Anatomical constraints on attention: hemifield independence is a signature of multifocal spatial selection

Abstract

Previous studies have shown independent attentional selection of targets in the left and right visual hemifields during attentional tracking (Alvarez & Cavanagh, 2005) but not during a visual search (Luck, Hillyard, Mangun, & Gazzaniga, 1989). Here we tested whether multifocal spatial attention is the critical process that operates independently in the two hemifields. It is explicitly required in tracking (attend to a subset of object locations, suppress the others) but not in the standard visual search task (where all items are potential targets). We used a modified visual search task in which observers searched for a target within a subset of display items, where the subset was selected based on location (Experiments 1 and 3A) or based on a salient feature difference (Experiments 2 and 3B). The results show hemifield independence in this subset visual search task with location-based selection but not with feature-based selection; this effect cannot be explained by general difficulty (Experiment 4). Combined, these findings suggest that hemifield independence is a signature of multifocal spatial attention and highlight the need for cognitive and neural theories of attention to account for anatomical constraints on selection mechanisms.

Figure 1

Example bilateral and unilateral search displays. Participants fixed the “+” at the center of the display and searched for the target letter T. The task was to indicate the orientation of the T as quickly and as accurately as possible. On bilateral trials, items appeared in either the upper or the lower visual field. On unilateral trials, items appeared in either the left or right visual field. The absolute location of the target in the visual field was balanced across these conditions (e.g., a target was equally likely to appear at the location just above and to the right of fixation in both the bilateral and unilateral displays).

Figure 2

Displays and results of Experiment 1. The top panel illustrates sample trials of the spatial subset search task and the standard search task. The bottom panel shows reaction time (ms) versus set size for correct responses on each task. Error bars show within-subject standard error of the mean. (a) In the spatial subset visual search task, reaction times are longer in the unilateral condition than in the bilateral condition, with a reliable difference emerging at set size 4. (b) In the standard visual search task, there was no difference between unilateral and bilateral displays, except at the largest set size.

Figure 3

Displays and results of Experiment 2. The top panel illustrates sample trials of the feature subset search task and the standard search task. The bottom panel shows reaction time (ms) versus set size for correct responses on each task. Error bars show within-subject standard error of the mean. (a) In the feature subset visual search task there was no consistent difference between unilateral and bilateral displays. (b) In the standard visual search task, there also was no consistent difference between unilateral and bilateral displays.

Figure 4

Displays and results of Experiment 3A. The top panel illustrates sample trials of the spatial subset search task and the standard search task. In the spatial subset search task, observers knew that the target would appear in the middle ring of the display. There were additional foil targets in the other rings to further encourage observers to focus on the middle ring. Placeholders also appeared on the midlines during the precue and test display in order to enhance the spatial-filtering demands. In the standard search, precues alerted the participant to the locations that would be task relevant, but no irrelevant items or placeholders were ever presented. The bottom panel shows reaction time (ms) versus set size for correct responses on each task. Error bars show within-subject standard error of the mean. (a) In the spatial subset visual search task, reaction times are longer in the unilateral condition than in the bilateral condition, with a reliable difference emerging at set size 4. (b) In the standard visual search task, there was no difference between unilateral and bilateral displays.

Figure 5

Displays and results of Experiment 3B. The top panel illustrates sample trials of the feature subset task and the standard search task. The displays were identical to Experiment 3A except that the task-relevant items were always white. The bottom panel shows reaction time (ms) versus set size for correct responses on each task. Error bars show within-subject standard error of the mean. (a) In the feature subset visual search task there was no reliable difference between unilateral and bilateral displays. (b) In the standard visual search task, there also was no reliable difference between unilateral and bilateral displays.

Figure 6

Displays and results of Experiment 4. The top panel illustrates sample trials of the easy and difficult search tasks. The easy search condition was the same as the standard search conditions of Experiments 3A and 3B. The difficult search displays were identical except that the line segments of the distractor Ls were offset to make them appear more similar to Ts. Both the easy search and difficult search conditions were standard search tasks in the sense that they did not have task-irrelevant items in the display. However, both tasks also had precues to alert observers to which locations would be relevant on a given trial. The bottom panel shows reaction time (ms) versus set size for correct responses on each task. Error bars show within-subject standard error of the mean. (a) In the easy search condition there was no overall effect of hemifield, although there was a significant interaction suggesting an increasing hemifield effect as set size increased (consistent with previous experiments). (b) In the difficult search task, although the numerical trend is towards faster reaction time in the unilateral condition, there was no main effect of hemifield (bilateral vs. unilateral) and no interaction between hemifield and set size.