Minimal use of working memory in a scene comparison task

Eye movement behaviour in hand-eye tasks suggests a preference for a “just in time” processing strategy that minimizes the use of working memory. In the present study, a scene comparison task was introduced to determine whether the preference holds when the task is primarily visual and when more complex naturalistic scenes are used as stimuli. In two experiments, participants made same or different judgements in response to simultaneously presented pairs of scenes that were identical or differed by one object. The number of fixations per scene glance and the number of fixations intervening between glances to corresponding objects suggest that frequently one object at a time is encoded and maintained in visual working memory. The same pattern of results was observed in a third experiment using word and object arrays. Overall, the results suggest a strong general bias toward minimal use of visual working memory in complex visual tasks.     

Task switching based on externally presented versus internally generated information

A number of observations from the task switching literature suggest that task preparation based on internally generated knowledge is less efficient than task preparation based on externally presented information. In the present study, we investigated task switching based on internally generated versus externally presented information and additionally varied the reliability of foreknowledge. Source and reliability of foreknowledge were varied between groups of participants. With reliable foreknowledge, the relevant task always conformed to foreknowledge, even when the features of the imperative stimulus called for an alternative task. With unreliable foreknowledge, the relevant task was determined by the imperative stimulus and foreknowledge was sometimes misleading. Apart from measuring switch costs, we examined the effectiveness of establishing a task set by measuring interference exerted by conflicting stimuli (conditions with reliable foreknowledge) or misled expectancies (conditions with unreliable foreknowledge). In terms of switch costs, we observed a slight superiority of externally presented over internally generated information when foreknowledge was reliable, but this relationship strongly reversed with unreliable foreknowledge. This conclusion was corroborated by observations regarding the effectiveness of task-set establishment in terms of proneness to interference.

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Tachistoscopic exposure and masking of real three-dimensional scenes

Although there are many well-known forms of visual cues specifying absolute and relative distance, little is known about how visual space perception develops at small temporal scales. How much time does the visual system require to extract the information in the various absolute and relative distance cues? In this article, we describe a system that may be used to address this issue by presenting brief exposures of real, three-dimensional scenes, followed by a masking stimulus. The system is composed of an electronic shutter (a liquid crystal smart window) for exposing the stimulus scene, and a liquid crystal projector coupled with an electromechanical shutter for presenting the masking stimulus. This system can be used in both full- and reduced-cue viewing conditions, under monocular and binocular viewing, and at distances limited only by the testing space. We describe a configuration that may be used for studying the microgenesis of visual space perception in the context of visually directed walking.     

Feature bindings endure without attention: Evidence from an explicit recall task

Are integrated objects the unit of capacity of visual working memory, or is continued attention needed to maintain bindings between independently stored features? In a delayed recall task, participants reported the color and shape of a probed item from a memory array. During the delay, attention was manipulated with an exogenous cue. Recall was elevated at validly cued positions, indicating that the cue affected item memory. On invalid trials, participants most frequently recalled either both features (perfect object memory) or neither of the two features (no object memory); the frequency with which only one feature was recalled was significantly lower than predicted by feature independence as determined in a single-feature recall task. These data do not support the view that features are remembered independently when attention is withdrawn. Instead, integrated objects are stored in visual working memory without need for continued attention.     

Gaze direction and the extraction of egocentric distance

The angular declination of a target with respect to eye level is known to be an important cue to egocentric distance when objects are viewed or can be assumed to be resting on the ground. When targets are fixated, angular declination and the direction of the gaze with respect to eye level have the same objective value. However, any situation that limits the time available to shift gaze could leave to-be-localized objects outside the fovea, and, in these cases, the objective values would differ. Nevertheless, angular declination and gaze declination are often conflated, and the role for retinal eccentricity in egocentric distance judgments is unknown. We report two experiments demonstrating that gaze declination is sufficient to support judgments of distance, even when extraretinal signals are all that are provided by the stimulus and task environment. Additional experiments showed no accuracy costs for extrafoveally viewed targets and no systematic impact of foveal or peripheral biases, although a drop in precision was observed for the most retinally eccentric targets. The results demonstrate the remarkable utility of target direction, relative to eye level, for judging distance (signaled by angular declination and/or gaze declination) and are consonant with the idea that detection of the target is sufficient to capitalize on the angular declination of floor-level targets (regardless of the direction of gaze).     

