Perception

Successive discrimination of suprathreshold gratings of different orientations by two-month-old infants was tested with the use of a type of familiarization–novelty paradigm. Infants showed clear discrimination between horizontally and vertically oriented gratings. Infants failed, however, to discriminate mirror-image obliques from each other and also failed to discriminate nonmirror-image obliques. Considered along with previous demonstrations in children and adults of a greater confusability of oblique grating pairs relative to that of the horizontal and vertical pair, these findings indicate that this differential confusability of orientation is, at least in part, manifest in humans by two months of age.

The study tested the hypothesis that movement and self-referent information differ, and that children can use both to code unseen locations. In one experiment blindfolded sighted children made more errors with changed than with unchanged accessing movements, despite unchanged reference conditions. Rotation decreased accuracy further, although most responses were to the correct side. Age did not interact with experimental conditions. In another experiment blind children and matched blindfolded controls were less accurate with changed movements; and rotations were more difficult still. Visual experience, but not age, interacted with conditions. The blind showed significantly more self-reference (responses to the original side in rotations), and were somewhat worse also with changed movements alone but did not differ from controls when movements and references were unchanged.

The results support the hypothesis that movement and self-referent information differ. Visuospatial experience and task conditions rather than age seem to determine the extent to which children use either form of information.

An oblique line superimposed on a vertical or horizontal grating appears to rotate when the whole pattern is expanded or contracted. The apparent rotation occurs with head movements towards or away from a stationary grating, or with zooming a grating relative to a stationary observer. The magnitude and direction of the apparent rotation is dependent upon the relative inclination of the line to the grating and is most pronounced with a relative orientation of 45°.

An adaptation method was used to determine the specificity of orientation-selective channels in the human visual system at different retinal eccentricities (up to 16 deg) in both hemifields of each eye. For a vertical test grating, the elevation in contrast threshold produced by adapting to a high-contrast grating of the same spatial frequency but variable orientation was equated with the contrast levels of a vertical adapting grating that produced equivalent effects (equivalent-contrast transformation). This enabled comparisons to be made between the orientation tuning of the aftereffect at different retinal loci. For the spatial frequency employed (3 cycles deg⁻¹), no systematic change in orientation selectivity was found as a function of either retinal eccentricity or the hemifield (and hence the cerebral hemisphere) stimulated.

Binocular rivalry was induced between two orthogonal square-wave gratings of the same spatial frequency, luminance, contrast, and field size, presented dichoptically. One of the gratings could be instantly replaced by a third grating differing only in orientation. In one experiment subjects were required to respond as soon as an orientation change was noticed, and to withold response to catch trials (no orientation change). When orientation changes were made to the visible grating, reaction time was found to be a U-shaped function of the magnitude of orientation change. When orientation changes were made to the grating undergoing binocular-rivalry suppression, an overall increase in reaction time was found with the increase being greater for large orientation changes (an asymmetrical U-shaped function). In another experiment subjects were required to detect the direction of a change in orientation in a two-alternative forced-choice procedure. Thresholds were thus obtained for 75% correct performance. It was found that thresholds for orientation changes made to the visible and invisible fields were identical from 20° to 70° orientation change. Outside this range thresholds were higher when orientation changes were made to the field suppressed by binocular rivalry. It is argued that the orientation functions obtained in the two experiments may represent incomplete suppression of either form or transient information during binocular rivalry.

The ability to generate voluntary pursuit eye movements in the absence of retinal-contour motion cues was assessed on the basis of observers' perceptions of depth and motion when they viewed dynamic visual noise with a filter over one eye. The results indicated that the depth-movement phenomenon yielded robust pursuit with the velocity an inverse function of filter density. These data suggest that retinal-contour motion cues are not necessary and that perceived motion is sufficient to drive pursuit.

