The experimental paradigm.

<p>(A) The experiment was composed of eight five-minute blocks. The gender of the voice to be attended in the first block (Attend A) was counterbalanced across subjects, and it alternated in the subsequent blocks. During a given block, male and female standard vowel sounds were presented synchronously every 300 ms and participants had to detect infrequent deviant sounds in the attended voice. Only standard sounds were used in the analysis. (B) shows the spectra of the standard sounds. On the left is the male standard sound with a F0 of 170 Hz and on the right, the female sound with a F0 of 225 Hz.</p

Frequency-following response reflects neural phase-locking to the fundamental frequency of male and female voice.

<p>Grand average brainstem frequency-following response (FFR) for each experimental condition, plotted in the time (upper panel) and frequency (lower panel) domain. Red lines indicate the response while attending to the female voice; black lines indicate the response while attending to the male voice. FFR obtained during dichotic presentation are shown on the left, diotic presentation on the right. Both vowels’ fundamental frequencies (170 Hz male, 225 Hz female) yielded clearly identifiable maxima in the individual and average FFR spectra.</p

Attentional modulation of the brainstem response correlates with behaviour.

<p>The individual neural attentional modulation indices and target discriminability measures (d′) are negatively correlated in both the dichotic and diotic condition (respectively r = −.56, p = 0.037 and r = −.58, p = 0.031). Each triangle represents data from one participant in either the dichotic (solid triangle) or the diotic (hollow triangle) condition. Participants correspond to numbers 1, 5, 7, 9, 12, 13 and 14 as shown on Fig. 4.</p

Selective attention modulates brainstem response at individual level, both in dichotic and diotic presentation.

<p>Normalized spectral power at the fundamental frequencies of the male (A,B) and female stimuli (C,D) for each participant in the dichotic (A,C) and diotic (B,D) condition. White bars indicate the response amplitude while attending to the female voice; grey bars indicate the response amplitude while attending to the male voice. Error bars indicate the standard error of the mean calculated by bootstrap resampling. Stars indicate a significant difference of two standard errors of the mean (p<0.05) between attending to the male and female voice.</p

Spatial presentation modulates behavioural discriminability.

<p>Average behavioral discriminability of the stimulus streams, expressed as d′ in each condition. The difference between attending to the male versus the female voice is significant in the dichotic condition (p = 0.001), but not in the diotic condition. Error bars indicate the standard error of the mean.</p

Both spatial and frequency cues contribute to attentional modulation of brainstem response.

<p>The average normalized modulation index across participants is significantly higher in the dichotic condition than in the diotic condition (p = 0.006). Error bars indicate the standard error of the mean.</p

Experiment 2 behavioural results: accuracy identifying matches and mismatches in trials of each condition.

<p>Accuracy was significantly above chance in each listening condition. Subjects had significantly better scores in the f0 condition. Mean accuracy across conditions was used for further analysis. Error bars indicate +/-SEM; individual data are superimposed to illustrate the distribution.</p

Melodic task.

<p>Half the melodies were in-tune, 25% were out-of-tune and 25% were out-of-key. The out-of-tune and out-of-key melodies were created by altering the last note of the in–tune melodies, by shifting them by either 50 cents (out-of-tune) or 100 cents (out-of-key). In the melodic task, participants judged whether a melody contained an incongruous note. Their judgment as well as their level of confidence were recorded for each trial on a four point scale (1-congruous, sure; 2-congruous, not sure; 3-incongruous, not sure; 4-incongruous, sure).</p

Experiment 1 experimental paradigm.

<p>(A) Behavioural testing of each listener’s perceptual bias. A sample tone pair is shown schematically; two complex missing fundamental tones comprised of the 8th-10th and 7th-9th harmonics were played sequentially after a short silent pause. This was repeated once, then subjects were asked to record whether they perceived the second tone to be higher or lower than the first. Tones were constructed such that spectral and fundamental perceptions lead to opposite responses, and a measure of overall perceptual bias was calculated from responses on 20 tone pairs. (B) Stimuli used in ABR testing; MF and FP stimuli differed only in the absence (MF: missing fundamental) or presence (FP: fundamental present) of energy at the fundamental frequency (f0).</p

Experiment 1 results.

<p>(A, B) Fundamental perception bias was correlated with f0 peak magnitude in response to missing fundamental and fundamental present tones in each condition (the effect of age is controlled). (C) Histogram of PLV differences at f0 between conditions. While many subjects showed a statistically significant distinction between conditions as determined by resampling (14/39 subjects), subjects did not show consistency in the condition with greater amplitude; 9 had a greater amplitude in the FP condition and 5 had a greater amplitude in the MF condition. (D, E) The PLV of the f0 in each condition was positively correlated with the mean PLV in the low gamma frequency range in both conditions. Both the relationships between f0 PLV and perceptual bias and between f0 PLV and gamma PLV appear more clearly in the MF condition (red), when the fundamental frequency is not present in the stimulus and must be computed. FP: fundamental present condition; MF: missing fundamental condition.</p