Perception

A briefly flashed line can be identified more accurately when it is part of certain types of pattern than in others (the ‘object-superiority effect’). Three experiments were designed to investigate what aspects of these patterns determine the facilitatory effect of context. Subjects identified which of four line segments was present in various briefly flashed figures. Other subjects rated the figures for three-dimensionality, connectedness, and ‘structural relevance’ of the target line. Little relationship was found between connectedness ratings and accuracy in the identification task, but accuracy was highly correlated with mean depth rating (accounting for 95% of the variance) and with mean structural-relevance rating (88%). Because of the high correlation (r = 0·98) between these two judgments in the present experiments, and confounding with other stimulus variables in previously published studies, the relative importance of these two global attributes cannot yet be determined definitively (though there was some evidence that for these patterns depth judgments were primary and structural-relevance judgments derivative). A reexamination of pertinent research suggests that comparisons between well-matched stimuli (as in the object-superiority effect) are likely to be more robust and informative than comparisons between lines alone and in context (the ‘object-line effect’).

The aim of the study was to establish whether monaural auditory stimulation (a nonretinal perceptual variable) affects the class 1 oblique effect (a behavioural manifestation of simple cells). The left or right monaural stimulus was a pure tone, 1000 Hz, 70 dB(A), delivered continuously throughout the experimental session. The left or right monocular stimulus was a thin red phosphorescent bar the orientation of which was manipulated. In order to determine the oblique effect differential orientation thresholds for principal meridians were compared to those for oblique orientations. The results, indicating an interaction effect of the monaural and monocular stimulation on the magnitude of the oblique effect, are a further demonstration that the oblique effect is not as simple as some theories (derived from extrapolation from neurophysiological findings) would imply.

Previous investigations of temporal resolution have shown that performance is influenced by a number of stimulus parameters. The interstimulus interval needed for accurate two-pulse discrimination has been shown to (i) decrease monotonically with flash duration, luminance, and contrast; and (ii) increase monotonically with the spatial frequency of the target.

A signal-detectability two-alternative forced-choice procedure was employed to reexamine the effects of spatial frequency on temporal resolution. Also assessed was the effect of grating orientation on such performance. The results confirm that temporal resolution declines with increases in spatial frequency. Furthermore, temporal resolution was significantly lower when oblique, as opposed to vertical, grating targets were used. This ‘oblique effect’ in temporal resolution was observed only with the highest-spatial-frequency target (15 cycles deg⁻¹), and not with stimuli of lower spatial frequency (0·9 and 3·8 cycles deg⁻¹). These findings suggest that stimulus parameters which elicit greater transient channel activity, as opposed to sustained channel activity, enhance temporal resolution. When transient activity is at a minimum, meridional differences in temporal resolution are likely to be attributable to sustained channel activity.

Moving visual phantoms look like real gratings and generate movement aftereffects as do real gratings. Three experiments have been carried out to test whether phantoms resemble real gratings in other ways. Changes in the spatiotemporal frequency of an inducing grating affect the minimum contrast at which it will induce phantoms, but only insofar as the changes affect the visibility of the inducing gratings. Changes in the areal extent of the inducing grating and in the extent of the gap across which phantoms must be induced alter the visibility of the phantoms but, again, only to the extent that the changes affect the visibility of the inducing gratings themselves. Phantoms induced by high-contrast gratings may be cancelled by gratings of near-threshold contrast when the latter are presented 180° out of phase with the inducing gratings. It is concluded that the appearance of phantoms is governed solely by the detectability of their inducing gratings, but that the effects of phantoms on pattern-sensitive channels are not the same as are the effects of real gratings. The pattern characteristics of phantoms thus do not represent a spread of energy from inducing to phantom regions of visual space or their neural representations.

Following an earlier report, the article offers additional points concerning visual images observed after enucleation. These concern: (i) the relationship between field-cut due to the lesion before surgery and corresponding blank areas within the post-enucleation images; (ii) similarities in motion between the foveal images and foveally-fixated afterimages; (iii) possible correspondence of time course of the images to transneuronal atrophy and cell loss in human and monkey studies.

