Perception

The lateral eyes of the female Copilia are exceptionally large for a creature of its size but have very few receptors. These receptors appear to scan the image plane of the anterior lenses of the eye, but until now there has been no behavioural evidence linking their movement positively with the functioning of the eyes, though there is some (disputed) evidence that it is merely a side-effect of peristalsis. On channel-capacity grounds we might not have expected optical scanning to occur in biological systems, so any positive evidence for scanning, in the engineering sense, is particularly interesting.

Optical measurements of the position of the image plane, in live intact female Copilia quadrata, generally corroborate Exner's (1891) finding that an image is in focus at the plane of the distal ends of the receptors, though the position of best focus seems slightly in front of a second lens, behind which the receptor structure lies. An alternative optical schema of Wolken and Florida (1969) is disproved. The position of focus satisfies minimal optical conditions for scanning.

Movements of a coloured stripe pattern, used as a test object for the optical measurements, elicited the putatively scanning movements of the receptor structure, suggesting strongly that these really are an intrinsic part of normal visual functioning, not an epiphenomenon. It was also discovered that intermittent swimming movements of the limbs tend to be preceded by these photoreceptor movements. Both findings support the hypothesis that Copilia's eye is an optical scanning system.

Indices of human performance are less clear-cut than analogous measures in the physical sciences for several reasons. Variables which can alter performance include the noise upon which sensory signals are imposed, the bias shown by an observer towards one or another alternative, the use of an optional-stopping strategy for examining sensory input, and the accumulation of statistical information over time concerning the source of the input. The development of a theoretical appreciation of these variables is traced and, on this basis, a measure of the noise, which sets a limit to discriminative capacity, is suggested. The proposed index is simply the standard deviation of the best fitting normal ogive, calculated for the psychometric function obtained in a forced-choice discrimination task by the method of constant stimuli, the discriminanda being presented for 100 ms, in random order, and followed by appropriate backward masking. The index is thus closely related to traditional psychophysical measures, but is distinguished by the detailed specification of conditions under which it should be obtained. Analysis of data from previous experiments gives some indication of the order of magnitude that might be expected from a carefully controlled determination of the measure. In addition, three experiments were carried out to evaluate this suggestion, and to test its underlying rationale. In the first two, observers were required to discriminate, by pressing one of two keys, between two lines of markedly different length, exposed in random order for ten different durations. In the third, stimulus exposure was held constant, while ten different stimulus differences were presented in random order. Results from the first two experiments yielded estimates of minimum inspection time close to 100 ms, and were inconsistent with the view that observers can abandon an optional-stopping procedure of processing sensory information in favour of responding by a deadline. Measures of noise calculated in the third experiment were of the order expected on the proposed rationale, while response latencies were again inconsistent with the notion of deadline responding. Further analysis of the results of the three experiments suggests that measures of inspection time and noise, together with a third index related to the degree of caution exercised by an observer, appear to be stable and consistent descriptors of performance. The wider implications of the successful use of this kind of perceptual index of performance are discussed with reference to the measurement of visual acuity, as a means for detecting effects of environmental stress, and as a conceptual framework for the understanding of individual differences.

Vickers et al. (1972) suggested that a useful index of noise in the visual system should be provided by a measure s_D, defined as ‘the standard deviation of the best fitting normal ogive calculated for the psychometric function obtained in a forced-choice discrimination task using the method of constant stimuli, the discriminanda being presented for 100 ms, in random order, and followed by appropriate backward masking’. The sensitivity of s_D to anxiety produced by painful electric shocks, arriving at unpredictable times, is examined. Results suggest that, although increased anxiety does not produce consistent or significant changes in heart rate or in degree of caution, it does cause a marked increase in noise.

