Working memory is not fixed-capacity: More active storage capacity for real-world objects than for simple stimuli

Abstract

Visual working memory is the cognitive system that holds visual information active to make it resistant to interference from new perceptual input. Information about simple stimuli-colors and orientations-is encoded into working memory rapidly: In under 100 ms, working memory ‟fills up," revealing a stark capacity limit. However, for real-world objects, the same behavioral limits do not hold: With increasing encoding time, people store more real-world objects and do so with more detail. This boost in performance for real-world objects is generally assumed to reflect the use of a separate episodic long-term memory system, rather than working memory. Here we show that this behavioral increase in capacity with real-world objects is not solely due to the use of separate episodic long-term memory systems. In particular, we show that this increase is a result of active storage in working memory, as shown by directly measuring neural activity during the delay period of a working memory task using EEG. These data challenge fixed-capacity working memory models and demonstrate that working memory and its capacity limitations are dependent upon our existing knowledge.

Keywords: contralateral delay activity; visual long-term memory; visual memory; visual short-term memory; working memory capacity.

Fig. 1.

Effects of encoding time on colors vs. realistic objects. (A) Participants saw either colors or objects and saw them for either 200, 1,000, or 2,000 ms each. While they performed each trial, they also performed a verbal interference task (rehearsing two digits). (B) The number of colors, objects, and objects with detail remembered as a function of encoding time. Note that the zero encoding time point is theoretical, because people cannot remember anything if they did not see any stimuli. The number of colors remembered (Left) plateaus with less than 200-ms encoding time; the number of objects (Middle) remembered and number of detailed objects remembered (Right) both continue to increase with more encoding time.

Fig. 2.

Active storage of detailed object representations. (A) Participants were cued to remember the objects on either the left or right side of the screen. The objects were presented for 200 ms in the short encoding time condition and 1,000 ms in the long encoding time condition. After a brief delay, a forced-choice comparison assessed detailed object memory. (B) Contralateral-minus-ipsilateral waveforms for the short (Top) and long (Bottom) encoding time conditions. The CDA is measured from 300 ms after offset until the cue appears (gray shaded rectangle, labeled CDA). (C) Mean CDA amplitude for short and long encoding times. Error bars represent within-subject SEMs.

Fig. 3.

More active storage for objects than for colors. (A) ERP waveforms for showing the contralateral-minus-ipsilateral waveforms, separately for when participants are asked to remember five colors (in blue) or five objects (in red). (B) Mean CDA amplitude for the five colors and five objects conditions. Error bars represent within-subject SEMs.

Fig. S1.

EEG data for experiment 2, for short encoding time (Top) and long encoding time (Bottom). After the initial perceptual response, the waveforms over the contralateral hemisphere to the objects to be remembered are more negative than the waveforms over the ipsilateral hemisphere to the objects to be remembered, and this activity maintains throughout the delay period. The CDA is measured as the contralateral-minus-ipsilateral difference from 300 ms after offset until the cue appears (gray shaded rectangle). Notice that in the long encoding condition we observe an ERP to the stimulus offset, as is typical for long stimulus presentations, but this is present equally at both ipsilateral and contralateral sites.

Fig. S2.

Data from the set size 3 condition of experiment 3. (A) Contralateral-minus-ipsilateral waveforms for the color (blue) and object (pink) conditions. The CDA is measured from 300 ms after offset until the cue appears (gray shaded rectangle, labeled CDA). (B) Average CDA in each condition.

Fig. S3.

Ipsilateral and contralateral ERP waveforms for experiment 3, for color memory condition (A) and object memory condition (B). The CDA was measured as the contralateral-minus-ipsilateral difference from 1,300 ms until 1,700 ms after the onset of the memory display (gray shaded rectangle).

Fig. S4.

Data from all conditions of experiment 3, showing average CDA in each condition.

Fig. S5.

(Left) Alignment of the three-object condition in experiment 2. (Right) Alignment of the three-object condition in experiment 3.

Fig. S6.

HEOG traces from all experiments, where 0 time is the onset of the items to be remembered, and −1,000 ms is the onset of the cue to which side to attend. the memory display. A shows the HEOG traces for the main analysis reported in the paper, where eye movements were rejected −200 ms prior to the onset of the memory display until the test display. B shows the HEOG traces of an additional analysis in which artifacts were rejected over the entire period of −1,000 ms (onset of the arrow cue) until the test display. Negative numbers (plotted up) reflect drift of the eyes toward the left side; positive numbers reflect drift toward the right side. In all conditions, participants kept their eyes fixated and we observed no significant drifts toward the attended side throughout the trial or before the onset of the stimuli.

Fig. S7.

Scalp distributions of the CDA component while remembering objects (Left) and colors (Right) in experiment 3. Topographical voltage maps show the contralateral-minus-ipsilateral amplitude differences projected on the right side of the scalp.