Robert B. Post

ADAPTATION OF TONIC ACCOMMODATION

Abstract We compared the resting (dark) focus of accommodation before and after adapting to accommodative stimuli placed nearer or farther from an initial baseline resting focus. Short‐term monocular adaptation (< 2 min) did not result in consistent after‐effects that were correlated with the adaptation stimulus. After short‐term adaptation, accommodation returned to its resting level in 2–15s. Long‐term monocular adaptation (30 min) to a 6‐D near stimulus resulted in a small (0.5‐D) average increase in the resting focus of accommodation beyond the normal 2–15‐s short‐term decay. These observations illustrate a tonic adaptation of accommodation that is small and requires longer durations of adaptation than an analogous adaptation of the fusional vergence system to prism.

Circular Vection is Independent of Stimulus Eccentricity

The sensation of self-rotation induced by viewing a surround rotating about the observers vertical axis (circular vection or CV) was investigated with equal-area stimuli located in either the central, the mid-peripheral, or the far-peripheral visual field. Magnitude estimation responses indicated greater CV with larger stimulus area, but no significant differences in CV sensations as a function of stimulus eccentricity. This pattern of results does not support the belief that CV is dominated by peripheral stimulation when equal-area stimuli are compared.

Induced motion and optokinetic afternystagmus: Parallel response dynamics with prolonged stimulation

Fixation of a stationary target during motion of background contours attenuates optokinetic nystagmus (OKN), while illusory induced motion (IM) of the fixated target occurs opposite the direction of contour motion. It is proposed that IM owes to a perceptually registered efferent signal for ocular pursuit which opposes an unregistered signal for OKN to achieve stable fixation. This proposal predicts parallel changes in the magnitudes of IM and optokinetic reflexes during and after optokinetic stimulation. Accordingly, leftward IM magnitude and rightward slow-phase velocity of optokinetic afternystagmus (OKAN) increased at similar rates across 90 and 160 sec of 60 deg/sec motion of background contours, and decayed at similar rates after stimulus termination. Both responses decayed more deeply following stimulation with, rather than without fixation. Neither retinal image motion nor vection can explain the IM obtained.

Does performance of tasks affect the resting focus of accommodation

ABSTRACT Measurements of the resting focus of accommodation were obtained with an infrared optometer while subjects engaged in four tasks: (1) passive viewing in darkness; (2) passive viewing of a laser speckle pattern; (3) active judgment of motion in a laser speckle pattern; and (4) mental arithmetic in darkness. The results showed resting focus measures to be comparable under all four conditions for some subjects. However, other subjects exhibited consistent increases or decreases in the resting focus position when asked to judge the direction of speckles within the laser test pattern or perform mental arithmetic. This finding suggests that “effort to see” may influence the resting focus of some subjects. Implications for measurement procedures and population estimates of the resting focus are discussed.

Induced motion considered as a visually induced oculogyral illusion.

The possibility that nystagmus suppression contributes to illusory motion was investigated by measuring perceived motion of a stationary stimulus following the removal of an optokinetic stimulus. This was done because optokinetic nystagmus typically outlasts cessation of an optokinetic stimulus. Therefore, it would be expected that a stationary fixated stimulus should appear to move after removal of an optokinetic stimulus if illusory motion results from nystagmus suppression. Illusory motion was reported for a stationary fixation target following optokinetic stimulation. This motion was reported first in the same direction as the preceding induced motion, then in the opposite direction. The two directions of illusory motion following optokinetic stimulation are interpreted as resulting from the use of smooth ocular pursuit to suppress first one phase of optokinetic afternystagmus and then the reverse phase. Implications for the origins of induced motion are discussed.

Short-term variability of the resting focus of accommodation.

Abstract Previous investigations have shown that, in dim illumination and empty visual fields, accommodation in most observers is for an intermediate distance referred lo as the dark focus or resting focus of accommodation. Dynamic properties of the resting focus were examined in the present study with a high‐speed infrared optometer. All subjects displayed fluctuations of accommodation in darkness that were primarily characterized by frequency components below 0.5 Hz. These fluctuations were substantially attenuated during cycloplegia or viewing a bright, high‐contrast target. Both the mean resting‐focus position and the magnitude of fluctuations varied from one subject to another and from day‐to‐day. A weak association was found between the mean and standard deviation of accommodation responses in the dark. The present findings suggest that the accommodation mechanism in most observers is somewhat unstable under degraded viewing conditions. In addition, the mean resting‐focus value for a large sample of subjects was found to be lower for i.r. optometer measurements than for previous investigations employing a laser optometer.

