Simultaneous control of attention by multiple working memory representations

Abstract

Working memory representations play a key role in controlling attention by making it possible to shift attention to task-relevant objects. Visual working memory has a capacity of three to four objects, but recent studies suggest that only one representation can guide attention at a given moment. We directly tested this proposal by monitoring eye movements while observers performed a visual search task in which they attempted to limit attention to objects drawn in two colors. When the observers were motivated to attend to one color at a time, they searched many consecutive items of one color (long run lengths) and exhibited a delay prior to switching gaze from one color to the other (switch cost). In contrast, when they were motivated to attend to both colors simultaneously, observers' gaze switched back and forth between the two colors frequently (short run lengths), with no switch cost. Thus, multiple working memory representations can concurrently guide attention.

Figure 1

Example search arrays and trial events for Experiment 1. Landolt-C objects were 0.33° in diameter, had a line width of 0.10°, and a gap measuring 0.07°. Half were red (u’=0.479, v’=0.514, 8.12 cd/m²) and half were blue (u’=0.180, v’=0.158, 8.96 cd/m²), presented on a light grey background (42.31 cd/m²). Objects were assigned randomly to locations within a 5 × 5 grid (excluding the center location) and jittered within each cell by ±0.96° vertically and ±0.82° horizontally. Observers began each trial by directing gaze to a central fixation region (1.55°) for 300–500 ms, followed by the presentation of the cue square (0.65°) and search array until the observer’s response. There was a 1000-ms inter-trial delay.

Figure 2

A) Mean time to the manual response (dark grey) and mean time to target fixation (light grey). B) Stacked bars indicate mean number of uncued- (dark grey), cued-color (light grey), or total (dark grey + light grey) objects fixated per trial. Since neither color was cued in the 50% condition, the columns represent mean number of blue (right diagonal) or red (left diagonal) objects fixated per trial. Error bars represent within-subject confidence intervals (Morey, 2008).

Figure 3

Mean fixation duration for fixations immediately prior to a switch from fixating one color to fixating the other color. In the 80/20 condition (circles), this is restricted to a switch from a run of cued-color items to a run of uncued-color items. In the 50/50 condition (triangles), this represents a switch from a run of one color (e.g., red) to a run of the other color (e.g., blue). A run is defined by 2+ objects of the same type fixated consecutively. Error bars represent within-subject confidence intervals (Morey, 2008).

Figure 4

Example search arrays and trial events for Experiment 2. There were 32 objects total: 8 red (u’=0.414, v’=0.443, 18.75 cd/m²), 8 blue (u’=0.193, v’=0.259, 18.60 cd/m²), 8 yellow (u’=0.305, v’=0.535, 18.67 cd/m²), and 8 green (u’=0.141, v’=0.510, 18.60 cd/m²), presented on a grey background (39.65 cd/m²). Objects were assigned randomly to locations within a 20.85° × 15.82° region with a minimum inter-item distance of 2.10° and a minimum distance from center of 1.96°. The cue square subtended 0.79° and was composed of four smaller squares each subtending 0.36°. On single-cue trials, all four squares in the cue were the same color; on dual-cue trials, two diagonally opposed squares were presented for each of the two cued colors. Observers began each trial by directing gaze to a central fixation region (1.55°) for 300–500 ms, after which the cue appeared in the center of the screen for 500 ms, beginning 1000 ms prior to search array onset. There was a 1200-ms inter-trial delay.

Figure 5

A) Mean time to manual response (solid-fill bars) and to target fixation (diagonal-fill bars). B) Stacked bars indicate mean number of uncued-color (solid-fill bars), cued-color (diagonal-fill bars), or total (solid-fill + diagonal-fill) objects fixated per trial. Error bars represent within-subject confidence intervals (Morey, 2008).

Figure 6

A) Mean number of cued-color (solid-fill bars) and uncued-color (diagonal-fill bars) objects fixated per trial, collapsed across separable and nonseparable dual-cue trials. B) Mean fixation duration for fixations on cued-color items, collapsed across separable and nonseparable dual-cue trials. Error bars represent within-subject confidence intervals (Morey, 2008).

Figure 7

Shown here are sample scanpaths for a sequential trial (above) and a simultaneous trial (below) for which the observer was cued to search both red and green. The order of fixations is indicated by the number next to each fixation. The sequential trial shows a run of seven red items followed by a run of five green items. The simultaneous trial shows the same observer switching more frequently between the red and green items (run of two green items, run of four red items, one green item, then one red item).

Figure 8

Mean fixation duration for fixations immediately prior to a switch from searching one cued color to searching the other cued color. This analysis was restricted to trials on which the target color was not initially search, and it was limited to a switch from a run of non-target cued-color items to a run of target-color cued-color items. A run is defined by 2+ objects of the same type fixated consecutively.