Neuronal Measures of Threshold and Magnitude of Forward Masking in Primary Auditory Cortex

Psychophysical forward masking is an increase in the threshold of detection of a brief sound (probe) when preceded by another sound (masker). These effects are reminiscent of the reduction in physiological responses following prior stimulation. However, previous studies of the response of auditory nerve fibers (Relkin and Turner, 1988) found probe threshold shifts following stimulation that were considerably smaller than those found perceptually. Although such threshold shifts are larger in some units of the cochlear nucleus (Ingham et al., 2006), these are either inhibitory interneurons or project to inhibitory neurons. A better account is obtained at the level of the IC in the awake marmoset (Nelson et al., 2009). In the present study, we measure responses of neurons in the primary auditory cortex of the anesthetised guinea pig to forward masked pure tones. Signal detection theory methods are used to infer probe detection thresholds. The objective is to determine whether forward masked thresholds in cortical neurons are higher than those of sub-cortical neurons. Changes in the neurometric function (the computed % correct against probe level) due to prior stimulation are diverse: for some units the function is shifted towards higher probe levels; for others the slope of the function becomes shallower. Thresholds shifts (e.g., 50 dB for a 60-dB SPL masker) calculated for individual units are on average much larger than seen in sub-cortical nuclei. Across the population, the minimum thresholds are also larger than the thresholds observed psychophysically. There is little evidence that persistent activity in response to the masker is contributing to masking.