Neutral auditory words immediately followed by painful electric shock may show reduced next-day recollection

Abstract

In this study, we investigated the effect of experimentally delivered acute pain on memory. Twenty-five participants participated in experimental sessions on consecutive days. The first session involved a categorization task to encourage memory encoding. There were two conditions, presented in randomized order, in which participants listened to a series of words, which were repeated three times. In one condition, one-third of the word items were immediately followed by a painful electrical shock. This word-shock pairing was consistent across repetition and the pain-paired items were presented unpredictably. In the other condition, all word items were not associated with pain. Response times over these repeated presentations were assessed for differences. Explicit memory was tested the following day, employing a Remember-Know assessment of word recognition, with no shocks employed. We found evidence that recollection may be reduced for pain-paired words, as the proportion of correct Remember responses (out of total correct responses) was significantly lower. There were no significant reductions in memory for non-pain items that followed painful stimulation after a period of several seconds. Consistent with the experience of pain consuming working memory resources, we theorize that painful shocks interrupt memory encoding for the immediately preceding experimental items, due to a shift in attention away from the word item.

Keywords: Auditory; Electric nerve stimulation; Memory; Pain; Recognition.

Figure 1.

Graphical depiction of experimental design in nested layers of detail, labelled on the left. Time is represented along the x-axis. Increasing level of experimental detail is shown going from bottom to top, with dashed lines indicating expanded detail of items within a larger hierarchical part of the experiment. The two main portions of the experiment, Encoding and Testing, are shown as blocks at the bottom, these always occurred consecutively. The Encoding portion has two conditions: Pain and No Pain, and the order of these was randomized. Each Encoding condition consisted of three repetitions of a word list, examples of which are shown expanded above the Pain condition. Both Pain and No Pain conditions contained 3 occurrences of the same 90-word list. As shown in the Item level of detail, words were delivered auditorily, over 0.75 s. In the Pain Condition (only) a painful electric shock immediately followed one third of the words (30 out of 90). No shocks occurred in the No Pain condition or in the Testing portion of the experiment. Abbreviations: RT= reaction time, RKN= Remember, Know, New, Alt= alternating.

Figure 2.

Graphical depiction of temporal bidirectionality of the effect painful shocks may have on memory. Paired items refer to the Pain word that the shock immediately follows. Following items, labelled n+1 and n+2, may be pain-paired or non-pain words that may have their own immediately-associated shock events. Abbreviation: s= seconds

Figure 3.

Response times over the three encoding repetitions for each word-type. Error bars represent standard error.

Figure 4.

Response times for Remember (R), Know (K), and New (N) responses during the RKN testing portion, including only correct responses. Error bars represent standard error. Significant differences reported in Results section.

Figure 5.

Recognition memory performance across word-type. Panel A compares average dʹ values for overall recognition (combined Remember and Know responses) and specifically for recollection and familiarity (Remember and Know responses separately). Panel B similarly shows the Response Criterion for the same results. Panel C compares a composite measure of familiarity (see text for details). Error bars display standard error. No differences were statistically significant.

Figure 6.

Proportion of Remember hits out of total hits for each word type. Error bars display standard error. Significant differences are indicated with an asterisk (*).

Figure 7.

Panel A represents hit rates for Remember and Know combined, separated by percent of the time that a word immediately followed a pain word (33%, 67%, and 100% separately). Panel B shows hit rates for Remember only responses, separated by percent of the time that a word immediately followed a pain word (33%, 67%, and 100% separately).