Development of the cone ERG in infants

PURPOSE. To assess cone photoreceptor and cone-mediated postreceptoral retinal function in infants. METHODS. ERG responses to a 1.8-log unit range of long-wavelength flashes on a white, rod-saturating background were recorded in 4-week-old ( n = 22) and 10-week-old ( n = 28) infants and control adults and children, 8 to 40 years of age ( n = 13). A model of the activation of cone phototransduction was fit to the a-waves. Sensitivity (S-CONE) and saturated-response amplitude (R-CONE) were calculated. The amplitude and implicit time of the b-wave were examined as a function of stimulus intensity. The cone photoresponse parameters were compared to the rod photoresponse parameters (S-ROD and R-ROD) in the same subjects. RESULTS. S-CONE and R-CONE in infants were significantly smaller than in the mature control subjects. The mean S-CONE was 64% and 68%, and the mean R-CONE was 63% and 72% in 4- and 10-week-olds, respectively. The mean rod photoresponse parameters were considerably less mature, as the mean S-ROD was 35% and 46%, and the mean R-ROD was 39% and 43% of mature values at 4 and 10 weeks. The b-wave stimulus - response functions in the 4- and 10-week-old infants did not show the photopic hill that was characteristic of the children's and adults' photopic b-waves. CONCLUSIONS. Peripheral cone function is relatively more mature than rod function in young infants. The lack of a photopic hill is hypothesized to result from immaturity in the relative contributions of ON and OFF bipolar cell responses.