Perception

Human beings possess a remarkable ability to recognise familiar faces quickly and without apparent effort. In spite of this facility, the mechanisms of visual recognition remain tantalisingly obscure. An experiment is reported in which image processing equipment was used to displace slightly the features of a set of original facial images to form groups of modified images. Observers were then required to indicate whether they were being shown the “original” or a “modified” face, when shown one face at a time on a TV monitor screen. Memory reinforcement was provided by displaying the original face at another screen position, between presentations. The data show, inter alia, the very high significance of the vertical positioning of the mouth, followed by eyes, and then the nose, as well as high sensitivity to close-set eyes, coupled with marked insensitivity to wide-set eyes. Implications of the results for the use of recognition aids such as Identikit and Photofit are briefly discussed.

Matching and cancellation techniques were used to measure the relative strength of the Ehrenstein illusion in dark figures on a light background (negative contrast) and light figures on a dark background (positive contrast). Brightness enhancement on the former was shown to be maximally 0.28 log unit (relative to the detection threshold), and darkness enhancement on the latter 0.43 log unit. Values differed little with figure–ground contrast (down to a minimum of ±0.5), but decreased with decreasing level of illumination. The luminance increment (decrement) needed to match the illusory brightness (darkness) was similar in size to the luminance decrement (increment) needed to cancel the illusion. The increment threshold for a small test flash measured in three locations relative to the subjective contour delineating the illusion did not differ systematically. The results are compatible with a neurophysiological explanation of the Ehrenstein illusion in terms of line-induced lateral interaction in hypercomplex receptive fields.

The Fuchs phenomenon (ie the perception of transparency when the object seen through transparency lies on a homogeneous background, and does not jut out from under the transparent layer) has been studied by the numerical-rating method in two experiments, with the use of achromatic colours. The results of the first experiment show that the degree of transparency depends on the difference in colour between the object and the background. The results of the second experiment suggest that the processes producing the phenomenon are different in nature from the processes producing the kind of transparency most frequently studied in which the object juts out from under the transparent layer.

The effect of 6 Hz uniform-field flicker masking of visible persistence at a range of spatial frequencies was investigated in 12-year-old children with specific reading disabilities and a control group of average readers. This reduced differences in visible persistence between the two groups. The results suggest that children with specific reading disabilities experience a deficit in their transient system.

Rotating or stationary targets were presented simultaneously with a random-dot mask which was itself either rotating or stationary. Observers were required to judge both the form of the target and its motion; these target attributes were manipulated orthogonally in the experiment. The results support the hypothesis that under these conditions figural information is encoded independently from motion information. On a trial-by-trial basis within conditions of target and mask motion combinations, performance in the two tasks was independent. Across conditions, different effects were observed in the two dependent measures. Form judgments were generally more accurate for rotating than for stationary targets, particularly when the mask was stationary. Motion judgments showed frequent intrusions from the motion of the mask, so that most errors occurred when target and mask were dissimilar in terms of their motion. The results show differential interference of the random-dot mask on the encoding of the two independent stimulus attributes, and support the possibility of separate processing of figural and motion information.

When the dynamic visual noise of an untuned television set is viewed with image defocusing (positive lenses) and with a narrow vertical obstruction partially blocking the pupil of one eye, the video ‘snow’ seems to separate into two stable surfaces at different depths, divided by a vertical discontinuity. The main features of this illusion can be quantitatively accounted for in terms of the optics of defocused images and the retinal disparities predicted from blur circles. A residual component of the illusion, however, which was perceived by a majority of subjects, cannot be readily explained by geometrical optics; it apparently reflects a more subtle aspect in the processing of visual images, corresponding to the Anstis–Howard–Rogers stereo-effect, in which local depth configurations can bias global stereopsis. Several novel aspects of that effect are described, based on use of this obstructed-pupil illusion as the evoking stimulus.

The ability to make egocentric distance estimates of a single point source of light, seen in darkness and without the cues of changing size and luminance, was investigated in sixteen observers. The attenuation required to maintain constant luminance, when the target was viewed from different distances, was shown to follow the inverse square law providing the angle subtended by the light was less than 20 s arc. Distance changes were also simulated by means of a split mirror which produced vergence cues, or by test lenses to provide accommodation cues.

