Paul Thagard
Philosophy Department
University of Waterloo
pthagard@uwaterloo.ca
The functioning of brains in humans and other animals involves dozens of chemical messengers, including neurotransmitters, hormones, and other molecules.Yet almost all computational models of the mind and brain ignore molecular details.Symbolic models such those based on production rules abstract entirely from neurological details.Neural-network computational models typically treat neuronal processing as an electrical phenomenon in which the activation of one neuron affects the activation of all neurons connected to it by excitatory and inhibitory links.The role of neurotransmitters and other molecules in determining this electrical activity is rarely discussed.
The neglect of neurochemistry in cognitive science would be appropriate if the computational properties of brains relevant to explaining mental functioning were in fact electrical rather than chemical.But there is considerable evidence that chemical complexity really does matter to brain computation.I will review that evidence by discussing the role of proteins in intracellular computation, the operations of synapses and neurotransmitters, and the effects of neuromodulators such as hormones.Attending to the ways in which the brain is a chemical as well as an electrical computer provides a qualitatively different view of mental computation than is found in traditional symbolic and connectionist models.I hope that computational modeling of brain activity at the molecular level will discover emergent properties that are important for understanding some of the most currently intractable problems in cognitive science, such as the origins of emotional consciousness.Hence it may be time for psychology and the philosophy of mind to become, like current biology and medicine,molecular.