Limited capacity for contour curvature in iconic memory

We measured the difference threshold for contour curvature in iconic memory by using the cued discrimination method. The study stimulus consisting of 2 to 6 curved contours was briefly presented in the fovea, followed by two lines as cues. Subjects discriminated the curvature of two cued curves. The cue delays were 0 msec. and 300 msec. in Exps. 1 and 2, respectively, and 50 msec. before the study offset in Exp. 3. Analysis of data from Exps. 1 and 2 showed that the Weber fraction rose monotonically with the increase in set size. Clear set-size effects indicate that iconic memory has a limited capacity. Moreover, clear set-size effect in Exp. 3 indicates that perception itself has a limited capacity. Larger set-size effects in Exp. 1 than in Exp. 3 suggest that iconic memory after perceptual process has limited capacity. These properties of iconic memory at threshold level are contradictory to the traditional view that iconic memory has a high capacity both at suprathreshold and categorical levels.     

Short-term visual memory for contour curvature in a delayed discrimination task

Abstract: A two-interval forced-choice of constant stimuli was used to measure the point of subjective equality (PSE) and discrimination threshold for standard contour curvature (1.91, 3.24 deg⁻¹) held in short-term visual memory (STVM). At both standard curvatures, the PSE for remembered curvature was nearly constant for standard curvature from 2 s to 16 s retention intervals, while the discrimination threshold increased as a linear function of retention interval. These results show that the decay in STVM for contour curvature is due to the noisy representation of curvature, neither to fading of the represented curvature nor to converging to the constant curvature. Furthermore, the Weber fraction was nearly constant for both standard curvatures at any delay from 2 to 16 s.     

Limitations for Change Detection in Multiple Gabor Targets

We investigate the limitations on the ability to detect when a target has changed, using Gabor targets as simple quantifiable stimuli. Using a partial report technique to equalize response variables, we show that the log of the Weber fraction for detecting a spatial frequency change is proportional to the log of the number of targets, with a set-size effect that is greater than that reported for visual search. This is not a simple perceptual limitation, because pre-cueing a single target out of four restores performance to the level found when only one target is present. It is argued that the primary limitation on performance is the division of attention across multiple targets, rather than decay within visual memory. However in a simplified change detection experiment without cueing, where only one target of the set changed, not only was the set-size effect still larger, but it was greater at 2000 msec ISI than at 250 msec ISI, indicating a possible memory component. The steepness of the set-size effects obtained suggests that even moderate complexity of a stimulus in terms of number of component objects can overload attentional processes, suggesting a possible low-level mechanism for change blindness.     

A feature-segmentation model of short-term visual memory

A feature-segmentation model of short-term visual memory (STVM) for contours is proposed. Memory of the First stimulus is maintained until the second stimulus is observed. Three processes interact to determine the relationship between stimulus and response: feature encoding, memory, and decision. Basic assumptions of the model are twofold: (i) the STVM system divides a contour into convex parts at regions of concavity; and (ii) the value of each convex part represented in STVM is an independent Gaussian random variable. Simulation showed that the five-parameter fits give a good account of the effects of the four experimental variables. The model provides evidence that: (i) contours are successfully encoded within 0.5 s exposure, regardless of pattern complexity; (ii) memory noise increases as a linear function of retention interval; (iii) the capacity of STVM, defined by pattern complexity (the degree that a pattern can be handled for several seconds with little loss), is about 4 convex parts; and (iv) the confusability contributing to the decision process is a primary factor in deteriorating recognition of complex figures. It is concluded that visually presented patterns can be retained in STVM with considerable precision for prolonged periods of time, though some loss of precision is inevitable.     

Set-size effects in simple visual search for contour curvature

In a visual-search paradigm, both perception and decision processes contribute to the set-size effects. Using yes - no search tasks in set sizes from 2 to 8 for contour curvature, we examined whether the set-size effects are predicted by either the limited-capacity model or the decision-noise model. There are limitations in perception and decision-making in the limited-capacity model, but only in decision-making in the decision-noise model. The results of four experiments showed that the slopes of the logarithm of threshold plotted against the logarithm of set size ranged from 0.24 to 0.32, when the curvature was high or low, contour convexity was upward or downward, and stimulus was masked or unmasked. These slopes were closer to the prediction of 0.23 by the decision-noise model than that of 0.73 by the limited-capacity model. We interpret this that in simple visual search for contour curvature, the decision noise mainly affects the set-size effects and perceptual capacity is not limited.     