The effects of age and set size on the fast extraction of egocentric distance

ABSTRACT

Angular direction is a source of information about the distance to floor-level objects that can be extracted from brief glimpses (near one's threshold for detection). Age and set size are two factors known to impact the viewing time needed to directionally localize an object, and these were posited to similarly govern the extraction of distance. The question here was whether viewing durations sufficient to support object detection (controlled for age and set size) would also be sufficient to support well-constrained judgments of distance. Regardless of viewing duration, distance judgments were more accurate (less biased towards underestimation) when multiple potential targets were presented, suggesting that the relative angular declinations between the objects are an additional source of useful information. Distance judgments were more precise with additional viewing time, but the benefit did not depend on set size and accuracy did not improve with longer viewing durations. The overall pattern suggests that distance can be efficiently derived from direction for floor-level objects. Controlling for age-related differences in the viewing time needed to support detection was sufficient to support distal localization but only when brief and longer glimpse trials were interspersed. Information extracted from longer glimpse trials presumably supported performance on subsequent trials when viewing time was more limited. This outcome suggests a particularly important role for prior visual experience in distance judgments for older observers.     

Ginkgo biloba and memory for a maze.

A number of sources suggest that the natural herb ginkgo biloba enhances mental sharpness by increasing blood flow to the brain. A preliminary study was designed to examine whether memory for a maze would be enhanced in 5 mice who received a dietary supplement containing ginkgo biloba. The mice showed an improved memory for the maze as evidenced by a decrease in the number of errors in reaching the goal box when they received gingko biloba as a dietary supplement.     

Pending intentions: Effects of prospective task encoding on the performance of another task

A number of recent studies casts doubt on the empirical separability of processes of task preparation from the actual execution of the respective task. We report two experiments in which we separated the precueing of a task from its execution by an intervening task. In addition, for the precued task we employed imperative stimuli that in most cases were associated with a competing task, making the execution of this task dependent on an internal representation of the information provided by the precue. Under these conditions, the performance of the intervening task suffered specific interference as a function of its relation to the precued task. This suggests that the encoding of the precue resulted in specific preparation for the precued task. Furthermore, the transition between the intervening task and the precued task was associated with relatively small costs that did not vary as a function of the interval that separated the two tasks. This also suggests that a great deal of preparation for the second task already took place during the encoding of the precue. Further observations suggest that the resolution of the interference between a memory-based task representation, as well as a perceptually induced task representation, and the representation of the currently relevant task took place on the level of individual task features.     

The role of top-down knowledge about environmental context in egocentric distance judgments

Judgments of egocentric distances in well-lit natural environments can differ substantially in indoor versus outdoor contexts. Visual cues (e.g., linear perspective, texture gradients) no doubt play a strong role in context-dependent judgments when cues are abundant. Here we investigated a possible top-down influence on distance judgments that might play a unique role under conditions of perceptual uncertainty: assumptions or knowledge that one is indoors or outdoors. We presented targets in a large outdoor field and in an indoor classroom. To control visual distance and depth cues between the environments, we restricted the field of view by using a 14-deg aperture. Evidence of context effects depended on the response mode: Blindfolded-walking responses were systematically shorter indoors than outdoors, whereas verbal and size gesture judgments showed no context effects. These results suggest that top-down knowledge about the environmental context does not strongly influence visually perceived egocentric distance. However, this knowledge can operate as an output-level bias, such that blindfolded-walking responses are shorter when observers’ top-down knowledge indicates that they are indoors and when the size of the room is uncertain.

     

The futility of explicit knowledge of a sequence of tasks

In an experiment that combined the task-shifting methodology with a serial reaction-time task, three groups of participants had to shift among a set of four tasks that were arranged in regular sequences of length eight. Two groups of participants were given explicit knowledge about the sequence of tasks. To ensure that the knowledge of the sequence was referred to in each trial, one group was asked to verbalise the forthcoming task (verbalisation group), whereas the other group had to detect trials that deviated from the learned sequence (detection group). Participants of a control group were given no information about the sequence of tasks. In addition, for all experimental groups in half of the blocks of trials external precues were presented which indicated the forthcoming task. The control group exhibited substantial task-shifting costs that were greatly reduced by the presentation of precues. Participants of the explicit-knowledge groups showed no indication that explicit knowledge resulted in advance preparation of the forthcoming task. In addition, the effect of the precues depended on the secondary task that had to be performed on the basis of explicit knowledge. Whereas for the detection group precues reduced shift costs to the same degree as for the control group, the verbalisation group showed no precueing benefits at all.