Afferent response parameters derived from postmasker probe-detection thresholds: 'the decay of sensation' revisited

The classical model of forward masking postulates that the detection threshold for a tone probe that follows a stimulus of similar frequency content is elevated relative to the quiet threshold because the probe must evoke a just-detectable increment in a decaying postmasker sensation. That postmasker decay is charted by probe-detection thresholds if the sensation increment is small and constant. This model was examined for a 2-kHz Gaussian-shaped probe and a 2-kHz forward masker, based on the model's assumption that a just-detectable increment in sensation results from a just-detectable increment in level. Psychometric functions for detection were obtained at 2.5-30 ms postmasker. Their means and standard deviations generally decreased with delay. It was assumed that standard deviation is related to the putative just-detectable level increment by a simple monotonic transformation. Thus, if the standard deviation of the psychometric function for probe detection is neither small nor constant, then the corresponding just-detectable increment in level is neither small nor constant, and the just-detectable increment in sensation is neither small nor constant. The classical model also fails to allow for the variability of internal events. The concept of detection threshold as a sensation increment was preserved in a Signal Detection model, that does allow for internal variability. In this model the postmasker residual is the input to a probe detector. The new model produces an equation for the just-detectable level increment as a function of probe delay. Comparison data were generated by again assuming some relation between the standard deviation of the psychometric function for detection, and the just-detectable increment in level. The fit of equation to data yields robust values for the probe detector's maximum firing rate, dynamic range, and spike-counting time. All that is required to account for the decay of sensation, for a pure tone, is a single neuron operating at some higher center.