Short-term visual memory and pattern masking

The results of two experiments involving the matching of unfamiliar, nameless shapes (Gibson forms) indicated that a visual representation of a brief (30-100 ms) stimulus survives in a Short-Term Visual Memory (STVM) for 6 s or more after the onset of a pattern mask. On the basis of these results a possible alternative to Sperling's (1967) model for short-term memory for visual stimuli was presented. In this model it is assumed that recognition processes occupy several hundred milliseconds and continue after the arrival of the mask using the information available in STVM.     

Role of the preprobe delay in memory-scanning tasks

In a variation of Sternberg's (1966, 1969) memory-scanning task not requiring an explicit negative response, Diener (1988) found that a preprobe delay was necessary to produce the usual set-size effect. In Experiment 1 of the present study, the effect of the preprobe delay was investigated in the typical two-response task. In the absence of a preprobe delay, the function relating response latency to set size was virtually flat for negative responses, but was described by a slope of about 18 msec/item for positive responses. Further research suggested that the reduced set-size effect in the absence of a preprobe delay is the result of expectancy effects usually controlled by the preprobe delay. Informing the subject of the size of the memory set before it was presented (Experiment 2) produced a set-size effect of the usual magnitude in the absence of a preprobe delay. Experiment 3 was designed to assess the effects of expectancy in the absence of a memory search. A task similar in stimulus arrangement to the memory-scanning task but requiring the subject to indicate whether the last digit in the set was odd or even produced a decrease in response latency with set size of 29 msec/item in the absence of a preprobe delay.     

The contents of visual working memory reduce uncertainty during visual search

Information held in visual working memory (VWM) influences the allocation of attention during visual search, with targets matching the contents of VWM receiving processing benefits over those that do not. Such an effect could arise from multiple mechanisms: First, it is possible that the contents of working memory enhance the perceptual representation of the target. Alternatively, it is possible that when a target is presented among distractor items, the contents of working memory operate postperceptually to reduce uncertainty about the location of the target. In both cases, a match between the contents of VWM and the target should lead to facilitated processing. However, each effect makes distinct predictions regarding set-size manipulations; whereas perceptual enhancement accounts predict processing benefits regardless of set size, uncertainty reduction accounts predict benefits only with set sizes larger than 1, when there is uncertainty regarding the target location. In the present study, in which briefly presented, masked targets were presented in isolation, there was a negligible effect of the information held in VWM on target discrimination. However, in displays containing multiple masked items, information held in VWM strongly affected target discrimination. These results argue that working memory representations act at a postperceptual level to reduce uncertainty during visual search.     

Interactive visual imagery and memory search for words and pictures

In a short-term recognition memory experiment with words, subjects: (1) subvocally rehearsed the words, (2) generated a separate visual image for each word, (3) generated an interactive scene with such images, or (4) composed a covert sentence using the words in the memory set. Contrary to Seamon's (1972) results in a similar study, a serial memory search was found in all conditions, instead of the simultaneous scan which was expected when items were combined in interactive images. In a second study with pictures as stimuli, subjects who generated imaginal interactions between separate pictures, viewed interacting pictures, or viewed separate pictures also showed a serial search, i.e., longer RTs were obtained when more stimuli were held m memory. Since interactive imagery facilitated performance in an unexpected paired-associate task with memory set stimuli, one can argue that subjects actually processed or generated such interactions. It was suggested that memory search might not be simultaneous in tasks where the test stimulus constitutes only part of a memory image.     

The effects of practice with one positive set in a memory scanning task can be completely transferred to a different positive set

Recognition tasks in which the items have been assigned to one and only to one response category throughout an experiment typically reveal that the functions relating reaction time and positive set size are negatively accelerated and have slopes which decrease significantly with practice. The present experiments were designed to determine whether this practice effect is specific to the individual items practiced or if it depends, at least partially, on relations built up among the positive set items. The results permit rejection of both alternatives. In the extreme case tested, it was found that, after prolonged practice with the same items composing nested positive sets, these items could be replaced with unpracticed ones without producingany significant change in the effect of set size, reaction times, or error levels. Absence of change in overall reaction times in combination with stability of slope values leads to the conclusion that not only do those practice effects which are related to the set-size effect transfer exceedingly well to unpracticed items, but practice effects on all other stages involved in item recognition also show almost total transfer to unpracticed items. It is concluded that, under the conditions of these experiments, the gain (through practice) in efficiency in processing the stimulus is independent of the stimulus.     

