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Word Imagery Effects on Explicit and Implicit Memory Nicholas Bube, Drew Finke, Darcy Lemon, and Meaghan Topper
Memory research has primarily focused on the distinction between implicit and explicit memory. The most pressing question now is whether or not these two types of long-term memory are maintained by different brain systems. Researchers in favor of this distinction have used ERP data to support their claims. Background • Paller (1990) suggested that a difference in subsequent memory effects between stem completion (an implicit task) and free & cued recall (explicit tasks) meant that explicit and implicit memory rely on different encoding processes. • Gardiner and associates (Gardiner, 1988; Gardiner and Java, 1990, 1991; Gardiner and Parkin, 1990) developed a method called the remember/know task (R/K), which they believed would show a difference between the two memory systems. Following positive recognition of a word, subjects indicated whether the word evoked an explicit memory (“R” or Remember) or if it was only familiar and there was no recollective experience (“K or Know/Familiar). • Smith (1993) used ERP data paired with the R/K task. In items identified correctly, it was found that explicit memory (R responses) evoked larger old/new effects than those of familiarity (K responses). It was believed that this difference supported the claim that explicit and implicit memory existed in separate brain systems
Reasons for Further Study Many of the previous experiments found that low frequency words are more likely than high frequency words to be recalled using the explicit memory system. This may occur because low frequency words are more distinct and are therefore easier to remember. In much the same manner, we propose that high imagery words will be easier to recall than low imagery words because they elicit a visual representation. High imagery words are, therefore, more likely to be recalled using the explicit memory system, while low imagery words use the implicit memory system. Previous experiments did not analyze data spatially within the brain, only temporally. This experiment makes use of both of these methods in order to provide for a better data analysis. • Rugg, Wells, and Doyle (1993) replicated Smith’s (1993) experiment, restricting analysis of the R/K task to those words that were judged old. They found that a recognition advantage for low frequency words was restricted to items that were given an R response. Given the findings of Smith (1993) and Rugg, Wells and Doyle (1993), it is possible that late old/new effects occur as a result of being aware that an item was previously experienced.
Methods & Stimuli • 1 subject (male) • 2 categories of stimuli: Low Imagery and High imagery words (for example, high imagery = apple; low imagery = fraternity) • 200 Low and 200 High imagery words were presented during an initial study period • Each word was presented for 1000 ms • Breaks were given between 50-word sets • During testing, 600 words were presented in random order: • 300 high imagery words (200 targets/100 distractors) and 300 low imagery words (200 targets/100 distractors) • Breaks were given between 50-word sets • TASK: Indicate whether a word is remembered, familiar, or new by pressing 1, 2, or 3 on a keypad, respectively • EEG data was recorded from 16 electrodes positioned at various areas of the brain
Spatial Analysis Examination of spatial data reveals that brain activity did not differ significantly when comparing high to low imagery words, and differed only slightly between the Remember and Know responses (Figure 1). Overall brain activity was observed to increase during the 300-500 ms time frame in the frontal and right temporal lobes (Figure 2). Temporal Analysis ERPs recorded for the Remember response in the 300-700 ms time frame are consistently more intense for high imagery words than low throughout all areas of the brain (Figure 3). ERPs recorded for the Remember and Know responses, although differing in intensity, exhibit waveforms of nearly identical shape in most regions of the brain (Figure 4). Results and Data Analysis Both spatial and temporal aspects of the ERP data obtained for this experiment were examined for evidence of separate brain systems at work in explicit and implicit memory processes.
Conclusion The data in this study provides neither strong evidence that implicit and explicit memory occupy different brain systems, nor proof that high imagery words are recalled using the explicit memory system and low imagery words using the implicit memory system. When examining spatial activity data, little variation is seen, and there is very little evidence to suggest the existence of separate implicit and explicit memory systems. Temporal ERP data showed that high imagery words elicited a stronger, more positive Remember response than low imagery words, suggesting that high imagery words may be more strongly remembered. One further related observation of the data was that the waveforms of the Remember and Know responses differed significantly in their potentials within the 200-600 ms time frame, but were nearly identical in the shape of the waveforms. The temporal data paired with the spatial data suggest that implicit and explicit memory occupy the same regions within the brain, but that these areas of the brain are more highly activated during explicit recollection. It is possible that implicit and explicit memory do not differ during recall, but differ greatly in encoding. Perhaps it is the way that memory is stored and not the manner by which it is retrieved that is the important difference between the two types of memory. Because ERPs were not recorded during encoding, however, a conclusion of this sort can not be drawn.
Figure 1. Spatial Activity Comparison of Remember and Know Responses Low Imagery Remember Response Low Imagery Know Response High Imagery Remember Response High Imagery Know Response Note: Numbers above images refer to time elapsed after presentation of stimulus (in milliseconds).
Figure 2. Intensity Comparison of Spatial Activity Between Remember and Know Responses • Increased neural activity was observed in nearly all areas of the brain when the stimuli triggered a Remember response, but most notably in the right frontal and temporal lobes. • ERPs observed following a Remember stimulus were of increased intensity compared to Know responses, as evidenced by the larger potentials recorded in the 300 ms to 500 ms time frame. Overall Activity: Remember Overall Activity: Know
Figure 3. Temporal ERP Data Demonstrating Increased Signal Intensity for High vs. Low Imagery Words T6 T4 F4 P4 Note: Plots show Potential (V) vs. Time (ms) data for recorded ERPs at various areas of the brain
Figure 4. Remember vs. Know Comparison of ERP Waveforms in Various Regions of the Brain • Notice that Remember and Know waveforms follow nearly identical patterns that differ significantly only in their measured potentials.
References • Gardiner, J.M., 1988. Functional aspects of recollective experience. Mem. Cognition 16:309-313. • Gardiner, J.M., and R.I. Java, 1990. Recollective experience in word and non-word recognition. Mem. Cognition 18:23-30. • Gardiner, J.M., and R.I. Java, 1991. Forgetting in recognition memory with and without recollective experience. Mem. Cognition 19:617-623. • Gardiner, J.M., and A.J. Parkin, 1990. Attention and recollective experience in recognition memory. Mem. Cognition 18:579-583. • Paller, K.A., 1990. Recall and stem-completion have different electrophysiological correlates and are modified differentially by directed forgetting. J.Exp.Psychol.[Learn. Mem.Cogn.] 16:1021-1032. • Rugg, M.D., T. Wells, and M.C. Doyle, 1993. Event-related potentials, word frequency, and recollection-based recognition. Unpublished. • Smith, M.E., 1993. Neurophysiological manifestations of recollective experience during recognition memory judgements. J. Cognitive Neurosci. 5:1-13