Adaptive Repulsion of Long-Term Memory Representations Is Triggered by Event Similarity

Abstract

We tested whether similarity between events triggers adaptive biases in how those events are remembered. We generated pairs of competing objects that were identical except in color and varied the degree of color similarity for the competing objects. Subjects (N = 123 across four experiments) repeatedly studied and were tested on associations between each of these objects and corresponding faces. As expected, high color similarity between competing objects created memory interference for object-face associations. Strikingly, high color similarity also resulted in a systematic bias in how the objects themselves were remembered: Competing objects with highly similar colors were remembered as being further apart (in color space) than they actually were. This repulsion of color memories increased with learning and served a clear adaptive purpose: Greater repulsion was associated with lower associative-memory interference. These findings reveal that similarity between events triggers adaptive-memory distortions that minimize interference.

Keywords: episodic memory; forgetting; long-term memory; open data; open materials.

Conflict of interest statement

Declaration of Conflicting Interests: The author(s) declared that there were no conflicts of interest with respect to the authorship or the publication of this article.

Figures

Fig. 1.

Experimental design. In each of four experiments, subjects learned object–face associations (a) that contained pairs of competing objects (object images that were identical except for their color values). The similarity (color distance) between competing objects was parametrically manipulated within and across experiments. In Experiments 1, 2, and 4, there were three similarity conditions: low similarity (72° apart), moderate similarity (48° apart), and high similarity (24° apart). In Experiment 3, the conditions were moderate similarity (48° apart), high similarity (24° apart), and ultra similarity (6° apart; not shown). Each experiment began with eight training rounds (b). Each training round contained a study phase, a color-memory test, and an associative-memory test. During study, subjects viewed each object–face pair. During color-memory tests, subjects were shown a face and a gray-scale version of the associated object. Using a continuous color wheel, subjects selected (recalled) the color of the object. During associative-memory tests (Experiments 1–3 only), an object image was presented, and subjects selected the associated face from a set of four options. The four face options always included the correct face (target) and the face that had been paired with the competing object (competitor). Procedures for Experiment 4 are described in Figure 5. For all experiments, subjects completed a posttraining test (c) that probed only color memory. The procedure was identical to that of the color-memory tests from the training rounds. The critical performance measure in the posttraining test was the percentage of color-memory responses that were biased away from the color of the competing object.

Fig. 2.

Results from Experiment 1 (top row) and Experiment 2 (bottom row). Mean color-memory error (a; absolute distance between reported and target color values from the color-memory tests) and accuracy on the associative-memory test (b; percentage of trials for which the target face was selected; chance = 25%) are shown as a function of training round and similarity condition. The percentage of responses away from the competitor on the posttraining test (c) is shown as a function of training day and similarity condition. Large dots represent means, and small dots represent data from individual subjects. Symbols represent a marginally significant difference (†p < .10) and significant differences (*p < .05, **p < .001) from 50%. Error bars represent standard errors of the mean.

Fig. 3.

Ultra-similarity condition and results from all conditions of Experiment 3. Experiment 3 dropped the high-similarity condition used in the previous experiments and added an ultra-similarity condition (a; 6° apart). An example of competing images from the ultra-similarity condition is shown. Mean color-memory error (b; absolute distance between reported and target color values from the color-memory tests) and accuracy on the associative-memory test (c; percentage of trials for which the target face was selected; chance = 25%) are shown as a function of training round and similarity condition. The percentage of responses away from the competitor on the posttraining test (d) is shown as a function of training day and similarity condition. Large dots represent means, and small dots represent data from individual subjects. The asterisk represents a significant difference (p < .05) from 50%. Error bars represent standard errors of the mean.

Fig. 4.

Analysis of data from Experiments 1 through 3. The scatterplot in (a) shows across-subject correlations between mean percentage of posttraining test (Day 1) color-memory responses that were away from the competitor and mean percentage of interference errors during the last three rounds of the associative-memory test (high-similarity condition only). The scatterplot in (b) shows across-subject correlations for the same variables, except that each measure was z-scored within experiment, allowing for a single correlation to be calculated for the pooled data. Interference errors were defined as selecting the face associated with the competitor object. Lines show best-fitting regressions, and shaded areas around the lines indicate 68% confidence intervals. The graphs in (c) show the distribution of correct and swap responses both toward and away from the competitor, separately for participants who showed a high number and a low number of interference errors during the last three rounds of the associative-memory tests. Asterisks indicate a significant difference (p < .001) between “correct” responses. Error bars represent standard errors of the mean.

Fig. 5.

Inference test and results from Experiment 4. All procedures of Experiment 4 were identical to those of Experiment 1 except for a subtle change to the associative-memory test during the training rounds. Instead of requiring subjects to discriminate between competing colors (as in Experiment 1), the associative-memory test consisted of an inference test that required subjects to generalize across competing objects. On each trial of the inference test, a probe face was presented, and subjects had to select which face, from a set of four options, was associated with the same object as the probe face (irrespective of color). Thus, what was previously the competitor face in Experiments 1, 2 and 3 was now the correct response. Mean color-memory error (b; absolute distance between reported and target color values from the color-memory tests) and accuracy on the inference test (c; percentage of trials for which the target face was selected; chance = 25%) are shown as a function of training round and similarity condition. The percentage of responses away from the competitor on the posttraining test (d) is shown as a function of training day and similarity condition. Large dots represent means, and small dots represent data from individual subjects. Asterisks represent significant differences (*p < .05, **p < .001) from 50%. Error bars represent standard errors of the mean.