Perception

Induced motion (IM) is illusory motion of a stationary test target opposite to the direction of the real motion of the inducing stimulus. We define egocentric IM as an apparent motion of the test target relative to the observer, and vection-entrained IM as an apparent motion of a stationary object along with an apparent motion of the self (vection) induced by the same stimulus. These two forms of IM are often confounded, and tests for distinguishing between them have not been devised. We have devised such tests. Our test for egocentric IM relies on evidence that this form of IM is due mainly to a misregistration of eye movements when optokinetic nystagmus (OKN) is inhibited, and on evidence that OKN is evoked only by stimuli in the plane of convergence. Our test for vection-entrained IM relies on evidence that vection is evoked only by the more distant of two superimposed inducing stimuli. Thus we found egocentric IM to be induced without vection or vection-entrained IM when subjects converged on a foreground moving display with a stationary display in the background, and vection-entrained IM to be induced without egocentric IM when subjects converged on a stationary-foreground display with a moving display in the background. The two types of IM were evoked in opposite directions at the same time when subjects converged on a foreground moving display while a background display moved in the opposite direction. The two forms of IM showed no signs of interaction, and we conclude that they rely on independent motion mechanisms that operate within distinct frames of reference. A control experiment suggested that the depth adjacency effect in IM is determined by the depth adjacency of the inducing stimulus to convergence, not just to the test target.

Distance disparity is a strong cue to element correspondence in apparent motion. Using a 2-AFC paradigm we have previously shown that shape similarity also plays a role. We now demonstrate a small gender difference in these effects: women are more sensitive to distance disparity, whereas men are more sensitive to differences in shape. Furthermore, in the competing presence of a shape cue, women's sensitivity to distance decreases while men's sensitivity is unaffected. These observations may be related to putative gender differences in the ‘form’ and ‘motion—spatial relations’ cortical pathways.

Shape discrimination was measured for: (i) two-dimensional rectangular targets that were perfectly camouflaged within a stationary pattern of random dots and rendered visible by relative motion of the dots, and (ii) similar dotted rectangles that were rendered visible by luminance contrast. Shape discrimination was disconfounded from size discrimination by requiring subjects to discriminate the aspect ratios of rectangles whose areas were altered independently of aspect ratio. When dot speed and contrast were both high, the aspect-ratio discrimination threshold was as acute for motion-defined (MD) rectangles as for contrast-defined (CD) rectangles and, at 2–3%, corresponded to a change of side length of about 24 s arc compared to a mean dot separation of 360 s arc. Discrimination of MD rectangles collapsed at low dot speeds and could not be measured at speeds less than about 0.03–0.08 deg s⁻¹, but discrimination of CD rectangles was almost unaffected by dot speed. The aspect-ratio discrimination threshold was lowest for a square and progressively increased as the rectangle became more asymmetric. It is suggested that the visual system contains a mechanism that compares the separations of pairs of contours along different azimuths, and that, during visual development, this shape-discrimination processing of MD and CD targets is driven by the same environmental and behavioural pressures towards a common end point. The human equivalent of a pathway that includes the cortical area MT is thought to be important for shape discrimination of MD forms.

It has been reported that an annulus patterned with two equally-spaced radial lines elicits slower induced rotary movement than an annulus patterned with sixteen equally-spaced radial lines. This has been attributed to the quantity of inducing stimulation. However, the number of pattern elements also affects the time course of induced rotary movement: it can be intermittent with two-radii inducers, but persistent with sixteen-radii inducers. Displacement of a two-radii inducer may be more salient to the subject than displacement of a sixteen-radii inducer: from time to time the displacement of the former may come to dominate perception, suppressing induced movement. Hypotheses invoking quantity of stimulation and salience of inducer displacement were tested by way of inducers with different numbers and spacings of pattern elements. Subjects timed induced rotary movement (experiment 1) and a subsequent aftereffect (experiment 2). The results were consistent with both the quantity of stimulation (experiments 1 and 2) and the salience of inducer displacement (experiment 2) having an effect. This suggests that at least two mechanisms may be involved in induced rotary movement.

The optic flow field can be described in terms of the local differential measures, divergence, deformation, and rotation, which are informative about observer motion and the 3-D structure of the environment. Does an explicit representation of these measures exist in human visual processing in the form of a feature map? Triesman's criteria were used to investigate this; ie is there ‘pop-out’ for a target defined as different in local divergence or deformation from surrounding elements, or is a serial search necessary? The stimulus arrays contained 3, 5, or 9 square or rectangular elements, which each underwent repeated cycles of expansion, contraction, or deformation. The time required to detect a target undergoing the opposite transformation increased steeply with the number of elements, implying very slow serial search. (The mean time was 210 ms per element for divergence targets and 542 ms per element for deformation.) The process was clearly still serial when the density and number of elements was increased up to 48 in an array 2.16 deg × 2.16 deg. In contrast, a single line element undergoing the opposite direction of translation motion to the rest of the display did show pop-out.

It is concluded that no parallel processes seem to exist which are sensitive to the spatial uniformity of divergence and of deformation of optic flow. These differential properties may be derived as conjunctions of signals from a primary process which extracts local velocity. This result contrasts with our findings for targets defined by stereo disparity gradient, which show parallel processing in analogous experiments.

