The present work investigates the validity of evoked coherences of the EEG
as an indicator of mental load. Previous studies have shown that coherences
of EEG, generally, react sensitively to subtle changes of a subject's ment
al date. Instead of the usually calculated ,,steady-state" values, the pres
ent study used evoked coherences, that, like evoked potentials, reflect neu
ral information processing in the context of discrete events. Considering t
he well-known difficulties with the induction of mental load in laboratory
conditions, especially in connection with EEG recording, chess playing was
chosen as a model of mental activity. Chess diagrams of different complexit
y were presented for solution to 25 active members of chess clubs. For a di
fferentiation between cognitive and perceptive load, a number of diagrams w
as provided with visual noise, which hampered recognition. Evoked coherence
s of EEG showed highly significant differences, nor only between perceptive
and cognitive load, but also between different cognitive operations. In ou
r experiment, perceptive load was reflected in the Theta frequency range, w
hereas cognitive load was reflected in Beta. A two-staged factor analysis p
rocedure, in connection with a topological analysis, allowed the definition
of cortical areas participating significantly in the discrimination of dif
ferent conditions. The results were confirmed in a re-test study. Evoked co
herences of EEG, thus, represent a sensitive tool for the investigation of
mental load.