R. H. Day

The effect of accommodation retinal image size

The apparent size of an object is diminished when accommodation of the eye moves inward to a position closer to the observer than to a viewed object. This phenomenon is called accommodation micropsia. Using schematic eyes, we investigated change in retinal image size caused by a change in accommodation. The use of schematic eyes is also discussed and is justified. The calculated magnitude of this diminution for four schematic eyes ranged from unity at infinity to a maximum of 0.98 (- 2%) at about 12.0 diopters (D). For distances at which accommodation micropsia is typically observed (about 2.0 D), retinal minification is less than 0.997 (- 0.3%). Thus changes in the size of the retinal image attributable to accommodation are virtually negligible when compared with the observed reduction of 3% to 33%. This suggests that accommodation micropsia is mediated almost entirely by processes other than those involving the optics of the eye.

Visual perception of human activity and gender in biological-motion displays by individuals with mental retardation

By examining perception of biological-motion in three experiments, we investigated whether limitations in perceptual ability by individuals with mental retardation may extend to learning perceptual categories based on elements of actions. In two experiments individuals with mental retardation and age- and gender-matched controls reported gender and activity in randomly ordered point-light displays. In Experiment 2 decision times were measured and in Experiment 3, the effect of training with feedback on discriminating gender was assessed. Individuals with mental retardation identified walking, running, and stair-climbing without error but were less proficient in identifying throwing, had slower decision times for activity identification, and could not identify gender. For both groups feedback training improved gender identification in the short- and long-term. Improvements represented above-chance responding for the retarded group. Experimental outcomes are discussed in terms of capacity to classify perceptual stimuli as actions.

Visual Discrimination and Motor Reproduction of Movement by Individuals with Mental Retardation.

Visual discrimination and motor reproduction tasks involving computer-simulated arm movements were administered to 12 adults with mental retardation and a gender-matched control group to examine whether inadequacies in visual perception account for the poorer motor performance of individuals with mental retardation. In the discrimination phase subjects judged whether simulated arm movements were either of greater or lesser extent or shorter or longer in duration, respectively, than those of a standard display. In the reproduction phase accuracy in reproducing the movement in the standard display was measured. Results indicate that error in discriminating extent and duration was significantly greater for the individuals with mental retardation, who were also less accurate and more variable in matching the extent and duration of the standard displays. These outcomes implicate both perceptual and motor-reproduction inadequacies in skill acquisition for these individuals.

Sine of an Illusion

In an informally observed sine-wave figure in which the vertical extent between contours was constant, apparent extent in the crest and trough (the ‘turns’) appeared greater than in the straight oblique sections of the figure. This observation was confirmed in two experiments in which the vertical extents were matched by two vertically arranged dots. It was found that in a turn the apparent extent was greater than the true extent, but in a straight section both extents were about equal. These outcomes were confirmed when the two sections were each separated from the figure and presented alone. The illusion is explained in terms of a perceptual compromise between the vertical extent and the greater overall dimensions of the section at the turn of the sine-wave figure and is thereby held to be the same in principle as the Müller-Lyer illusion.

Two principles of perception revealed by geometrical illusions

Although perceptual illusions have been studied since the middle years of the 19th century there is no generally acceptable explanation of specific effects or of illusions in general. The central thesis proposed here is that illusions might be more fruitfully approached in terms of the principles of perception that they reveal and that apply also to normal, veridical perception. Recent experiments involving the conventional form of the Muller-Lyer figure presented progressively in a narrow aperture and a much simplified collinear form devised for presentation over space or time in both the visual and auditory modes suggested two such principles, that of whole-part determination of perceived size and that of space- time reciprocity. The first refers to the determination of the size of intrinsic parts by the whole figure or object and the second to the modulation of extent by time and vice versa. The broader implications of these principles are discussed.

The roles of static depth information and object-image relative motion in perception of heading.

