Distance adaptation depends upon plasticity in the oculomotor control system

Maintaining binocular fixation on a target at 20 cm in the absence of secondary cues to distance produced changes in apparent distance and lateral phoria. Positive lenses of 0, .5, 2.0, 3.5, and 5.0 spherical diopters (SD) were used to manipulate the level of accommodative convergence in force during the period of maintained fixation. An inverse relationship was found between the stimulus to accommodation and the magnitude of the induced esophoria, the phoria being linearly related to an increase in apparent distance. The distance aftereffect obtained in the condition with the lowest net accommodative stimulus li.e., 0 D) equaled that typically produced by base-out prism adaptation with full secondary cues to distance available. In a second experiment, subjects walked through a well-lit hallway while viewing through a pair of 5h base-out prisms. It was shown that increasing the stimulus to accommodation by adding negative lenses of 0, 1.5, 3.5, and 5.5 SD reduced the adaptive change in apparent distance, as well as the change in phoria produced by the conventional base-out prism adaptation paradigm. It was concluded that a change in the resting tonus of the disparity vergence system underlies such adaptation, rather than recalibration of the oculomotor cues to distance. Monocular exposure data indicated that a small change in the tonus control for the accommodative system may be present as well.     

Construct validity of perceptual style: Role of stimulus size in the embedded-figures test and the rod-and-frame test

Three experiments were performed in order to test the construct validity of perceptual field dependence (FD) and field independence (FI). In Experiment 1, performance of subjects on two sizes of Form A of Witkin’s Embedded Figures Test (EFT) with a size-ratio of 1 to 4 was compared. The size of the standard EFT was taken as unity. A nonsignificant size effect was found for all subjects as a group, and a significant rank correlation was found between performances on the two sizes of the EFT. Furthermore, the performance of field-dependent subjects (as defined by performance on the standard EFT) and field-independent subjects did not interact significantly with the size factor. In Experiment 2, the physical size of the EFT changed from 1 to 8. A nonsignificant, overall size effect was again found, but the EFT performance of fielddependent and field-independent subjects interacted significantly with the size factor. In Experiment 3, the performance of subjects on two sizes of the EFT (i.e., 1 to 8) and two sizes of the standard rod-and-frame test (RFT) (i.e., 1 to 4.5) were compared. A significant stimulus-size effect was found in the EFT, with all subjects becoming morefield-independent, and a significant stimulus-size effect was found in the RFT with the same subjects becoming morefield-dependent. Finally, the EFT performance of the field-dependent and field-independent subjects again displayed a significant interaction with the size of the embedded figures. A parallel-serial processingstyles hypothesis is proposed to account for the EFT data.     

Additivity of aftereffects of maintained head and eye rotations: An alternative to recalibration

Seven groups of 10 subjects each were exposed to various combinations of left and right head and eye rotations for a period of 10 min. Both head and eye produced significant aftereffects of prior position as measured by pointing at a visible target with the unseen hand, but there was no significant interaction. Thus, aftereffects of sustained head and eye rotation were shown to be additive and to account fully for the results of Craske and Crawshaw (1975). Eye muscle potentiation rather than recalibration may be assumed to be the cause of the altered direction of gaze resulting from exposure to displacing prisms.     

Determinants of the rod and frame effect: The role of retinal size

In Experiment 1, base-out prisms were used to alter perceived size and distance to a luminous rod and frame while the retinal size remained unchanged. The rod-and-frame effect (RFE) was the same, whether the display was viewed directly or through the prisms. In Experiment 2, one large and one small rod-and-frame display were placed at distances such that they produced identical retinal angles. This was replicated at three different sets of distances. Perceived size and distance of the large and small frame of identical retinal angle interacted with the observation distance, such that at near distances the large frame was perceived as larger and farther than the small frame while, at far distances, both types of estimates converged to a constant value. In contrast, the RFE was identical for the large and small frames matched in retinal angle, but diminished with distance. In both experiments, the RFE varied precisely with variation in retinal angle. Implications of the role of retinal angle in the RFE and for the interpretation of individual differences were discussed.     

Effects of optical pitch on oculomotor control and the perception of target elevation

In two experiments, we used an ISCAN infrared video system to examine the influence of a pitched visual array on gaze elevation and on judgments of visually perceived eye level. In Experiment 1, subjects attempted to direct their gaze to arelaxed or to ahorizontal orientation while they were seated in a room whose walls were pitched at various angles with respect to gravity. Gaze elevation was biased in the direction in which the room was pitched. In Experiment 2, subjects looked into a small box that was pitched at various angles while they attempted simply to direct their gaze alone, or to direct their gaze and place a visual target at their apparent horizon. Both gaze elevation and target settings varied systematically with the pitch orientation of the box. Our results suggest that under these conditions, an optostatic response, of which the subject is unaware, is responsible for the changes in both gaze elevation and judgments of target elevation.