In ROP subjects, as in controls, detection thresholds depend on the total energy of the stimulus up to a critical duration, t
CRIT (
Fig. 1). We found that t
CRIT was significantly longer in subjects who had ROP, whether mild or severe, than in subjects who were born prematurely but never had ROP or in term-born control subjects. In this sample, t
CRIT did not vary significantly with the severity of ROP (
Fig. 2). There was no difference in t
CRIT between those who never had ROP and term-born control subjects; their values agree with those reported in other studies of temporal summation in healthy adults.
1–3,5,6
Classic ERG studies suggested that temporal processes are limited by events at the level of the photoreceptor.
8–10 As phototransduction became understood in molecular terms, Lamb and Pugh
12 presented a model of the biochemical steps in the activation of rod phototransduction. Rod photoreceptor sensitivity derived from the a-wave of the ERG is proportional to the amplification constant,
11 which in turn depends on the time constants of events in the rod outer segment.
12,21 Harris et al.
13 found low rod photoreceptor sensitivity in ROP subjects many years after their active disease had resolved. In 10- to 11-year-old children with a history of mild or severe ROP, median rod sensitivity (6.9 isoms
−1 · s
−2) was approximately two-thirds of the normal adult value (10.2 isoms
−1 · s
−2).
13 In the models of activation, photoreceptor sensitivity (S
ROD) is related to the time constants (
τ) in the outer segments by the relation S
ROD = 1/
τ2. For the above values of S
ROD, the corresponding time constants are 381 ms in ROP compared to 312 ms in our healthy adult control subjects. Thus, the long t
CRIT values in ROP subjects may be a consequence of changes in the function of the rod outer segment. Our data do not permit exact comparison of critical duration and the time constants of activation of phototransduction. Proximal to the rod outer segment, the visual pathway further integrates information. The present results, however, suggest that the rod outer segment has a role in setting the critical duration for complete temporal summation.
We conclude that ROP has a significant long-term effect on temporal summation, perhaps as a consequence of altered rod photoreceptor function. Taken together with our recent report that altered postreceptor retinal circuitry underlies abnormal scotopic spatial summation,
22 the temporal summation results reported herein add to the evidence that ROP has long-term effects on vision.