In a series of 6 experiments, two hypotheses were tested: that nominal heading perception is determined by the relative motion of images of objects positioned at different depths (R. F. Wang & J. E. Cutting, 1999) and that static depth information contributes to this determination. By manipulating static depth information while holding retinal-image motion constant during simulated self-movement, the authors found that static depth information played a role in determining perceived heading. Some support was also found for the involvement of R. F. Wang and J. E. Cuttings (1999) categories of object-image relative motion in determining perceived heading. However, results suggested an unexpected functional dominance of information about heading relative to apparently near objects.

Visual accommodation as a cue for size

Accommodation micropsia is examined in the general context of ocular accommodation as a cue for object size. The nature and limits of accommodation micropsia and arguments dealing with the possible contribution of accommodation to the perception of size are reviewed. Literature on the anomalous myopias, the intermediate-resting hypothesis, and theories of ciliary muscle innervation is examined critically in so far as it bears on the accommodation-micropsia hypothesis. The anomalous myopias and evidence for the intermediate-resting hypothesis are well documented, but without a mechanism for proprioceptive feedback from the ciliary complex about the state of accommodation it can only be concluded that such feedback would have to be indirect, either via the reflex link with vergence, or possibly through the agency of efference-copy neurones.

On apparent misalignment of collinear edges and boundaries

In two experiments, subjects adjusted various pairings of the top and bottom boundaries of two obliquely oriented outline bars (Experiment 1) and those of two similarly oriented complete and incomplete squares (Experiment 2) to apparent alignment. The data from the first experiment showed that the misalignment effects were determined jointly by the directional properties of the bar ends (vertical, oblique, and semicircular) and the pair of boundaries that were aligned (both top boundaries, top of upper bar with bottom of lower bar, bottom of upper bar with top of lower bar). The results from the second experiment showed that the misalignment effects were the same for the oblique boundaries of solid and outline squares and persisted when the squares were reduced to two parallel lines. The effect was undiminished when the ends of the parallels were aligned, but was markedly reduced when pairs of parallels themselves were aligned. The outcomes of the two experiments are explained in terms of the apparentpositions of the oblique boundaries. It is proposed that these vary with the positions of the elements (bar or square) relative to the visual field, the position of the boundaries relative to the stimulus elements, and the positions of the boundaries relative to axes that are delineated by the parallel adjacent ends of bars and sides of squares. This relative-position basis for apparent misalignment is held to be the basis of misalignment effects in other figures.

The role of lightness, hue and saturation in feature-based visual attention

Visual attention is used to select part of the visual array for higher-level processing. Visual selection can be based on spatial location, but it has also been demonstrated that multiple locations can be selected simultaneously on the basis of a visual feature such as color. One task that has been used to demonstrate feature-based attention is the judgement of the symmetry of simple four-color displays. In a typical task, when symmetry is violated, four squares on either side of the display do not match. When four colors are involved, symmetry judgements are made more quickly than when only two of the four colors are involved. This indicates that symmetry judgements are made one color at a time. Previous studies have confounded lightness, hue, and saturation when defining the colors used in such displays. In three experiments, symmetry was defined by lightness alone, lightness plus hue, or by hue or saturation alone, with lightness levels randomised. The difference between judgements of two- and four-color asymmetry was maintained, showing that hue and saturation can provide the sole basis for feature-based attentional selection.

Pseudophakes Experience Apparent Minification in an Imaging Display

Apparent minification of virtual images has been attributed to accommodation micropsia, that is, a reduction in the apparent size of viewed objects accompanying an inward accommodation of the eyes. Thirteen bilateral pseudophakes incapable of accommodation but with good residual vision were tested monocularly for apparent minification in the viewfinder of a single-lens reflex camera. Apparent minification of about the same order as that typically found with normals occurred with the pseudophakic observers, demonstrating that pseudophakes are subject to distortions in apparent size with virtual-imaging displays in the same way as normals, thus establishing that visual accommodation is not a necessary condition for the effect.