Perception

We aimed to address two issues: first, to describe how the perception of motion differs in elderly observers as compared to younger ones; and, second, to see if these changes in motion perception could be accounted for by the known changes in the ability of elderly observers to detect patterns (as indexed via contrast sensitivity). The lower threshold of motion, motion coherence, and speed discrimination were measured, alongside contrast sensitivity, in a group of thirty-two older (mean age 61.5 years) and thirty-two younger (mean age 23.2 years) subjects. The older observers showed losses in their ability to detect slow motions as indexed via the lower threshold of motion for random-dot patterns and for gratings of a range of spatial frequencies. They also were impaired on a test of motion coherence, but only for stimuli of a slow to medium speed, whereas faster speeds showed no decline with age. Finally, at all speeds tested the older observers required greater differences in speed in order to discriminate between patterns moving at different speeds. The pattern of losses on motion perception tasks was not predicted by the deficits of the older groups, such as loss of detection thresholds for high spatial and/or temporal frequencies. It is concluded that these hypotheses do not provide an adequate account of the data, and therefore that the losses occurring with age are complex and probably are a result of the loss of several types of cell.

When holding a small-scale model of Ames's trapezoidal window with the arms fully extended, several observers experience a striking proprioceptive distortion (eg one hand appears farther from the other, or one arm appears longer than the other). However, data from a matching experiment suggest that the proprioceptive misalignment of the hands is, in fact, rather less than the apparent slant of the window when this is not held. This finding argues against a ‘visual-capture’ account, supports an explanation in terms of bimodal integrative processes, and underscores the importance of supplementing phenomenological observations with objective measures.

An isolated dot appears double outside a small disparity range called Panum's fusional area. In random-dot stereograms (RDSs), however, this doubling, or diplopia of dot elements is not evident at any disparity. Nevertheless, depth is perceived up to disparities that greatly exceed Panum's fusional limit. Either one is unaware of dot diplopia at disparities exceeding Panum's fusional limit or the fusion limit is extended. To examine these possibilities, we developed a novel RDS in which dichoptically color-coded dots have a distinctive color when fused, and return to their intrinsic colors when diplopic. We measured the fusion limit of dots in this RDS, and compared it to the patent stereopsis limit of the perceived surface in similar RDSs. We found that the fusional area of dots in the RDS was comparable to Panum's fusional area. Furthermore, there was clear dissociation between the fusion limit and the patent stereopsis limit in the RDS. We conclude that the elements composing a surface are not necessarily fused when a large disparity surface is perceived in depth.

In an attempt to understand how low-level visual information contributes to object categorisation, previous studies have examined the effects of spatially filtering images on object recognition at different levels of abstraction. Here, the quantitative thresholds for object categorisation at the basic and subordinate levels are determined by using a combination of the method of adjustment and a match-to-sample method. Participants were asked to adjust the cut-off of either a low-pass or high-pass filter applied to a target image until they reached the threshold at which they could match the target image to one of six simultaneously presented category names. This allowed more quantitative analysis of the spatial frequencies necessary for recognition than previous studies. Results indicate that a more central range of low spatial frequencies is necessary for subordinate categorisation than basic, though the difference is small, at about 0.25 octaves. Conversely, there was no effect of categorisation level on high-pass thresholds.

The effects of visual field color and spatial complexity on self-motion perception were investigated by placing observers inside a large rotating cylinder (optokinetic drum). Under optokinetic-drum conditions visually induced self-motion (vection) is typically perceived within 30 s, even though all forms of sensory input (eg vestibular, proprioceptive, auditory), except vision, indicate that the observer is stationary. It was hypothesized that vection would be hastened and vection magnitude increased by adding chromatic colors and spatial complexity to the lining of an optokinetic drum. Addition of these visual-field characteristics results in an array that shares more visual-field characteristics with our typical environment that usually serves as a stable frame of reference regarding self-motion perception. In the color experiment, participants viewed vertical stripes that were: (i) black and white, (ii) various gray shades, or (iii) chromatic. In the spatial complexity experiment, participants were presented with: (i) black-and-white vertical stripes, or (ii) a black-and-white checkerboard pattern. Drum rotation velocity was 5 rev. min⁻¹ (30° s⁻¹), and both vection onset and magnitude were measured for 60 s trials. Results indicate that chromaticity and spatial complexity hasten the onset of vection and increase its perceived magnitude. Chromaticity and spatial complexity are common characteristics of the environments in which our visual system evolved. The presence of these visual-field features in an optic flow pattern may be treated as an indicator that the scene being viewed is stationary and that the observer is moving.

