  The relation between performance and stress is described by the Yerkes-Dodson
Law but varies significantly between individuals. This paper describes a method
for determining the individual optimal performance as a function of
physiological signals. The method is based on attention and reasoning tests of
increasing complexity under monitoring of three physiological signals: Galvanic
Skin Response (GSR), Heart Rate (HR), and Electromyogram (EMG). Based on the
test results with 15 different individuals, we first show that two of the
signals, GSR and HR, have enough discriminative power to distinguish between
relax and stress periods. We then show a positive correlation between the
complexity level of the tests and the GSR and HR signals, and we finally
determine the optimal performance point as the signal level just before a
performance decrease. We also discuss the differences among signals depending
on the type of test.