Vergence responses were recorded from practised observers viewing narrow-band spatial-frequency-filtered planar random-dot stereograms. It was found that low spatial frequencies of 1·75–3·5 cycles deg⁻¹ could trigger appropriate vergence responses to larger disparities than could the relatively high spatial frequency of 7·0 cycles deg⁻¹. Nevertheless, appropriate vergence shifts were observed reliably for spatial-frequency/disparity combinations well outside the range predicted by Marr and Poggio's (1979) model of stereo vision. It was also found that for large-disparity/high-spatial-frequency combinations which the subjects could not fuse, the vergence system went into oscillation with the eyes diverging and converging at a frequency of about 1·5 Hz and with an amplitude of about 10–20 min arc. Finally, it was demonstrated that when a prominent monocular cue was superimposed upon a large-disparity/high-spatial-frequency stereogram then a speedy vergence response occurred which resulted in successful fusion. This latter finding supports the hypothesis advanced earlier that monocular cues can facilitate stereopsis by triggering appropriate vergence shifts.

The range of pictorial depth perception was tested with four pictures from the repertoire of European art, rather than the customary line drawings or photographs. These pictures included those rendered in linear perspective and inverse perspective, as well as those with different degrees of depth. Using Pandora’ Box, the subjects were asked to place a lamp at the same apparent depth as objects in the pictures. The subjects did so without regard to the depiction technique. The results suggest that depth is seen in pictures both where the rules of linear perspective hold and where they have been violated.

Textbook accounts of perception frequently contain grossly wrong perspective drawings and/or statements about perspective which misrepresent the available monocular information about distance and size, as well as giving the unfortunate impression that perspective alone elicits only a weak impression of recession. An early example of such a false perspective is the ‘Ames window’, which is not a correct rendering of a slanted window.

It is sometimes stated that Wundt believed in the primacy of ‘sensations of innervation’ in the control of eye and limb position, to the exclusion of afferent feedback. Wundt's own statements on the subject are traced through the six editions of the Grundzüge: he believed that sensations of innervation were a contributory factor along with peripheral feedback; but in the fourth edition he dropped the term and subsumed them under ‘central sensations of senses’, because increased neurophysiological knowledge made a literal interpretation of the original term impossible. In the fifth edition he developed a more precise model of central processes, described as ‘sensory costimulation’, and an additional idea of ‘reproduced’ or conditioned sensations. He mentions clinical evidence on the perception of eye and limb position and of weight by paralysed and other subjects. This evidence is discussed in relation to modern theories. It is concluded that Wundt's later theories have some similarities with modern ‘hybrid’ theories of efference-contingent afference.

A tone was played to neonates in one of four conditions: in an unstructured visual field; spatially concordant with a single visual target; spatially discordant with a single visual target; or spatially concordant with one of two identical visual targets at different locations. Auditory–oculomotor spatial coordination, contingent on sound, was enhanced by the presence of a target in the visual field. Audition and vision are mutually supportive in auditory localisation from birth.

The indication of subjective states during steady visual stimulation by means of key depressions has been assumed to be free of systematic bias. However, it is shown that the reported visibility of simple stimuli may be determined by the level of categorisation employed before the initiation of an overt response. In particular, the duration of unitary disappearance of a stimulus is found to be greater when this is the only category of change to be reported than when the same measure is derived from concurrent reports of the disappearance of its parts. The correspondence of findings both with afterimages and with steady fixation shows that peripheral factors such as eye movements cannot provide an adequate explanation. Experiments comparing part/whole categorisation of the visibility of single lines suggest caution in the acceptance of previous studies designed to indicate underlying neurophysiological mechanisms in subjective disappearances.

Some tests are reported on a young woman who was believed to have unusual powers of eidetic imagery and voluntary control of vivid hallucinations. No evidence was found of abnormal eidetic ability. Measurements of detection thresholds for light of different wavelengths, while the subject tried to hallucinate various colours, suggested that the supposed hallucinations acted like coloured filters held over the eye.