Purpose

The electrical response of the human retina to light stimuli can be recorded non-invasively as the electroretinogram (ERG). The light adaptation effect (LAE) refers to the increase, over several minutes, in the amplitude of the ERG elicited in response to repeated flashes delivered superimposed on a light-adapting background, following a prior period of dark adaptation.  Previous studies investigating the LAE measured the a-wave and b-wave elicited by brief flash stimuli. We used long duration stimuli to explore the LAE of the response to stimulus onset and offset.

Methods

Bilateral ERG recordings were made to 200ms flash stimuli over the course of light adaptation in 3 healthy adult male participants (6 eyes), aged 21, 21 and 32. Participants gave written informed consent, and the study had Research Ethics Committee approval and conformed to the tenets of the Declaration of Helsinki. Pupils were dilated pharmacologically and conductive fibre electrodes were placed in the lower conjunctival fornix. Participants were fully dark adapted for 20 minutes, and then exposed to a white rod-suppressing background (40 cd m^-2), and 50 white 200 ms flashes (250 cd m^-2) were delivered in quick succession. The train of flashes was repeated at 2-minute intervals for up to 20 minutes; responses to each series of 50 flashes were averaged. The amplitude and peak time of the response to stimulus onset (a-wave, b-wave) and offset (d-wave) were measured.

Results

In all participants, the a-wave, b-wave, and d-wave increased in amplitude during the course of light adaptation, reaching a plateau after 10-12 minutes following the onset of the background. The peak times were found to shorten during this period. A time point of 14 min following background onset was chosen for comparison: the amplitudes of all components were significantly larger (p<0.013) and peak times significantly shorter (p<0.01) compared with the corresponding values measured immediately following background onset (paired t test). The mean (SEM) proportional increases in amplitude were 43 (19)%, 38 (5.5)% and 73 (20)% for a-wave, b-wave and d-wave respectively. Mean (SEM) advancements in peak time were 1.1 (0.2), 3.0 (0.6) and 2.8 (0.6) ms respectively.

Conclusions

We consistently found an increase in amplitude for all 3 components, indicating that the response to both stimulus onset and offset increases in amplitude over the course of light adaptation. Response kinetics also appear to become more rapid over this time period. The mechanisms underlying the LAE are still unknown, but our findings indicate that they apply to electrical activity elicited by both onset and offset of light stimuli.