[Eeglablist] Different results for erpplot depending on channels

[Lourdes Machin]

Hi,
I am relatively new to data analysis and EEGLAB.
I am analysing differences in ERP between two conditions for a
vigilance task (Psychomotor Vigilance Test), with a repeated measures
design. I am looking mainly at differences in P3. Number of participants
n=23.
I ran the analysis below, and plotted independent electrodes.

STUDY = pop_statparams(STUDY,
'condstats','on','mode','fieldtrip','fieldtripnaccu',10000,'fieldtripmethod','montecarlo','fieldtripmcorrect','cluster','fieldtripalpha',0.05);
STUDY = pop_erpparams(STUDY, 'plotconditions','together','timerange',[-200
1000] );
STUDY =
std_erpplot(STUDY,ALLEEG,'channels',{'FP1','FP2','F7','F3','FZ','F4','F8','FT7','FC3','FCZ','FC4','FT8','T3','C3','CZ','C4','T4','TP7','CP3','CPZ','CP4','TP8','T5','P3','PZ','P4','T6','O1','OZ','O2','FT9','FT10','PO1','PO2'},
'design', 1);
>>
STUDY = std_erpplot(STUDY,ALLEEG,'channels',{'FC3','FCZ','FC4'}, 'design',
1);
STUDY = std_erpplot(STUDY,ALLEEG,'channels',{'C3','CZ','C4'}, 'design', 1);
STUDY = std_erpplot(STUDY,ALLEEG,'channels',{'CP3','CPZ','CP4'}, 'design',
1);
STUDY = std_erpplot(STUDY,ALLEEG,'channels',{'FP1','FP2'}, 'design', 1);

First, I plotted all electrodes, to get an overview, then I plotted
separately the frontocentral ({'FC3','FCZ','FC4'), central 'C3','CZ','C4',
centro-parietal 'CP3','CPZ','CP4'and frontal ('FP1','FP2') electrodes.
I was surprised to find that, when plotting all electrodes, there appear to
be no differences between conditions for the central electrodes. However,
when plotting the central electrodes separately from the rest
('C3','CZ','C4'), there was an ERP difference in the 300-400ms, for each
electrode. I repeated the measurements with 10000 and 20000 permutations,
to the same results.
My questions are: what would be a plausible explanation for this behaviour?
Are the differences between conditions found in the central electrodes
reliable? Is there any variation of the parameters that you recommend?
Many thanks, anticipated,
Regards,
Lourdes