Christina J. Howard

Tracking the changing features of multiple objects : Progressively poorer perceptual precision and progressively greater perceptual lag

To measure the limits on attentive tracking of continuously changing features, in our task objects constantly changed smoothly and unpredictably in orientation, spatial period or position. Observers reported the last state of one of the objects. We observed a gradual decline in performance as the number of tracked objects increased, implicating a graded processing resource. Additionally, responses were more similar to previous states of the tracked object than its final state, especially in the case of spatial frequency. Indeed for spatial frequency, this perceptual lag reached 250ms when tracking four objects. The pattern of the perceptual lags, the graded effect of set size, and the double-report performance suggest the presence of both serial and parallel processing elements.

Position representations lag behind targets in multiple object tracking

In the multiple object tracking (MOT) task, observers can typically keep track of up to four moving objects. Little is known however about the extent to which object motion is used by observers during MOT. For example, direction and speed might be used to anticipate future positions. We here ask to what extent position reports lag behind targets or instead correspond to extrapolated positions. Using a range of different motion trajectory patterns, observers tracked 1-4 targets among distracters and reported the final position of one of the targets. On average, reports corresponded to previous positions rather than the final position. This lag varied across conditions from around 10 to 70ms of the objects trajectory. Although some have suggested that extrapolation occurs during MOT, we find no evidence of anticipation of future positions of targets. The significant increase in lag with speed of the object is consistent with slow or intermittent updating of object positions during tracking.

Visual search in the real world: evidence for the formation of distractor representations.

Visual search in the real world often requires that we search the same environment a number of times for different targets. What is the fate of information about fixated distractor objects during these searches? Here, participants searched the same array of real objects on a tabletop twice for two different targets successively whilst wearing a head-mounted eye-tracker. We found that fixating an object when it was a distractor in the first search facilitated search for that same object when it became the target in the second search. The results suggest that the location and identity of fixated distractor objects are represented to a level that guides subsequent searches, even when this information is not required at the time of fixation.

Distractors slow information accumulation in simple feature search

M. Carrasco and B. McElree (2001) presented a speed-accuracy trade-off experiment, investigating covert attention in visual search. One of the conclusions from Carrasco and McElree was that adding distractors to a single feature search does not decrease the speed with which information is accumulated about target identity. We present a reanalysis of the relevant data from Carrasco and McElree in which we demonstrate that their conclusion was incomplete and we demonstrate a processing speed advantage for single feature search displays with no distractors compared with displays with distractors. This finding is confirmed in a new speed-accuracy trade-off experiment presented here. Further, we demonstrate that increasing the display duration increases the processing speed of displays with distractors but not for displays without distractors. We discuss these results in relation to theories of visual attention and the debate between graded and fixed architecture accounts for attentional allocation.

Task relevance predicts gaze in videos of real moving scenes

Low-level stimulus salience and task relevance together determine the human fixation priority assigned to scene locations (Fecteau and Munoz in Trends Cogn Sci 10(8):382–390, 2006). However, surprisingly little is known about the contribution of task relevance to eye movements during real-world visual search where stimuli are in constant motion and where the ‘target’ for the visual search is abstract and semantic in nature. Here, we investigate this issue when participants continuously search an array of four closed-circuit television (CCTV) screens for suspicious events. We recorded eye movements whilst participants watched real CCTV footage and moved a joystick to continuously indicate perceived suspiciousness. We find that when multiple areas of a display compete for attention, gaze is allocated according to relative levels of reported suspiciousness. Furthermore, this measure of task relevance accounted for twice the amount of variance in gaze likelihood as the amount of low-level visual changes over time in the video stimuli.

Acutely induced anxiety increases negative interpretations of events in a closed-circuit television monitoring task

In two experiments we measured the effects of 7.5% CO2 inhalation on the interpretation of video footage recorded on closed circuit television (CCTV). As predicted, inhalation of 7.5% CO2 was associated with increases in physiological and subjective correlates of anxiety compared with inhalation of medical air (placebo). Importantly, when in the 7.5% CO2 condition, participants reported the increased presence of suspicious activity compared with placebo (Experiment 1), a finding that was replicated and extended (Experiment 2) with no concomitant increase in the reporting of the presence of positive activity. These findings support previous work on interpretative bias in anxiety but are novel in terms of how the anxiety was elicited, the nature of the interpretative bias, and the ecological validity of the task.

