Orientation Illusions Research Articles

Modification of visual orientation illusions by drugs which influence dopamine and GABA neurones: differential effects on simultaneous and successive illusions.

The effects of haloperidol, nomifensine and lorazepam on the visual tilt illusion were studied in normal volunteers. Haloperidol and nomifensine produced no significant changes in the illusion, although in previous work they had been found to reduce and enhance, respectively, a closely related illusion, the tilt aftereffect. By contrast, lorazepam produced a dose-related increment in the size of the tilt illusion, but had no effect on the tilt aftereffect. The results are discussed in relation to proposed mechanisms which may underlie the two kinds of illusion. The differential effects of individual drugs on the two illusions may reflect their differing actions on two processes: lateral inhibition and adaptation in visual channels.

An orientation illusion analog to the rod and frame: relational effects in the magnitude of the distortion.

Using a printed configuration of a rod enclosed by a tilted frame, an orientation illusion analog to the classical effect was obtained. In the first experiment it was shown that the distortion ob­ tained in this array varies in direction and magnitude as a function of degree of frame tilt, in a manner that is globally similar to that usually obtained in more traditional methods of mea­ surement. The second experiment indicates that relational factors determine the magnitude of the effect, with increasing rod and frame separation resulting in diminished apparent tilt of the rod. Possible interactions between such an orientation illusion effect and visual-vestibular con­ tributions to the rod and frame are discussed. The rod-and-frame effect is a well-known phenomenon in which a line or rod surrounded by a tilted frame under­ goes a shift in apparent orientation (Witkin & Asch, 1948). There are two general classes of explanation that have been offered for the occurrence of this effect. The first views the effect as a visual-vestibular interaction; the sec­ ond views it as a purely visual interaction analogous to many angular illusions. The visual-vestibular interaction explanations for this effect depend upon fairly automatic processes that are free of cognitive mediation (cf. Howard, 1982, for a review). In general, this type of theory suggests that peripheral receptive fields may be assumed to process the image of the tilted frame, and that these in turn stimulate static orientation detectors (presumably at the vestibular nucleus). These then instigate a sequence of events which eventually produces the perception of inclination in a phys­ ically vertical line (Daunton & Thomsen, 1979; Dichgans & Brandt, 1978; Ebenholtz & Glaser, 1982; Ebenholtz & Utrie, 1983; Waespe & Henn, 1977). However, although vestibular factors may play an important role in the formation of the percept, there appears to be a more purely visual component present. This has been demon­ strated by Rock (1966), who was able to obtain the rod­ and-frame illusion with supine observers, a manipulation that eliminates vestibular mechanisms as the sole deter­ minant of the effect. In contrast, explanations based on visual interactions view the rod-and-frame illusion as analogous to many other visual-geometric illusions of orientation, such as the Zollner or the Lipps illusions (Coren & Girgus, 1978a). In these explanations it is the relationship between the con

Another visual illusion involving orientation

The orientation of a grating may be misperceived when that grating appears superimposed within one-dimensional visual noise of a different orientation. An adjustment procedure was employed to measure the size of this tilt illusion and its dependence on noise orientation. The illusion is maximum when the noise and test differ by 20 to 40 deg, depending on the spatial frequency of the test grating. Noise composed of spatial frequencies several octaves removed from the test spatial frequency can still cause illusory tilt, although the degree of tilt under these conditions depends on test spatial frequency. This orientation illusion, like the tilt aftereffect simultaneous tilt illusion and angle expansion illusion, may reflect inhibitory interactions between orientation selective neurons in human vision.

Spatial orientation in weightlessness and readaptation to earth's gravity.

Unusual vestibular responses to head movements in weightlessness may produce spatial orientation illusions and symptoms of space motion sickness. An integrated set of experiments was performed during Spacelab 1, as well as before and after the flight, to evaluate responses mediated by the otolith organs and semicircular canals. A variety of measurements were used, including eye movements, postural control, perception of orientation, and susceptibility to space sickness.

The roles of axes of symmetry in orientation illusions.

