Frequency and location specificity of the human vertex N1 wave

Test tones of 1000 Hz subjectively located in the middle of the head were randomly presented with equiprobable intervening tones. The latter stimulus was constant within a stimulus block. The frequency of the intervening stimulus varied between different blocks from 578 Hz to 1728 Hz and its location varied in parallel with the frequency along the left-right dimension through 7 different locations. The constant inter-stimulus interval was 460 msec. The EEG was recorded at Cz and Fz. The N1 wave elicited by the test stimuli was smaller the smaller was the separation between the two stimuli in frequency or location. These results were interpreted in terms of stimulus-specifically adapted detector activity. The more the test and intervening stimuli resemble each other the greater is the overlap between the respective feature-detector populations activated and, therefore, the smaller is the N1 amplitude. Thus the sensory-specific component of the N1 wave generated in the primary auditory areas at least in part reflects detector activity. The selective frequency adaptation was much more specific in the present study than in the previous ones and suggested that the N1 component recorded was generated by highly frequency-specific neurons. The frequency and location effects were independent, i.e., the frequency effect was rather similar for different location separations, and vice versa. Thus, evidence for separate detectors for frequency and location of an auditory stimulus was obtained.