asked to sit on a comfortable chair placed in front of a computer screen and
they had a cap containing 64 electrodes fitted to their head. Their scalp was
further degreased using small amounts of alcohol, and small amounts of
electrolyte gel were deposited at each electrode site to establish a conductive
bridge between scalp and electrodes.
In each experimental trial, participants were presented with a picture
stimulus (e.g. a red book) on a computer screen either to the left or the right
of a fixation cross for 200 ms. After a 500 ms interval participants were pre-
sented with a sentence (constructed as described above) in chunks of isolated
words and small phrases (The/red/book/was/on the right). Each of the
chunks was presented for 200 ms. Nouns and adjectives were followed by
intervals of 800 ms, while each of the remaining chunks was followed by an
interval of 500 ms. At the end of sequence, that is after the final 500 ms
interval, a question mark was presented at the centre of the screen for
2000 ms during which participant response was expected. Inter-trial inter-
vals lasted 2000 ms.
Participants were asked to respond only when either the colour (adjective)
or the object (noun) mentioned in the sentence matched the picture’s char-
acteristics, and to refrain from responding if neither the colour nor the object
matched the picture. For example, following the picture of a red book, par-
ticipants were required to respond after seeing the sequences a, b, c, d, e, f and
i listed under (4), but not sequences g, h and j. In trials that required a
response, participants were instructed to respond by pressing keyboard keys
set under their left and right index fingers depending on whether the posi-
tion mentioned in the sequence (left/right) matched the position of the pic-
ture on the screen or not. For example, upon viewing the picture of a red
book on the left side of the screen followed by the sequence The red book was
on the left (or any of the following: The book red/red car/car red/blue book/book
blue/book was on the left), participants had to press the designated yes key.
Response side was counterbalanced between participants.
There were 15 practice trials involving feedback, with a 70% accuracy cri-
terion. If 70% response accuracy was not achieved at the end of the 15 trials,
practice was repeated until threshold accuracy was achieved. During practice,
participants were told that some of the sentences would be ‘a bit strange’, but
they ‘needn’t worry about that’. The experiment was divided in six blocks of
80 trials each. Trial order was randomised between participants.
ERP recording and data analysis
Electrophysiological data were recorded in reference to Cz at a rate of
1 kHz from 64 Ag/AgC1 electrodes placed according to the 10–20 convention.
Impedances were kept below 7 kΩ. EEG activity was filtered on-line band pass
between 0.1 Hz and 200 Hz, and off-line low pass at 30 Hz using a zero
phase shift digital filter. Ocular artefacts were monitored using vertical
Juggling Two Grammars 223