On the generality of the contingent orienting hypothesis

The contingent-orienting hypothesis states that attentional capture by a task-irrelevant stimulus is contingent on whether that stimulus shares a feature property that is critical to the task at hand [Folk, C. L., Remington, R. W., & Johnston, J. C. (1992). Involuntary covert orienting is contingent on attentional control settings. Journal of Experimental Psychology: Human Perception and Performance, 18, 1030-1044]. Studies supporting this hypothesis have mostly used set size four displays throughout the experiment and thus constrict its ecological validity, since conclusions drawn from experiments using fixed set-size displays may not be generalized to other conditions with different set sizes. We used a spatial cueing paradigm in which a non-informative onset or color cue preceded an onset or a color target, and manipulated set size as a within- or between-subject factor. In four experiments, the original finding of Folk et al. (1992) was replicated only when a fixed set size (four) was used throughout. When both set-size four and eight were used in an experiment, stimulus-driven capture by onset in search of a color target was found even for set-size four displays. These results raise doubts as to the generality of the contingent-orienting hypothesis and help to delineate the boundary conditions on this hypothesis.     

Hemispheric asymmetry in memory search for four-letter names and human faces

Two memory search experiments were conducted using vertically oriented four-letter names and human faces as stimuli. Subjects were required to indicate as quickly and as accurately as possible whether or not a single probe stimulus (presented for 150 msec to either the left or right visual field) was contained in a set of 2, 3, 4, or 5 items being held in short-term memory. The probe stimuli were presented alone (clear condition) or centrally embedded in a matrix of dots (degraded condition). In Experiment 1 (involving names), a right visual field/left hemisphere advantage was obtained and pinpointed at the encoding stage rather than at the memory comparison stage of the information-processing system. For Experiment 2 (involving human faces), no hemispheric advantage was readily observed. In each experiment, both the left hemisphere and the right hemisphere employed an abstract memory comparison operation from which the effects of probe degradation have been removed. These results are discussed in terms of their implications for various models of hemispheric asymmetry.     

The time course of figure-ground reversal

We report the results from five experiments employing a modified version of the short-term visual matching (STVM) method introduced by Driver and Baylis (1996). In STVM, participants see a study display with ambiguous figure-ground relations. After the study display, participants have to decide which of two shapes in a match display was seen before in the study display. STVM has been used by Vecera, Vogel, and Woodman (2002) to demonstrate that the lower region is a figure-ground cue. In our modified version of STVM, the study stimulus was preceded by a brief prime. This caused a biasing of the figural interpretation of the ambiguous figure-ground displays that contained the lower region cue. We show that 100-msec priming with an unambiguous display is enough to affect the subsequent interpretation of the ambiguous figure-ground display. It takes maximally 350 msec to complete a transition from the nondominant interpretation to the dominant interpretation of an ambiguous figure-ground display that contains the lower region cue.     

Absence tasks of the set-size effect in memory-search in the absence of a preprobe delay

In the traditional Sternberg (1966) paradigm, response latency increases linearly with increases in the size of the positive set (the set-size effect). The results of four experiments converge on the conclusion that this set-size effect depends on a delay before the presentation of the memory probe. In Experiment 1, subjects were required to respond as soon as a repetition occurred in a series of digits. Despite the similarity of this task to memory-search tasks that invariably show set-size effects, there was no increase in response latency with increasing series length. Neither the inclusion of negative trials (Experiment 2) nor the explicit designation of the test digit (Experiment 3) resulted in the typical set-size effect. However, the introduction of a 1-sec preprobe delay (Experiment 4) resulted in a set-size effect of 31 msec/item.     

The influence of visual short-term memory on size perception

We show that perceived size of visual stimuli can be altered by matches between the contents of visual short-term memory and stimuli in the scene. Observers were presented with a colour cue (to hold in working memory or to merely identify) and subsequently had to indicate which of the two different-coloured objects presented simultaneously on the screen appeared bigger (or smaller). One of the two objects for size judgements had the same colour as the cue (matching stimulus) and the other did not (mismatching stimulus). Perceived object size was decreased by the reappearance of the recently seen cue, as there were more size judgement errors on trials where the matching stimulus was physically bigger (relative to the mismatching stimulus) than on trials where the matching stimulus was physically smaller. The effect occurred regardless of whether the visual cue was actively maintained in working memory or was merely identified. The effect was unlikely generated by the allocation of attention, because shifting attention to a visual stimulus actually increased its perceived size. The findings suggest that visual short-term memory, whether explicit or implicit, can decrease the perceived size of subsequent visual stimuli.     

