David E. Fencsik

Virtually Perfect Time Sharing in Dual-Task Performance: Uncorking the Central Cognitive Bottleneck

A fundamental issue for psychological science concerns the extent to which people can simultaneously perform two perceptual-motor tasks. Some theorists have hypothesized that such dual-task performance is severely and persistently constrained by a central cognitive “bottleneck,” whereas others have hypothesized that skilled procedural decision making and response selection for two or more tasks can proceed at the same time under adaptive executive control. The three experiments reported here support this latter hypothesis. Their results show that after relatively modest amounts of practice, at least some participants achieve virtually perfect time sharing in the dual-task performance of basic choice reaction tasks. The results also show that observed interference between tasks can be modulated by instructions about differential task priorities and personal preferences for daring (concurrent) or cautious (successive) scheduling of tasks. Given this outcome, future research should investigate exactly when and how such sophisticated skills in dual-task performance are acquired.

Tracking unique objects

Is content addressable in the representation that subserves performance in multiple-object-tracking (MOT) experiments? We devised an MOT variant that featured unique, nameable objects (cartoon animals) as stimuli. There were two possible response modes: standard, in which observers were asked to report the locations of all target items, and specific, in which observers had to report the location of a particular object (e.g., “Where is the zebra?”). A measure of capacity derived from accuracy allowed for comparisons of the results between conditions. We found that capacity in the specific condition (1.4 to 2.6 items across several experiments) was always reliably lower than capacity in the standard condition (2.3 to 3.4 items). Observers could locate specific objects, indicating a content-addressable representation. However, capacity differences between conditions, as well as differing responses to the experimental manipulations, suggest that there may be two separate systems involved in tracking, one carrying only positional information, and one carrying identity information as well.

Fixational Eye Movements Are Not an Index of Covert Attention

The debate about the nature of fixational eye movements has revived recently with the claim that microsaccades reflect the direction of attentional shifts. A number of studies have shown an association between the direction of attentional cues and the direction of microsaccades. We sought to determine whether microsaccades in attentional tasks are causally related to behavior. Is reaction time (RT) faster when microsaccades point toward the target than when they point in the opposite direction? We used a dual-Purkinje-image eyetracker to measure gaze position while 3 observers (2 of the authors, 1 naive observer) performed an attentional cuing task under three different response conditions: saccadic localization, manual localization, and manual detection. Critical trials were those on which microsaccades moved away from the cue. On these trials, RTs were slower when microsaccades were oriented toward the target than when they were oriented away from the target. We obtained similar results for direction of drift. Cues, not fixational eye movements, predicted behavior.

The role of location and motion information in the tracking and recovery of moving objects

Observers in a multiple object tracking task can track about four to five independently moving targets among several moving distractors, even if all of the stimuli disappear for a 300-msec gap. How observers reacquire targets following such a gap reveals what kind of information they can maintain for targets. Previous research has suggested that participants maintain minimal information about a set of moving objects—namely, just their present spatial locations. We report five new experiments that demonstrate retention of location information for at least four objects, and extrapolated motion information for around two objects.

How do we track invisible objects

We previously demonstrated that observers in multiple object tracking experiments can successfully track targets when all the objects simultaneously vanish for periods lasting several hundred milliseconds (Alvarez, Horowitz, Arsenio, Dimase, & Wolfe, 2005). How do observers do this? Since observers can track objects that move behind occluders (e.g., Scholl & Pylyshyn, 1999), they may treat a temporal gap as a case of complete occlusion. If so, performance should improve if occlusion cues (deletion and accretion) are provided and items disappear and reappear one by one (asynchronously), rather than simultaneously. However, we found better performance with simultaneous than with asynchronous disappearance (Experiment 1), whereas occlusion cues were detrimental (Experiment 2). We propose that observers tolerate a gap in tracking by storing the current task state when objects vanish and resuming tracking on the basis of that memory when the objects reappear (a task-switching account).

