Basic HTML Version
Participants responded over a voice-operated switch (VOX). Responses
were timed from stimulus presentation to voice onset using a micro-
phone mounted on a headband worn by the participants.
Each trial consisted of the presentation of one of four words (
RED
,
BLUE
,
GREEN
, and
YELLOW
) appearing on the screen in an incongruent
color (fromamong the corresponding colors red, blue, green, and yellow).
Utilizing stimuli based on the method of Sheehan et al. (1988), words
were presented in upper case letters 2 cm high with a letter thickness of
4mm. All stimuli were 11.75 cm in length and centered on the screen
with a viewing distance of 120 cm. The visual angle subtended by each
stimulus was then 0.9
8
vertically and 5.9
8
horizontally.
Each trial began with the instruction ‘‘Word’’ or a bar outline. The
instructions were 4–5 cm long, 1 cm high, and displayed in white in the
center of the screen for 500ms. A blank screen then appeared for a
further 400ms, followed by the stimulus, which was displayed until the
experimenter signaled the computer that the response was correct or
incorrect by pushing one of two buttons. The screen was blank for
1 second before the next trial.
Word-naming and color-naming trials occurred in series of one, two,
or three trials each before a change in response type occurred. The
sequence of variation in the size of the trial blocks was designed so as to
appear random to participants. A brief auditory tone was presented
with instructions when they represented a change from word-naming
to color-naming, or vice versa, in order to clearly signal the need to
change preparation strategies. Each set of stimuli consisted of 140 trials.
The first 30 trials were considered practice trials and discarded for
analysis. Of the remaining 110 trials, there were 32 color-naming no-
change (from instruction on the previous trial); 32 color-naming
change; 15 word-naming no-change; and 31 word-naming change
trials. Color was equally distributed over both types of color-naming
trials. Since a primary aim of the present study was to reproduce the
finding of Sheehan et al
.
(1988), the version of the Stroop task used in
that experiment was adopted here.
Procedure
Participants were told on arrival for the session that they would be
required to perform a short version of the Stroop task during both
hypnotized and nonhypnotized conditions. They were given a set of
written instructions to read describing the requirements of the task.
Any questions regarding the task were clarified at this point.
Participants were then shown the apparatus and tried on and
adjusted the headset and microphone to be worn during the task. Half
the participants performed the Stroop task first in the nonhypnotized
condition and then immediately repeated the task in the hypnotized
AN EMPIRICAL TEST OF DISSOCIATED CONTROL
237
Downloaded by [ ] at 05:16 26 March 2012