One of the greatest challenges to students of the neurological correlates of behavior in humans is interpreting the dramatic degree of difference observed between individuals. Such differences characterize virtually all sophisticated measures, from behavioral assessments of psychophysical skills, to noninvasive imaging of neuroanatomical and neurophysiological correlates of behavior. Yet this rich diversity of detail is often either ignored, or considered experimental noise and actively suppressed through use of statistical methods which emphasize group rather than individual results.

Our research is based on the assumption that accounting for the ‘repertoire’ of individual patterns is a prerequisite for understanding the variety of ways in which human brain organization underlies behavior. In this presentation, we will describe the design and use of the ‘auditory-system cross-section battery’, a relatively inexpensive noninvasive test battery which makes it possible to describe a representative ‘neurological fingerprint’ for each individual. Just as with forensic uses of real finger prints, the dependent variables yielded by the battery offer not only means of uniquely specifying each individual, but also an objective basis for classifying individuals into groups, to support formulations of universal principles of brain/behavior relations. Such classification may not only be important in and of itself, but also crucial as a background for understanding individual differences observed in experiments based on neuroimaging. The minimal battery consists of a combination of behavioral and physiological measures. Behavioral tests include: (1) an audiometric assessment, and (2) a set of measures of fine motor control. Physiological tests include: (1) otoacoustic emissions (OAEs), (2) the Repeated Evoked Potentials version of the Auditory Brainstem Response (REPs/ABR), and (3) quantitative electroencephalography (qEEG) collected under resting conditions. Physiological protocols assess right/left and afferent/efferent relations characterizing each level of the system. When possible, the minimal battery can be supplemented by MRI studies to document the anatomical integrity of cerebral structures, and quantify volume asymmetries of specific regions such as perisylvian cortex, and/or functional imaging to corroborate the electrophysiological findings on asymmetries.

The ‘bottom-up’ approach represented by the design of this battery derives from the conviction that a focus on the range of individual expressions may suggest principles of brain/behavior classification that could be derived in no other way. One example is our Trimodal Model of brain organization, which proposes a new formulation of functional asymmetries, and posits a continuum of individual differences regarding access to functional-asymmetry skills, dependent on prenatal hormonal conditions, and trimodally distributed in the larger population. The resulting three major variants of human brain are considered to be adaptive for life in pre-industrial human and proto-human societies. Their features have implications for explaining many types of individual psychological and physiological differences including special skills such as athletic ability and musical aptitude, the etiology and nature of psychiatric and developmental disorders, and differences in response to and recovery from acquired injury. Examples of applications of the auditory-system cross-section battery for studying aspects of the Trimodal Model will be presented, to illustrate use of the battery as well as the potential utility of the Trimodal Model for understanding individual differences in human brain/behavior relations.