Differences in influence between pitched-from-vertical lines and slanted-from-frontal horizontal lines on egocentric localization

The visual field exerts powerful effects on egocentric spatial localization along both horizontal and vertical dimensions. Thus, (1) prism-produced visual pitch and visual slant generate similar mislocalizations of visually perceived eye level (VPEL) and visually perceived straight ahead (VPSA) and (2) in darkness curare-produced extraocular muscle paresis under eccentric gaze generates similar mislocalizations in VPEL and VPSA that are essentially eliminated by introducing a normal visual field. In the present experiments, however, a search for influences of real visual slant on VPSA to correspond to the influences of visual pitch on VPEL failed to find one. Although the elevation corresponding to VPEL changes linearly with the pitch of a visual field consisting of two isolated 66.5°-long pitched-from-vertical lines, the corresponding manipulation of change in the slant of either a horizontal two-line or a horizontal four-line visual field on VPSA did not occur. The average slope of the VPEL-versus-pitch function across 5 subjects was +0.40 over a ±30° pitch range, but was indistinguishable from 0.00 for the VPSA-versus-slant function over a ±30° slant range. Possible contributions to the difference between susceptibility of VPEL and VPSA to visual influence from extraretinal eye position information, gravity, and several retinal gradients are discussed.     

Change in visually perceived eye level without change in perceived pitch

Both the physical elevation that appears to correspond to eye level and the visually perceived pitch of a visual field are linear functions of the physical pitch of a normally illuminated, complexly structured visual field. One of the possible bases for the large effect of physical pitch on the elevation of visually perceived eye level (VPEL) is that the visual field generates a mental representation which specifies spatial coordinates and these determine the VPEL elevation ('implicit-surface model'; ISM). The influence on the elevation of VPEL is nearly as large when the visual field contains either one or two long pitched-from-vertical or rolled-from-vertical lines in otherwise total darkness as when it consists of a well-illuminated and complexly structured pitched room (L Matin and W Li, 1994 Vision Research 34 311-330), and, in order to examine the ISM, we employed a rolled-from-vertical, two-line configuration within a frontoparallel plane viewed in otherwise total darkness. Measurements of visually perceived pitch were made by a manual matching procedure and VPEL measurements were made by the psychophysical setting of the elevation of a small visual target to appear at eye level while each of three subjects viewed the two-line configuration at each of three horizontal eccentricities with the configuration at each of seven roll orientations. In direct contradiction to the ISM, the perceived pitch of the two-line configuration did not deviate significantly from the erect orientation ('vertical') for any roll at any eccentricity, but the elevation of VPEL changed systematically with the roll of the configuration both at left and at right eccentricities, and did not change at all with the two-line configuration centered on the median plane. Consistent with our previous work and with our previous interpretation regarding the basis for VPEL (L Matin and W Li, 1994 Vision Research 34 2577-2598), the variation of VPEL for the two-line visual field equals the average of the VPEL variations produced by viewing each of the single lines separately.     

Visually perceived eye level: changes induced by a pitched-from-vertical 2-line visual field.

The physical elevation corresponding to visually perceived eye level (VPEL) changes linearly with the pitch of a visual field. Deviations from true eye level average more than 0.5 times the angle of pitch over a 65 degrees pitch range. A visual field consisting of 2 dim, isolated vertical lines in darkness is more than 4/5 as effective as that of a complexly structured visual field; 2 horizontal lines have a small and inconsistent effect. Differences in influence on VPEL between pitched-from-vertical and horizontal lines were predicted from an analysis that extracted differences in retinal perspective resulting from changes in pitch. The Great Circle Model (GCM), based on a spherical approximation to the erect, stationary eye, predicts the present results and results of 8 other sets of experiments. The model treats the influence of a single line on VPEL as systematically related to the elevation of the intersection between the great circle containing the image of the line and the central vertical retinal meridian; generalized GCM combines visual inputs with inputs from the body-referenced mechanism and maps onto the central nervous system.     

Why do pitched horizontal lines have such a small effect on visually perceived eye level?

In two experiments, visually perceived eye level (VPEL) was measured while subjects viewed two-dimensional displays that were either upright or pitched 20 degrees top-toward or 20 degrees top-away from them. In Experiment 1, it was demonstrated that binocular exposure to a pair of pitched vertical lines or to a pitched random dot pattern caused a substantial upward VPEL shift for the top-toward pitched array and a similarly large downward shift for the top-away array. On the other hand, the same pitches of a pair of horizontal lines (viewed binocularly or monocularly) produced much smaller VPEL shifts. Because the perceived pitch of the pitched horizontal line display was nearly the same as the perceived pitch of the pitched vertical line and dot array, the relatively small influence of pitched horizontal lines on VPEL cannot be attributed simply to an underestimation of their pitch. In Experiment 2, the effects of pitched vertical lines, dots, and horizontal lines on VPEL were again measured, together with their effects on resting gaze direction (in the vertical dimension). As in Experiment 1, vertical lines and dots caused much larger VPEL shifts than did horizontal lines. The effects of the displays on resting gaze direction were highly similar to their effects on VPEL. These results are consistent with the hypothesis that VPEL shifts caused by pitched visual arrays are due to the direct influence of these arrays on the oculomotor system and are not mediated by perceived pitch.     

