1. Apparatus for generating a topographic display of information on the electrical activity of the brain, said apparatus comprising a plurality of electrical-activity transducers adapted to be placed at spaced apart locations on the skull of a patient, stimulus means for repeatedly providing a sensory stimulus for activating the brain to produce EP responses to said transducers, response averaging means connected to be responsive to said transducers, to produce average responses for each transducer, processing means connected to be responsive to said averaging means for processing said average responses to generate a time sequence of matrices, display means connected to be responsive to said processing means, for displaying said matrices as a time sequence of topographic maps of the skull, said matrice having elements defining discrete points of said maps, said display means including means for displaying said topographic maps at a variable frame rate, said maps corresponding to different portions of said average responses being displayed respectively at different selected frame rates.