The lines of a surrounding figure can induce illusory distortion in the apparent orientation of an enclosed line. The axes of symmetry of the figure have been implicated in this distortion. A series of experiments showed generally that the effects are attributable to the axes of bilateral symmetry rather than to less restrictively defined axes of topological symmetry, although those axes may produce other illusory effects. Furthermore, the distortions affect the whole line rather than only parts of it. These results are consistent with interactions of the lines in the orientation rather than position domain. It is suggested that axes of symmetry may play an important role in pattern recognition.

Techniques for the measurement of the Poggendorff and Zoellner illusions in large-group situations

Although validated procedures exist for group measurement of illusions of extent and area, few exist for the measurement of illusions of direction and orientation. Two new group- measurement techniques, for the Poggendorff and Zoellner illusions, are presented. These are shown to be sensitive enough to detect the usually obtained parametric variations of illusion magnitude with specific changes in stimulus configurations.

The angular function of orientation illusions induced by projected images of tilted real object scenes.

Tilt aftereffects induced by line gratings are maximal when the test and inducing stimuli have an angular separation of 10 degrees-15 degrees. Similar effects induced by prismatically tilted real-object scenes have been found to increase monotonically with scene tilt. The difference between these two angular functions of aftereffect (that induced by gratings and that induced by real objects) has been attributed to the 'meaning' inherent in the real-object scenes. The preliminary experiments described here suggest that tilt aftereffects and illusions induced by projected slides of tilted real-object scenes have angular functions similar to that induced by a line grating. Hence, the monotonically increasing angular function obtained in the prism studies is not necessarily determined by the use of real-object scenes.

Component analysis of orientation illusions.

Orientation illusions occur when the inducing figure is a line or grating (the tilt illusion) or a square outline frame (the rod-and-frame illusion). In the range of inducing figure tilts between vertical and horizontal, the tilt illusion describes one cycle of positive (direct) and negative (indirect) effects but the rod-and-frame illusion describes two such cycles. In two experiments, angular functions of illusions were measured with the six possible inducing figures which result when two of the four sides of a square inducing frame are deleted. As expected, the parallel-side frame amputations induced angular functions similar to the tilt illusion and these functions differed from those induced by the orthogonal-sided amputations. In agreement with previous findings on the nonadditivity of tilt illusions, the sum of angular functions induced by frame amputations, which together form a complete frame, were not always equivalent to the angular function induced by a complete frame, and there were asymmetries in the data fro which neither of two simple hypotheses could adequately account. The discussion focuses upon properties of inducing figures which psychophysical hypotheses might need to consider in order to account for the shapes of angular functions of orientation illusions and, in particular,a distinction is drawn between the global orientation of the inducing figure and the orientations of it (local) component features. It is suggested that it might be fruitful if the tilt illusion and the rod-and-frame illusion were conceived of as illusions resulting from inducing figures composed of all or part of n gratings of spatial frequency fn intersecting at angles of 180 degrees/n.

The tilt illusion: Repulsion and attraction effects in the oblique meridian

It is commonly believed that the same neural mechanism underlies the tilt illusion and the tilt aftereffect. Recently, it has been demonstrated that tilt aftereffects induced on oblique stimuli are similar, in magnitude and direction, to those induced on stimuli which are vertical or horizontal. If the mechanisms of the illusion and aftereffect are the same, then illusions induced on oblique stimuli should also be similar to those induced on vertical or horizontal stimuli. The six experiments reported here confirmed this prediction by suggesting that both repulsion (direct) and attraction (indirect) tilt illusions occur in the oblique meridian. The data are considered in relation to both psychophysical (normalisation) and neural (lateral inhibition) theories of orientation illusions and aftereffects.

Orientation illusions and after-effects: Inhibition between channels

The apparent tilt from vertical has been examined for vertical sinusoidal gratings in the presence of an inducing grating which was tilted 12° from vertical. The amount of apparent tilt depended on the contrast of the test grating. At high test contrasts, the grating appeared to be tilted about 2° clockwise; at low contrasts, near threshold, there was little or no apparent tilt. If the inducing grating and test grating were not contiguous but were separated by about 0.4°, there was no apparent tilt at any test contrast. The detection threshold for the test grating was elevated by the inducing grating only when the whole of the test grating was close to the inducing grating. It is argued that these results can be explained if there is an inhibitory interaction between detectors responding to similar orientations and subserving similar parts of the visual field.

