Psychophysical Experiments on “Tuning Curves” and “Two-Tone Inhibition”

Two characteristic features of the neural coding of acoustic stimuli are clearly evident in current literature: high frequency selectivity and an effect known as two-tone inhibition. We investigated whether these phenomena can be revealed by psychophysical methods. Masking-like paradigms, in which a test tone (probe) is applied to study the “internal representation” of a stimulus (masker) seemed particularly appropriate. Maskers consisting of one or two frequency components were used. It was felt essential to avoid confounding interactions between the masker and the probe tone (combination tones, beats). Therefore, in the main part of the experiments a paradigm was used in which masker and probe were presented in continuous alternation (4 Hz); a probe “threshold” was determined by the level of the probe bursts at which their pulsating character just became audible. Results on frequency selectivity and two-tone inhibition obtained in this way appeared to be in excellent agreement with primary-neuron data. Additional experiments, with a number of more traditional masking paradigms, revealed that these results are typical for paradigms using a non-simultaneous probe; with a superimposed probe (direct masking) no effect of two-tone inhibition could be demonstrated. The possibilities and limitations of the several probe-tone methods used here are discussed in some detail.