Comparison of laser and infrared techniques for measurement of the resting focus of accommodation: mean differences and long-term variability.

Abstract The resting focus of accommodation was measured in the same subjects with both a laser optometer and a high‐speed infrared optometer. Although i.r. optometer and laser optometer measures produce similar estimates of the mean resting‐focus level in some subjects, others were found to have different levels of resting focus with the two techniques. Control studies, demonstrated that these differences were not due to the temporal sampling characteristics of the laser optometer, but resulted instead from making judgments about the direction of speckle motion during the laser optometer procedure. The temporal stability of the resting focus of accommodation was also investigated with i.r. measures obtained several minutes, 1 day, and 1 and 2 weeks apart. Results indicated long‐term variability similar to that previously reported with the laser optometer.

Implications of OKN suppression by smooth pursuit for induced motion

Induced motion refers to the illusory movement of a stationary stimulus that results from the opposite movement of other stimuli in the visual field (for a historical review, see Duncker, 1929). A familiar example is provided by the apparent motion of the moon viewed through a surround of moving clouds. Although the moon is objectively stationary, it appears to move opposite to the cloud motion. A variety of explanations have been offered to account for induced motion. Duncker (1929) proposed that changes in the apparent location of a stimulus relative to its surround produced the illusory movement. Specifically, movement of the surround was presumed to result in the perception of changed relative locations of the induced motion stimuli, which in tum would give rise to perceived movement. More recently, Brosgole (1968) and Bridgeman and Klassen (1983) suggested that induced motion resulted from changes in the perceived direction of the fixated stimulus relative to the observer. Specifically, the motion of the inducing stimulus was assumed to alter the direction of the apparent straight-ahead, which in tum would alter the perceived direction of the stationery stimulus. Changes in perceived direction are, in tum, inferred as movement of the fixated stimulus. These two accounts are similar in that the induced movement percept is mediated indirectly, that is, the illusory movement results from changes in perceived location or perceived direction. To date, the possibility that oculomotor mechanisms contribute to induced motion has been rejected on the basis of experiments in which eye movements were measured during observation of the illusory movement. These studies (Mack, 1970; Brosgole, Cristal, & Carpenter, 1968; Bassili& Farber, 1977) consistently found that there was no significant amount of eye movement during fixation of induced motion stimuli. The absence of significant retinal image motion eliminates the possibility that

Induced motion and apparent straight ahead during prolonged stimulation

Induced motion (IM) of a fixation target and the location of the apparent straight ahead (ASA) were measured during and after 2 min of exposure to a moving contoured background. The magnitude of IM increased to an asymptotic level during the exposure period. Following termination of moving contours, illusory motion of the fixation target occurred first in the same direction and then in the direction opposite to that of previous IM. The change in location of ASA followed a similar time course, shifting first in the direction of background motion, reaching an asymptotic position during contour exposure, and then moving in the opposite direction after contour exposure. The similar time courses displayed by IM magnitude change and ASA position change suggest that these two phenomena may be related through a third, causal factor. The hypothesis that IM is caused by motion of ASA is disconfirmed, however, by the fact that IM persists while the ASA is nearly stationary at its asymptotic position.

Nighttime Driving and Visual Degradation

Recent developments which provide new insights into night driving accidents are reviewed. Selective Degradation: The mechanisms subserving steering are different from those underlying hazard recognition and these two modes of processing visual information are selectively impaired at night. Although it is possible to steer a vehicle as well at night as during the day, the ability to recognize and respond to infrequent hazards is seriously degraded. Night Myopia: Many individuals become nearsighted under reduced illumination. The finding that this is a normal consequence of the passive return of the accommodative system to an intermediate resting posdition has led to a procedure to ameliorate this effect. By determining the value of an individuals dark focus it is possible to provide a special night driving prescription which effectively eliminates night myopia.