Perception

After enucleation of the right eye the author observed persistent and continuous visual images featuring angular groups of parallel-line segments which have continued for two years, to the present time, though considerably diminished. These phenomena are reported along with results of measurements of the images and of several simple experiments. Some quantitative comparisons are made to results from studies of macaque monkeys, and to human migraine fortification images.

Smooth tracking across an oblique grid pattern produced hallucinations of vertical and/or horizontal striations which moved with the eyes. The effect was produced by single or multiple gratings, monocularly or binocularly, but in the latter case it appeared to lie stereoscopically in the plane of fixation. Gratings containing thin lines or sawtooth edges of moderate contrast were particularly effective stimuli but sine or square waves were not. The subjective stripes had an apparent edge polarity which was opposite to that of the inducing edges or, with thin inducing lines, was determined by the lines' polarity and movement direction. Conventional explanations (eg strobe, afterimage, or moiré effects) can be ruled out. Neither the present effect nor the ‘pincushion-grid illusion’ are due to the presence of spurious Fourier components in the stimulus pattern. An extension of a previous model, involving disinhibitory interaction between movement and pattern channels, accounts for many aspects of this elaborate phenomenon. The detailed dependence of polarity on spatial waveform and movement direction implies: (i) the spatial second harmonic is a necessary component, (ii) the generating mechanism is approximately linear and has 90° phase preference, and (iii) there is a movement-induced phase lag of about 45° in the response to the second harmonic.

The perceived spatial frequency of low-frequency sinusoidal gratings is elevated when the gratings are temporally modulated sufficiently rapidly. Although this phenomenon is usually called ‘spatial-frequency doubling’, the magnitude of this spatial-frequency elevation depends on the spatial and temporal parameters of the grating and may be less than or greater than doubling. Adaptation to such a pattern produces contrast-threshold elevation whose spatial-frequency tuning is centred upon the grating's real rather than perceived spatial frequency. These results suggest that spatial-frequency elevation occurs later in the visual system than adaptation or that these two processes are independent of one another, at least under the conditions of these experiments.

Duration of visual persistence for sine-wave gratings of 1, 2, 4, 8, 12, and 16 cycles deg⁻¹ was determined for seven-year-old, ten-year-old, and thirteen-year-old children. It was found that there was a decrease in persistence duration with age but the slope of this function for the different age groups did not change.

Five psychophysical functions for visual density are obtained with the use of the magnitude-estimation method. Because of apparent rarefaction, a different psychophysical function ensues for each size of square used. It is found that apparent rarefaction is barely detectable for sides longer than 10–15 cm (6–9 deg). It is also found that apparent rarefaction occurs to the greatest extent when short interlinear spacings are used.

It was observed by chance that illusions in patterns consisting of criss-crossing lines, such as the Orbison and Hering illusions, are markedly reduced when the patterns are blurred. In the first of two experiments the reduction with blurring was confirmed for four patterns but failed to occur in patterns lacking criss-crossing lines. In the second experiment the reduction was shown to be due mainly to the reduced luminance contrast in blurred patterns, thus confirming earlier findings on the effect of contrast on this group of illusions. Theoretical implications are discussed.

Young infants withdraw their heads when shown a display simulating a rectangle rotating towards their faces. The head withdrawal is specific to that transformation and is not elicited by a similar transformation which does not specify approach. This is possible evidence of an unlearned ability to pick up information about change of position in the third dimension.

When shown the names of two objects, subjects determine which object is larger more slowly as the difference in the sizes of the objects decreases. This might result from variations in the time taken to access sufficient information to perform the task; information which crudely specifies size is accessed first and can be used when the sizes differ greatly; information which specifies size on a more finely graded scale must be accessed when they do not. This hypothesis was tested. Subjects shown the names of three objects, determined which object was intermediate in size. Immediately thereafter the name of another object was shown, the task then being to decide whether the object previously judged intermediate was larger than this object. In this second task reaction times increased with decreasing differences in size between the two objects; this increase was smaller, however, when the sizes of the objects in the first task were similar. The results were predicted from the assumption that when the specification of an object's size in terms of fine discrimination is accessed for comparison in the first task it remains available for use in the second task; thus the time normally required for accessing that information in the second task is reduced. Some implications of the results are discussed.

