The reference frame for encoding and retention of motion depends on stimulus set size

The goal of this study was to investigate the reference frames used in perceptual encoding and storage of visual motion information. In our experiments, observers viewed multiple moving objects and reported the direction of motion of a randomly selected item. Using a vector-decomposition technique, we computed performance during smooth pursuit with respect to a spatiotopic (nonretinotopic) and to a retinotopic component and compared them with performance during fixation, which served as the baseline. For the stimulus encoding stage, which precedes memory, we found that the reference frame depends on the stimulus set size. For a single moving target, the spatiotopic reference frame had the most significant contribution with some additional contribution from the retinotopic reference frame. When the number of items increased (Set Sizes 3 to 7), the spatiotopic reference frame was able to account for the performance. Finally, when the number of items became larger than 7, the distinction between reference frames vanished. We interpret this finding as a switch to a more abstract nonmetric encoding of motion direction. We found that the retinotopic reference frame was not used in memory. Taken together with other studies, our results suggest that, whereas a retinotopic reference frame may be employed for controlling eye movements, perception and memory use primarily nonretinotopic reference frames. Furthermore, the use of nonretinotopic reference frames appears to be capacity limited. In the case of complex stimuli, the visual system may use perceptual grouping in order to simplify the complexity of stimuli or resort to a nonmetric abstract coding of motion information.     

The size of the Simon effect depends on the nature of the relevant task

Four experiments were conducted to investigate contextual modulations of the Simon effect. The results showed that the Simon effect was quantitatively different depending on which kind of task needed to be performed. Importantly, this effect did not depend on the relative processing time of the relevant dimension, nor on a direct or indirect overlap between the relevant and irrelevant stimulus part. To account for the data, we refer to the neural overlap hypothesis, which extends the definition of dimensional overlap (Kornblum, Hasbroucq, & Osman, 1990) with similarity of processing regions as the key factor for the interaction between relevant and irrelevant information processing.     

The effect of set size on the latency to name words

The experiment attempted to determine whether or not Ss' verbal reaction time to name words was influenced by the number of possible word stimuli in a set. It was suggested that a discrepancy between the results of previous investigators was an artefact of the manner in which some of the data had been analysed. The results of the experiment indicate that set size does have a small but significant effect on reaction time to this task.     

The effect of set size on hemifield asymmetries in letter recognition

Single letters were presented tachistoscopically in the left or right hemifield to right-handed observers. Superior recognition of letters on the right was found for letters chosen from subsets containing four items, but no differences were found when any letter, or any symmetrical letter, could appear. In a second experiment, the right hemifield advantage was found under conditions requiring the subject to determine the subset only when the subset chosen was appropriate. Results were discussed in terms of the adequacy of language-based explanations of hemifield asymmetries in letter recognition.     

The effect of feedback stimuli on contextual fear depends upon the length of the minimum intertrial interval

The present experiments investigated the effects of manipulation of the minimum intertrial interval (ITI) on the ability of a feedback stimulus to reduce the amount of fear conditioned to the shock context. In Experiment 1, we found that animals exposed to yoked shock with a feedback stimulus were equivalent to animals which received escapable shock, and both these groups showed less fear of the shock context than did yoked animals not given feedback, when a minimum ITI of 60 s was used. This replicated the effect of feedback on contextual fear conditioning. However, animals exposed to yoked shock with a feedback stimulus but for whom the minimum ITI was 5 s showed as much fear of the context as yoked animals not given feedback. Experiment 2 assessed whether this difference was solely due to the lower minimum ITI resulting in more fear than the 60-s minimum ITI, or whether the lower minimum ITI precluded the effect of feedback. The former possibility was discounted. Thus, these studies demonstrate that a relatively long minimum ITI is essential for the observance of a feedback effect on fear of the shock context.     

Holistic processing in the composite task depends on face size

Holistic processing is a hallmark of face processing. There is evidence that holistic processing is strongest for faces at identification distance 2–10 metres from the observer. However, this evidence is based on tasks that have been little used in the literature and that are indirect measures of holistic processing. We use the composite task—a well validated and frequently used paradigm—to measure the effect of viewing distance on holistic processing. In line with previous work, we find a congruency x alignment effect that is strongest for faces that are close (2 m equivalent distance) than for faces that are further away (24 m equivalent distance). In contrast, the alignment effect for same trials, used by several authors to measure holistic processing, produced results that are difficult to interpret. We conclude that our results converge with previous findings providing more direct evidence for an effect of size on holistic processing.     

The effect of visual angle on global and local reaction times depends on the set of visual angles presented

It has been shown that there is a transition from a global to a local advantage in reaction time as visual angle increases (Kinchla & Wolfe, 1979), and it has been assumed that this transition reflects lower level (e.g., retinal) processes. In three experiments, we examined whether higher level (e.g., attentional) processes play a role in this transition. In each experiment, subjects received a different stimulus set in each of two blocks of trials. In Experiment 1, stimuli subtending 1.5 degrees, 3 degrees, 4.5 degrees, or 6 degrees of visual angle vertically (small-stimuli set) were randomly presented in one block, while the other block consisted of random presentations of 3 degrees, 6 degrees, 9 degrees, or 12 degrees stimuli (large-stimuli set). The subjects' task was to identify targets that appeared randomly at either the local or the global level. It was found that the transition from a global to a local reaction-time advantage took place at a larger visual angle for the large-stimuli set than for the small-stimuli set. The same effects of stimulus set were found in Experiment 2, in which the small-stimuli set included 1.5 degrees, 3 degrees, or 6 degrees stimuli while the large-stimuli set included 3 degrees, 6 degrees, or 9 degrees stimuli. In Experiment 3, eye position was monitored to rule out the possibility that subjects adopted different fixation strategies depending on which stimulus set was being presented. The findings suggest that attention plays a major role in determining the relative speed of processing of local-and global-level information.     

