Human evoked cortical activity to signal-to-noise ratio and absolute signal level

Abstract

The purpose of this study was to determine the effect of signal level and signal-to-noise ratio (SNR) on the latency and amplitude of evoked cortical activity to further our understanding of how the human central auditory system encodes signals in noise. Cortical auditory evoked potentials (CAEPs) were recorded from 15 young normal-hearing adults in response to a 1000 Hz tone presented at two tone levels in quiet and while continuous background noise levels were varied in five equivalent SNR steps. These 12 conditions were used to determine the effects of signal level and SNR level on CAEP components P1, N1, P2, and N2. Based on prior signal-in-noise experiments conducted in animals, we hypothesized that SNR, would be a key contributor to human CAEP characteristics. As hypothesized, amplitude increased and latency decreased with increasing SNR; in addition, there was no main effect of tone level across the two signal levels tested (60 and 75 dB SPL). Morphology of the P1-N1-P2 complex was driven primarily by SNR, highlighting the importance of noise when recording CAEPs. Results are discussed in terms of the current interest in recording CAEPs in hearing aid users.

Figure 1

Noise spectra of the white noise background recorded at the output of the transducer. The original unfiltered white noise (top) and filtered white noise (bottom) are shown. Golay methods resulted in filtered white noise that had a flatter spectrum, larger bandwidth (out to 5 kHz), and a steeper drop in spectrum level.

Figure 2

Scalp topography and waveform morphology across the scalp for the Quiet, 60 dB tone condition (above). Electrode Cz is circled at the center. Below is the resulting global field power waveform demonstrating the instantaneous activity across the scalp calculated as the standard deviation of all channels as a function of time. The N1, P2, and N2 peaks are indicated.

Figure 3

Grand mean waveforms (n=15) of electrode Cz for the 60dB tone (left) and the 75 dB tone (right). Waveforms are displayed for a Quiet condition and 5 SNR conditions. In general, increases in amplitude and decreases in latency occur with increasing SNR. Components P1, N1, P2, and N2 are labeled for the 60dB Quiet condition.

Figure 4

Signal-to-noise growth functions for electrode Cz for P1, N1, P2, and N2 components. Latency (left) and amplitude (right) measures are displayed for the 60 dB tone (dotted line, open circles) and 75 dB tone (solid line, closed squares). Error bars represent 1 standard error of the mean. In general, increases in amplitude and decreases in latency occur with increasing SNR with no significant difference between tone levels.

Figure 5

Grand mean waveforms (n=15) for global field power for the 60dB tone (left) and the 75 dB tone (right). Waveforms are displayed for a Quiet condition and 5 SNR conditions. In general, increases in amplitude and decreases in latency occur with increasing SNR. Components N1, P2, and N2 are labeled for the 60dB Quiet condition.

Figure 6

Signal-to-noise growth functions for global field power for N1, P2, and N2 components. Latency (left) and amplitude (right) measures are displayed for the 60 dB tone (dotted line, open circles) and 75 dB tone (solid line, closed squares). Error bars represent 1 standard error of the mean.