Anisotropic perception of visual angle: Implications for the horizontal-vertical illusion,overconstancy of size,and the moon illusion

Three experiments investigated anisotropic perception of visual angle outdoors. In Experiment 1, scales for vertical and horizontal visual angles ranging from 20° to 80° were constructed with the method of angle production (in which the subject reproduced a visual angle with a protractor) and the method of distance production (in which the subject produced a visual angle by adjusting viewing distance). In Experiment 2, scales for vertical and horizontal visual angles of 5°–30° were constructed with the method of angle production and were compared with scales for orientation in the frontal plane. In Experiment 3, vertical and horizontal visual angles of 3°-80° were judged with the method of verbal estimation. The main results of the experiments were as follows: (1) The obtained angles for visual angle are described by a quadratic equation, θ′=a+bθ+cθ² (where θ is the visual angle; θ′, the obtained angle;a, b, andc, constants). (2) The linear coefficientb is larger than unity and is steeper for vertical direction than for horizontal direction. (3) The quadratic coefficientc is generally smaller than zero and is negatively larger for vertical direction than for horizontal direction. And (4) the obtained angle for visual angle is larger than that for orientation. From these results, it was possible to predict the horizontal-vertical illusion, over-constancy of size, and the moon illusion.     

Visual and haptic perception of the horizontal-vertical illusion

The horizontal-vertical illusion consists of two lines of the same length (one horizontal and the other vertical) at a 90 degree angle from one another forming either an inverted-T or an L-shape. The illusion occurs when the length of a vertical line is perceived as longer than the horizontal line even though they are the same physical length. The illusion has been shown both visually and haptically. The present purpose was to assess differences between the visual or haptic perception of the illusions and also whether differences occur between the inverted-T and the L-shape illusions. The current study showed a greater effect in the haptic perception of the horizontal-vertical illusion than in visual perception. There is also greater illusory susceptibility of the inverted-T than the L-shape.     

Handgrip maximum force and the visual horizontal-vertical illusion

The visual horizontal-vertical illusion (HVI) refers to the tendency to overestimate vertical distances relative to horizontals in both 2-D and 3-D presentations. Although the HVI is evident across a wide range of different stimuli, no general theoretical account fully explains the illusion. Some recent authors have proposed the 'effort' account of HVI, contending that vertical overestimation is mediated by effort assessment of gravitational challenges offered by the stimulus. The theory has been supported by a set of studies showing that the height overestimation of large-scale 3-D objects is inversely related to perceivers' fitness and strength. We explored if the large-scale HVI/strength dependence extends to the evaluation of small-scale 2-D line stimuli, traditionally used in HVI studies. We measured the maximum handgrip strength, and assessed the HVI with a computerised line-adjustment task in thirty-two individuals. Compatible with earlier findings in the context of large-scale 3-D stimuli, a significant negative correlation was found between the strength of the dominant hand and amount of HVI. In addition, the variability of HVI was negatively correlated with maximum grip strength of both hands. The results are discussed with reference to the 'effort' account of HVI.     

The size illusion: visual and kinaesthetic information in size perception

Earlier studies have shown the size of kinaesthetically presented two-dimensional movement patterns to be significantly overestimated. Whether this size overestimation is characteristic of the kinaesthetic system alone has not been established. Two experiments are reported which were designed to investigate size judgment made after kinaesthetic and visual pattern presentation and the effect of environmental cues on the perception of movement patterns. In experiment 1 patterns were presented kinaesthetically (experimenter guided hand movements around the outline of the pattern) or in combination with visual information given by a moving light (pinpoint light attached to the stylus which was moved around the pattern); visual and kinaesthetic cues were either congruent or conflicting with each other; and environmental cues were either present or absent. In experiment 2 static visual display was compared with visually traced pattern presentation, again with or without environmental cues. Overall the results showed that, regardless of experimental manipulation, in all cases where the information was given over time the subject perceived the pattern larger than reality. After static visual display, overestimation of size did not occur.     

Effect of the standard on size of the horizontal-vertical illusion

70 subjects participated in a study designed to replicate the 1968 claim of Gardner and Long that the horizontal-vertical illusion is larger with the vertical than with the horizontal as standard. The effect did not appear in Exp. 1 when subjects made judgments under both the reproduction and graded-series methods but did in a pilot study and in Exp. 2, although more strongly with the method of reproduction.     

EYE LINES: A tool for studying visual perception of size and angle

EYE LINES is a set of programs for preparing and running experiments that measure the perception of the size, position, or angle of orientation of visual stimuli. A drawing program builds images as collections of linked line-segment vectors. The experiments involve the method of adjustment. Subjects use a mouse to adjust the lengths or angles of specified line segments. Designed for use both in research and instructional laboratories, the programs run on the Macintosh.     

The role of figure orientation and apparent depth in the perception of the horizontal-vertical illusion.

An experiment was performed which examined the role of figural orientation directly, and the role of an inappropriately invoked size-constancy mechanism indirectly, in the actuation and magnitude of the horizontal-vertical illusion. When the vertical line of the stimulus figure was aligned above the horizontal line, the illusory effect was significant and positive; in contrast, when the vertical line was located below the horizontal line, the illusion was negative. Under the assumption that a vertical line can appear as a foreshortened line in depth, these findings support an explanation based on the operation of a misapplied size-constancy mechanism.     

