
The
Effects of Task Requirements . . . continued
Method
Subjects:
Thirty male and 20 female undergraduate
psychology students aged between 17 and 30 years (M = 19.49, SD
=
2.41 years) participated. Each subject was allocated at random to either
Group Number or Group Longer.
Apparatus and Procedure:
Subjects were told that a series of
two shapes would be presented as well as a tone. They were instructed to
perform two tasks concurrently^{8}:

The counting task. Group Number
were instructed to count the number of presentations of one shape (circle/ellipse)
and to ignore presentations of the other shape (ellipse/circle). Group
Longer were instructed to count the number of longerthanusual presentations
of one shape (circle/ellipse) and to ignore presentations of the other
shape (ellipse/circle). The shape to be counted is designated as taskrelevant,
whereas the shape to be ignored is designated as taskirrelevant irrespective
of its duration.

The RT task. Subjects were asked to press a button on
a microswitch a quickly as possible whenever a tone was presented, including if it was presented
during shape presentations or between shape presentations.
Subjects in both groups received the same
stimulus arrangements. Shape presentations were arranged into four blocks
of 24. Each block consisted of 12 presentations of a circle and 12 presentations
of an ellipse (see Figure 2). Seventyfive percent of the presentations
of each shape lasted for 5 s, whereas the remaining presentations lasted
for 7 s (i.e., longerthanusual). Shape presentations were separated by
intertrial intervals that varied among 9, 12, and 15 s with a mean of 12
s.
The RT probe was a tone presented for
500 ms at a frequency of 1000 Hz. Fortyeight of the RT probes were
presented during the intertrial intervals (12 in each block). The remaining
48 probes were presented during the shapes. Within each block, six probes
were presented during the circle and six during the ellipse. For each shape,
two of the probes were presented each at 50, 150, and 250 ms following
shape onset.
Dependent measures:
The dependent measures consisted of:

Number of shapes counted as per the instructions
for each group.

The presence of a button press during
shapes that did not contain a RT probe stimulus (referred to here as "false
alarms").

Secondary RT to probes presented during
the shapes at the three probe positions of 50, 150, and 250 ms.