Microsaccades and Attention: Does a Weak Correlation Make an Index?: Reply to Laubrock, Engbert, Rolfs, and Kliegl (2007)

In the recent literature on microsaccades and attention, two questions have been conflated. There is a broad question of whether microsaccades are related to attention, and there is a more specific question about whether microsaccades serve as an index of attention. We are happy to agree that microsaccades are related to attention. However, the claim that ‘‘microsaccades are an index of covert attention’’ (in the title of Laubrock, Engbert, Rolfs, & Kliegl, 2007, this issue) depends on a strong correlation. Were this claim true, a researcher might be able to conduct a study relying entirely upon microsaccade direction as a measure of attentional deployment. Although we would be delighted to be able to conduct such an experiment, both our data and those of Laubrock et al. suggest that microsaccades cannot be used as a reliable marker of covert attention. Laubrock et al. make two arguments. First, reviewing our experiment (Horowitz, Fine, Fencsik, Yurgenson, & Wolfe, 2007, this issue), they criticize our selection of trials on which the directions of the cue and microsaccade disagreed, arguing that this selection would inevitably lead to a negative microsaccade-congruency (MC) effect. Second, they report new data demonstrating a correlation betweenMC and reaction time (RT). We deal with these points in reverse order. Laubrock et al. have demonstrated a statistically reliable relationship between MC and RT. An incongruent microsaccade was associated with a 6-ms slowing of RT. This is a weak effect, an order of magnitude smaller than the 81-ms effect associated with an invalid cue. This effect may be statistically significant, but it suggests that microsaccade direction provides very little useful information about the spatial distribution of attention. Also, Laubrock et al. argue, under a seemingly reasonable set of assumptions, that in our study, trials on which the microsaccade direction diverged from the cue direction were dominated by trials on which the microsaccade did not follow attention, even if microsaccades usually did follow attention. The argument is as follows. Assume that observers direct attention toward the cue with probability w, and that the microsaccade reflects the direction of attention with probability x. Let v denote cue validity. There are two kinds of trials on which the cue is valid but the cue direction and microsaccade direction disagree: (a) valid trials (v) on which attention does not follow the cue (1 w) and the microsaccade follows attention (x) and (b) valid trials (v) on which attention follows the cue (w) but themicrosaccade does not reflect attention (1 x). The proportion of trials of the first type is given by p15 v(1 w)x, and the proportion of trials of the second type is given by p25 vw(1 x). Laubrock et al. note that if w5 v and x5 .75 (i.e., the microsaccade is almost as good an index of attention as the cue), the predictions would be qualitatively consistent with our results. However, this scenario is not quantitatively consistent with our results. Although w and x are not directly observable, one can observe the proportion of trials on which the cue direction and microsaccade direction disagree, p5 p1 1 p2 5 v(w1 x 2wx). Because v is known (arrow cues were 80% valid, so v 5 .80), any observed p is compatible with a line through wx space. Figure 1 plots the wx curves that could produce the observed ps in the manual-detection condition of our experiment for all 3 observers (data from the other two conditions lead to similar conclusions). The diamond represents the hypothetical point on which Laubrock et al. base their argument (w 5 .80, x 5 .75); this point is clearly not consistent with the data. In fact, if we assume that observers frequently shifted attention in the direction of the cue (i.e., w .60), then the probability that the microsaccade followed attention must have been less than .55 (note that if x 5 .50, then the direction of the microsaccade is independent of attention). If observers were at least probability matching (i.e., w .80), then x would have been less than .52. Thus, the predictive power of microsaccades is, for practical purposes, negligible. Address correspondence to Todd S. Horowitz, Visual Attention Laboratory, Brigham and Women’s Hospital, 64 Sidney St., Suite 170, Cambridge, MA 02139, e-mail: toddh@search.bwh.harvard.edu. PSYCHOLOGICAL SCIENCE

Target reacquisition strategies in multiple object tracking

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Reducing visual stress symptoms of VDT users with prescription eyeglasses

The use of Video Display Terminals (VDT’S) in modem information processing environments is associated with a variety of visual stress symptoms including eye strain, headaches, blurry vision and necldshoukler pain. This study evaluates a method of prescribing computer specillc glasses to improve vision and reduce symptoms. Thirty-seven computer users who suffer from VDT related visual symptoms wore two pairs of prescription eyeglasses for two weeks each in a double blind crossover experiment. The experimental glasses were prescribed using a Gaussian image testing device and the control glasses were prescribed using a standard paper Snellen chart. Data was collected on the frequency and severity of symptoms in their regular work environment as well as in a controlled laboratory setting. Results indicate that the experimental eyeglasses were preferred overall (p<.05). Visual stress symptoms were significantly reduced in frequency and severity for the eyeglasses prescribed using the Gaussian image testing device.

Faster is not necessarily better in visual search

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