Are there separate explicit and implicit memory systems? Dawn Beeker, Aaron Nuetzman, Kirk Stage Q301, Cognitive Science, Indiana University Presented on November 21, 2000

Background The separate system model is based on findings that: It has been proposed that memory has two separate systems, explicit and implicit. Explicit memory refers to the conscious recollection of facts and events. Implicit memory refers to cases when previous experiences affects behavior without conscious awareness of the connection between the two. (1) The separate system model is based on findings that: I. Research done on animals with specific lesions displays deficits of either explicit or implicit memory (1) II. Memory tests targeting specific elements of explicit or implicit memory shows disassociations between the two memory systems. (3) III. Amnesic patients show different impairments and abilities based on their particular injuries that support the existence of explicit and implicit memory (1,2) Here we test the the claims that two separate memory systems exist and that this separation is recordable by EEG technology.

Do two different memory systems exist? We can test for the existence of two separate memory systems by presenting a subject with two different tasks that will require the usage of the hypothesized explicit and implicit systems. To test this idea, we presented the subject with a task that required the use of both implicit and explicit systems, we: Conducted a word recognition task, where the subject had to categorize previously studied words as ‘remember’ or ‘know’. If categorized as ‘remember”, the subject remembers the context of studying the words and therefore is utilizing the explicit memory system. If categorized as ‘know’, the subject only feels familiar with the word, does not remember the context of previous studying, and is therefore utilizing the implicit memory system. If indeed two different memory systems exist, we should see spatial or temporal differences in ERP data.

Methods and Stimuli 1 Subject (Male, age 23) 2 categories of stimuli: Condition 1: Low Imagability Condition 2: High Imagability Huzza Banana Word lists were obtained from a Psycholinguistic database. Words were selected according to an imagability index. The highest and lowest 300 words were selected. During the study period, the subject was presented with the 200 target words of each condition combined together in a random order with a one second delay. During this study period, no recording took place. During the test period, the subject was presented with the 400 target words from each condition as well as 100 additional distractor (new) words from each condition in random order with a one second delay. The subject was required to indicate whether each word was a distractor (new) word or if it was an original target (old) word presented during study. If the word was judged to be an old word, the subject was required to indicate if they knew for sure (remember) or if the word seems familiar (know). Brain activity was recording to the test session, and then was averaged according to whether the word was labeled as ‘remember’, ‘know’, or ‘new’.

Results Low Imagability, Remember Response Condition one shows more overall activity across the test period with the greatest concentration in the occipital lobe across. High Imagability, Remember Response Condition two exhibits a notable difference in activity levels with much less intensity than condition one until later in the test period.

Results Low Imagability, Know Response The “know” response in condition one and condition two show similar activity patterns across the test period. Activity is also seen in the left medial temporal lobe. High Imagability, Know Response The level of activity is slightly higher in condition one than in condition two.

Discussion Previous research suggests that explicit memory utilizes the medial temporal lobe (4, 5). We should therefore expect to see some such activity in the ‘remember’ responses; however, the spatial data does not suggest any overtly strong medial temporal activity. The most discernable difference occurs with words of high imagability as compared with low imagability words. A lack of activity occurs up to 600 milliseconds in high imagability words. High imagability words do show greater negative waveform responses than do those of low imagability in the remember response. The know responses do show some activity in the temporal lobe of the left hemisphere around 200 milliseconds.

Conclusion Our data suggests that there are differences in high and low imagability word recognition. It does not clarify the existence of implicit or explicit memory systems. It would be beneficial to reproduce the unusual results of high imagability words in the ‘remember’ category. This could point to how or why of such results. Repetition of the experiment could also help to clarify the greater negative waveform responses for high imagability words than low imagability words in the remember category. High imagability words were expected to result in explicit memory activity. The greater negative response in the remember category is suggestive of system differences; however, lacking a similar response for remember ‘know’ ERP data, it is not conclusive whether there differences point to different memory systems or differences inherent in the choice of high or low imagability words.

References 1. Gazzange, Michael S., Richard B. Ivry, George R. Mangun, Cognitive Neuroscience, W.W. Norton & Company. New York 1998 p. 256, 257, 262-266, 271, 273-278. 2. Squire, Larry R. and Barbara J. Knowlton Memory Hippocampus and Brain Systems, Q301 Packet, Busey 2000 pg. 161-162. 3. Tuluins, Endel Organization of Memory: Quo Vadis? Q301 Packet Busey 2000 pg. 175-176. 4. Purves, Dale and others. Neuroscience. Sinauer Associates, inc. Massachusetts. 1997. 554-555. 5. Squire, Larry R., and Barbara J. Knowlton. Memory, Hippocampus, and Brain Systems. Q301 packet. 2000. 168-170. 6. Rugg, Michael D. Event-Related Potential Studies of Human Memory. Q301 Packet. 2000 189 – 193.