Control by past and present stimuli depends on the discriminated reinforcer differential

The extent to which a stimulus exerts control over behavior depends largely on its informativeness. However, when reinforcers have discriminative properties, they often exert less control over behavior than do other less reliable stimuli such as elapsed time. We investigated why less reliable cues in the present often overshadow stimulus control by more reliable cues presented in the recent past, by manipulating the reliability and duration of stimulus presentations. Five pigeons worked on a modified concurrent schedule in which the location of the response that produced the last reinforcer was a discriminative stimulus for the likely time and location of the next reinforcer. In some conditions, either the location of the previous reinforcer, or the location of the next reinforcer, was signaled by a red key light. This stimulus was either Brief, occurring for 10 s starting a fixed time after the most recent reinforcer, or Extended, being present at all times between food deliveries. Brief and Extended stimuli that signaled the same information had a similar effect on choice when they were present, but control by Brief stimuli weakened as time since stimulus offset elapsed. Control was divided among stimuli in the present and recent past according to the apparent reliability of the information signaled about the next reinforcer. More reliable stimuli in the present degraded, but did not erase, control by less reliable stimuli presented in the recent past. Thus, we conclude that less reliable stimuli in the present control behavior to a greater degree than do more reliable stimuli in the recent past because these more reliable stimuli are forgotten, and hence their relation to the likely availability of food cannot be discriminated.     

Determining the degree of inconsistency in a set of paired comparisons

Consistency in paired comparison data is defined. Two types of inconsistency which may arise are defined. Computational formulas for these types of inconsistency are derived, and examples illustrating the use of these formulas are presented.These ideas were developed while the first author was on the staff of the Research Center for Human Relations. The work was made possible by the ONR contract NONR 285(10). The authors are indebted to Jack Moshman for his helpful critical suggestions. The United States Government is authorized to reprint this article in whole or in part.     

Intramodal and crossmodal pairing and anchoring in comparisons of successive stimuli

Two experiments were conducted to study effects of modality, temporal position, and their interaction on comparisons of successive stimuli. In Experiment 1, intramodal (tone–tone and line–line) and crossmodal (tone–line and line–tone) stimulus pairs, with two interstimulus intervals (ISIs), 400 and 2,000 ms, were presented. Participants indicated which stimulus was the “stronger.” Time-order errors (TOEs) were assessed using the D% measure and were found in all types of pairs. Variation in TOEs across conditions was well accounted for by changes in parameters (stimulus weights, reference levels) in an extended version of Hellström’s sensation weighting (SW) model. With an ISI of 2,000 ms, the first stimulus had a lower weight (less impact on the response) than did the second stimulus. More negative TOEs were found with the longer ISI in all pair types except tone–line. In Experiment 2, participants indicated which of two lines was the longer or which of two tones was the louder. An intra- or crossmodal anchor, or no anchor, was interpolated between the stimuli. Anchoring tended to reduce the weight of the first stimulus, suggesting interference with memory, and to yield negative TOEs. Intramodal anchors yielded reduced weights of both stimuli, most dramatically for tones, suggesting an additional effect of stimulus interference. Response times decreased with crossmodal anchors. For line–line pairs, strong negative TOEs were found. In both experiments, the variation in TOE across conditions was well accounted for by the SW model.     

Perceptual-cognitive explorations of a toroidal set of free-form stimuli

A polar-coordinate analogue of Fourier synthesis generates organic-appearing “free forms” that can be continuously deformed along any desired number of difficult-to-verbalize dimensions. Since the dimensions are also circular, the forms correspond to points on the surface of a torus which, though conveniently finite, is free of bounding edges. Two experiments explore a particular two-dimensional set of 81 such forms. The first shows that perceived pair-wise similarities among the individual forms are well explained purely in terms of the distances among their corresponding points in the toroidal parameter space. The second, however, establishes that forms that tend to be grouped together as having the same cognitive interpretation define regions in parameter space that are variously shaped or even bimodal and, hence, that cannot be explained solely on the basis of the fixed set of pair-wise similarities. The stimuli appear to offer a novel combination of cognitive richness and low-dimensional parametric control.     

Fixed set in the perception of size in relation to lightness

When control subjects compared the sizes of two circles of different lightness, the lightness-size illusion was observed, i.e., the darker circle was perceived to be smaller. However, after experimental subjects were shown a large, light circle and a smaller, darker circle repeatedly, the subjective size of the dark circle increased. It decreased after repeated exposures to a small, light circle and a large, dark one. These changes in perception were assumed to be contrast effects produced by an experimentally fixed set and were similar to changes observed when the same method was previously applied by this author to the size-weight illusion. Despite differences in modality and dimension of perception, every application of the fixed-set method resulted in analogous patterns. When the situation of the set-fixing experiment and that of the critical experiment were similar to each other, the fixed set was activated more and greater contrast effects were produced.