Evoked responses to frequency shifted tones: tonotopic and contextual determinants.

Event-related potentials were recorded from 13 normal adults in response to a wide range of frequency shifts of a constant amplitude tone, in four separate experiments. Based on the tonotopic organization of the auditory system, we predicted a logarithmic relationship between amount of shift (delta f) and response amplitude. As predicted, the amplitudes of the exogenous N100 and P200 components increased with progressively larger shifts. Log (delta f) predicted 38.6% of the within-subjects variance for N100-P200 amplitude. In addition, an unexpected contextual effect was observed: the largest shift in each experiment tended to produce similar amplitude responses, despite large differences in absolute magnitude of delta f. A two-factor model including a contextual measure of delta f accounted for 48.7% of the variance, indicating that both physical and contextual stimulus parameters determine the amplitude of exogenous components. N100 latency was also influenced by both physical and contextual parameters, whereas P200 latency was almost constant over a wide range of delta f.