Neural population coding and auditory temporal pattern analysis

Over the 2 decades that have elapsed since Robert Erickson first published his pioneering work on across-fiber patterns in the gustatory system, the idea that information is represented by a population code has become almost universally accepted among neuroscientists. Although the concept of a population code is an implicit theoretical assumption underlying most of the work done in neuroscience today, the details of how population codes operate in specific systems remain unclear in many respects. This article reviews electrophysiological studies of the auditory system of echolocating bats that show that information about sound is initially represented across both space and time by relative amounts of activity in populations of excitatory and inhibitory neurons with different discharge patterns, different sensitivity functions, and different latencies. At the next level, each neuron in the auditory midbrain receives convergent input from a specific population of these lower brainstem neurons and acts as a "readout" of activity within this population. As a result, midbrain neurons become selectively tuned to stimulus features, for example, signal duration, to which neurons at lower levels respond indiscriminately. Intracellular recordings from auditory midbrain neurons show some of the mechanisms by which population input is processed. The known projection patterns of the midbrain "readout" neurons indicate that they, in turn, must become part of a new spatio-temporal population code that is transmitted to neurons at the thalamus, where additional forms of selectivity and patterns of output arise.