Neurochemical regulation of auditory information ... - Helda - Helsinki.fi

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Neurochemical regulation of auditory information ... - Helda - Helsinki.fi

paradigm was set before the passive paradigm. Each paradigm consisted of two blocks

with 1- min interval.

P300 recording

In the P300 recording paradigm, pure tones were binaurally presented via headphones

at an intensity of 60dB. The standard tone had a frequency of 1000 Hz and duration of

50 ms. Deviant tones had the same duration but they were of 2000 Hz. The probability

of tones was 80% for standards and 20% for deviants. During the recording, at least 30

deviant stimuli were presented. Subjects were asked to silently count all deviant stimuli

and then to report the number. To ensure counting accuracy subjects were asked to count

the stimuli out loud at the beginning of the trial. In this way, we were able to make sure

that subjects understood the procedure. Afterwards, subjects were allowed to count the

stimuli silently; however, with the interval of 2-10 stimuli the subjects were periodically

checked by being asked to report the calculated number. Only when we were completely

sure of the subjects’ required accuracy did we start the actual recording. Meanwhile, the

subjects were aware that they were participating in the actual recording. Consequently,

the subjects were ready to report the number at any time and stayed alert throughout the

entire procedure, anticipating our possible checks on them. Subjects were motivated by

the fact that we explained that a precise calculation of the final number was of utmost

importance to the interpretation of the results of the entire procedure. Most of the time

subjects performed very accurately. There were no significant differences between

schizophrenia and healthy control groups in the reported number of calculated deviants.

The ISI was 1500 ms. The analysis period was 1000 ms.

MMN recording

During the MMN recording, subjects were reading a self-selected text. They were

instructed not to attend to tones (for more details, see Kraus et al. 1992; Naatanen et al.

1978). Pure tones were binaurally presented via headphones at an intensity of 60dB. The

standard tone was 1000 Hz and 75 ms in duration. Deviant tones had a frequency of 1000

Hz and they were of 25 ms in duration. The probability for standards was 90% and for

deviants 10%. During the recording, at least 60 deviant tones were presented with an ISI

of 1 s. The analysis period was 500 ms.

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