Discriminability of sound contrasts in the face of speaker variation quantified

How does a naive language learner deal with speaker variation irrelevant to distinguishing word meanings? Experimental data is contradictory, and incompatible models have been proposed. Here, we examine basic assumptions regarding the acoustic signal the learner deals with: Is speaker variability a hurdle in discriminating sounds or can it easily be ignored? To this end, we summarize existing infant data. We then present machine-based discriminability scores of sound pairs obtained without any language knowledge. Our results show that speaker variability decreases sound contrast discriminability, and that some contrasts are affected more than others. However, chance performance is rare; most contrasts remain discriminable in the face of speaker variation. We take our results to mean that speaker variation is not a uniform hurdle to discriminating sound contrasts, and careful examination is necessary when planning and interpreting studies testing whether and to what extent infants (and adults) are sensitive to speaker differences

Average time course of the hemoglobin changes following auditory stimulation in individual channels revealed by a general linear model.

<p>The trace in red represents oxyHb, in blue deoxyHb; the error bars indicate standard error (over participants). The zero level or baseline is defined as the intercept of the linear model. The black dotted lines show the standard HRF model convolved with the average duration of stimulation, scaled to the maximum average concentration. The scale as well as the timing of stimulation (green box) are shown in the reference axes.</p

Comparison of amount of data between the newborn results reported here, and previous comparable infant work.

<p>Min/Max trial N stands for the minimum and maximum number of stimulation blocks or trials. Max Tot is calculated by multiplying the sample size by the maximum number of trials. White cells indicate unavailable information. Except for Shultz*, all studies focused on newborns and used fNIRS (see main text for details).</p><p>Comparison of amount of data between the newborn results reported here, and previous comparable infant work.</p

Location of the 10 shallow channels (1–10) and 4 deep ones (4a,b and 7a,b) on a model of a newborn's head.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115162#pone-0115162-g004" target="_blank">Fig. 4</a> for an estimation of the point of maximal sensitivity for each channel.</p

Channels responding significantly to auditory stimulation.

<p>The three channels indicated had significant activation in the analysis where all stimulation was declared together against the silent baseline.</p

Comparison of procedure between the newborn results reported here, and previous comparable infant work.

<p>Duration is the average stimulation duration. Min and Max rest indicate the minimum and maximum duration of the silence following a block. When only the minimum rest duration is noted, only average and not the range was reported. The other white cells indicate unavailable information. Except for Shultz*, all studies focused on newborns and used fNIRS (see main text for details).</p><p>Comparison of procedure between the newborn results reported here, and previous comparable infant work.</p

Comparison between the newborn results reported here, and published results from human 4-month-olds [11], adult humans and adult macaques [12].

<p>The symbol indicates significance level with * at a corrected level (through resampling in the present work, FDR in the 4-month-old study, and FWE in the adult work) and # at an uncorrected level (.05 for the infant work,.005 for the adult work); ‘none’ indicates None at p<.05 uncorrected for the infants, and at p<.001 for the macaques. The Native vs. Foreign contrast in the 4-month-olds was significant at uncorrected p  = .05 in an ANOVA including both channels. A negative sign indicates that a difference was counter to the stated order (e.g., more activation for macaque calls than human emotional sounds in the newborns). Empty cells were not reported; those with (–) involve stimuli not presented to that population. The channels in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115162#pone.0115162-MinagawaKawai2" target="_blank">[11]</a> have been renamed a through c here for ease of reference. LS  =  lateral sulcus, STS  =  superior temporal sulcus, STG  =  superior temporal gyrus, F = frontal regions, IFG  =  inferior frontal gyrus. All activations in adult monkeys and humans were present in both hemispheres, with various degrees of dominance (not represented here).</p><p>Comparison between the newborn results reported here, and published results from human 4-month-olds <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115162#pone.0115162-MinagawaKawai2" target="_blank">[11]</a>, adult humans and adult macaques <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115162#pone.0115162-Joly1" target="_blank">[12]</a>.</p

Approximate localization of channels in the 4-month-old study (in blue) overlaid over our shallow (in green) and deep (in pink) channels.

<p>Approximate localization of channels in the 4-month-old study (in blue) overlaid over our shallow (in green) and deep (in pink) channels.</p

Examples of acoustic characteristics of the stimuli.

<p>The top left is an example from the emotion stimuli; the middle left, from monkey vocalizations; and the bottom left, from speech. The three examples on the right are the scrambled counterparts of the corresponding stimulus on the left.</p

Channels activated in the sound versus silence contrast.

<p>Ch  =  channel (channels with a number followed by a subscript are deep); <i>β</i> =  beta recovered from the GLM in mM.mm; SE <i>β</i>  =  standard error of the <i>β</i>, N  =  number of children contributing data for that channel and condition, t value, p unc(orrected), and p cor(rected through resampling). Only channels whose <i>β</i> value was significantly different from zero at p ≤ 0.05 uncorrected, for the relevant condition, are shown. Channels with significant <i>β</i> after correction through resampling are marked with *.</p><p>Channels activated in the sound versus silence contrast.</p