One set of sounds, two tonotopic maps : exploring auditory cortex with amplitude-modulated tones

Abstract

Objective: The possibility of simultaneously observing activation of primary and secondary auditory cortices has been demonstrated byEngelien et al. [Hear Res 2000;148:153 60].Methods: Such a dual monitoring by means of neuromagnetic recordings can be achieved when a subject is stimulated by brief pulses of40 Hz-modulated tones. Depending on the frequency filter applied, either the steady-state field (SSF) or the N1m can be extracted from theevoked magnetic field complex.Results: Using this combined (two-maps) paradigm with 4 carrier frequencies, we show that it is possible to synchronously screen twotonotopic maps one map each reflected either by the SSF or the N1m. Indicators are the systematic variation in the location (higherfrequencies are more posterior) and orientation (higher frequencies oriented differently in the saggital plane) of the equivalent current dipole(ECD). These parameters were compared with those obtained from classic (one map) paradigms in which either a pure tone elicits an N1mor a 40 Hz continuous (3 s) stimulation produces an SSF. Overall the results were similar, however, systematic differences between theparadigms were found for ECD localization, dipole strength, amplitude, and phase.Conclusions and Significance: One possible interpretation of these results is that different tonotopically arranged cortical fields wereinvolved in the generation of the components.