Abstract: Introduction
Implicit memory and explicit memory are two distinct memory systems underlying different neural substrates. Encoding and retrieval are two important phases of memory. For explicit memory, the relation of encoding and retrieval has been investigated by many researches with divided attention paradigm. These researchers have found that the performance of a secondary task during encoding reduces the later memory performance, but dividing attention in the same way during retrieval has virtually no effect on memory performance, which confirms asymmetry between encoding and retrieval processes. But with respect to implicit memory, we know little about these issues. It is well known that implicit and explicit memories are dissociated not only at encoding but also at retrieval. Concerning the relations between encoding and retrieval, we proposed that there might be asymmetry between encoding and retrieval processes in implicit memory, but the asymmetry might be different in explicit memory.
Method
Fifty undergraduate students (30 in experiment 1 and 20 in experiment 2) participated in the study. Two behavioral experiments were conducted with Chinese characters as stimuli. These experiments adopted a study-to-test paradigm, in which participants performed a “shallow” (color) study task or a “deep” (pleasant) study task, followed by either a lexical decision (implicit) test (experiment 1) or a recognition (explicit) test (experiment 2). An interference task was used to ask participants to account the total number of “+” in a regulated orientation which appeared with word, which was performed concurrently with either the encoding or the retrieval phase of the memory task for encoding interference condition or retrieval interference condition. The data analysis, ANOVA using Greenhouse-Geisser corrections, was performed by SPSS software with the reaction time and accuracy which was recorded at two tests.
Results
The experiments showed that the effects of interference at encoding on the performance in implicit memory test were different from that at retrieval. The performance of a concurrent task during encoding had little effects on later lexical decision performance, but interference during retrieval disrupted priming, and this was the same case for shallow or deep encoding items. The effects of interference at encoding on the performance in explicit memory test were different from those at retrieval. The performance of a concurrent task during encoding reduced later recognition accuracy. But dividing attention in the same way during retrieval had no or little effects on recognition performance. For shallow encoding items, the different effect of encoding or retrieval interference on recognition was more evident.
Conclusions
The results not only confirmed the asymmetry between encoding and retrieval processes in explicit memory, but also showed the asymmetry between encoding and retrieval processes in implicit memory. These two forms of asymmetry were dissimilar. In other words, interference during encoding had effects on explicit memory, but left implicit memory intact. Interference during retrieval affected implicit memory, but had little effects on explicit memory. So the relation between encoding and retrieval was different between implicit and explicit memory, thus providing further evidence on the dissociation between implicit and explicit memory

Key words: implicit memory, explicit memory, encoding, retrieval, asymmetry