Visually perceived eye level with reversible pitch stimuli: implications for the great circle and implicit surface models

Visually perceived eye level (VPEL) and perceived pitch were measured while subjects viewed two sets of stimuli that were either upright or pitched top-toward or top-away from them. The first set of stimuli, a pair of vertical lines viewed at various angles of pitch, caused systematic changes in perceived pitch and upward and downward VPEL shifts for the top-toward and top-away pitches, respectively. Neither the perceived pitch nor the VPEL measures with these stimuli differed between monocular and binocular viewing. The second set of stimuli was constructed so that, when viewed at the appropriate pitch angle, the projected orientations of the lines in the retinal image of each stimulus were similar to those generated by a pair of vertical lines pitched by a lesser amount in the opposite direction. When viewed monocularly, these stimuli appeared pitched in the direction opposite their physical pitch, yet produced VPEL shifts consistent with the direction of their physical pitch. These results clearly demonstrate a dissociation between perceived pitch and VPEL. The same stimuli, when viewed binocularly, appeared pitched in the direction of their physical pitch and caused VPEL shifts indistinguishable from those obtained monocularly. The retinal image orientations of these stimuli, however, corresponded to those of vertical line stimuli pitched in the opposite direction. This finding is therefore consistent with the hypothesis that VPEL and perceived pitch are processed independently, but inconsistent with the specific version of this hypothesis which states that differences in VPEL are determined solely on the basis of the orientation of lines in the retinal image.     

Accuracy and adaptation of reaching and pointing in pitched visual environments

Visually perceived eye level (VPEL) and the ability of subjects to reach with an unseen limb to targets placed at VPEL were measured in a statically pitched visual surround (pitchroom). VPEL was shifted upward and downward by upward and downward room pitch, respectively. Accuracy in reaching to VPEL represented a compromise between VPEL and actual eye level. This indicates that VPEL shifts reflect in part a change in perceived location of objects. When subjects were provided with terminal visual feedback about their reaching, accuracy improved rapidly. Subsequent reaching, with the room vertical, revealed a negative aftereffect (i.e., reaching errors that were opposite those made initially in the pitched room). In a second study, pointing accuracy was assessed for targets located both at VPEL and at other positions. Errors were similar for targets whether located at VPEL or elsewhere. Additionally, pointing responses were restricted to a narrower range than that of the actual target locations. The small size of reaching and pointing errors in both studies suggests that factors other than a change in perceived location are also involved in VPEL shifts.     

Effect of structured visual environments on apparent eye level

Each of 12 subjects set a binocularly viewed target to apparent eye level; the target was projected on the rear wall of an open box, the floor of which was horizontal or pitched up and down at angles of 7.5 degrees and 15 degrees. Settings of the target were systematically biased by 60% of the pitch angle when the interior of the box was illuminated, but by only 5% when the interior of the box was darkened. Within-subjects variability of the settings was less under illuminated viewing conditions than in the dark, but was independent of box pitch angle. In a second experiment, 11 subjects were tested with an illuminated pitched box, yielding biases of 53% and 49% for binocular and monocular viewing conditions, respectively. The results are discussed in terms of individual and interactive effects of optical, gravitational, and extraretinal eye-position information in determining judgements of eye level.     

The field dependence/independence cognitive style does not control the spatial perception of elevation