Objects, raised lines, and the haptic horizontal-vertical illusion

Experiments were conducted to determine whether the haptic horizontal-vertical illusion occurs with solid, three-dimensional objects as well as with tangible lines. The objects consisted of round or square bases, with dowel rods projecting above them at heights equal to the widths of the horizontal bases. A negative illusion, with overestimation of horizontals, was found with free haptic exploration, but not with tracing with the fingertip. The negative illusion occurred when subjects felt wooden Ls and inverted Ts with a grasping, pincers motion of the index finger and thumb. The presence or absence of illusory misperception was dependent upon exploration strategy, since the negative illusion vanished with finger tracing. A negative illusion was also found when subjects adjusted a vertical dowel so that it was judged to be equal in extent to a round or square base. A general overestimation of judged size derived from the pincers response measure, but was not found with the use of a tangible ruler. Comparable illusory results are most likely when drawings and objects promote similar haptic scanning methods. The results were consistent with the idea that the orientation of an edge or line is more important than whether one explores a tangible line or a three-dimensional object.     

The influence of figure size and orientation on the magnitude of the horizontal-vertical illusion

Two experiments were performed to examine the effect of display size and figure orientation on the horizontal-vertical illusion. According to the visual field hypothesis of Künnapas (1957a, 1957b) if the relation of the figure components to the surrounding frame is held invariant neither experimental manipulation should exert an appreciable influence. However, both manipulations produced significant effects indicating that the visual field hypothesis is untenable as the primary determinant of the horizontal-vertical illusion. An alternative explanation was discussed.     

Comparing effects of the horizontal-vertical illusion on grip scaling and judgment: relative versus absolute, not perception versus action

The discovery that the prehension component of an open-loop, two-fingered reach is largely immune to certain salient pictorial illusions has been used to suggest that humans possess 2 distinct visual systems, 1 that subserves perceptual judgment and 1 that mediates visually controlled action. In this article, the authors present evidence that suggests that the critical distinction is not that of reaching and judgment but of relative and absolute perception. Experiment 1 extends the findings of S. Aglioti, J. F. X. DeSouza, and M. A. Goodale (1995) and suggests that the manual prehension component of open-loop reaching is affected by the horizontal-vertical illusion to a much smaller degree than perceptual size judgments. In Experiments 2 and 3, however, when perceptual size judgment is directed at a single element of the display, this difference vanishes. Experiment 4 demonstrates that grip scaling is strongly affected by the illusion when a single reach is scaled to both the horizontal and vertical components of a triangular figure.     

Masking,aftereffect, and illusion in visual perception of curvature

Masking, aftereffect, and illusion paradigms were used to establish the spatial selectivity of curvature detectors in human vision. Arcs with the same chord orientation mask each other maximally when they are identical in radius and direction of curvature. There is gradual reduction in masking over an extensive spatial range as arcs diverge in curvature. The transition from convexity to concavity does not produce discontinuity in the masking function. The extent to which a straight line appears curved also depends on the curvature of arcs shown previously (aftereffect) or at the same time (illusion). It is suggested that these effects could occur through selective adaptation of detectors responsive to either global curvature or the orientation of local straight-line approximations within an arc. Evidence is reviewed in support of the latter interpretation.     

Acquisition and retention of perceptual learning and the horizontal-vertical illusion.

The present study examined procedure-specific differences in the acquisition and retention of perceptual learning using four forms of the Horizontal-Vertical illusion. Training to criterion was conducted using intertrial feedback, continued visual inspection, or yoked-control procedures. Retention of perceptual learning was assessed at posttraining intervals ranging from 1 minute to 1 month. Subjects trained with feedback achieved criterion in fewer trials and showed greater accuracy and short-term retention of perceptual learning on the inverted-T figure and the 1-in. vertical line-production task. The present results show that the correction of inaccurate strategic factors most likely represents the temporary acquisition of compensatory strategies that facilitate performance on simple perceptual-motor skills tasks.     

Affective stimulus properties influence size perception and the Ebbinghaus illusion

In the New Look literature of the 1950s, it has been suggested that size judgments are dependent on the affective content of stimuli. This suggestion, however, has been 'discredited' due to contradictory findings and methodological problems. In the present study, we revisited this forgotten issue in two experiments. The first experiment investigated the influence of affective content on size perception by examining judgments of the size of target circles with and without affectively loaded (i.e., positive, neutral, and negative) pictures. Circles with a picture were estimated to be smaller than circles without a picture, and circles with a negative picture were estimated to be larger than circles with a positive or a neutral picture confirming the suggestion from the 1950s that size perception is influenced by affective content, an effect notably confined to negatively loaded stimuli. In a second experiment, we examined whether affective content influenced the Ebbinghaus illusion. Participants judged the size of a target circle whereby target and flanker circles differed in affective loading. The results replicated the first experiment. Additionally, the Ebbinghaus illusion was shown to be weakest for a negatively loaded target with positively loaded and blank flankers. A plausible explanation for both sets of experimental findings is that negatively loaded stimuli are more attention demanding than positively loaded or neutral stimuli.