High reward makes items easier to remember, but harder to bind to a new temporal context

Abstract

Learning through reward is central to adaptive behavior. Indeed, items are remembered better if they are experienced while participants expect a reward, and people can deliberately prioritize memory for high- over low-valued items. Do memory advantages for high-valued items only emerge after deliberate prioritization in encoding? Or, do reward-based memory enhancements also apply to unrewarded memory tests and to implicit memory? First, we tested for a high-value memory advantage in unrewarded implicit- and explicit-tests (Experiment 1). Participants first learned high or low-reward values of 36 words, followed by unrewarded lexical decision and free-recall tests. High-value words were judged faster in lexical decision, and more often recalled in free recall. These two memory advantages for high-value words were negatively correlated suggesting at least two mechanisms by which reward value can influence later item-memorability. The ease with which the values were originally acquired explained the negative correlation: people who learned values earlier showed reward effects in implicit memory whereas people who learned values later showed reward effects in explicit memory. We then asked whether a high-value advantage would persist if trained items were linked to a new context (Experiments 2a and 2b). Following the same value training as in Experiment 1, participants learned lists composed of previously trained words mixed with new words, each followed by free recall. Thus, participants had to retrieve words only from the most recent list, irrespective of their values. High- and low-value words were recalled equally, but low-value words were recalled earlier than high-value words and high-value words were more often intruded (proactive interference). Thus, the high-value advantage holds for implicit- and explicit-memory, but comes with a side effect: High-value items are more difficult to relearn in a new context. Similar to emotional arousal, reward value can both enhance and impair memory.

Keywords: context; explicit memory; free recall; implicit memory; lexical decision; reward; value.

Figure 1

Value-learning task results in Experiments 1 (A), 2a (B), and 2b (C). Performance is shown as the probability of choosing the high-value word over the low-value word in each of the learning blocks 1 to 13. Chance probability of choosing the high-value word is indicated by the dashed line. Error bars are 95% confidence intervals around the mean, corrected for inter-individual differences (Loftus and Masson, 1994).

Figure 2

Performance in the memory tasks in Experiment 1. (A) Response times from the lexical decision task. (B) Proportion of total words recalled from the free recall task. “High” and “Low” represent the high- and low-value words, respectively. “New” represents words first used in the lexical decision task, that were not present in the value-learning task. Error bars are 95% confidence intervals, corrected for inter-individual differences (Loftus and Masson, 1994).

Figure 3

Correlation between lexical decision and free recall tasks in Experiment 1 [ρ(93) = −0.20, p < 0.05]. The lexical decision measure was the facilitation of high-value words compared to low-value words (difference in response time) divided by the participants’ average response time. The free recall measure was the proportion of recalled words that were high value, divided by the total number of words recalled from the value-learning task. Each dot represents an individual participant.

Figure 4

Correct recall rates for Experiments 2a (A) and 2b (B). “High” and “Low” represent high- and low-value words, respectively, from the value-learning task. “New” represents words that were not present in the value-learning task, but only in study-test free recall. Error bars are 95% confidence intervals, corrected for inter-individual differences (Loftus and Masson, 1994).

Figure 5

Output positions in the free recall task of Experiments 2a and 2b. Probability of recall of each word type for vincentized output position bins in the free recall task of Experiments 2a (A) and 2b (B). “High” and “Low” represent high- and low-value words, respectively, from the value-learning task. “New” represents words first that were not present in the value-learning task. Solid lines and markers represent correct responses; dashed lines with hollow markers represent intrusion responses. Error bars were omitted for visual clarity. (C) Plots of the Z-transformed U-statistics comparing median output positions for high- versus low-value words in both Experiments 2a and 2b. Larger values represent later output positions. Error bars are 95% confidence intervals.

Figure 6

Intrusion rates during free recall in Experiments 2a (A) and 2b (B). “High” and “Low” represent high- and low-value words, respectively, from the value-learning task. “New” represents words first that were not present in the value-learning task. Error bars are 95% confidence intervals, corrected for inter-individual differences (Loftus and Masson, 1994).