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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 concurrently8:

  1. 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 longer-than-usual presentations of one shape (circle/ellipse) and to ignore presentations of the other shape (ellipse/circle). The shape to be counted is designated as task-relevant, whereas the shape to be ignored is designated as task-irrelevant irrespective of its duration. 
  2. 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). Seventy-five percent of the presentations of each shape lasted for 5 s, whereas the remaining presentations lasted for 7 s (i.e., longer-than-usual). Shape presentations were separated by intertrial intervals that varied among 9, 12, and 15 s with a mean of 12 s.

Figure 2. The circle and ellipse shapes used as the visual orienting stimuli in the counting task

The RT probe was a tone presented for 500 ms at a frequency of 1000 Hz.  Forty-eight 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:

  1. Number of shapes counted as per the instructions for each group.
  2. The presence of a button press during shapes that did not contain a RT probe stimulus (referred to here as "false alarms").
  3. Secondary RT to probes presented during the shapes at the three probe positions of 50, 150, and 250 ms.
Correspondence to: D.Neumann@ccrn.uwa.edu.au
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