Subjects were advised that the purpose of the experiment was to establish how
well they could perceive briefly presented visual stimuli in different conditions. They
were tested individually in a I -hr session. The sequence of events in a trial was explained
to the subjects with the help of a flow diagam. Each subject was trained with two 20-trial
blocks of trials before the experiment. These practice blocks served to allow the subjects
to get used to the brief stimulus duration and to be familiar with their experimental task.
A trial began with a warning tone. A fixation point (i.e., the [+] symbol) was
presented in the center of the CRT 1/2 s after the end of the warning tone. The duration of
the fixation point was 500 ms. A seven-item array appeared for a predetermined period
1/2 s after the offset of the fixation point. The fourth item of the stimulus array occupied
the location previously occupied by the fixation point. The partial-report probe (i.e., the
[?] symbol) was presented for 1/2 s at a predetermined ISI after the offset of the stimulus
array. The probe was presented in the row above the seven-item array. Had the array and
the probe been presented simultaneously, the probe would be above one of the seven
letters in the array. When applicable, a masking array appeared at a predetermined delay
after the offset of the stimulus array. The masking array stayed on until the subject had
responded. The temporal relation between the partial-report probe and the masking array
will be described individually for each experiment.
The subjects were required to recall the letter indicated by the probe as well as the
letter's intra-array location. For example, if (?) appeared in the location above the fifth
one of the array, MBDHZLG, the correct responses were Z and 5. The subjects typed in
their responses. They were advised to take their time in order to be as accurate as
possible. (The fact that subjects might not be equally proficient in typing was not a
problem because reaction time was not measured. Moreover, it is reasonable to assume
that typing proficiency would not interact with the two independent variables of interest.)
There were 240 trials in the experiment proper. Data from the first 30 trials were
discarded because they were warm-up trials. The remaining 210 trials consisted of 21
trials of each of the 10 (2 masking levels X 5 ISI levels) treatment combinations. Among
the 21 trials devoted to a treatment combination, each of the seven locations was probed
three times. The 210 experimental trials were randomly selected, with replacement, from
the 10 treatment combinations for individual subjects.
From the subjects' viewpoint, the session was divided into four blocks of 50 trials
and one block of 40 trials (the last block). Within each block, the intertrial interval was 5
s. The duration of the break between blocks was controlled by the subjects.
A uniform level of Type I error, namely, 5%, will be adopted throughout this
report when the significance of statistical tests is reported.
Experiment 1