  Neural circuits often face the problem of classifying stimuli into discrete
groups and making decisions based on such classifications. Neurons of these
circuits can be distinguished according to their correlations with different
features of stimulus or response, which allows defining sensory or motor
neuronal types. In this study we define the third class of neurons, which is
responsible for making decision. We suggest two descriptions for contribution
of units to decision making: first, as a spatial derivative of correlations
between neural activity and the decision; second, as an impact of variability
in a given neuron on the response. These two definitions are shown to be
equivalent, when they can be compared. We also suggest an experimental strategy
for determining contributions to decision making, which uses electric
stimulation with time- varying random current.
