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O.J. Braddick, D. Birtles, J. Atkinson, J. Wattam-Bell; Temporal Dependence of Direction-Reversal VEP in Infants . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1997.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: VEPs elicited by reversal of motion direction in a random dot pattern are an indicator of development of directional selectivity in infants' visual cortex (Wattam-Bell 1991). Comparison of direction-reversal (DR-)VEPs with orientation-reversal VEPs in the same infants suggest that directional selectivity has a relatively late onset in development (Atkinson et al, ARVO 2002). However, a complete picture of the development of direction selectivity requires information about how temporal properties of the system change with age. Methods: A group of infants were tested longitudinally between 6-18 weeks of age for DR-VEPs, with cross sectional data in age groups between 5-18 weeks from a larger group. Steady-state DR-VEPs were elicited by a moving random pixel pattern (velocity 5 deg/sec) with periodic reversals of direction. The random pattern was replaced at 2x reversal frequency to isolate direction-selective responses at the reversal frequency. Rates of 2 and 4 reversals/sec were tested in counterbalanced order in each session. The presence of a VEP response at the stimulus frequency with consistent phase was tested by Hotelling's T-squared test. Results: Significant responses at 2 rev/sec were first seen in the longitudinal at a median age of 10 weeks, and for 4 rev/sec at 12 weeks. The first individual significant responses were seen at 6 weeks. Over all infants, signal amplitude at the reversal rate rose from 5 to 13 weeks, and up to 11 weeks was consistently higher for 2 rev/sec than 4 rev/sec. Mean amplitudes fell between 13-18 weeks and in this age group 4 rev/sec elicits the stronger response. Conclusions: Motion processing revealed by the DR-VEP shows relatively increasing high-frequency response as the system develops from 6-18 weeks. Testing at 4 rev/sec may underestimate motion processing capability in the infant visual system. However, even at the lower frequency of 2 rev/sec few infants show directional cortical responses before 8 weeks of age. This can be compared with orientation reversal responses which also show earlier emergence of low-frequency responses, but at 3 weeks of age (Braddick, 1993. The result can also be related to behavioural measures at low reversal rates, showing a median age of onset around 7 weeks of age (Wattam-Bell 1996). The present results support the idea that while temporal response is an important aspect of functional cortical development, directional selectivity is consistently later to develop than orientation selectivity.
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