A common discrete resource for visual working memory and visual search

Abstract

Visual search, a dominant paradigm within attention research, requires observers to rapidly identify targets hidden among distractors. Major models of search presume that working memory (WM) provides the on-line work space for evaluating potential targets. According to this hypothesis, individuals with higher WM capacity should search more efficiently, because they should be able to apprehend a larger number of search elements at a time. Nevertheless, no compelling evidence of such a correlation has emerged, and this null result challenges a growing consensus that there is strong overlap between the neural processes that limit internal storage and those that limit external selection. Here, we provide multiple demonstrations of robust correlations between WM capacity and search efficiency, and we document a key boundary condition for observing this link. Finally, examination of a neural measure of visual selection capacity (the N2pc) demonstrates that visual search and WM storage are constrained by a common discrete resource.

Keywords: ERP; N2pc; evoked potentials; visual attention; visual memory; visual search; working memory.

Fig. 1

Illustration of the tasks and search displays in Experiment 1. In the working memory task (a), participants were instructed to maintain fixation and to remember the orientation of all six objects presented in the display. After a short delay, six probe rings were presented, and participants clicked on the thicker ring to report the position of the gap in the sample item that had appeared in that position. In the visual search task (b), participants maintained fixation and were instructed to identify the direction (right or left) of the target letter L. Set sizes ranged from 1 to 8. Participants used the left or right arrow key to indicate whether the vertical segment of the L was on the left or right, respectively. Target-distractor similarity and distractor variability were manipulated (c). In Experiment 1a and one condition of Experiment 1c, both target-distractor similarity and distractor variability were high (Example 1). In Experiment 1b, target-distractor similarity was lower than in Experiment 1a (the relative position of the vertical segment was always different in targets than in distractors), and distractor variability was low (Example 2). Finally, in Experiment 1c, the vertical segment of the L was shifted inward, so that target-distractor similarity was even higher than in Experiment 1a, but grouping was still encouraged by low distractor variability (Example 3). Distances are not to scale. ITI = intertrial interval.

Fig. 2

Results from Experiment 1. The graphs on the left show mean reaction time (RT) as a function of set size in (a) Experiment 1a, (c) Experiment 1b, and (e) Experiment 1c. Also shown is the slope (m) of each function. Error bars represent 95% confidence intervals. The graphs on the right show individual participants’ search slopes (with best-fitting regression lines) as a function of working memory (WM) capacity in (b) Experiment 1a, (d) Experiment 1b, and (f) Experiment 1c. Illustrations of the stimulus configurations for the experiments are displayed to the right of the scatter plots; in (f), stimuli for the uniform- and variable-distractor conditions are illustrated in gray and black, respectively.

Fig. 3

Results from Experiment 2: (a) reaction time (RT) as a function of set size, (b) individual participants’ search slopes (with best-fitting regression line) as a function of working memory (WM) capacity, (c) grand-averaged difference waves (contralateral amplitude – ipsilateral amplitude) from the OL and OR electrodes at each set size (“SS”), (d) N2pc amplitude as a function of set size (with a fitted bilinear function), (e) individual participants’ search slopes (with best-fitting regression line) as a function of the inflection point of the N2pc-by-set-size function, and (f) WM capacity (with best-fitting regression line) as a function of the inflection point of the N2pc-by-set-size function. In (a) and (d), error bars represent 95% confidence intervals. In (c), the gray shading indicates the temporal window used to measure N2pc amplitudes.