Subjects were able to discriminate photographs of their own hands from other hand photographs but were unable to recognize their hands amongst a group of other objects when they were unaware that their hands were in the photographed set. It was concluded that recognition of even this highly familiar stimulus is dependent upon selection of the appropriate identifying cues, and that this process depends on expectation.

Pictograms were created in which the outline of a word denoting an object was shaped to be the same as the object itself. A number of objects were presented, some drawn as pictograms, some as outline shapes, and some as normally printed words. The experiment was designed to test if recognition memory was superior for the pictograms as compared to outline pictures or words, and if this would be true whether the subjects were asked to attend to the form or only the content of the stimuli. One group of subjects was trained to respond OLD only if the test item was the same object in the same form, and NEW otherwise. The other group responded OLD if the test item was the same object in any form, and NEW only to objects never before shown in any form. Recognition accuracy (a signal detection analysis) was greatest for the pictograms, and poorest for the words in both groups. Though the subjects could disregard form, they were most accurate when probed with the same form as presented. But in all comparisons subjects were most accurate when forced to recall both the form and the content. These and other results were taken to be mildly supportive of a dual coding hypothesis, and of the utility of these new stimuli.

A mathematical scheme is proposed for the classification of impossible objects. Impossible objects are usually defined from a psychological point of view as mental images of ‘solid objects’ which cannot exist in a real three-dimensional world. From the mathematical point of view, however, various kinds of impossibility can be observed in such objects, and they are classified according to the kinds of impossibility they possess. Since impossible objects are treated in a purely mathematical way, the results afford a powerful basis for the psychological study of human visual perception of impossible objects. Several psychological problems arising from the present results are also discussed.

Relations among percepts in the organized perceptual world are called percept—percept couplings. If these couplings are assigned a causal interpretation they present a challenge to a theory of direct perception. Experimental evidence of percept-percept coupling is reviewed and the case for a causal interpretation of these relations is examined. It is concluded that the causal interpretation is plausible and that the facts of percept—percept couplings need to be addressed by advocates of a theory of direct perception.

The effect of familiar size as a distance cue was tested with familiar objects at familiar distances. Experiment 1 showed that there were no uncontrolled distance cues available and that in their absence the retinal image did not affect depth or size perception. Under these conditions, size and distance judgments were essentially indeterminate and independent of each other. In experiment 2 a paradigm was employed which allowed a direct determination of whether equivalent changes either in size of a familiar object or in its true distance produced equivalent changes in its perceived distance. The results showed that there were no uncontrolled distance cues, and that subjects perceived the familiar object as having its familiar size. Moreover, changing the retinal image of the objects had almost exactly the same effect on their perceived distance as did changing their true distance. Hence, familiar size does effectively govern the perception of distance when there are no competing cues.

An experiment is reported the object of which was to check whether a small amount of nonspecific experience in perceiving random-dot stereograms could facilitate the perception of a previously unseen stereogram. The mean stereopsis perception time of a group of totally naive subjects was found to be significantly slower than that of a group who had previously been shown two different stereograms. Closer inspection of the data showed that this difference was primarily due to approximately one third of the naive group who were much slower than the ‘experienced’ group. It is therefore suggested that nonspecific experience provides most initial help for relatively slow perceivers, since many naive subjects can perform as well as those with prior experience of other stereograms.

Underwood (1975) has used judgments of duration to compare the attentional demands of learning and recall. His study is critically discussed and two further experiments are reported. In experiment 1 subjects spent 50 s learning a list of words and a further 50 s either trying to recall them or learning another list. In contrast to Underwood's subjects both groups judged the first interval to have been longer than the second. In experiment 2 three separate groups of subjects spent 50 s learning a list of items (nonsense syllables, unrelated words, or related words, as in Underwood's experiment) and then a further 50 s trying to recall them. Once again the first interval was judged to have been of greater duration. The implications of these findings for the use of duration judgments as a measure of attentional demand are discussed.

The relative frequencies of ‘bright’, ‘dark’, and ‘neither’ reports were similar for diffuse, as compared to sharp-edged illusory circles and rings, whether viewed directly or peripherally, suggesting in this case a shared source of at least part of their illusory brightness alteration.