Abstract:

Studies have shown that choline is a substance that is closely related to memory. Previous studies focused on the effect of cholinergic drugs on explicit memory, and those results revealed that explicit memory is sensitive to most cholinergic drugs. However, relatively few studies have discussed the effect of cholinergic drugs on implicit memory. Furthermore, whether the effect of cholinergic drugs on implicit memory is consistent with explicit memory is still uncertain.
The effect of cholinergic drugs on memory was investigated by drawing a comparison between the participants with nicotine condition and those without. We used lexical decision and lexical recognition tasks to test implicit and explicit memory, respectively. In experiment 1, 30 subjects participated in two occasions, 2 days apart. They participated once in memory tasks after receiving 12 mg/ml body weight of nicotine and once after receiving 0 mg/ml placebo. Experiment 2 examined whether receiving treatment before encoding or before the retrieval phase would moderate the cholinergic effect in explicit and implicit memory. In experiment 2, 19 subjects participated in two experimental occasions, 2 days apart, as follows: after receiving 12 mg/ml body weight of nicotine before the encoding phase; after receiving nicotine before the retrieval phase. In addition, we adopted event-related potential (ERP) technology to observe the affected ERPs. Participants were instructed to response to corresponding items by pressing keyboard. The Reaction Time and Accuracy data on retrieval phase of the two memory tasks were recorded and analyzed.
Implicit and explicit memory performance declined under nicotine condition in both experiments. It reflected that receiving nicotine not only impacted explicit memory but also implicit memory. Furthermore, nicotine effects are moderated by the level of processing at the encoding phase. Such impact only occurred on the deep processing level. Moreover, memory retrieval after receiving nicotine was affected. These effects were more remarkable on implicit memory retrieval than on explicit memory. The results of ERP data also showed that related ERPs of memory were affected by nicotine.
In conclusion, results from the current study revealed that effects of cholinergic drugs were similar on implicit and explicit memory. The rest of the segregated results might have been due to the discrepancy of memory tasks rather than the differences in physiological mechanisms of the two memory types. Implicit memory and explicit memory might not belong to two extremely independent memory systems, because there are some covariant effects existing between them.

Key words: nicotine, implicit memory, explicit memory, processing level