Depth perception after prolonged usage of night vision goggles.

The present study was initiated following a report that a few helicopter pilots had failed a test of stereoscopic depth perception after a prolonged training flight employing night vision goggles (NVGs). In order to determine the cause of the loss, 12 helicopter pilots/copilots were assessed for depth perception, lateral and vertical phoria, and contrast sensitivity before and after training flights requiring the pilots to wear night vision goggles for the duration of the flight. Pilots flew one to three missions while wearing either PVS-5A or AN/AVS-6 goggles. Mission duration ranged from 1 to 4 h. The results indicate that contrast sensitivity and depth perception when monocular cues are present did not degrade over the course of the mission. Lateral phoria, however, did demonstrate an average exophoric shift of 1.5 prism diopters for 12 out of the 24 missions. The results indicate that the original report of a loss of depth perception based on a test of depth requiring stereopsis might have been caused by a shift in lateral phoria. It would be expected that as additional fusional effort is required, the minimum resolvable disparity degrades due to the increase in accommodation brought about through vergence accommodation. Possible causes for the phoria shift and future testing are discussed.

Role of vergence accommodation in correcting for night myopia.

Although the dark-focus or resting position of accommodation provides a parsimonious interpretation of the anomalous myopias, in correcting for night myopia the role of vergence accommodation must be considered. Both contrast sensitivity (CS) and accommodation were determined for monocular and binocular observation without and with optical corrections based on the individual dark-focus. The data suggest that the availability of a stimulus for vergence accommodation may modify the magnitude of an optical correction for night myopia.

The effect of spectrally selective filters on visual search performance.

The effect of five spectrally selective filters on the performance of an acuity-dependent visual search task was evaluated. The filters were: A) a neutral density filter (control condition); B) a 5200A green interference filter; C) a 3215-250 red filter; D) a neodymium visor; and E) a holographic visor. The observers were presented with 5 blocks of 10 slides per filter. Each slide projected a 6 degrees X 6 degrees field of 900 letter O's--each 10' of arc--which contained a single Landolt C. The observers were required to find the C and indicate the position of the opening in the C. The opening in the C subtended 2.64' corresponding to an acuity of 0.38. Response time, error rate, accommodative accuracy, and the number and duration of fixations were recorded for each slide presentation. The results demonstrated that filter type had no effect on any of the response measures. During the first three trial blocks, the observers appeared to optimize their search strategies, after which they began to revert to their initial performance levels. However, this effect was not supported statistically.

Reaction times to different spatial frequencies as a function of detectability.

Simple reaction time to sine-wave gratings of 1, 4 and 10 c/deg was measured as a function of the detectability of the gratings. Reaction time increased with spatial frequency over a range of detectabilities. Assuming that equally detectable gratings produce equivalent levels of response in the visual system, this increase in reaction time reflects an increase in perceptual latency as spatial frequency is increased.