Recovery of the Rod Photoresponse in Infants

Abstract

Abstract: : Purpose:Test the hypothesis that the kinetics of recovery of the rod photoresponse in young infants differ significantly from those of adults. During development, the gain of the processes involved in the activation of phototransduction is scaled by rhodopsin content, and the ratio of gain to saturated amplitude is invariant with age. Infants have short rod outer segments and low rhodopsin content. Methods:Recovery of the rod photoresponse was studied using a paired flash paradigm. Subjects were infants aged 4-weeks (N=11), 10-weeks (N=11), and adults (N=8). Full field ERG a-wave responses to a bright, saturating probe flash (+3.6 log scot td sec) were recorded at 8 selected intervals (1 to 120 sec) after a test flash (+3.6 log scot td sec). Between each test-probe pair, 2 minutes in the dark were allowed. At each interval, the amplitude of the response to the probe was expressed as the percent of the response to the probe alone. The time (t₅₀) at which the amplitude was 50% was determined. Results: For every subject, an exponential function describes the recovery of the photoresponse (% amplitude vs inter-flash interval). The t₅₀ values of infants are significantly longer (F=13.9, df: 2,27; p<0.01) and more variable than those of adults. The median t₅₀ values are: 4- weeks, 11 (range 7 to 23) sec; 10 weeks, 7 (range 4 to 14) sec; adults, 3 (range: 2 to 5) sec. Furthermore, across all ages, t₅₀ values are inversely proportional to the gain of activation of phototransduction. Conclusions:Recovery of the photoresponse inferred from ERG responses to full field stimuli is slower and more variable in infants than adults. Possibly the randomness of inactivation of R* is greater in immature than in mature rods.

Keywords: electroretinography: non-clinical • visual development: infancy and childhood • photoreceptors