Action is not usually considered to play a role in colour perception. However, sensorimotor theories of perception (eg O'Regan and Noë, 2001 Behavior and Brain Science 24 939–1011) suggest that, on the contrary, the transformations created by action in the sensory input are a necessary condition for all perception. In the case of colour vision, eye movements may explain how a retina with significant irregularities in resolution and cone arrangement (Roorda and Williams, 1999 Nature 397 520–522) could permit the perception of a richly coloured world (Clark and O'Regan, 2000 15th International Conference on Pattern Recognition volume 2: Pattern Recognition and Neural Networks pp 503–506; Skaff et al, 2002 16th International Conference on Pattern Recognition volume 2, pp 681–684). We provide evidence that perception of colour is modified when an artificial coupling is introduced linking eye movements and colour changes. After 40 min of wearing left-field-blue/right-field-yellow spectacles, observers' colour vision adapts so that, after removing the spectacles, white patches seem to become bluer when the eyes move rightwards and yellower when the eyes move leftwards. This induced dependence of colour perception on the direction of eye saccade is shown to be related to the amount of eye movements during exposure. This result, which cannot be explained either by retinal adaptation, or by a conditioned association between colour and side, constitutes first clear evidence for a role of eye movements in perceived colour and argues for the involvement in colour perception of neural mechanisms continuously tuned to sensorimotor contingencies.

We report two experiments in which we used animated averaged faces to examine infants' ability to perceive and discriminate facial motion. The faces were generated by using the motion recorded from the faces of volunteers while they spoke. We tested infants aged 4–8 months to assess their ability to discriminate facial motion sequences (condition 1) and discriminate the faces of individuals (condition 2). Infants were habituated to one sequence with the motion of one actor speaking one phrase. Following habituation, infants were presented with the same sequence together with motion from a different actor (condition 1), or a new sequence from the same actor coupled with a new sequence from a new actor (condition 2). Infants demonstrated a significant preference for the novel actor in both experiments. These findings suggest that infants can not only discriminate complex and subtle biological motion cues but also detect invariants in such displays.

What is it like to see the world in black and white? In the pioneer days of cinema, when movies displayed grey worlds, was it true that no ‘colours’ were actually seen? Did every object seen in those projections appear grey in the same way? The answer is obviously no-people in those glorious days were seeing a world full of light, shadows, and objects in which colours were expressed in terms of lightness. But the marvels of grey worlds have not always been so richly displayed. Before the invention of photography, the depiction of scenes in black-and-white had to face some technical and perceptual challenges. We have studied the technical and perceptual constraints that XV-XVIII century engravers had to face in order to translate actual colours into shades of grey. An indeterminacy principle is considered, according to which artists had to prefer the representation of some object or scene features over others (for example brightness over lightness). The reasons for this lay between the kind of grey scale technically available and the kind of information used in the construction of 3-D scenes. With the invention of photography, photomechanical reproductions, and new printing solutions, artists had at their disposal a continuous grey scale that greatly reduces the constraints of the indeterminacy principle.

The present study was aimed at evaluating whether the very high accuracy of memory for familiar faces, demonstrated by Ge et al (2003, Perception 32 601–614) with a very familiar famous person, generalises to faces of personally known individuals. The accuracy of participants' perceptual memory for a close colleague's face and for their own face was evaluated by presenting original and manipulated pictures of these two targets. The manipulation consisted of increasing or decreasing the interocular distance. As in Ge et al's study, results indicated that proportions of correct recognition of the original faces, and just noticeable differences for the detection of alterations in the recognition task, were not significantly different from the corresponding measures in a perceptual discrimination task performed by a sample of participants who did not know the target persons at all. High accuracy of memory generalises to faces of personally known individuals.

Congenially blind, late-blind, and blindfolded-sighted participants performed a pointing task at proximal memorised proprioceptive targets. The locations to be memorised were presented on a sagittal plane by passively positioning the left index finger. A ‘go’ signal for matching the target location with the right index finger was given 0 or 8 s after left-hand demonstration. Absolute distance errors were smaller in the blind groups, with both delays pooled together; signed distance and direction errors were underestimated with the longer delay, and were overestimated by blind groups, whereas the blindfolded-sighted group underestimated them. Elongation of the scatters was stretched but not affected by delay or group. The surface scatter was greater with the longer delay; and orientation of the main axis of the pointing ellipses shows the use of an egocentric frame of reference by the congenitally blind group for both delays, the use of egocentric (0 s) and exocentric (8 s) frame of reference by the blindfolded-sighted group, with the late-blind group using an intermediate frame of reference for both delays. Therefore, early and late visual-deprivation effects are distinguished from transient visual-deprivation effects as long-term deprivation leads to increased capabilities (absolute distance estimations), unaltered organisation (for surface and elongation), and altered organisation (amplitude and direction estimations, orientation of pointing distribution) of the spatial representation with proprioception. Besides providing an extensive exploration of pointing ability and mechanisms in the visually deprived population, the results show that cross-modal plasticity applies not only to neural bases but extends to spatial behaviour.

A discrimination task was used to examine how locations on the glabrous skin of the terminal and middle phalanges of the index finger affect the perceived shape of tactile patterns. On each trial, a pair of same-shape or different-shape patterns was presented successively on the distal half, on the proximal half, or on both halves of a phalanx. Observers responded “same” or “different” depending on the perceived pattern shape. Performance was compared between the two phalanges, with two different pattern sets. For patterns at separate locations, performance was uniformly poor. For patterns at the same location, performance was better on the distal halves than on the proximal halves of both phalanges for one pattern set but not for the other. Performance was best on the distal half of the terminal phalanx. The results are discussed in terms of the densities of innervation of first-order afferents.