Feature-based attentional interference revealed in perceptual errors and lags.

According to a limited-resource account of feature-based attention, dividing feature-based attention by selecting targets on the basis of different features dilutes its power. Multiple-feature costs have been documented previously, but it is not clear whether the multiple-feature cost arose at the selection (segregating targets from non-targets) stage predicted by the limited-resource account. The cost might instead result from a post-selection difficulty in processing or accessing the contents of the targets. By defining the targets with a selection attribute (color) that is very distinct from the attribute participants must access and report (spatial period), we were able to manipulate the selection process independently from the access stage. We still found a cost for different selection features (colors), suggesting that multiple-feature costs can arise at the selection stage. The cost was only significant however when distracters were present that shared the selection features. The cost manifested not only as greater errors or less precision in reporting the access attribute (spatial period), but also as an increased temporal lag between the physical stimuli and the reported percept. In summary, splitting selection among different features incurred little or no penalty by itself, but selection interference by distracters sharing target features could be large and could slow processing.

Eye-response lags during a continuous monitoring task.

We measured the temporal relationship between eye movements and manual responses while experts and novices watched a videotaped football match. Observers used a joystick to continuously indicate the likelihood of an imminent goal. We measured correlations between manual responses and between-subjects variability in eye position. To identify the lag magnitude, we repeated these correlations over a range of possible delays between these two measures and searched for the most negative correlation coefficient. We found lags in the order of 2 sec and an effect of expertise on lag magnitude, suggesting that expertise has its effect by directing eye movements to task-relevant areas of a scene more quickly, facilitating a longer processing duration before behavioral decisions are made. This is a powerful new method for examining the eye movement behavior of multiple observers across complex moving images.

Suspiciousness perception in dynamic scenes: a comparison of CCTV operators and novices

Perception of scenes has typically been investigated by using static or simplified visual displays. How attention is used to perceive and evaluate dynamic, realistic scenes is more poorly understood, in part due to the problem of comparing eye fixations to moving stimuli across observers. When the task and stimulus is common across observers, consistent fixation location can indicate that that region has high goal-based relevance. Here we investigated these issues when an observer has a specific, and naturalistic, task: closed-circuit television (CCTV) monitoring. We concurrently recorded eye movements and ratings of perceived suspiciousness as different observers watched the same set of clips from real CCTV footage. Trained CCTV operators showed greater consistency in fixation location and greater consistency in suspiciousness judgements than untrained observers. Training appears to increase between-operators consistency by learning “knowing what to look for” in these scenes. We used a novel “Dynamic Area of Focus (DAF)” analysis to show that in CCTV monitoring there is a temporal relationship between eye movements and subsequent manual responses, as we have previously found for a sports video watching task. For trained CCTV operators and for untrained observers, manual responses were most highly related to between-observer eye position spread when a temporal lag was introduced between the fixation and response data. Several hundred milliseconds after between-observer eye positions became most similar, observers tended to push the joystick to indicate perceived suspiciousness. Conversely, several hundred milliseconds after between-observer eye positions became dissimilar, observers tended to rate suspiciousness as low. These data provide further support for this DAF method as an important tool for examining goal-directed fixation behavior when the stimulus is a real moving image.

Going the distance: spatial scale of athletic experience affects the accuracy of path integration

Evidence suggests that athletically trained individuals are more accurate than untrained individuals in updating their spatial position through idiothetic cues. We assessed whether training at different spatial scales affects the accuracy of path integration. Groups of rugby players (large-scale training) and martial artists (small-scale training) participated in a triangle-completion task: they were led (blindfolded) along two sides of a right-angled triangle and were required to complete the hypotenuse by returning to the origin. The groups did not differ in their assessment of the distance to the origin, but rugby players were more accurate than martial artists in assessing the correct angle to turn (heading), and landed significantly closer to the origin. These data support evidence that distance and heading components can be dissociated. Furthermore, they suggest that the spatial scale at which an individual is trained may affect the accuracy of one component of path integration but not the other.