A compensation for field expansion caused by moving forward

Attention, Perception, & Psychophysics - When an object increases in size and its retinal image expands, it is perceived to grow. But image expansion caused by one’s approaching an object...     

The compensation for movement-produced changes of object orientation

When one moves forward, one views objects to the side of one’s path successively from different directions. In the mover’s visual field, such objects change their orientation; relative to him they undergo a partial rotation. Although this rotation is given in several ways, it is hardly ever perceived. This is due to a compensating process that takes O’s change in position relative to the object into account. We demonstrated the existence of this compensating process and measured the accuracy with which it operates by means of a device that made an object turn in response to O’s position change so that the normal rotation of a stationary object relative to the moving O could be augmented or diminished to various degrees.     

Differences in the dissipation of the effect of adaptation to two kinds of field displacement during head movements

When Ss were simultaneously adapted to horizontal and to vertical target displacements of equal rate during head turning about a vertical axis, the adaptation effects measured by one-trial tests immediately after the adaptation period were about equal. But retests after a time lapse of 10 and 20 min, during which S sat immobile and with eyes closed, showed a greatly different rate of dissipation of the two adaptation effects. After a lapse of 20 min, the effect of adaptation to horizontal target displacements had been reduced to 37%, whereas the effect of adaptation to vertical displacements at this final test still stood at 80% of the initial measurement. The decline over 20 min in the latter case was so smail that it could readily be ascribed to an effect of the two tests that preceded the final test. These two tests represented an effective exposure to natural viewing conditions and hence caused an unlearning of the adaptation, an effect whose existence we had demonstrated in previous work with the one-trial test.     

Target distance and adaptation in distance perception in the constancy of visual direction

Hay and Sawyer recently demonstrated that the constancy of visual direction (CVD) also operates for near targets. A luminous spot in the dark, 40 cm from the eyes, was perceived as stationary when S nodded his head. This implies that CVD takes target distance, as well as head rotation, into account as a stationary environment is perceived during head movements. Distance is a variable in CVD because, during a turning or nodding of the head, the eyes become displaced relative to the main target direction, the line between the target and the rotation axis of the head. This displacement of the eyes during head rotation causes an additional change in the target direction, i.e., a total angular change greater than the angle of the head rotation. The extent of this additional angular displacement is greater the nearer the target. We demonstrated that the natural combination of accommodation and convergence can supply the information needed by the nervous system to compensate for this additional target displacement. We also found that wearing glasses that alter the relation between these oculomotor adjustments and target distance produces an adaptation in CVD. An adaptation period of 1.5 h produced a large adaptation effect. This effect was not entirely accounted for by an adaptation in distance perception. Measurements of the alteration between oculomotor cues and registered distance with two kinds of tests for distance perception yielded effects significantly smaller than the effect measured with the CVD test. We concluded that the wearing of the glasses had also produced an adaptation within CVD.     

Two kinds of adaptation in the constancy of visual direction

Adaptation in the constancy of visual direction had previously been obtained by causing a large or a small visible area representing the environment to be objectively displaced in dependence on head movements. No stationary objects were permitted to be visible. Now experiments are reported in which displacements of a large patterned field, with the subject fixating a stationary mark in its center, led to adaptation. In these experiments, objective displacements of the environment were given by image displacements on the retina. Adaptation also resulted when the large field was stationary and only the fixation mark was displaced. Here the objective displacement was given by the rate of pursuit eye movements.     

Viewing direction and pictorial representation

During prolonged monocular observation from afar, an upright wire cube (Necker cube) or a drawing of such a cube inverts continuously so that the lower of the two frontal faces appears to be either in front (version A) or in back (version B). Version A is perceived as if it were viewed obliquely from above and version B as if seen from below. In our experiments, timing the durations of the versions showed that version A lasted longer than version B. When a wire cube was shifted upward so that it was actually viewed slightly from below, version B lasted longer than version A. The actual viewing direction was apparently taken into account by the subjects, which was not the case when the cube was replaced by a drawing showing the projection of the cube. In that case, version A always lasted longer, regardless of the actual viewing direction. This finding conforms with picture viewing in general, where a three-dimensional object’s orientation does not change when the observer’s viewing direction changes.     

Some consequences of stereoscopic depth constancy

Retinal disparity decreases in proportion to the square of the distance of the corresponding objective depth interval from the eyes. Up to a distance of 2 m, stereoscopic depth perception compensates well for this decrease in disparity with observation distance; for a given disparity, experienced depth increases approximately in proportion to the square of the observation distance. When disparities are artificially produced, by anaglyphs or vectograms, or by spectacles, they decrease only in proportion to the first power of the observation distance. The same is true when the Pulfrich effect gives rise to equivalents of disparity. Depth perception, however, compensates for the normal disparity loss. As a result, there should be a net gain in perceived depth approximately in proportion to the first power of observation distance. When perceived depth caused by horizontal magnification in one eye or by the Pulfrich effect was measured, it was found to increase approximately in proportion to observation distance.     

Familiar size and linear perspective as distance cues in stereoscopic depth constancy

Both the image size of a familiar object and linear perspective operate as distance cues in stereoscopic depth constancy. This was shown by separating their effects from the effect of the oculomotor cues by creating cue conflicts between either the familiar size cue or linear perspective, on the one hand, and accommodation and convergence, on the other. In the case of familiarsize, this cue was used deceptively. In the case of linear perspective, spectacles caused nonveridical oculomotor adjustments.