The effects of posthypnotic suggestion on the perception of egocentric distance were evaluated by two methods, one direct and the other indirect. The direct method was the verbal report of the perceived distance of the stimulus. The indirect method used a measure based on apparent concomitant motion of the stimulus with a lateral movement of the head. The indirect method, unlike the direct method, has been shown to be insensitive to cognitive biases. Two highly trained hypnotic observers received posthypnotic suggestions to see a point of light at approximately 2, 4, or 6 ft (61, 122, or 183 cm) from themselves. For each suggested distance the light was physically at 2, 4, or 6 ft. As a control, trials were completed using these same physical distances without the posthypnotic suggestions. Nine experimental trials, one for each combination of suggested distance and physical distance, constituted a trial set. One observer completed three sets, and the other completed two sets. It was found that the posthypnotic suggestions affected the direct but not the indirect measures of the perceived distances of the light.

Threshold function and suprathreshold performance for nonacuity stimuli are compared for an amblyope who exhibits a low as well as a high spatial frequency threshold abnormality.

The results show that (i) a suprathreshold abnormality involving discrimination and localization of easily visible stimuli occurs in amblyopia and (ii) suprathreshold abnormalities occur in amblyopia independent of any threshold dysfunction.

Binocular sensitivity to the movement or flicker visible in a phase-reversing grating pattern is higher by a factor of about 1·7 to 2·0 compared with monocular sensitivity, at low rates of phase reversal. This factor decreases as the temporal frequency of the phase reversal increases, especially above 10 Hz. For different observers, the ratio may be higher or lower overall with gratings of spatial frequency 5 cycles deg⁻¹ than with 0·5 cycle deg⁻¹ gratings.

Visual and haptic judgments of stimulus orientation were examined intramodally and cross-modally by having subjects reproduce standard stimulus orientations simultaneously with their inspection or after a delay. For all conditions, an oblique effect was obtained, i.e. vertical and horizontal orientations were reproduced reliably more accurately than oblique orientations. Although intramodal differences were large, with haptic errors being greater than visual errors, cross-model differences were small. Furthermore, while for intramodal conditions simultaneous visual reproductions were reliably more accurate than delayed reproductions but haptic reproductions were more accurate when delayed, cross-modal errors were reliably greater with simultaneous reproductions, regardless of whether the standard orientation was visual or haptic. The modality differences reflect basic differences in stimulus information processing and the stability of the oblique effect across the experimental conditions suggests that perceptual spatial anisotropic effects are strongly influenced by experiential factors.

Two experiments are reported which demonstrate the role of contextual factors in the rat's perception of orientation. Designed to eschew criticisms of an earlier study by McGonigle and Jones, the first experiment shows that the rat's perception of linear Gestalten is conditional upon the set of alternative patterns within which the (linear) pattern is embedded. The second experiment shows that the ecological framework of an orientational cue (rather than its position relative to retinocortical coordinates) determines the rat's perception of vertical and horizontal stimuli.

Subjects viewed the Kanizsa triangle under conditions of visual masking. Depending on the type of mask employed, the triangle could be perceived when the inducing discs were not phenomenally present, or the discs could be seen with the triangle not present. These findings suggest that no single set of mechanisms, physiological or cognitive, will suffice to explain perception of the illusory triangle.

A version of the face—vase pattern was constructed with the use of subjective contours. The McCollough effect was viewed on this pattern and it was found that perceptual alteration (face to vase) does not weaken the strength of the effect's hues while a complete perceptual rearrangement (faces/vases to concentric rectangles) does.