The specificity of stimulus - specific adaptation in human auditory cortex increases with 1 repeated exposure to the adapting stimulus

19 The neural response to a sensory stimulus tends to be more strongly reduced when the 20 stimulus is preceded by the same, than a different, stimulus. This stimulus-specific adaptation 21 (SSA) is ubiquitous across the senses. In hearing, SSA has been suggested to play a role in 22 change detection as indexed by the mismatch negativity. This study sought to test whether 23 SSA, measured in human auditory cortex, is caused by neural fatigue (reduction in neural 24 responsiveness) or by sharpening of neural tuning to the adapting stimulus. For that, we 25 measured event-related cortical potentials to pairs of pure tones with varying frequency 26 separation and stimulus onset asynchrony (SOA). This enabled us to examine the relationship 27 between the degree of specificity of adaptation as a function of frequency separation and the 28 rate of decay of adaptation with increasing SOA. Using simulations of tonotopic neuron 29 populations, we demonstrate that the fatigue model predicts independence of adaptation 30 specificity and decay rate, whereas the sharpening model predicts interdependence. The data 31 showed independence and thus supported the fatigue model. In a second experiment, we 32 measured adaptation specificity after multiple presentations of the adapting stimulus. The 33 multiple adapters produced more adaptation overall, but the effect was more specific to the 34 adapting frequency. Within the context of the fatigue model, the observed increase in 35 adaptation specificity could be explained by assuming a 2.5-fold increase in neural frequency 36 selectivity. We discuss possible bottom-up and top-down mechanisms of this effect. 37 38