David O. Robbins

A method for producing foveal retinal exposures in an awake, task-oriented, rhesus monkey*

A method for producing retinal exposures from a laser source in an awake, task-oriented S is presented. The technique allows for the measurement of rhesus monkey visual acuity immediately following exposures.

Subthreshold functional additivity occurring at the transition zone between temporary and permanent laser-induced visual loss

Damage criteria from laser irradiation have relied on fundoscopic and/or histological evidence. These methodologies have provided limited information regarding the functional impact of any observed damage and more importantly, do not lend themselves to assessments of the transition zone between temporary and permanent effects. Using a behavioral techniques, we have explored this transition zone with CW and Q-switched lasers. Our results demonstrate that daily exposure within the power levels of the transition zone become additive resulting in longer recovery times for successive exposures at the same power levels. Below this zone the impact of daily exposures appears non-additive; i.e., baseline acuity recovers to pre-exposure levels and both the magnitudes and durations of the recoveries appear unaffected by previous exposures. The additivity of successive exposures at the transition zone was most easily observed when relatively large-diameter (> 100 (mu) ), prolonged CW (100 msec) retinal exposures were made. When relatively small diameter (< 50 (mu) ), Q-switched (15 nsec) exposures were presented, significant decrements in acuity were not easily detected even with relatively intense exposures presented as single or multiple pulses within or across test sessions. These findings may reflect the contributions of different damage mechanisms evident in the transition zone.

Visual acuity changes in rhesus following low-level Q-switched exposures

Previously we have shown that visual deficits can be produced by long duration pulses at or slightly below traditional threshold levels for retinal injury. Initially the deficits produced were only transient shifts in baseline acuity that lasted less than 30 min, but successive exposures over a period of days at these same power levels were shown to be cumulative and their impact on visual acuity lengthened and became permanent. The present investigation extended these exposures to Q-switched, 532 nm Nd/YAG pulses presented to awake, task-oriented nonhuman primates performing Landolt ring discriminations. At and above the ED50, single pulses of minimal spot diameter produced only minor, transient shifts in visual acuity while repeated exposures produced significant shifts in acuity that became permanent over time. At lower energies, minimal spot, single-pulsed exposures again produced little observable consequence until either retinal spot sizes or number of pulses were increased. At these lower energy levels, however, no permanent functional loss was observed. Hence, the functional impact of single Q-switched pulses was more difficult to assess than longer time domain exposures. Multiple, low level Q-switched pulses, and/or larger spot sizes produced visual deficits similar to those observed for msec time domain exposures, suggesting both temporal and spatial summation at energy levels where no permanent effects have been noted.

Changes in Spectral Acuity following Laser Irradiation

Maximal visual acuity and normal color vision are closely associated with foveal receptor function. When the fovea is destroyed or altered by intense laser irradiation, severe and sometimes permanent losses in visual acuity ensue. In this paper we have shown that the threshold for long-term functional alterations in the fovea is determined not only by the wavelength and corneal exposure power of a laser flash but also by the types of performance criteria used to assess visual functioning. Chromatic acuity provides a much lower threshold for permanent alterations (Krypton — 6 mW; Argon — 3 mW) as opposed to achromatic targets (HeNe — 11 mW). Further, the use of chromatic acuity to assess post-exposure functioning can more accurately delineate the specific cone process most affected by the exposing source. The data from these studies also suggest that some relatively long-term (24 h) cumulative process within the retina is occurring which obviously will affect any threshold value determined.

Effects of small spot foveal exposure on spatial vision and ERG spectral sensitivity

In previous experiments, we observed that small spot foveal laser exposure could produce morphological retinal damage while having only transient or delayed effects on spatial visual function. These effects are markedly different from those obtained with large spot foveal exposures, where permanent function change was observed at exposure levels below the retinal damage threshold. In this paper, we have examined functional effects below and above laser retinal damage thresholds for small spot foveal exposures. At subthreshold levels we have observed a dependency of recovery on spatial frequency of the acuity test target, and no significant difference in recovery from exposure levels below or significantly above retinal damage levels. At levels that produce frank retinal damage, long term recovery involves complex retinal receptor system reorganization and inner retinal neural change.

