Comparing Direct (Explicit) and Indirect (Implicit) Measures to Study Unconscious Memory
Philip M. Merikle and Eyal M. Reingold
Page 5
Source: http://psych.utoronto.ca/users/reingold/publications/Merikle_&_Reingold_1991/
Figure 1.
Mean sensitivity (A') of the recognition and contrast tasks at each trial block in Experiment 1. (Bars indicate standard errors.)
Inspection of Figure 1 indicates that both the recognition and the contrast tasks were sensitive measures of the old/new discrimination and that the sensitivity of both tasks decreased slightly across trial blocks. A 2 X 3 analysis of variance with task and trial block as factors revealed that the recognition task was superior to the contrast task, F (1,94) = 369.25, p < .001, but that the small decrease in performance across trial blocks was neither significant, F (2,188) = 1.56, nor did it interact with task, F (2,188) < 1.
To evaluate the sensitivity of the contrast task, performance in each trial block was compared with the chance level of performance. These comparisons revealed that the sensitivity of the contrast task significantly exceeded the chance level of performance both in Block 1 (.57), t (47) = 3.68, p < .001, and in Block 2 (.55), t (47) = 2.53, p < .025. However, the sensitivity of the contrast task in Block 3 (.53) only approached significance, t (47) = 1.85, p < .10.
The results of Experiment 1 demonstrate that the contrast task is a sensitive indirect measure of memory, at least during initial blocks of trials. Although the contrast measure was less sensitive than the direct recognition measure, the results indicate that judgements of perceptual contrast between a word and the background mask are influenced by memory for previously attended or cued words. Given these results, the contrast measure may also be a sensitive indirect measure of memory for the uncued words.
Experiments 2A and 2B
The major goal of Experiments 2A and 2B was to compare the relative sensitivity of the contrast and recognition tasks as measures of memory for the uncued words. According to the logic underlying comparisons of comparable direct and indirect measures, unconscious memory would be demonstrated if the indirect contrast measure is more sensitive than the direct recognition measure.
The two experiments were quite similar in that Experiment 2B was basically a replication of Experiment 2A with a) a different set of stimulus materials and b) a different density mask to degrade the visibility of the words in the contrast and recognition tasks. Given these methodological differences, successful replication across experiments would increase the generality of any findings demonstrating unconscious memory for the uncued words.
Method
Experiment 2A. The general methodology was the same as used in Experiment 1 except that memory for the uncued rather than the cued words was tested. In addition, as a consequence of this changed procedure, the subjects assigned to the direct task group were instructed explicitly that an "old" word was one of the uncued words presented during the study trials and that a "new" word was any word not presented during the study trials.
Experiment 2B. This experiment was very similar to Experiment 2A. However, two aspects of the methodology were different. First, the word pool contained 160 five-letter and 160 six-letter low frequency nouns with a Kucera and Francis (1967) frequency of one/million. Second, the density of the eight background masks used during the test trials was always 45%. Even though mask density was constant, each mask had a different appearance, given the different random arrangements of pixels, and each word had a different appearance against each mask, depending upon how the letters in the word overlapped the locations of the selected pixels in the mask. In all other respects, the general methodology was the same in Experiments 2A and 2B.3
Subjects. The 96 subjects in each experiment were undergraduate students at the University of Waterloo. Each subject was paid $5 for participation and had normal or corrected to normal vision. In both experiments, 48 subjects were assigned on the basis of an alternating sequence to the direct and indirect groups when they arrived at the laboratory to participate in the experiment.
Results and Discussion
The data in Experiments 2A and 2B were analyzed in the same manner as the data in Experiment 1. Briefly, the 96 test trials for each subject were divided into three blocks of 32 trials, and proportions of hits and false alarms in each trial block were computed. These mean proportions are shown in Table 1. In addition, A' values were also computed, and the mean values for each experiment are shown in Figure 2.
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