Figure 1.
Stimulus description and behavioral performance.
(A) Cartoon spectrogram of a typical stimulus. The stimulus consists of a repeating target note embedded in random interferers. A spectral protection region surrounds the target frequency with a spectral width of twice the minimal distance between the target note and nearest masker component (orange band). In the target task, participants were instructed to detect a frequency-shifted (ΔF) deviant in the repeating target notes. In the masker task, participants were instructed to detect a sudden temporal elongation (ΔT) of the masker notes. (B) Behavioral performance results for target and masker tasks, as measured by d-prime as a function of spectral protection region width. Orange (respectively, light-blue) lines show the mean performance in task detection in the target task (respectively, masker task) in the psychoacoustical study. Red (respectively, dark-blue) points show the mean performance in task detection in the target task (respectively, masker task) in the MEG study (eight-semitone condition only). Error bars represent standard error.
Figure 2.
(A) Power spectral density of MEG responses for a single participant (participant 14 in Figure 2B below) in target (left) and masker (right) tasks, averaged over 20 channels. Insets: the MEG magnetic field distributions of the target rhythm response component. Red and green contours represent the target magnetic field strength projected onto a line with constant phase. (B) Normalized neural response to the target rhythm by participant (individual bars) and task (red for target task, blue for masker task). The normalized neural response is computed as the ratio of the neural response power at the target rate (4 Hz) to the average power of the background neural activity (from 3–5 Hz; see Materials and Methods). Bar height is the mean of the 20 best channels; error bars represent standard error. Light-pink background (respectively, light-blue) is the mean over participants for the target task (respectively, masker task).
Figure 3.
Power and phase enhancement during target task.
(A) Normalized neural response of target task relative to masker task shows differential enhancement exclusively at 4 Hz (the frequency of the target rhythm). Each data point represents the difference between normalized neural response of target relative to masker task; error bars represent standard error The asterisk at 4 Hz shows that only that particular frequency yields a statistically significant enhancement. (B) Phase coherence between distant MEG channels of target relative to masker task. The difference between the number of long-range channel pairs with robust increased coherence in target task, and channel pairs with decreased coherence, is normalized over the total number of long-range channel pairs. The phase enhancement is significant (shown with asterisk) only at 4 Hz. (C) Channel pairs with robust coherence difference at target rate for single participant, overlaid on the contour map of normalized neural response at target rate. Each channel pair with enhancement coherence is connected by a red line, whereas pairs with decreased coherence are connected by a blue line. Coherence is only analyzed for the 20 channels with the best normalized response to target rhythm. (D) Neural responses to target across hemispheres. The 20 channels with the strongest normalized neural response at target rate were chosen from the left and right hemispheres, respectively, to represent the overall neural activity of each hemisphere. Neural responses were averaged across the 20 channels, and 14 participants were compared across hemispheres and tasks. The left hemisphere shows stronger differential activation at target rate in target task, whereas the right hemisphere shows stronger activation in masker task (asterisks indicate that the differences are significant).
Figure 4.
The effect of bottom-up acoustic saliency on behavior and neural responses.
(A) Normalized neural response to target rhythm, and behavioral performance, as a function of target frequency in target task (left) and masker task (right), averaged over participants. Error bars represent standard error. (B) Correlation of behavioral and neural responses as a function of target frequency. The ratio of the neural to behavioral response differences as a function of target frequency, interpreted as a slope angle, is averaged across participants yielding a mean slope angle of 55.1° for target (left) task and −36.3° for masker (right) task (yellow line). Bootstrap estimates (overlying green lines) and their 95% confidence intervals (gray background) confirm the positive (respectively, negative) correlations for target (respectively, masker) task. (C) Phase coherence between distant MEG channels of target relative to masker task for high-frequency targets over low-frequency targets. High- versus low-frequency targets show significant enhancement only for target task (indicated by the asterisk).
Figure 5.
Buildup over time of behavioral and neural responses in target task.
(A) Normalized neural response to target rhythm, and behavioral performance, as a function of time in target task, averaged over participants. Error bars represent standard error. Insets: the MEG magnetic field distributions of the target rhythm response component for a single participant at representative moments in time (participant 10 from Figure 2B). (B) Correlation of behavioral and neural responses as a function of time. The ratio of the neural to behavioral response trends as a function of time, interpreted as a slope angle, is averaged across participants, yielding a mean slope angle of 34.3° (yellow line). Bootstrap estimates (overlying green line) and the 95% confidence intervals (gray background) confirm the positive correlation between the psychometric and neurometric buildup curves.
Figure 6.
Analysis of neural buildup over time in target task for different duration windows, both for data and in a model of the data.
(A) Normalized neural response to target rhythm as a function of time, in target task, averaged over participants. The solid curve is identical to the red curve in Figure 5. The dashed curve is the result of identical analysis except that the normalized neural response is calculated for every 250-ms cycle of the target rhythm, rather than over the 750-ms window of three-cycles used above. Only the longer window shows buildup, implying that it is not power per cycle that is growing, but phase coherence over several cycles. Error bars represent standard error over all participants. (B) Model results for 750 ms (three cycles) windows, solid curve, and for 250 ms (one cycle) windows, dashed curve. The modeled normalized neural response rises as temporal phase jitter decreases, but only in the three-cycle case, since the power per cycle is constant but the temporal phase coherence across cycles increases. Error bars represent standard deviation over 30 simulation runs.