Over the range 0.5 to 9.2 m distance estimates were surprisingly accurate, although there was some overestimation of near and underestimation of far distances. Most observers made good judgements when only convergence cues were varied, whereas no observers made consistently good judgements when only accommodation cues were varied. The difficulties are discussed in terms of the accommodation–convergence link. When distance was simulated by changing convergence and accommodation cues, estimates were not as good as when real distance was changed.

Since good estimates were made with brief target exposures, these judgements were not based on subsequent convergence or accommodation changes. It is suggested that the metric or reference against which the apparently absolute judgements were made was the efferent demand signal associated with a ‘resting’ position of convergence in darkness.

The hypotheses that in Braille learning (i) coding strategies change with reading level, and (ii) coding differs between normal and retarded readers were tested with oddity judgments by blind children. Experiment 1 showed that strategy choices varied with reading level only in association with mental age. By contrast, shape neglect and preference for phonological strategies were shown by retarded readers rather than by matched normal readers. Experiment 2 showed that under instructions to use given coding strategies the retarded were as accurate as normal readers. Accuracy for all forms of coding increased with reading level, but coding word shape was significantly less accurate than other forms of coding, and even correct coding of shape was no faster than semantic or phonological coding. It is concluded that (i) coding the shape of Braille words is unlikely to be a major factor in producing faster Braille reading, and (ii) retarded Braille readers differ from normal readers in their spontaneous choice of strategy rather than in the ability to use the relevant codes.

Eight subjects were required to localise a sound source (white noise through a speaker) which varied in position on both sides of the head over a range of elevations (-40° to +40°) and azimuths (0° to 180°) at 10° intervals. The perceived position of the source was indicated by pointing a special gun. Depression of the trigger activated a photographic system which recorded two views of the subject, the sound source, and the gun. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. In a second experiment, four of the same subjects performed the same task but in this case visually located the sources. This experiment provided an estimate of inherent motor error in the pointing task. No differences in localisation acuity between sides were found, but there were significant differences between front and back regions. Azimuth and elevation error were well matched and low in the front. However, azimuth error increased in the regions behind the head, particularly for azimuth positions 120° to 160°. Larger increases were found for positions in the upper elevations of this region. Elevation error also increased in the upper elevations behind the head. A comparison of the auditory and visual data indicates that this pattern of error is not due to motor factors. The results are discussed in relation to the structural characteristics of the pinnae and modifications that they impose on incoming sound energy.

The acuity of azimuth and elevation discrimination was measured under conditions in which the cues to localisation provided by the pinnae were removed. Four subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40° to +40°) and azimuths (0° to 180°), at 10° intervals, on the left side of the head. Pinna cues were removed by the insertion of individually cast moulds in both pinnae. Each mould had an access hole to the auditory canal. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. The performance of the subjects was compared to their performance under normal hearing conditions. Insertion of the pinnae moulds was found to increase substantially elevation error and the number of front/back reversals. The importance of the cues provided by the pinnae in these discriminations was thus confirmed. However, the increase in elevation error did not result in a corresponding increase in azimuth error. These findings provide support for the proposition that azimuth and elevation discrimination are coded independently.

The role of focused attention in vision is examined. Recent theories of attention hypothesize that serial search by focal attention is required for discrimination between different combinations of features. Experiments are reported which show that the mixture of a few (less than five) horizontal and vertical line segments embedded in an aggregate of diagonal line segments can be rapidly counted (also called ‘subitizing’) by a parallel (preattentive) process, while the discrimination between horizontal and vertical orientation requires serial search by shifting focal attention to each line segment. Thus detecting and counting targets that differ in orientation can be done in parallel by a preattentive process, whereas knowing ‘what’ the orientation of a target is (horizontal or vertical, ie of a single conspicuous feature) requires a serial search by focal attention.

Micronesian navigators routinely make voyages across large expanses of open ocean. To do this, a navigator must judge both the direction in which he is sailing and the distance he has travelled. The rising and setting points of the stars (and other cues) provide instantaneous information about direction, but distance can only be judged by integrating velocity-related information over time. Micronesian navigators judge distance in a way that seems odd. When they are out of sight of land, they imagine that the canoe is stationary and that the islands move back past them. For each voyage, they ‘attend’ to an island off to the side of the course which is out of sight over the horizon. As they sail, they imagine the island moving back along the horizon changing in bearing until it is imagined to be under the bearing it is known to have from the destination island. Then they know they are near their destination. There is good reason for using a frame of reference whose origin is defined by the boat. We show how it finesses a perceptual paradox–the rising and setting points of the stars do not exhibit motion parallax.