Rugg, M.D., Mark, R.E., Walla, P., Schloerscheidt, A.M., Birch, C., and Allan, K. (1998). Dissociation of the neural correlates of implicit and explicit memory Nature, 392, 595-598. 

Cognitive neuroscientists use electrical recording and neuroimaging techniques to understand how the brain and the nervous system work during cognitive processing. One area that has benefited greatly from the cognitive neuroscience approach is the study of memory. Cognitive psychologists have divided memory into two broad classes of explicit and implicit memory based on different types of performance on a number of memory tasks. Explicit memory is specific, verbalizable memory, such as what you had for breakfast and what you have learned in your psychology course. Implicit memory is memory that can't be verbalized but that can affect performance on some task without conscious awareness. For example, sometimes you can recognize a multiple choice answer without ever realizing you learned the information in the first place. Eich, Macaulay, Loewenstein, and Dihle (1997) used a measure of implicit memory to study memory in people with dissociative identity disorder. 

Rugg, Mark, Walla, Schloerscheidt, Birch, and Allan (1998) used electrical recording of brain activity to investigate implicit and explicit memory. Neuroscientists have developed a recording technique called event-related brain potentials (ERPs) to measure brain activity. ERPs are measured by placing electrodes along different points on the scalp and recording electrical activity while the person is performing some task. ERPs provide information about the timing of mental processes but, by comparing activity from different electrodes, researcher can obtain a little information about the location of the activity. If implicit and explicit memory are truly different memories, then they might show different patterns of electrical activity in different areas of the brain. 
 

Participants were presented with a series of words in the study phase. For each word, they had to either determine whether the first and last letter in the word were in alphabetical order (e.g., the first and last letters in apple are in alphabetical order but they aren't in battle) or use the word in a sentence. The first task did not require much processing of the actual word and was considered a shallow processing task. The second task required more processing to be able to use it in a sentence and this was considered a deep processing task. 

Participants then completed a recognition task for the test phase. They were presented with some study words and some new words and had to respond, as quickly as possible, whether they had seen the word in the study phase. Electrical activity was recorded using ERPs while the participants completed the test phase. 

Three different patterns of electrical activity were obtained from the test phase. Rugg et al. found one pattern for explicit memory, that is for words that were correctly recognized, another for new words that were not presented during the study phase, and a third pattern for words that participants did not recognize from the study phase. This last pattern provides evidence for implicit memory; participants responded that they had not seen the words before but their brain activity was different for the words they truly had not seen before. 

Old words, regardless of whether they were correctly recognized or not, showed greater electrical activity in the parietal electrodes than new words from about 300 - 500 ms after the word was presented. This pattern appears to identify a neural correlate of implicit memory. Rugg et al. also found that words from the deep processing task showed greater activity over the parietal electrodes at 500 - 800 ms after the word was presented. The deeply processed words were almost always recognized. This finding of increased parietal activation for recognized words appears to identify a neural correlate for recollection. Finally, Rugg et al. found that explicitly recognized words had a greater pattern of positive electrical activity from the electrodes located over the frontal lobe between 300 - 500 ms after the word was presented. This pattern may reflect familiarity for the word that will be consciously recognized several hundred milliseconds later. 

Rugg et al. repeated their experiments using a different test. Following studying the words through either shallow (determining whether the first and last letters were in alphabetical order) or deep (using the word in a sentence) processing, the test phase consisted of determining whether the test word was animate or inanimate. The researchers found very similar ERP patterns, replicating their findings with a different implicit memory task. 

Based on Rugg et al.'s results, it appears that brain activity associated with memory is something like this: First, information is implicitly recognized in the parietal region of the brain. At the same time, information that is later explicitly recalled identified as familiar in the frontal region. Finally, the information is explicitly recalled in the parietal region of the brain. These different processes occur in different parts of the brain within less than a second! 

This study is important because, by combining neuroscience methods with cognitive tasks, it provides somewhat more direct evidence that implicit memory is a distinct form of memory, and has a different time course of activation in different brain regions than explicit memory. 

Links to the Lecture

• Depth of Processing. Chaffin and Herrmann (1983) describe a demonstration of depth of processing that can be adapted to demonstrate the tasks used by Rugg et al. Present a series of words one at a time for about 1 sec each. Instruct students to determine silently whether the first and last letters of the word are in alphabetical order. Then have students write down as many words as they remembered seeing. Score as a class. Repeat the task using a new set of words and instruct the students to think of the word in a sentence. Score and compare correct recall of the shallow versus deeply studied words. This demonstration works well with as few as 20 study words. 
Chaffin, R., & Herrmann, D. J. (1983). A classroom demonstration of depth of processing. Teaching of Psychology, 10, 105-107.
• Implicit Memory. Schoen (1988) describes a demonstration of implicit memory using a word stem completion task for the test phase. Schoen describes using a computer to drive the task but it can just as easily be conducted using overheads. 
Schoen, L. M. The word fragment completion effect: A computer-assisted classroom exercise. (1988). Teaching of Psychology, 15, 95-97.

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Links to Life

This handout from Grinnell College provides a good, brief summary of implicit and explicit memory 

Here is a fascinating news report of recent research demonstrating implicit memory for information presented while undergoing surgery! 

Don't rely on your implicit memory to get you through those psychology tests, improve your memory. Check out Mind Tools to learn some techniques for improving your memory. The Memory Page has links to many free and commercial sites for memory improvement.