Earlier work described the presence of a significant connection between an individual's ability to disregard distracting aspects of a visual field in the classical rod-and-frame test (RFT), in which a subject is required to set a rod so that it will appear vertical in the presence of a square frame that is roll tilted from vertical, and in paper-and-pencil tests, in which the subject is required to find a hidden figure embedded in a more complex figure (the Embedded Figures Test [EFT]; see, e.g., Witkin, Dyk, Faterson, Goodenough, & Karp, 1962; Witkin et al., 1954; Witkin, Oltman, Raskin, & Karp, 1971). This has led to a belief in the existence of a bipolar dimension of cognitive style that is utilized in such disembedding tasks--namely, the extent to which an individual is dependent on or independent from the influence of a distracting visual field. The influence of an inducing visual field on the perception of elevation measured by the setting of a visual target to appear at eye level (the visually perceived eye level [VPEL] discrimination) has also been found to be correlated with the RFT. We have thus explored the possible involvement of the dependence/independence cognitive style on the VPEL discrimination. Measurements were made on each of 18 subjects (9 of them female, 9 male) setting a small target to the VPEL in the presence of a pitched visual field across a range of six pitches from -30 degrees (topbackward) to +20 degrees (topforward) and on each of three tests generally recognized as tests of cognitive spatial abilities: the EFT, the Gestalt Completion Test, and the Snowy Pictures Test (SPT). Although there were significant pairwise correlations relating performance on the three cognitive tests (+.73, +.48, and +.71), the correlation of each of these three with the slope of the VPEL-versus-pitch function was not significant, as it was with the slope of the perception of visual pitch of the field (PVP)-versus-pitch function. VPEL, PVP, and a cognitive factor separated into three essentially independent factors in a multiple-factor analysis, with the three cognitive tests clustering at the cognitive factor, and with no significant loading on either of the other two factors. From the above considerations and a multiple-factor analytic treatment including additional results from this and other laboratories, we conclude that the cognitive-processing style held to be involved in the performance on the EFT and the perception of vertical as measured by the RFT is not general for egocentric space perception; it does not involve the perception of elevation.     

Orientation-specific aftereffects and illusions in the perception of brightness

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.     

The human sense of the head's polarity is influenced by changes in the magnitude of gravity

The present investigation concerns the integrity of a primary mental function, the egocentric frame of reference and the sense of polarity of one's own head. The visually perceived eye level (VPEL) and the subjective antero-posterior axis of the head were measured by means of a visual indicator in darkness during two stimulus conditions: static pitch (sagittal-plane) tilting in the 1-g environment and gondola centrifugation (2G). It is demonstrated that an increase in the magnitude of the gravitoinertial (G) force, acting in the direction of the head and body long (z) axis, causes a substantial change not only in the VPEL but also in the perceived direction of the antero-posterior axis of the head.     

Decay of visual adaptation to tilt and displacement

Persistence in the dark following 48 min of visual adaptation to tilt and displacement was compared in two experiments to determine if same or different processes are involved in the two kinds of adaptation. Decay of tilt adaptation occurred rapidly, all within about 16 min. However, it was not complete and some residual tilt adaptation persisted for at least as long as 56 min. Decay of displacement adaptation occurred more slowly but was clearly complete after at most 56 min in the dark. Displacement adaptation appears to be entirely subject to decay, while tilt adaptation involves an additional, more long-term component. Results are interpreted in terms of independent systems for the perception of location and orientation.     

The eyes have it: Exposure times and saccadic movements in visual half-field experiments

Several tachistoscopic visual half-field experiments using exposure times in excess of 150 msec have been reported and arguments have been put forth justifying this procedure. An experiment was done investigating visual field accuracy under conditions where eye movement was allowed, following parafoveal exposure. Two control experiments were done to evaluate the viewing conditions. When eye movement is permitted, accuracy in both visual fields reaches 100%. It is concluded that visual field differences found with exposure times greater than 150 msec are due to the active cooperation of the subjects and not due to the justifications advanced by experimenters using long exposure times.     

Additivity in prism adaptation as manifested in intermanual and interocular transfer

Level of adaptation was assessed in both exposed and unexposed eye and/or hand for visual shift (VS), proprioceptive shift (PS), and the eye-hand coordination, negative after effect (NA) measure of both visual and proprioceptive change, following 15-min and 20-diopter base-right displacement viewing of the active hand, under conditions of unconstrained head movement and terminal exposure feedback. Transfer was complete for the VS test, and significant, but incomplete for the PS and NA tests. For both exposed and unexposed eye/hand situations, level of adaptation was greater for the NA than for the PS test, which in turn showed greater adaptation than the VS test. Additivity was virtually perfect for the unexposed eye/hand (VS+PS = NA), but underadditivity appeared for the exposed eye/hand (VS+PS < NA). This underadditivity was approximately equal in magnitude to the amount that transfer on the NA test was less than on the PS test, suggesting that underadditivity was due to a nontransferable, assimilated corrective response in the NA test with the exposed eye/hand. Possible explanations for intermanual transfer are discussed.     

Restricted adaptation to prism rearrangement

Changes in eye-foot and eye-hand coordination were measured following 20 min of squint prism viewing (alternate monocular viewing of the movements of each leg with the contralateral eye at 1-min intervals: prism base right for right eye and left for left eye). In different sessions, response changes were measured following the viewing of the left leg with the right eye (prism base right) for periods of i min interspersed with 1-min blank periods (periodic viewing). Sensorimotor changes following the alternate exposure condition were smaller and restricted to eye-foot responses.     

