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/
Experiment 1
The primary purpose of Experiment 1 was to determine if the indirect contrast measure was a sensitive measure of memory. To evaluate this measure under optimal conditions, memory for the previously cued or attended words was assessed. During the test phase of the experiment, the cued words from the study phase were presented against a background mask which degraded the visual quality of the letters. For each word, the subjects were required to judge whether the contrast between the word and the mask was high or low. The subjects were told that high contrast meant that the word and the background were relatively distinct from each other, while low contrast meant that the word and the background tended to blend together.
The rationale underlying the contrast measure as a possible sensitive indirect measure of memory is based on recent findings indicating that memory influences subjective perceptual experience. Jacoby, Allan, Collins and Larwill (1988) reported that background white noise was perceived as less intense when the noise was judged in the context of previously heard sentences or words than when the noise was judged in the context of new materials that had not been presented previously to the subjects. Another way to describe these results is to state that the perceived contrast between sentences or words and the background noise was greater when the subjects had previously heard the sentences or words in the context of the experiment. If this change in subjective perceptual experience due to memory for the words or sentences reported by Jacoby et al. reflects a fairly general phenomenon, then it is reasonable to expect that memory for the cued words in the present experiment should influence subjects' subjective experience of the perceptual contrast between the words and the background mask.
Finally, possible changes in memory over time were evaluated. Erdelyi (1986) has discussed previously how different measures of memory may be differentially affected by either forgetting or hypermnesia (i.e., an improvement in memory over time). Consequently, the relative sensitivity of comparable direct and indirect measures may change over time following initial presentation. For this reason, sensitivity across trial blocks was assessed for both the contrast measure and a comparable measure of recognition memory.
Method
Subjects. The 96 subjects were undergraduate students at the University of Waterloo. Each subject had normal or corrected to normal vision and was paid $5 for participation. Forty-eight subjects were assigned on the basis of an alternating sequence to the direct and indirect tasks when they arrived at the laboratory to participate in the experiment.
Apparatus. All stimulus materials were displayed on a Zenith flat tension color monitor (Model ZCM-1490) that was driven by a Zenith microcomputer equipped with an 80286 processor. Subjects viewed the monitor while seated with their head position constrained by a chin rest adjusted to a comfortable height. The viewing distance was approximately 105 cm. During the study phase of the experiment, a microphone connected to a voice key was clipped to the chin rest. In addition, during both the study and the test phases of the experiment, a button box with three buttons was placed directly in front of the subjects. The center button was used to initiate each trial during both phases of the experiment, and the buttons on the left and right were used to indicate either "old/new" or "high/low contrast" decisions during the test phase of the experiment.
Procedure. Each subject was tested individually in a single session that lasted approximately 25 min. At the beginning of the session, subjects were informed that the experiment involved two different tests of word perception. The instructions implied that the two tests were unrelated, and no information concerning the test phase was given until after the study phase was completed.
Prior to the beginning of the study trials, all subjects were told that they would see two words on each trial and that their task was simply to read aloud the one word with arrows pointing towards it. The instructions emphasized that each cued word should be read into the microphone clipped to the chin rest and that both naming speed and naming accuracy were important. In addition, the voice key was demonstrated by showing subjects that it made a clearly audible click each time they spoke a word into the microphone.
The sequence of events on each of the 60 study trials began with the subjects viewing a fixation dot located in the center of the visual field. Subjects initiated a trial by pressing the center button on the button box, which led to the fixation dot being replaced by a 200-msec blank field. Following the blank field, a target display containing a pair of words was presented for approximately 500 msec. Each display also contained an arrow located at each end of one word. These arrows cued the subjects as to which word they should read aloud. Following each target display, the fixation dot reappeared after a brief untimed interval to indicate that the computer was ready for the next trial.
The test phase of the experiment began immediately after completion of the study trials. The microphone clipped to the chin rest was removed, and the subjects were told that they would now see a series of 96 words, with each word being presented against a mottled background. To illustrate the display conditions, a practice display was then presented. All subjects were instructed that their task on each trial was to read the word aloud and then to make a decision concerning the word. The subjects assigned to the direct task group were told to decide whether the word was "old" or "new." An "old" word was defined explicitly as one of the cued words presented during the study trials, and a "new" word was defined as any word not presented during the study trials. For the subjects assigned to the indirect task group, the required decision concerned whether the word was presented under "high" or "low" contrast conditions. High contrast conditions were defined as conditions in which the word appeared to "stand out" against the background, whereas low contrast conditions were defined as conditions in which the word appeared to "blend" into the background.
Before beginning the 96 test trials, all subjects in both groups were given eight practice trials. The sequence of events on each practice trial began with the presentation of the fixation dot. Once the start button was pressed, the fixation dot was replaced by a display containing a word presented against a background mask. Each word was presented until a decision was made. For the subjects assigned to the direct task group, the words "old" and "new" were presented four times each across the eight practice trials, and the subjects pressed the right button to indicate an "old" decision and the left button to indicate a "new' decision. For the subjects assigned to the indirect task group, the word "word" was presented on all eight practice trials, and the subjects in this group indicated their decisions on each trial by pressing the right button if the word had "low contrast" and the left button if the word had "high contrast" with the background. To ensure that the subjects were not confused as to which button to press during the test trials, the buttons on the right and left of the button box were labelled either "old" and "new" or "low contrast" and "high contrast," depending on the group to which a subject was assigned.
The sequence of events on each of the 96 test trials was the same as the event sequence on the practice trials. In addition, in each block of 16 trials, equal numbers of "old," "new," low contrast, high contrast, five-letter, and six-letter were presented. The subjects in both groups were told that the required decision was sometimes very difficult to make. It was emphasized that there were equal numbers of old and new words (direct task) or high and low contrast words (indirect task) and that for this reason, each of the two responses should be used approximately equally often. No feedback as to the correctness of responses was given during the test trials.
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