Comparing Direct (Explicit) and Indirect (Implicit) Measures to Study Unconscious Memory
Philip M. Merikle and Eyal M. Reingold
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Source: http://psych.utoronto.ca/users/reingold/publications/Merikle_&_Reingold_1991/
Stimulus Materials. A pool of words containing 384 five-letter and 384 six-letter nouns was selected from the Kucera and Francis (1967) norms. The frequency of the selected words ranged from 2 to 15/million.
For the study phase of the experiment, 60 pairs of words were selected randomly from the word pool. Thirty of these pairs consisted of two five-letter words, while the remaining 30 pairs consisted of two six-letter words. A different set of 60 pairs was selected for each subject. Each pair of words was presented with one word above and one word below a centrally located fixation dot. The vertical distance between the words was approximately 16 mm (0.9o).1 The words were presented in white, lower-case letters in EGA graphics mode against a black background, and the font was Borland Turbo C default font, double-sized. Each letter in each word when presented on the monitor was approximately 5 mm (0.3o) wide and 5 mm to 7 mm (0.4o) high, depending on the particular letter. The length of each word, considering the spacing between letters, was approximately 30 mm (1.6o) for the five letter words and 36 mm (2.0o) for the six letter words.
The order of presentation for the 60 pairs of words was randomized for each subject. The first six and the final six pairs presented to each subject were considered filler pairs and no words from these pairs were used in the test phase of the experiment. For the remaining 48 pairs, the cued word in each pair was presented equally often above and below the fixation dot and equal numbers of cued words contained five and six letters. In addition, the arrows used to cue subjects as to which words they should read aloud were approximately 7 mm (0.4o) long and located approximately 3 mm (0.2o) from each end of the cued words.
For the test phase of the experiment, 48 "new" words were selected randomly from the word pool. A different set of "new" words was selected for each subject and each set consisted of 24 five-letter and 24 six-letter words. The "new" words were combined with 48 "old" words from the study phase to form a single set of 96 words. For each subject, a different random order of presentation was determined for this set of words, within the constraint that in each block of 16 trials, there were equal numbers of "old," "new," five-letter, and six-letter words.
All words used in the test phase of the experiment were presented against a rectangular background mask. This mask was presented at the center of the monitor and measured approximately 45 mm (2.5o) horizontal by 10 mm (0.6o) vertical. The density of the mask was varied at two levels; either 50% or 60% of the pixels within the rectangular area were selected randomly and displayed whenever a word was presented. Prior to the beginning of the experiment, four different 50% density masks and four different 60% density masks were determined for each subject. Each of these eight masks was used with one "old" and one "new" word in each block of 16 trials. Within this constraint, the selection of a mask on each trial was random.
Results and Discussion
For each subject, the 96 test trials were divided into three blocks of 32 trials, and proportions of hits and false alarms in each of block of trials were computed. For the contrast task, a hit was defined as an old word judged to have high contrast against the background mask, and a false alarm was defined as a new word judged to have high contrast against the mask. The mean proportions of hits and false alarms for each task are shown in Table1.
To obtain a measure of memory for each task that was independent of response bias, the sensitivity of each task to the old/new dimension was expressed in terms of A'.2 This nonparametric measure of sensitivity can vary from 0.00 to 1.00, with A' = 0.50 indicating a complete absence of sensitivity (see Snodgrass & Corwin, 1988, for a detailed discussion of the A' measure). Figure 1 shows the mean A' values for each task across the three trial blocks.
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