Recently, we and others have demonstrated that speech recognition with broader current configurations can be equal to or better than that achieved with narrower configurations. This calls for us to reexamine these two assumptions. The relative sizes of the neural populations activated by various electrode configurations are predictable for the condition in which the same level of current is applied to all configurations .
Broad configurations excite a larger population of neurons than more restricted configurations.
However, broad configurations require less current than narrow configurations to achieve a given sensation of loudness. When currents are adjusted to achieve equal loudness for the various configurations, the sizes of the activated neural population are more difficult to predict because current level also affects the number of neurons activated. Contrary to this hypothesis, several types of psychophysical data suggest that the neural response patterns for various electrode configurations are in fact different, even at threshold or suprathreshold equal-loudness levels. An alternative hypothesis, consistent with the psychophysical observations, is that at equal-loudness levels the spike counts summed across the neural population are equal but that this is achieved in different ways for different configurations. Specifically, we hypothesize that narrower configurations produce a more spatially restricted pattern of activation but with greater discharge rates per neuron.