This study demonstrates the neural system potentially involved in the representation of, and choice between, stimulus classifications in an ambiguous, novel, decision‐making task. This difficult choice behaviour is taken as an example of a basic executive processing task. Subjects heard sounds that were consonant‐vowel combinations that had been distorted and were required to categorize each stimulus as speech‐like or not‐speech‐like. Cerebral activity was measured with positron emission tomography. A neural system (thalamic and medial prefrontal cortical regions) was demonstrated; there was greater activity involved in assigning the sound to the larger class of not‐speech‐like sounds than to the more restricted category of speech‐like sounds. We interpret this activity as reflecting process and representation in a simple central executive task.