Modulation of task demands suggests that semantic processing interferes with the formation of episodic associations

Abstract

Although episodic and semantic memory share overlapping neural mechanisms, it remains unclear how our pre-existing semantic associations modulate the formation of new, episodic associations. When freely recalling recently studied words, people rely on both episodic and semantic associations, shown through temporal and semantic clustering of responses. We asked whether orienting participants toward semantic associations interferes with or facilitates the formation of episodic associations. We compared electroencephalographic (EEG) activity recorded during the encoding of subsequently recalled words that were either temporally or semantically clustered. Participants studied words with or without a concurrent semantic orienting task. We identified a neural signature of successful episodic association formation whereby high-frequency EEG activity (HFA, 44-100 Hz) overlying left prefrontal regions increased for subsequently temporally clustered words, but only for those words studied without a concurrent semantic orienting task. To confirm that this disruption in the formation of episodic associations was driven by increased semantic processing, we measured the neural correlates of subsequent semantic clustering. We found that HFA increased for subsequently semantically clustered words only for lists with a concurrent semantic orienting task. This dissociation suggests that increased semantic processing of studied items interferes with the neural processes that support the formation of novel episodic associations. (PsycINFO Database Record

Figure 1. Methods and behavioral results

(A) Methods. During the encoding period, participants viewed words presented for 3000 ms and separated by a variable interstimulus interval. Following the last item on the list, participants recall the study items in any order. A subset of study items were semantic associates, e.g. cat and dog in this figure. Semantic relatedness was determined using Word Association Space (WAS) values (see Methods). Encoding items were divided into four conditions based on how they were recalled: temporally clustered (C_t, black) or recalled preceding or following a study neighbor, e.g. tree and lime; semantically clustered (C_s, dark grey) or recalled preceding or following a semantic associate; not clustered (NC, light grey) or recalled preceding and following non-neighboring and non-semantically related items; or not recalled (NR, white). (B) Behavioral results. Participants show a tendency to both temporally and semantically cluster their recalls. The lag contiguity analysis (first panel) shows that participants are more likely to make transitions between study neighbors, those items separated by a lag of +/− 1, than between non-neighboring study items. Likewise, the semantic contiguity analysis (second panel) shows that participants are more likely to make transitions between semantically associated items, where increased association corresponds to increased WAS values. These effects are consistent for both no-task (blue) and task (orange) lists. The third panel shows the quantification of these contiguity effects. Temporal clustering scores are reliably greater on no-task compared to task lists, whereas semantic clustering scores are reliably greater on task compared to no-task lists. Errorbars are standard error of the mean. (C) Regions of interest. We analyzed three a priori defined ROIs, left Anterior Superior (AS), left Anterior Inferior (AI) and left Posterior Inferior (PI).

Figure 2. Subsequent clustering effects

The top panel shows the subsequent temporal clustering effect (SCE_t) and the bottom panel shows the subsequent semantic clustering effect (SCE_s) for no-task (blue) and task (orange) lists. Each line shows the difference in Z-Power between subsequently clustered and subsequently recalled, but not clustered, items, for 6 frequency bands (θ_L, 3–4 Hz; θ_H, 6–8 Hz; α, 10–14 Hz; β, 16–26 Hz; γ_L, 28–42 Hz; γ_H, 44–100 Hz). Z-Power is averaged across the encoding interval (0 – 3000 ms). Errorbars are standard error of the mean.

Figure 3. HFA as a function of clustering and list type

Subsequent temporal and semantic clustering effects (SCE_t and SCE_s) separately for no-task (blue) and task (orange) lists. Each bar shows the difference in high frequency activity between subsequently clustered (temporal or semantic) and subsequently recalled, but not clustered words. There is a reliable interaction in AS and AI such that the SCE_t is specific to no-task lists and the SCE_s is specific to task lists. Error bars are standard errors of the mean.

Figure 4. Subsequent memory effect for semantically isolated items

Semantically isolated items are those items which do not have a strong semantic study associate (WAS > .4). The figure shows HFA for semantically isolated items that are subsequently recalled (recalled, black) or subsequently not recalled (not recalled, white), separately for no-task and task lists. There is a reliable interaction in AS such that HFA is increased for not recalled relative to recalled items specifically for task lists. Error bars are standard errors of the mean.