias at high contrast: downwards mechanisms appear to be more responsive at all contrasts above threshold.
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