Effects of gravitational and optical stimulation on the perception of target elevation

To examine the combined effects of gravitational and optical stimulation on perceived target elevation, we independently altered gravitational-inertial force and both the orientation and the structure of a background visual array. While being exposed to 1.0, 1.5, or 2.0 Gz in the human centrifuge at NASA Ames Research Center, observers attempted to set a target to the apparent horizon. The target was viewed against the far wall of a box that was pitched at various angles. The box was brightly illuminated, had only its interior edges dimly illuminated, or was kept dark. Observers lowered their target settings as Gz was increased; this effect was weakened when the box was illuminated. Also, when the box was visible, settings were displaced in the same direction as that in which the box was pitched. We attribute our results to the combined influence of otolith-oculomotor mechanisms that underlie the elevator illusion and visual-oculomotor mechanisms (optostatic responses) that underlie the perceptual effects of viewing pitched visual arrays.     

Brightness exponent as a function of retinal eccentricity in the peripheral visual field: Effects of dark and light adaptation

Using a method of direct magnitude estimation, the exponent of the brightness power function was determined under dark and light adaptation at luminance levels well above threshold. The exponent was estimated for functions describing the brightness of stimuli presented at the fovea and the following peripheral retinal loci: 10, 20, 30, 40, and 50 deg nasally eccentric to the fovea along the horizontal meridian of the right eye. The exponent for a 1-sec flash was found to be approximately .33 at the fovea and increased slightly with increasing retinal eccentricity.The effect of adaptation on the brightness exponent was not so large when the target luminance was set well above threshold.     

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.     

Do background luminances interact during binocular fusion?

The question investigated in the experiments reported here was whether monocular background luminances sum during binocular fusion. Fusion was made explicit by using a random-dot stereogram (RDS) as a background stimulus. In the presence of the RDS, differential luminance thresholds were somewhat higher than in the uniform field: a full-field, binocular dot array acted as a mask for a full-field luminance change, but global depth had no effect at threshold. The amount of the binocular advantage at threshold was compared to the basic "threshold response," that is, the change in threshold resulting from raising the background luminance by a factor of 2. It was found that the amount of the binocular advantage was equivalent, on the average, to some 75% of the threshold response--significantly less than the 100% predicted by "simple summation." The amount of the binocular advantage varied substantially among observers and eyes, whereas the threshold response obeyed Weber's law in all cases: the variability was eye-, rather than threshold-dependent. Monocular thresholds did not decrease when taken with the nontest eye occluded rather than viewing a fused background. The proposition that the adaptation state of the visual system is increased during binocular fusion (Cogan, 1982) was not supported. Yet occluding the nontest eye, rather than presenting the test stimulus monocularly against a fused background, did change monocular thresholds in some eyes and observers. These findings are interpreted as evidence for a complex binocular background interaction involving both summation and inhibition.     

Components of displacement adaptation in acquisition and decay as a function of hand and hall exposure

Tests of proprioceptive shift (PS), visual shift (VS), and negative aftereffect (NA) were made during 25-min exposure to 20-D displacement and during a subsequent 30-min dark decay period in two separate experiments. Different groups of subjects explored hallways or viewed their active hand during exposure. VS was greatest in hall exposure, while PS was greatest in hand exposure. Larger VS occurred in the second experiment, where test procedures were modified to minimize a tendency to center the target within the momentary or remembered field of view. Substantial and possibly complete VS decay occurred when the initial level of adaptation was high, but although PS decay was substantial, it was not complete. In all conditions, the sum of VS and PS numerically exceeded the NA, and this difference tended to be largest and significant in the hall exposure. Implications of this effect for the two-component additivity hypothesis are discussed.     

Effects of a visual frame and of low radial accelerations on the visually perceived eye level

The purpose of this study was to determine how the combined effects of a reference frame and of very low gravito-inertial forces produced by centrifugation affect the visually perceived eye level (VPEL). Twenty subjects were instructed to set a luminous target to the VPEL under various experimental conditions involving two main factors: (1) visual context (frameless, frame centered, frame moved down 50 mm, and frame moved up 50 mm) and (2) gravito-inertial context (motionless, Gi₁=9.81001 m/sec² and Gi₂ = 9.95 m/sec²). The visual context significantly reduced the lowering of VPEL in darkness as caused by radial acceleration; this confirms the prevailing role of vision versus propriosomesthesis. However, under condition Gi₂, there was a significant effect on the VPEL in spite of the presence of the luminous frame; this demonstrates that VPEL processing involves both visual and propriosomesthesic information. Furthermore, the VPEL varied linearly with the vertical shift of the luminous frame for any of the gravito-inertial conditions used in this study, but, under condition Gi₂, the VPEL was shifted downward.