ELECTROPHYSIOLOGICAL MEASURES OF HUMAN MEMORY FUNCTIONS

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The major objective of this research is to advance our understanding of the brain mechanisms responsible for human memory. The primary memory functions under study are those responsible for episodic recollection, the type of memory used to perform standard tests of recall and recognition. Results from neuropsychological studies of organic memory disorders have been used to (a) identify brain structures that are crucial for this memory function and (b) show that episodic recollection is biologically distinguishable from other types of memory - an insight that provides a key foundation for the proposed experiments. These experiments make use of a new technique that allows electrophysiological correlates of episodic recollection to be derived from EEG recordings in neurologically normal subjects. With this technique it is possible to monitor recollection in the absence of any concurrent behavioral indication to that effect. Recordings are made while words are presented under explicit or implicit memory testing conditions. Some of the words are also presented earlier in the experiment, but in implicit memory tests subjects are not informed that memory is being tested. Specialized performance measures in these implicit memory tests are sensitive to the influence of prior word presentations, an outcome referred to as "priming." In addition, subjects in these circumstances regularly engage in "incidental recollection," which is defined as recollection not required by task instructions. The extent to which subjects engage in conscious recollection can be systematically controlled using various experimental procedures. Behavioral measures demonstrate that certain procedures influence recall and recognition, but do not influence priming, whereas other procedures yield the opposite pattern. These memory dissociations support the conception of recollection and priming as two different types of memory. Furthermore, preliminary results showed that recollection was associated with a distinct pattern of brain electrical activity recorded from the scalp. Priming was associated with another pattern of brain electrical activity that occurred at an earlier latency and was more focally distributed over occipital brain regions, consistent with the fact that priming occurred within the visual modality. Further experiments will focus on three specific aims: (1) to provide converging evidence with several experimental methods supporting the contention that conscious recollection is associated with a measurable electrophysiological marker, (2) to determine whether this phenomenon is a general one that can be found using either implicit or explicit memory tests, auditory or visual stimuli, linguistic or pictorial information, and so on; and (3) to compare and contrast electrophysiological measures of recollective processes with those associated with priming so as to learn more about the distinction between these two types of memory and the responsible neural mechanisms. These new on-line measurements of the electrical activity accompanying recollection and priming will thus be useful for monitoring the time-course of memory functions and developing a better understanding of their cognitive and neural organization.