Perception & Psychophysics

The pitch of a visual field systematically influences the elevation at which a monocularly viewing subject sets a target so as to appear at visually perceived eye level (VPEL). The deviation of the setting from true eye level averages approximately 0.6 times the angle of pitch while viewing a fully illuminated complexly structured visual field and is only slightly less with one or two pitched-from-vertical lines in a dark field (Matin & Li, 1994a). The deviation of VPEL from baseline following 20 min of dark adaptation reaches its full value less than 1 min after the onset of illumination of the pitched visual field and decays exponentially in darkness following 5 min of exposure to visual pitch, either 30° topbackward or 20° topforward. The magnitude of the VPEL deviation measured with the dark-adapted right eye following left-eye exposure to pitch was 85% of the deviation that followed pitch exposure of the right eye itself. Time constants for VPEL decay to the dark baseline were the same for same-eye and cross-adaptation conditions and averaged about 4 min. The time constants for decay during dark adaptation were somewhat smaller, and the change during dark adaptation extended over a 16% smaller range following the viewing of the dim two-line pitched-from-vertical stimulus than following the viewing of the complex field. The temporal course of light and dark adaptation of VPEL is virtually identical to the course of light and dark adaptation of the scotopic luminance threshold following exposure to the same luminance. We suggest that, following rod stimulation along particular retinal orientations by portions of the pitched visual field, the storage of the adaptation process resides in the retinogeniculate system and is manifested in the focal system as a change in luminance threshold and in the ambient system as a change in VPEL. The linear model previously developed to account for VPEL, which was based on the interaction of influences from the pitched visual field and extraretinal influences from the body-referenced mechanism, was employed to incorporate the effects of adaptation. Connections between VPEL adaptation and other cases of perceptual adaptation of visual direction are described.

The role of element proximity in perceptual grouping was examined in tasks requiring speeded discrimination of two-element visual patterns. Grouping of two elements was defined as the failure of attention to be focused on one element selectively in filtering tasks where only that one element was relevant to the discrimination. Failure of selective attention was measured by the degree of interference caused by variation of the irrelevant element. Grouping was shown to diminish monotonically as the spacing between two elements was increased. At a given spacing, grouping could be reduced or eliminated by the introduction of a third element into the stimulus field, presumably because the addition of this element triggered a reorganization of the perceptual field into a new grouping structure. Grouping appeared to facilitate performance on condensation tasks requiring distributed attention, to the degree that the condensation tasks were actually easier than the filtering tasks at close proximities. Paradoxically, for some tasks, moving an irrelevant element away from a relevant one actually impaired performance, suggesting that paying attention to irrelevant information could be beneficial. This result, if generalizable, suggests that grouping be conceptualized not as an automatic process under preattentive control but as an optional process under strategic control.

Các đối tượng tham gia nghiên cứu quan sát các mảng kích thích màu sắc sao cho có hai hiện tượng tranh đấu màu sắc xảy ra đồng thời. Cụ thể, có hai mảng màu khác nhau, mỗi mảng nằm ở nửa bên trái của hai trường nhìn, và đồng thời, các mảng tranh đấu cũng có mặt ở hai nửa bên phải. Như dự kiến, các cảm nhận từ hai mắt thường tương ứng với sự chiếm ưu thế của một trong hai mắt. Tuy nhiên, trong một số trường hợp khác, các đối tượng đã báo cáo cảm nhận cho thấy sự chiếm ưu thế đồng thời của nửa mũi hoặc, thường xuyên hơn, của nửa thái dương của hai võng mạc.

A category-judgment experiment is the reported. Rating-scale judgments of loudness were made with five or eight intensity levels of a tone and with different frequencies of presentation of the stimuli. Obtained ROC curves for pairs of stimuli were judged to support application of signal-detection theory. However, certain common assumptions of the theory are called into question by the data. Discrimination was poorer when more categories were available. This is attributed to an increase in criterion variance with an increase in the number of criteria. The slope of the ROC curves for different stimulus pairs suggested that stimulus variance is neither constant nor does it necessarily increase with stimulus intensity It is argued that stimulus variance may depend upon the distribution of internal observations over all trials.

Two experiments investigated observers’ perceptions of 3-D structure when optical sources of information were contradictory. When motion and stereoscopic disparities specified different surfaces, the perceptual outcome depended strongly on the direction of curvature present within each modality. Previous research has shown that the perception of surface slant and curvature is anisotropic for both motion and stereo and that it depends on the direction in which it takes place. In the present experiments, the modality with the “effective” direction of curvature tended to dominate or suppress the perception of surfaces in the other modality with less effective curvatures. The results have implications for models which attempt to combine 3-D data from different optical sources.

Five experiments were carried out to investigate the effects of electrode size on threshold for current pulses that flowed between the ventral and dorsal sides of the right forearm. The main result was that the total current threshold decreases as the size of the cathode and anode increases at least up to 15 mm in diameter. To account for this finding, a neural summation model was proposed, assuming that the central nervous system sums up neural impulses discharged near the conduction paths under electrodes. It was also found that the current flowing from the dorsal to the ventral side provided lower thresholds and more stable localization than the opposite flow of current did. This finding suggested that current threshold and its perceived locus are determined by both the body site on which electrodes were placed and the relative polarity that was assigned to electrodes.

Lines of various orientations were viewed as prolonged afterimages in six experiments. The duration of unitary appearance was not influenced by line orientation for monocular afterimages, but there was a marginal effect for afterimages generated binocularly; vertical and horizontal lines tended to be visible for longer than did 45-deg lines. Measures of fragmentation frequency and the latency to the first disappearance did not vary reliably with orientation under any conditions. Binocular afterimages lasted longer than did monocular ones, but generally showed the same pattern of fragmentations. These results are compared with those from experiments using optical stabilization and steady fixation, in which orientation differences have been reported.

Previous research indicates that a tilted visual display is capable of inducing eye torsion and an illusion of self-tilt in objectively upright observers. These effects may contribute to performance errors on the rod-and-frame test by rotating the perceived axes of visual space toward the tilted frame. The kinesthetic-matching method was used in the present study to see whether an effect of the visual orientation contrast between red and frame sides might also contribute to rod-and-frame test performance. Observers aligned invisible hand-held rods with the visual rod at various tilts under a control condition when the frame was absent, and under experimental conditions with the frame upright or set at 45°. The frame induced matching errors in the direction away from the frame sides which were most nearly parallel to the rod. Since no rotation of apparent visual axes should occur under these conditions, the data suggest that an orientation contrast effect is involved in the rod-and-frame test.