Perception

A model is described for the processing of word-like (regular) and nonword-like (irregular) letter strings in a situation where either type of string may occur. It is proposed that each type of string is processed by specialised routines which are deployed contingently on an initial decision about the regularity of an item. Three experiments involving word/nonword and same/different responses investigate how the processing of a string is affected by the brief exposure of a prior string. It is shown that processing is interfered with less when both strings are of the same degree of regularity than when they are not.

The model is revised and extended in the light of the results obtained as well as some theoretical considerations.

Both Künnapas and Rock and Ebenholtz investigated the effects of surrounding-frame size upon line-length judgments. Whereas Künnapas obtained errors of the order of 10–15%, typical of those which occur in other geometrical illusion figures, Rock and Ebenholtz reported errors closer to 100%. Rock and Ebenholtz claimed that perceived size is largely relationally determined and that this fact was obscured in Künnapas' experiment by allowing observers to compare the test lines directly with each other and within a common framework provided by the wall of the room.

On the contrary, the experiments reported here suggested that errors in the Rock and Ebenholtz study may have been inflated by their use of nondirective instructions and by the confounded effects of other variables. When the Rock and Ebenholtz experiments were repeated with adequate controls over these variables, the effect of surrounding frames on line-length matches were of the order of 15–19%, similar to those of Künnapas but much smaller than those reported by Rock and Ebenholtz. The complexity of the stimuli in the Rock and Ebenholtz type of experiment is such, however, that the effects of a number of variables and possible cue conflicts remain to be investigated.

Four tests of visual perception were given to twenty-five men and twenty-five women. These were a test of acuity, threshold for four field positions, visual persistence, and a measure of comfortable brightness. Subjects also completed five personality questionnaires. In most measures, differences were found to be related to sex rather than to personality factors. In fact, the analyses performed suggest that personality tests do not measure equivalent processes in men and women. Correlational analysis showed all visual functions to be independent of one another with the exception of photopic acuity and scotopic threshold, which were highly correlated. Two new findings on the visual system emerged which have not been reported elsewhere: (i) Four distinct dark adaptation curves were produced, and have been labeled as exponential, flat-exponential, linear, and plateau. All subjects fell into one of these categories and showed a consistent trend to exhibit these curves for all field positions, (ii) Highly significant differences were found in sensitivity for the four visual fields, the upper field was superior, followed by the right, then left, with the lower visual field considerably poorer.

Sex differences were investigated in two experiments on visual persistence: the Ganzfeld and the afterimage. Males were found to hold visual sensation longer than females, particularly in the Ganzfeld where there was little overlap of scores. Variability of experience in the Ganzfeld was also greater for males and they commonly reported ‘blank-out’ effects while females did not. There was further evidence from both experiments that females are more responsive to the long-wave region of the frequency spectrum.

Potentials in relation to eye movements were studied by means of direct recording of the striate cortex in a waking man. In a lighted environment, the usual evoked potential—lambda response—was obtained and was clearly visible after each eye movement. In complete darkness no individual potential was observable by means of visual analysis after each eye movement, but a slow potential of low amplitude could be obtained by superimposition and averaging of the cortical striate activity time-locked to the start of a series of eye movements. This eye-movement potential showed a longer latency and a lower amplitude than the lambda response. These data are discussed in reference to those obtained in the cat and the monkey; the significance of this eye-movement potential in darkness as a ‘corollary discharge’ is considered.

When observers tracked moving stripes across a background either of stationary stripes, or of stripes moving in the opposite direction, they saw a clear motion aftereffect when the stripes stopped moving. The direction of this aftereffect was opposite to that of the previously tracked stripes, and was thus the same as the direction of the retinal movement of the non-tracked stripes. This aftereffect of tracking was shown not to depend upon slippage of the tracked contours on the retina during tracking, or upon the saccadic phase of optokinetic nystagmus. The effect showed storage over a period of time with the eyes shut. It appears that the effect is due to induced movement, and arises originally from stimulation of the retina by background contours in the tracking phase. This was shown by confining the view of the moving target to one eye, while permitting both eyes to be exposed to background stimulation during tracking. After such stimulation the magnitude of the aftereffect was equal in the two eyes.

It has been known for some time that children have particular difficulty drawing lines that are not perpendicular. But this difficulty has not been studied systematically. By systematically varying, in three experiments, the baseline, the response, and the kind of figures to be copied, we showed that the tendency to draw angles as more perpendicular than they actually are is a general one which occurs as much with abstract as with meaningful material and with radically different responses. We also discovered an unexpected effect, the vertical effect, which takes the form of the error occurring much less when a vertical baseline is involved than with other baselines.

Visual numerosity judgements were made for tachistoscopically presented linear arrays of dots or lines. The interelement interval (which could be specified in spatial frequency terms) was constant for a given array but varied across conditions. A clear limit in the accuracy of numerosity judgements was found to be set at 4 for regularly spaced elements with spatial frequencies below approximately 10 cycles/deg (element and interelement interval of 0·05 deg). This limit in terms of accuracy is accompanied by a fast and almost constant response time for arrays of 4 or less, compared to response times for arrays of more than 4 elements. The limit in accuracy falls to 2 elements rather than 4 for spacing narrower than 0·05 deg although with such spacing the elements are still easily resolved. The limit of 4 is found if the stimulus is a bright afterimage, lasting for approximately 60 s. This result suggests that the limit is independent of the time allowed for a single fixation and is a perceptual limit rather than a limit in some memory buffer. ‘Numerosity’ units are proposed to account for the results.

Visual numerosity judgements were made for tachistoscopically presented arrays of dots. The arrangement within the arrays was either linear or such that dots could be easily perceptually subdivided into two groups. Subdivision was either in terms of an orientation difference, a colour difference, or a spacing difference in the centre of the array. For a large difference in orientation between the two ‘arms’ of the array (90°), or a large central space (three times the interdot interval) up to 8 dots were accurately perceived. This numerosity limit was twice that found for equivalent linear arrays, with no grouping. Although in terms of accuracy it seems that in these conditions the two groups within each array can be counted independently, there is no evidence for independent processing in terms of response times. From the results of a subsidiary experiment it seems likely that the slow response times in the subgrouping conditions are due to the necessity of processes other than counting (such as judgements of symmetry). For arrays where subgrouping was in terms of a colour difference, or an orientation difference of between approximately 45° and 90°, or a small central space of twice the interdot interval, there was an improvement in accuracy compared to equivalent linear arrays, but no evidence of independent processing, up to a limit of 4, in each of the subgroups. From these preliminary results, tentative proposals concerning ‘numerosity’ units and their properties are made.

A group of Scottish schoolchildren were tested on a task intended to measure the effect of implicit-shape constancy, and the scores were compared with those obtained from African samples. It was found that both groups were influenced by the implicit-shape constancy although the influence was less in the African sample. The relationship of these findings to other published reports of cross-cultural research into pictorial perception and susceptibility to illusions is discussed.

With a constructive knowledge-based theory of perception as its foundation, this paper starts with a review and critique of some artificial-intelligence programs that purport to see. It is then argued that these computer programs for scene analysis offer the hope of providing a more adequate account of human competence in interpreting line drawings as polyhedra than do the current psychological theories. This thesis has several aspects. The one emphasized here is that those programs have explored a variety of methods of incorporating a priori knowledge of objects through the use of models. After outlining the range of models used, presenting a set of criteria for evaluating the use of model information, and sketching some psychological theories, the various proposals are contrasted. This discussion leads to two new proposals for exploiting model information that involve elaborations of an existing program, POLY.