Does the visual system represent stereoscopic depth purely as a map of local disparities, or does it explicitly represent local relationships of disparity, such as disparity gradients? Experiments are reported in which visual search for a target containing the same disparity range as other elements in the display, but differing in the relationship of the disparities (stereo slant), was used to determine whether the target showed ‘pop-out’ like a unitary feature, or the serial search characteristic of feature conjunctions. Each stereo pair of elements was selected randomly from a range of outline parallelograms leaning to the right or to the left, so that the target could not be identified using any monocular shape cue.

Response times for detection of the target (present on 50% of the trials) were independent of the number of elements in the display. This result was confirmed by varying element size and spacing, and by using oblique crosses rather than parallelograms as stimuli. It is concluded that stereoscopically defined slant, or disparity gradient, can be processed and compared in parallel across the display, and acts in this respect as an explicit unitary visual property. This contrasts with findings in analogous experiments on movement, which show that targets defined by divergence or deformation of optic flow can only be identified by serial search.

Training effects on dynamic visual acuity (DVA) were determined over a range of target durations and target velocities under the viewing condition of free-head movement. Dynamic acuity levels were lower than static acuity levels, and training was found to be very effective in improving performance, particularly under the most difficult conditions in which performance was poorest. Improvement was also most evident among those observers who initially exhibited the poorest DVA scores. Preliminary comparisons between DVA scores of a nonselect sample of college students and those of a small group of college athletes indicated that the athletes were not very different from the nonselect group and would also benefit from training on the task.

The contrast sensitivity functions of a large group of observers (N = 71) were determined for three orientations of test gratings: vertical, oblique, and horizontal. Comparison of group means indicated that, consistent with previous findings for the ‘oblique effect’, sensitivity was poorer for the oblique orientation—but only for the mid—high spatial frequencies. Correlation analyses indicated that contrast sensitivity for a particular spatial frequency at a given orientation was highly correlated with contrast sensitivity for that same frequency at the other two orientations. Factor analysis of the intercorrelations revealed two strong factors, a low frequency factor, and a mid-high frequency factor. Results are discussed in terms of: (a) the implications for contrast-sensitivity testing across orientations, (b) the basis for the oblique effect, and (c) a different type of evidence for a dichotomy among spatial-frequency channels that may reflect the distinction between X cells and Y cells or between the parvocellular and magnocellular systems.

Recently hatched domestic chicks control their upright bipedal posture, at least in part, with respect to the flow of visual information at the retina, as do human infants when they first acquire control of the head, of sitting, and of standing. Some implications of the similarity of the proprioceptive function of vision in chicks and in humans for the origins and development of postural control are discussed.

A geometrical illusion in which the horizontal spacing between adjacent parallel lines in a row is underestimated when the lines are tilted away from vertical in a chevron configuration was investigated in two experiments. The perceived spacing was found to decrease as the tilt angle increased, consistent with the idea that separation judgements are influenced by the normal spacing between lines ie at right angles to the line orientation. It is proposed that this illusion reveals an analogue in spatial perception to the well-known aperture problem in motion perception. In establishing the separation of nearby or overlapping shapes in an image, the visual system cannot only rely upon the normal separation of contours belonging to each shape (as would be visible through small spatial apertures or receptive fields), since this varies with contour orientation. The system is therefore faced with a spatial aperture problem. The spacing illusion may arise because information usually available to solve the problem is absent in the illusion figure, or it may reflect a bias in favour of the orthogonal, which is adopted in the face of the ambiguity.

To perceive a stable world, one must somehow be able to relate visual information from successive fixations. Little is known, however, about the nature of the integrative process. By using a task which requires the integration of spatial position information from different fixations, it is demonstrated that visual information from previous fixations is preserved in a world-centered representation which is precise enough to support judgements of geometric shape. It is also shown that successive views are aligned with respect to common visual features, indicating that visual stability may be normally accomplished by a visual matching strategy in combination with cancellation by an eye-position signal.

Two experiments were carried out to investigate the influence of structural and semantic information in the categorisation of visual forms. In experiment 1 the stimuli were pairs of ‘well structured’ or ‘poorly structured’ fragmented forms differing in structural properties including convexity, collinearity, and closure of the elements, and in their ‘name-ability’. The influence of structural and semantic information was tested by means of a task involving the presentation to both visual hemifields separately of stimuli having within-category physical similarity. For well structured forms, subjects were asked to judge if two sequentially presented forms belonged to the same semantic category. For poorly structured forms, subjects were required to decide if two stimuli belonged to the same previously learnt category of forms. For the two types of stimuli, one category was composed of physically similar items, and the other was composed of physically dissimilar items. The results show a marked advantage for the category composed of physically similar forms when the stimuli are well structured but no differences between the two categories when the stimuli are poorly structured. This suggests a facilitation in the computation of global shape information for forms having collinear and closed elements. The only effect of semantic information was a tendency towards a right visual field advantage for same-category pairs of well structured forms. The pattern of results for well structured fragmented forms (experiment 1) and forms in which fragments had been joined in order to complete the contour (experiment 2) were similar, suggesting that structured incomplete forms can be processed in the same way as forms having a continuous contour. Hypotheses about the organisational processes of forms are proposed in the discussion.

Measures of the size of various hand parts and their viewing distance when held at arm's length were made on one hundred and eighteen undergraduate students. A simple rule of thumb can be confirmed: Visual angle of the width of the thumb held at arm's length is about 2 deg. The thumbnail subtends about 1.5 deg and the index fingernail about 1 deg in width, when both are held at arm's length. These figures are good approximations for males and females, although a significant, direct, linear relationship exists between hand-part size and the visual angle of the part at arm's length.