The auditory P50 component to onset and offset of sound

Abstract

Objective: The auditory Event-Related Potentials (ERP) of component P50 to sound onset and offset have been reported to be similar, but their magnetic homologue has been reported absent to sound offset. We compared the spatio-temporal distribution of cortical activity during P50 to sound onset and offset, without confounds of spectral change.

Methods: ERPs were recorded in response to onsets and offsets of silent intervals of 0.5 s (gaps) appearing randomly in otherwise continuous white noise and compared to ERPs to randomly distributed click pairs with half second separation presented in silence. Subjects were awake and distracted from the stimuli by reading a complicated text. Measures of P50 included peak latency and amplitude, as well as source current density estimates to the clicks and sound onsets and offsets.

Results: P50 occurred in response to noise onsets and to clicks, while to noise offset it was absent. Latency of P50 was similar to noise onset (56 ms) and to clicks (53 ms). Sources of P50 to noise onsets and clicks included bilateral superior parietal areas. In contrast, noise offsets activated left inferior temporal and occipital areas at the time of P50. Source current density was significantly higher to noise onset than offset in the vicinity of the temporo-parietal junction.

Conclusions: P50 to sound offset is absent compared to the distinct P50 to sound onset and to clicks, at different intracranial sources. P50 to stimulus onset and to clicks appears to reflect preattentive arousal by a new sound in the scene. Sound offset does not involve a new sound and hence the absent P50.

Significance: Stimulus onset activates distinct early cortical processes that are absent to offset.

Fig. 1

A. Binaural stimuli used in this study. Top: Binaural continuous white noise with randomly distributed gaps; Bottom: Click pairs presented with inter stimulus intervals identical to the gap durations and inter-gap intervals. Potentials associated with noise offset, noise onset, first and second clicks in the pair were recorded. B. Grand averaged waveforms (13 subjects) of potentials to noise offsets and noise onsets. In addition to the marked differences in N₁₀₀, note the clearly defined P₅₀ to noise onset, particularly at midline central and frontal electrodes, and its diminution to an earlier inflection from baseline in response to noise offset. C. Potentials in response to noise onset, noise offset, first and second clicks of click pairs with the same 500 msec interclick intervals. Note the similar latencies and waveforms of potentials to noise onsets and to the first and second clicks in the pairs, comprising a P₅₀ followed by an N₁₀₀-P₁₆₀ sequence. In contrast, the potentials to noise offset, in addition to the bifid N-Complex, had no P₅₀ peak. The inset shows enlarged P₅₀ waveforms to the different stimuli. Note that waveforms evoked by clicks are intermediate between the waveforms to noise onset and offset. The definition of P₅₀ peak to clicks in this grand average is inferior to that of individual subjects, due to intersubject latency jitter.

Fig. 2

Average source current density distributions in the time period of P₅₀ to noise onset, offset and to clicks. In reponse to noise onset (top) activity involved bilateral superior central regions (Brodmann areas 18, 19, 20, 23, 38, 7 and 31) while to noise offset (middle) left inferior temporal and occipital regions (Brodmann areas 18, 20, 28, 6 and 13) were activated. The P₅₀ to clicks (bottom) involved all the above areas, but the most active areas (Brodman areas 47, 31 and 20) was in the left inferior frontal and temporal cortices. Note the different calibrations of the current density color bars, underscoring the significantly lower activity to noise offset.

Fig. 3

A. Statistical non-parametric t-value mapping of current density differences during P₅₀ between noise onset and offset. At the peak of P₅₀ current density was significantly higher in response to noise onset than to offset in the vicinity of Brodmann area 40 as well as the general location of Brodmann areas 39, 31 and 13. The t-value for statistical significance is indicated on the color bar. B. The time courses of activity in the areas most differentially activated between noise onset and offset. In response to noise onset, activity in BA 39 and 40 peaked slightly before the scalp recorded peak of P₅₀, in BA 31 - slightly after the peak, while in BA13 it peaked slightly before and slightly after P₅₀. In the plots to noise offset, ‘peak’ denotes the point of inflection from baseline. Note that in response to offset, activity in these areas was unchanged and low throughout the duration of P₅₀, increasing only toward the very end of this period.