In general terms, transduction occurs when one form of energy is converted to another. For instance, an element on an electric range is a transducer because it converts electricity into heat.
Cognitive science uses a more specialized notion of transduction: the conversion of energy into information, or into a code, that can be dealt with by an information processor. The notion of transduction was central to logical treatments of neurons (McCulloch & Pitts, 1943). This is because the physical states of neurons (i.e., whether they were generating an action potential or not) were translated into the notions of "true" or "false", or more generally into bits of information. So, if one were to use a McCulloch-Pitts neuron as a transducer, its inputs would be physical energy (e.g. photons of light) and its output would be a code that represented the presence or absence of detected energy.
- McCulloch, W. S., & Pitts, W. (1943). A logical calculus of the ideas immanent in nervous activity. Bulletin of Mathematical Biophysics, 5, 115-133.