a hypothetical model of the neurological and physiological functions of the nervous system that can be manipulated to provide analogies of behavioral activities. Critics claim that research should be concentrated on the actual nervous system rather than on this type of model. However, Donald O. Hebb pointed out that, over time, conceptual models of the nervous system have acquired more of the properties of the actual nervous system and that modeling properties of the nervous system has been fruitful in encouraging research.