Recent neurophysiological studies have suggested that orientation constancy might be achieved by the action of a compensatory signal from the vestibular system operating at the level of the visual cortex. Two experiments were carried out in an attempt to find psychophysical evidence for such a system. In the first the variation of visual acuity with stimulus orientation was measured as the subject's head was tilted through 45°, and in the second the orientation specific adaptation to gratings of Blakemore and Campbell (1969) was similarly studied. The results showed that both visual discriminations were dependent upon the retinal properties of the stimulus only; in other words no effect of phenomenal orientation could be detected.

The visual world remains perceptually quite stable during rotation of the head. There is some physiological evidence that in the cat this phenomenon of orientational constancy might result from changes in the orientation selectivity of neurones in the visual cortex during head tilt. In humans adaptation to a grating causes elevation of the threshold contrast for detecting gratings of similar orientation, and it can be argued that this aftereffect depends on neurones in the human visual cortex. If the head is tilted during such adaptation, but is vertical when the threshold is tested with a vertical grating, the maximum aftereffect occurs for an adapting grating that was vertical on retinal coordinates. We conclude that in humans the mechanism for orientational constancy occurs after the visual cortex.

Schrödinger once proposed a simple rule to describe how binocular brightness depends upon the inputs from left and right eyes in situations where the two eyes receive different light intensities. It is shown that Schrödinger's suggestion can account for the main features of Levelt's recent data if it is assumed that each monocular response varies linearly with the logarithm of the light intensity.

Previous studies on brain-damaged patients have shown impairments in orientation. When the results have been based on a route-finding test the impairment was found in both left and right parietal lesion groups, while when maze tests have been used, the impairment correlated clearly only with lesions of the right hemisphere.

In the present study the route-finding test was given to 77 right-handed patients with unilateral cortical lesions (33 right-sided, and 44 left-sided), and to 24 controls. In order to evaluate the possible influence of kinesthetic afferents, the test was carried out under two different conditions: active (walking) and passive (in a pushchair).

The results show the performances of subjects with lesions on the left side are significantly poorer than those of the controls, but better than those of subjects with lesions on the right side. For both groups the impairment associated with the lesion is the same under active and passive conditions, which does not support the hypothesis of the possible role of the kinesthetic afferents. Posterior lesions produce significantly more failures in both groups. The importance of the parietal lobe, however, was clearly seen only in subjects with lesions on the right side.

Two experiments on adaptation-level theory are reported which arise from informal predictions made by Helson (1964). They both involved interpolated anchors in comparative judgments, in order to avoid the sorts of response factors inherent in the category scaling technique normally employed in adaptation level studies. The first experiment indicated the operation of time error as interstimulus intervals are increased. The second showed that if the anchor stimulus is not judged it is less effective in producing contrast compared with when it is judged. In addition, response times and confidence ratings of judgments were shown to be potentially useful indices of differences in effectiveness among the various anchor stimulus levels used. The strong relationship between these two measures is discussed.

Contrast-modulated grating patterns were used to compare evoked responses and psychophysical thresholds of contrast modulation. The stimulus consisted of the successive presentation of a grating pattern at a higher and lower contrast. At a modulation rate of 8 Hz it was found that there is correspondence between the two kinds of data for a 6 cycle/degree grating. The just noticeable difference was not constant, and data approximated a Weber's law of contrast discrimination. However, it was found that there is a slight departure from this law, in that the ratio decreases with mean contrast. Both psychophysical and electrophysiological data exclude a model of simple luminance detection and are consistent with the operation of spatial contrast detectors in the human visual system.

Three experiments are described on induced motion in the third dimension. In experiment 1 two constant lines on a sheet of glass appear to move in depth as a consequence of the rapid displacements of a back surface (real inducing motion); this apparent motion is the more evident (i) the longer the inspection time and (ii) the faster the inducing stimulus. In experiment 2 a constant circle appears to move in depth as a consequence of the stroboscopic displacement of a surrounding circle (apparent inducing motion). Experiment 3 compares this last induced-motion effect with the aftereffect. The importance of a dynamic frame of reference is stressed.