Perception

Induced motion occurs when there is a misallocation of nonuniform motion. Theories of induced motion are reviewed with respect to the model for uniform motion recently proposed by Swanston, Wade, and Day. Theories based on single processes operating at one of the retinocentric, orbitocentric, egocentric, or geocentric levels are not able to account for all aspects of the phenomenon. It is therefore suggested that induced motion is a consequence of combining two different types of motion signals: one provides information by registering the motion with respect to the retina, orbit, and egocentre; the other provides information only on the relational motions between the pattern elements. Simple rules are given for defining a frame of reference for the relational motion process, which can result in a reallocation of the motion signals. It is proposed that the two signals in combination are weighted differentially, with the greater influence coming from the relational signals. Procedures for determining the weighting factors are described, and predictions from the model are examined.

Two large groups of inexperienced subjects (n = 208 and n = 50) and a small group of experienced subjects (n = 5) were tested using time-till-breakdown as a measure of long-range apparent motion across a range of temporal frequencies. One group of inexperienced subjects was retested after one week and demonstrated quite stable patterns of response. Large intersubject variability was observed in terms of the amount of motion seen, with most inexperienced subjects reporting very little apparent motion. A raster display produced a peak frequency 1 Hz higher than a standard tachistoscope display. The role of experience was also examined with a small group of inexperienced subjects (n = 8) tested once daily over five consecutive days. There was high intersubject variability and intrasubject consistency, demonstrating little influence of learning and experience. The results are discussed in terms of current ideas on the breakdown effect.

The encoding of positional relationships between pattern elements in eccentric vision was studied with different patterns consisting of the same short line segments in different positions. In each trial two stimulus patterns were flashed one above the other and the subject had to decide whether the patterns were identical or mirror symmetric (experiment 1) or whether the patterns were the same or different (experiments 2 and 3). The ability to discriminate between identical and mirror symmetric patterns was reduced in eccentric vision, even when the patterns were size-scaled according to the cortical magnification factor and thus the patterns were similarly visible at the different eccentricities. The results agree with the notion that eccentric vision is inferior to central vision in tasks which require proper encoding of information about the relative positions of pattern elements.

Exposure to a vernier offset can cause a subsequently viewed straight contour to appear offset in the opposite direction. The size of this vernier aftereffect (VAE) varies systematically with the size of the adapting offset. The VAE may be a version of the tilt aftereffect.

Mental imagery interferes with perception. This, an example of the ‘Perky effect’, was studied for vernier acuity. Mean accuracy for reporting the offset of vertical line targets declined from 80% to 65% when subjects were requested to imagine vertical lines near fixation. Images of horizontal lines or of a grey mist in the fixation region lowered accuracy to a similar extent. However, accuracy was barely affected when the image was requested 1.5 deg or more from the target. The Perky effect remained strong for at least 4 s after an instruction to ‘clear’ the image away. The results were not due to imagery-induced changes in fixation, pupil diameter, or accommodation, or (at least primarily) to central attentional or decisional factors. Rather, imagery produces a local, pattern-insensitive, and relatively long-lasting reduction in visual sensitivity. The sensitivity loss may be mimicked by a 0.24 log unit reduction in target energy.

The equiratio taste mixture model was originally developed for the prediction of psychophysical power functions of equiratio mixtures of substances that have a similar taste and that also exhibit mutual cross adaptation. Earlier studies have shown that the model is valid for mixtures of sugars and/or sugar alcohols. Two experiments are reported in which it is questioned whether the psychophysical functions of mixtures of higher physical complexity can be predicted by the model. In the first experiment the psychophysical power functions of binary and quaternary equiratio mixture types were determined experimentally and compared to those predicted by the generalized model. In the second, similar, experiment quaternary and eight-component mixture types were examined. The method of magnitude estimation, in combination with the sip and spit procedure, was used. The functions predicted by the model were almost identical to the functions established on the basis of the experimental data. These results reconfirm that the gustatory modality operates like an ‘averaging’ system when processing this kind of mixture. It is argued that for other kind of mixtures the model will predict incorrectly. The status of the equiratio mixture model is discussed.

