G. Sartori et al. /Neuroscience Letters 390 (2005) 139-144
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and this value was similar in all No responses (range 0.069-
0.076).
Instructions were presented to the subject on a computer
screen. In a trial a blank screen was displayed for 300 ms.
Next appeared the first word of the concept description. Every
word of a trial was presented sequentially for 300 ms with
200 ms of separation between one word and the following
one. The target word was displayed after a random interval
ranging between 0 and 1000 ms after the final word of the
sentence. The total 640 trials were presented in a single ses-
sion in two blocks, which lasted about 50 min with YES and
NO responses randomly intermixed.
Scalp voltages were collected using a 64-channel Elec-
troCap. The electrocap consists of 59 sintered Ag/AgCL
electrodes. A frontal electrode (AFZ) was connected to the
ground, and the vertex electrode was used as reference. Elec-
trode impedance has been kept under 10kΩ for all recordings.
Ocular movements have been monitored trough four elec-
trodes fixed close to the eyes: two for vertical movements
and two for the horizontal movements. Scalp voltage were
continuously recorded, digitised by a computer at a sam-
pling rate of 1000 Hz, and stored on the hard disk for off-line
analysis. Electrical signals were amplified with Synamps
amplifier (high pass = 0.10 Hz, 24-dB/octave attenuation; low
pass = 1000 Hz, 24-dB/octave attenuation; 50 Hz notch fil-
ter). The signal was recorded in all the scalp’s areas (frontal,
temporal, parietal, and occipital) and filtered using a low pass
filter for 30 Hz, 24-dB/octave attenuation. The continuous
EEG was segmented in epochs starting 100 ms before target
onset and lasting until 1500 ms after its onset. The epochs
were aligned to the 100 ms baseline before the onset of the tar-
get. EEG epochs were examined, and all trials contaminated
with ocular or movements artefacts were discarded. Approx-
imately, 5% of the trials were excluded from the average
because of ocular and movements artefacts. Consequently,
the reference channel has been replaced with an average-
reference. This procedure allows computing the mean signal
recorded in all active channels and then using this mean sig-
nal as reference. This step has been necessary because the
N400 has its maximum amplitude in the centre-parietal areas
Fig. 1. Grand average ERPs in response to descriptions of Living and Non-living concepts. No difference in the N400 windows at any site.