A memory-based model of Hick's law

We propose and evaluate a memory-based model of Hick’s law, the approximately linear increase in choice reaction time with the logarithm of set size (the number of stimulus–response alternatives). According to the model, Hick’s law reflects a combination of associative interference during retrieval from declarative memory and occasional savings for stimulus–response repetitions due to non-retrieval. Fits to existing data sets show that the model accounts for the basic set-size effect, changes in the set-size effect with practice, and stimulus–response-repetition effects that challenge the information-theoretic view of Hick’s law. We derive the model’s prediction of an interaction between set size, stimulus fan (the number of responses associated with a particular stimulus), and stimulus–response transition, which is subsequently tested and confirmed in two experiments. Collectively, the results support the core structure of the model and its explanation of Hick’s law in terms of basic memory effects.     

Terms of the debate on the format and structure of visual memory

Our ability to actively maintain information in visual memory is strikingly limited. There is considerable debate about why this is so. As with many questions in psychology, the debate is framed dichotomously: Is visual working memory limited because it is supported by only a small handful of discrete “slots” into which visual representations are placed, or is it because there is an insufficient supply of a “resource” that is flexibly shared among visual representations? Here, we argue that this dichotomous framing obscures a set of at least eight underlying questions. Separately considering each question reveals a rich hypothesis space that will be useful for building a comprehensive model of visual working memory. The questions regard (1) an upper limit on the number of represented items, (2) the quantization of the memory commodity, (3) the relationship between how many items are stored and how well they are stored, (4) whether the number of stored items completely determines the fidelity of a representation (vs. fidelity being stochastic or variable), (5) the flexibility with which the memory commodity can be assigned or reassigned to items, (6) the format of the memory representation, (7) how working memories are formed, and (8) how memory representations are used to make responses in behavioral tasks. We reframe the debate in terms of these eight underlying questions, placing slot and resource models as poles in a more expansive theoretical space.     

Haptic underestimation of angular extent

To what extent can individuals accurately estimate the angle between two surfaces through touch alone, and how does tactile judgment compare to visual judgment? Subjects' ability to estimate angle size for a variety of haptic and visual stimuli was examined in a series of nine experiments. Triangular wooden blocks and raised contour outlines comprising different angles and radii of curvature at the apex were used in experiments 1-4 and it was found that subjects consistently underestimated angular extent relative to visual baselines and that the degree of underestimation was inversely related to the actual size of the angle. Angle estimates also increased with increasing radius of curvature when actual angle size was held constant. In contrast, experiments 5-8 showed that subjects did not underestimate angular extent when asked to perform a haptic-visual match to a computerized visual image; this outcome suggests that visual input may 'recalibrate' the haptic system's internal metric for estimating angle. The basis of this cross-modal interaction was investigated in experiment 9 by varying the nature and extent of visual cues available in haptic estimation tasks. The addition of visual-spatial cues did not significantly reduce the magnitude of haptic underestimation. The experiments as a whole indicate that haptic underestimations of angle occur in a number of different stimulus contexts, but leave open the question of exactly what type of visual information may serve to recalibrate touch in this regard.     

Saved by a log: how do humans perform hybrid visual and memory search?

Could you find 1 of your 1,000 Facebook friends in a crowd of 100? Even at a rate of 25 ms per comparison, determining that no friends were in the crowd would take more than 40 min if memory and visual search interacted linearly. In the experiment reported here, observers memorized pictures of 1 to 100 targets and then searched for any of these targets in visual displays of 1 to 16 objects. Response times varied linearly with visual set size but logarithmically with memory set size. Data from memory set sizes of 1 through 16 accurately predicted response times for different observers holding 100 objects in memory. The results would be consistent with a binary coding of visual objects in memory and are relevant to applied searches in which experts look for any of many items of interest (e.g., a radiologist running through a mental checklist of what might be wrong in a car-crash victim or an airport screener looking for any of a list of prohibited items in a carry-on bag).     

Traveling economically through memory space: Characterizing output order in memory for serial order

How do people report the contents of short-term memory when information about order must be retained but items can be retrieved in any order? We report an experiment using an unconstrained reconstruction task in which people can report list items in any order but must place them in their correct serial positions. We found (1) a tendency to report recent items first in immediate but not in delayed reconstruction, (2) a tendency to recall temporally isolated items first, (3) a preference for forward recall order, and (4) a preference for output orders that minimize the length of the path that must be traversed through memory space during retrieval. The results constrain most current models of short-term memory in which retrieval is ballistic and is assumed to run to completion autonomously once initiated.