Two experiments are reported in which the perceptual interactions between oral pungency, evoked by CO₂, and the taste of each of four tastants–sucrose (sweet), quinine sulfate (bitter), sodium chloride (salty), and tartaric acid (sour)–were explored. In experiment 1 the effect of three concentrations of each tastant on the stimulus-response function for perceived oral pungency, in terms of both rate of change (slope) and relative position along the perceived pungency axis, was determined. In experiment 2 the effect of three concentrations of CO₂ on the stimulus-response function for the perceived taste intensity of each tastant was examined. Results show that the characteristics of the mutual effects of tastant and pungent stimulus depend on the particular tastant employed. Sucrose sweetness and CO₂ oral pungency have no mutual effect; sodium chloride saltiness or tartaric acid sourness and CO₂ oral pungency show mutual enhancement; and quinine sulfate bitterness abates CO₂ oral pungency, whereas CO₂ has a double and opposite effect on quinine sulfate bitterness–at low concentrations of bitter tastant CO₂ enhances bitterness, and at high concentrations of bitter tastant CO₂ abates bitterness. It is suggested that the perceptual attributes of saltiness and sourness are closer, from a qualitative point of view, to oral pungency than are the attributes of bitterness and sweetness.

The technique of uniform field flicker (UFF) masking has frequently been used to address issues concerning the relative performance of sustained and transient neural channels in the human visual system. Unfortunately there has been an artifact in the implementation of this method in most published experiments which has meant that the contrast of the target has been flickered in synchrony with the mean luminance. A study is reported in which the artifact was corrected and the effects of UFF masking on the contrast sensitivity function then examined. With this correction, masking was still restricted to low spatial frequencies but it was much weaker than reported originally. It is argued that the original evidence suggesting that UFF masking can be used to examine the functioning of transient and sustained channels has not been interpreted correctly and that the basis for such a claim is weak.

Although the ‘filling in’ of each blind spot by healthy retina in the other eye has long been described as an adaptive property of the spatial arrangement of the optic disks, an explanation of why the disks are specifically located where they are has yet to be proposed. A rationale for their horizontal position in humans is offered that is based on the projections of the blind spots in visual space in relation to fixation distance and to the protrusion of the bony facial occlusion of the nose bridge.

In two experiments subjects were asked to report the identity of a position-cued critical letter in an array of four letters. Four types of arrays were used: (i) unpronounceable nonwords; (ii) pronounceable nonwords (‘pseudowords’); (iii) words in which the critical letter was minimally constrained by the context letters; and (iv) words in which the critical letter was maximally constrained by the context letters. All four-letter stimuli were presented in two parts. A leading array in which the information from two quadrants of a vertical by horizontal division of each letter was presented, and, after intervals of 0, 20, 40, 80, 100, 120, 160, 320, and 480 ms and infinity (ie, no trailing array), a trailing array of the complementary letter parts. In experiment 1 a single group of eight subjects responded to the one hundred and sixty combinations of the four types of letter strings, the four serial positions, and the ten stimulus onset asynchrony values. In experiment 2 the stimulus onset asynchrony values were varied among subjects, with twelve subjects responding at each value. The results from these two studies were generally similar. Performance in the word conditions was consistently superior to performance in the nonword conditions, and the magnitude of this difference (ie, the word-superiority effect) increased with increasing stimulus onset asynchrony up to 120 ms, and then gradually declined. The fact that the magnitude of the word-superiority effect initially increased with the separation of leading and trailing arrays was interpreted as support for Johnston's suggestion that letters in words are represented during visual encoding both in the form of individual letter percepts and in a decay-resistant word percept, as opposed to letters in nonwords, which are represented only as decay-susceptible letter percepts. The experimental findings are discussed in relation to the ‘interactive activation’ model of word perception.

If a few parallel horizontal rows of dots are set diagonally, like steps, across the visual field, the inner rows appear not to be horizontal but sloping up to one side; the effect holds as long as the vertical distances between the rows do not exceed a given visual angle. This illusion, described by Vicario in 1978, was never explained. An experiment is reported in which the illusion was still visible at row separations well in excess of the spatial limits originally considered, provided the stimulus elements were enlarged. The maximum illusion was obtained for length ratios (interrow distance to size of dots) identical to those which have been shown to produce the largest effects in a number of illusions of area and length. This suggests that Vicario's illusion is similar to other illusions of extent, and that it can be explained by a neural extent-coding model.

Most people can correctly apply the concepts of horizontal and vertical in describing objects, but a simple demonstration shows that they are confused about how these concepts work. The nature of the confusion and its possible causes are briefly discussed.