Acute threshold damage, minimal spot, Q-switched exposure effects on focal contrast sensitivity

In previous investigations of minimal spot, q-switched laser visible (532 nm) on-line pulsed laser exposure, non-human primate (NHPs) required up to 13 times the retinal damage threshold for the emergence of permanent visual acuity dysfunction. In the present experiment, a Landolt ring contrast sensitivity task, employing 4 NHPs trained on a Landolt ring contrast sensitivity task, was employed to determine the effect of threshold retinal damage (532 nm, 3 microjoules, 20 Hz, PRF) on the slope of the NHP Landolt ring contrast sensitivity function measured under repeated threshold exposure conditions. All four animals initially showed uniform deficits in contrast sensitivity requiring about 6 to 16 min post exposure for complete recovery. Over several months of repeated exposures made in 3 NHPs, a steepening of the contrast sensitivity slopes appeared, manifested by an inability to measure sensitivity for the finest Landolt ring spatial frequencies and an associated enhancement in the mid and lower spatial frequencies. In one animal that had undergone a longer period of repeated threshold laser exposure, the slope of its contrast sensitivity function shifted to a uniform deficit relative to pre exposure with an inability to provide sensitivity measurements above 20 Hz/deg, demonstrating an abrupt loss in the ability to measure the higher spatial frequencies associated with maximal optimal visual acuity. Ophthalmic retinal observations demonstrated the presence of punctate lesions induced by minimal spot foveal exposure. These data support measurements of high contrast acuity undergoing repeated q-switched exposure at damaging levels. While the acuity in such exposure situations eventually undergoes a permanent deficits, measurements of the entire contrast sensitivity function, in which acuity function maybe obtained at and above 60 Hz/deg, may reveal enhanced lower spatial frequency sensitivity as well as permanent a permanent deficits in higher spatial frequencies.

Laser-induced acute and long-term alterations to visual fuction

We examined acute laser exposure effects in awake task- oriented non-human primates. These animals were trained to discriminate between various acuity targets that initially exceeded 1 minute of arc. They were exposed in the fovea and parafovea by aligning the output from a laser source with the gap of threshold Landolt ring. Parafoveal exposures were produced by offsetting the laser source and Landolt ring gap by 1 degree. For small spot exposures (< 100 μm) using repetitively pulsed (20 Hz) Q-switched laser (532 nm) pulses above the retinal damage threshold, initial acuity deficits returned to pre-exposure baseline acuity within 30 minute postexposure. Periodic ophthalmoscopic examinations revealed punctate lesions within the fovea as well as parafoveal region. Off-axis exposures produced a greater abundance of parafoveal punctate lesions. With repetitive exposures over a period of 6 months to one year, immediate postexposure recoveries in visual acuity lengthened from minutes to hours and eventually the initial deficits became permanent. These results suggest that Q-switched visible laser pulses may induced photoreceptor pathology prior to the observance of significant acuity changes and/or initiation of the photochemical transduction process. If this hypothesis is correct, Q-switched laser damage may be more selective to the outer segment of the photoreceptor where the transduction mechanism presumably is resident.

Small-spot laser-exposure effects on visual function

Laser field exposure effects on visual function involve produc tJon of minimal spot irradiation on or near the huntan fovea. Functional effects of such exposure may involve transient or perinanent change in visual function depending upon exposure dose. While Maximun Permissible Exposure (MPE) lirrtits define exposure in terins of threshold retinal niorphological change such limits are not applicable with regard to transient changes in visual function below MPE limits induced by alteration in retinal physiological processes. Mechanisms of transient and permanent functional change reported in these exper iments point out the need to examine laser safety limits in terms of both the functional as well as the morphological disturbance induced in retinal tissue. L

Transient Visual Effects of Prolonged Small Spot Foveal Laser Exposure

Development of a test of foveal function during and after small spot foveal exposure was the primary objective of this investigation. This objective was accomplished. At retinal damage levels, only a small focal foveal lesion was observed indicating the ability to utilize the fovea during such exposure. Postexposure recovery effects analyzed for target size and contrast conditions suggest retinal and possibly cortical saturation processes.