Dark-adaptation mechanisms of the long-wave foveal cones

The pupil photostress test

PURPOSE

An objective, pupillographic photostress test was developed to assess the initial sensitivity loss and its rate of recovery in healthy eyes and in eyes affected by optic neuropathy.

METHODS

Pupillary light reflexes were recorded with an infrared pupillometer after a binocular photostress that subtended 30 degrees of the central visual field. The initial loss of light sensitivity (in log units) and its recovery with time were derived by analyzing subsequent increases in pupil contraction amplitude to a series of alternating light stimuli. The effects of photostress duration and intensity were studied in ten control subjects. Photostress recovery also was compared between eyes of nine patients with unilateral optic neuropathy.

RESULTS

In the ten control subjects, increasing photostress duration caused significantly increased sensitivity loss at time zero after the photostress (analysis of variance), but did not significantly affect the exponential recovery rate. Intense photostress brightness caused more initial sensitivity loss than did dimmer photostress lights. In addition, half-time of sensitivity recovery was prolonged after the brightest photostress. In nine patients with unilateral optic neuropathy, a 120-second photostress caused significantly less initial sensitivity loss in the affected eye than in the healthy fellow eye.

CONCLUSIONS

The pupil photostress test objectively can measure the eye's loss of light sensitivity after a photostress and its exponential rate of recovery. Loss of optic nerve function diminishes the initial stress-induced loss of light sensitivity and does not significantly affect the poststress rate of recovery in the patients tested.

Mechanisms of long-term dark adaptation

It has previously been suggested that long-term dark adaptation is controlled by bleaching signals that regulate the activity of an allosteric, positively cooperative protein (Stabell et al., 1986a, b). Recent biochemical evidence strongly supports this assumption, indicating that the primary regulator of the light-sensitive channels in the plasma membrane of the outer segments of the photoreceptors is a homo-oligomeric, allosteric, positively cooperative protein. In this report, we discuss the possibility that signals from bleached photopigments may control the dark-adaptation process through the allosteric protein of the plasma membrane. It is suggested that the concentrations of the bleached photopigment and of the allosteric effector are reciprocal quantities.

Dark adaptation of the long-wave cones at different eccentricities

Using a Wright colorimeter the ordinary long-term, long-wave cone dark-adaptation curve was measured at 0, 2, 4, 7, 17, 25, 40 and 49 degrees nasally in the visual field. In opposition to previous findings, the results show that the dark-adaptation function of the long-wave cones changes markedly when the test field is moved outward from the rod-free fovea. It is suggested that the kinetics of the long-wave cone photopigment change with eccentricity. Also, at variance with previous findings, the present curves at all eccentricities may reasonably well be interpreted as consisting of three different sections; a first section where the threshold decreases rapidly, followed by a major, approximately linear section and a terminating section that converges asymptotically towards the final level of sensitivity. This finding suggests that the dark-adaptation process of the cone system, under the given experimental conditions, is based on three somewhat different processes.