Mean perceptual distance in relation to the stimulus modification and the familiarity of the motif.

<p>For familiar motifs the perceptual distances (MDS units = multidimensional scaling units) was significantly larger between the complete and the gap stimulus than between the complete and the noise stimulus (* indicates p<0.005), but there was no such difference between unfamiliar stimuli.</p

Spectrograms of an exemplary stimulus set.

<p>Spectrograms (spectral power indicated by darkness of shading plotted as a function of time vs. frequency) of an exemplary complete motif and its modifications; (A) complete motif, (B) the same motif as in (A) with 50 ms silent gaps introduced every 75 ms, (C) the same as in (B), but now the gaps are filled with band-passed noise to produce a noise motif.</p

Exemplary pooled half matrix.

<p>A pooled half matrix of response latencies to one stimulus set out of one experiment from four individual birds with mean±standard error values in ms for every background/deviator combination.</p

Exemplary multidimensional scaling result.

<p>An exemplary multidimensional scaling result derived from the data shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005974#pone-0005974-t001" target="_blank">Tab. 1</a>: the scaling result indicates a smaller distance between the perceived complete and noise stimuli than between the perceived complete and gap stimuli.</p

Comparison between observed and estimated masked thresholds.

<p>Observed and estimated masked thresholds (in dB SPL) were compared for each masker type, for all target frequencies and spatial conditions. The first four columns show the estimated and the observed masked thresholds as mean absolute values (in dB SPL) ± standard deviation. The last two columns show the estimated and the observed amounts of spatial release from masking (SRM) in dB that were calculated by subtracting the thresholds of the spatially separated conditions from the thresholds of the co-located conditions. Mean values for the observed thresholds were calculated from five individuals. Mean values for the estimated thresholds were calculated from 51 subjects of the LISTEN HRTF database <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0026124#pone.0026124-Warusfel1" target="_blank">[13]</a>. Details about the estimation of the thresholds can be found in the section <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0026124#s2" target="_blank">Materials and methods</a>: Model description.</p

Exploring binaural hearing in gerbils (<i>Meriones unguiculatus</i>) using virtual headphones

<div><p>The Mongolian gerbil (<i>Meriones unguiculatus</i>) has become a key species in investigations of the neural processing of sound localization cues in mammals. While its sound localization has been tested extensively under free-field stimulation, many neurophysiological studies use headphones to present signals with binaural localization cues. The gerbil's behavioral sensitivity to binaural cues, however, is unknown for the lack of appropriate stimulation paradigms in awake behaving gerbils. We close this gap in knowledge by mimicking a headphone stimulation; we use free-field loudspeakers and apply cross-talk cancellation techniques to present pure tones with binaural cues via “virtual headphones” to gerbils trained in a sound localization task. All gerbils were able to lateralize sounds depending on the interaural time or level difference (ITD and ILD, respectively). For ITD stimuli, reliable responses were seen for frequencies ≤2.9 kHz, the highest frequency tested with ITD stimuli. ITD sensitivity was frequency-dependent with the highest sensitivity observed at 1 kHz. For stimuli with ITD outside the gerbil's physiological range, responses were cyclic indicating the use of phase information when lateralizing narrow-band sounds. For ILD stimuli, reliable responses were obtained for frequencies ≥2 kHz. The comparison of ITD and ILD thresholds with ITD and ILD thresholds derived from gerbils’ free-field performance suggests that ongoing ITD information is the main cue for sound localization at frequencies <2 kHz. At 2 kHz, ITD and ILD cues are likely used in a complementary way. Verification of the use of the virtual headphones suggests that they can serve as a suitable substitute for conventional headphones particularly at frequencies ≤2 kHz.</p></div

Mean observed masked thresholds for each masker type.

<p>Mean masked thresholds from five individuals for the detection of a pure tone target in each of the five masker types are displayed in absolute values (in dB SPL). Thresholds are shown for the co-located configuration (filled symbols) and for the spatially separated configuration (open symbols) separated by the target frequency (left panel = 1 kHz, right panel = 8 kHz). Error bars represent the standard deviation (SD). The five masker types used in this experiment are: “Harm” = a harmonic masker, “Mistuned” = a harmonic masker with an inharmonic relation to the target frequency, “Inh/Sess” = inharmonic per session, i.e. a random frequency composition redrawn for each session, “Inh/Pres” = inharmonic per presentation, i.e. a different random frequency composition for each stimulus presentation, “Noise” = bandpass noise.</p

Amount of spatial release from masking for each masker type.

<p>Amount of spatial release from masking (difference between co-located and spatially separated configuration) for the 1 kHz target frequency (grey, squares) and for the 8 kHz target frequency (black, triangles) for each masker type. Error bars represent the standard deviation.</p

Mean ITD thresholds measured under virtual-headphone stimulation (vhp, orange) and derived from free-field stimulation (ff, blue).

<p>Only threshold values calculated from cumulative normal distribution functions that yielded an R²>0.875 with the raw data were included in the calculation of the average threshold values (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175142#pone.0175142.s010" target="_blank">S3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175142#pone.0175142.s011" target="_blank">S4</a> Tables, Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175142#pone.0175142.g002" target="_blank">2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175142#pone.0175142.g003" target="_blank">3</a>). Error bars show standard deviations.</p

Statistical results of the four planned contrasts for each of the four subgroups.

<p>A repeated measure ANOVA with planned contrasts was performed for each of the four subgroups and the F-value, the p-value and the effect size partial eta squared are displayed for each contrast. Details about the underlying hypotheses can be found in the results section.</p