Title of the article

2.2.Tasks and experimental procedures

For the discrimination task, which was the first task imposed on the subjects, we followed Ladd and Morton (1997) in using the AX discrimination paradigm. Stimuli were presented in pairs that were either the same or one step apart on the continuum. In the latter case, the second can be higher or lower than the first (hereafter AB and BA, respectively). The eight AB stimulus types ran from pair {1,2} to {8,9}; the eight corresponding BA types from {2,1} to {9,8}. This yielded 9 identical pairs and 2 x 8 = 16 different pairs, which occurred in random order, yielding a set of 25 trials in all, which was presented to each listener four times in different random orders, preceded by five practice trials. Stimuli within pairs were separated by a 500-ms silence, the pause between pairs was 3000 ms. A short warning tone was sounded after every tenth trial.

For the identification task listeners responded to individual stimuli from the 9-step continuum by classifying each either in terms of a binary or a ternary choice:

1. 
   ‘Command’ ~ ‘no command’. In one task the listeners were instructed to decide for each stimulus whether they interpreted it as a command or not.
   
2. 
   ‘Question’ ~ ‘no question’. An alternative task involved the decision whether the stimulus sounded like a question or not.
   
3. 
   ‘Command’ ~ ‘condition’ ~ ‘question’. The third task was identical to the task imposed in van Heuven & Kirsner (2002).
   

Half of the listeners first performed task (1), the other half of the listeners began with task (2). Task (3) was always the last identification procedure in the array of tests. For each task, the set of nine stimuli were presented five times to each listener, in different random orders, and preceded by five practice items, yielding sets of 50 identification stimuli per task.

Twenty native Dutch listeners, ten males and ten females, took part in the experiment on a voluntary basis. Participants were university students or members of their families. None of them reported any perceptual deficiencies.

The experiments were run with small groups of subjects, who listened to the stimuli at a comfortable loudness level over Quad ESL-63 electrostatic loudspeakers, while seated in a sound-treated lecture room. Subjects marked their responses on printed answer sheets provided to them, always taking the discrimination task first and the identification tasks last.