Orientation illusion and masking in central and peripheral vision.

Notes that a vertical line appears tilted when viewed following inspection of tilted lines (aftereffect) or against a background of tilted lines (illusion). In Exp. I with 16 undergraduates, the illusion obtained, as the orientation of the inducing lines was varied, differed between central and peripheral vision. The possibility that orientation-selective cells in the human visual system are more broadly tuned in peripheral than central vision was examined in Exp. II with 8 Ss using a contour masking paradigm. The detectability of a vertical line varied with the orientation of a grating exposed immediately before the line, but contrary to expectations, masking (threshold elevation) was obtained over a wider range of orientation values in central than peripheral vision. No evidence was obtained that perceptual facilitation (improved detection of the line following exposure to the grating) can occur in central vision in addition to masking. Results question whether aftereffect, illusion, and masking paradigms are equally effective in obtaining information about the tuning characteristics of feature analyzers in the human visual system. (23 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved).

Geometrical illusions and the response of neurones in the cat's visual cortex to angle patterns.

1. This report describes the responses of thirty-six single neurones in the primary visual area of the cat's neurologically isolated and unanaesthetized forebrain, to movements of thin white lines across the visual field. The experiments were designed to record the effects upon the response to a single test line of an added line, which was either parallel to the test line or joined it, making an angle-pattern of 30 degrees . Unit responses were measured in terms of the peak probability of firing derived from a post-stimulus histogram.2. All of the cortical neurones tested exhibited a preferred orientation for stimulation by the test line, i.e. an orientation of the line which produced a maximal response when the line passed through the centre of the unit's receptive field.3. There was no evidence that the orientation of a single test line preferred by cortical neurones was different from that preferred by the same cell when excited by an angle pattern, one arm of which was the original test line.4. The position of a test line (with preferred orientation) in the visual field that produced a maximal response from cortical neurones, was not always the same as the position for maximal response, when a second line was added to make either an angle pattern or to make a pattern of two parallel lines.5. Where the two lines of these patterns were close together and separated by less than the radius of the receptive field, the position for maximal response to the test line was shifted towards the added line. Where the two lines were further apart than this but separated by less than a receptive field diameter, the optimal position for the test line was displaced away from the added line.6. Some evidence was found of a lateral inhibition in the visual system, sufficient to account for the displacements described in paragraphs 4 and 5 above.7. It is concluded that the tip of an angle pattern of 30 degrees produces a distorted cortical image within the primary visual area.8. This neural distortion of sensory information seems adequate to explain the well known illusions of orientation that are associated with human perception of patterns containing acute angles.

Visual orientation illusion following judgments with a tilted visual field.

Asch and Witkin showed that when an observer adjusted a bar on the far wall of a miniature room tilted at 22° laterally, the apparent vertical of the bar was displaced about 10°–14° in the direction of room tilt1. We report here an experiment which shows that earlier exposure to a similarly tilted room produces the same direction of change in apparent verticality when a luminous bar is adjusted in darkness. This experiment included appropriate controls for the occurrence of a spatial after-effect2,3, and so the data suggest that illusory effects persist after the conditions inducing them are removed.

Some curious problems suggested by case studies of handedness.

The author believes the application of statistical methods to the study of unilateral dominance bids fair to clear up many obscurities. It should not, however, wholly supersede the clinical procedure. From a number of individual case studies, there were studied: (1) experiences of subjects who have been shifted from the preferred hand, from left to right or right to left--one case of such change shows relapse to the preferred activity in learning novel activities and creative drawing; (2) observations on a change in the use of the hands motivated by a curious experience in visualization, and an inhibition of visualization due to conflict; (3) orientation illusions, arising from definitely right or left motor sets; (4) experiences which suggest that in some cases handedness shifts with changes in organic tension. (PsycINFO Database Record (c) 2006 APA, all rights reserved)

Reverse illusions of orientation.

Reverse illusions of orientation.