Probability of responses to the right as a function of ITD at 750, 1000, and 1250 Hz. The left column shows the performance using virtual headphones. The right column shows the free-field performance depending on the ITDs occurring in the free-field experiments.

<p>Symbols indicate individual performance of animals [750 Hz: n = 6 (vhp, free-field), 1000 Hz: n = 6 (vhp, free-field), 1250 Hz: n = 5 (vhp) and 6 (free-field)]. Lines represent the cumulative normal distribution functions with four parameters (slope, inflection point, offset from 0, offset from 1) fitted to the raw data of individual animals. Identical colors and symbols represent data and fit of the same individual animal in the different panels. Thresholds (in μs) and identifiers of individual animals are given on the right.</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

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

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

Probability of responses to the right as a function of ITD at frequencies ≥2000 Hz. The left column shows the performance using virtual headphones. The right column shows the free-field performance depending on the ITDs occurring in the free-field experiments.

<p>Tones with frequencies of 2000 [n = 6 (vhp, free-field)], 2400 [n = 5 (vhp) and 4 (free-field)], 2673 (n = 5, vhp only), 2900 Hz (n = 5, vhp only), and 3000 Hz (n = 6, free-field only) were presented. Symbols and lines as in the previous figure. Thresholds (in μs) and identifiers of individual animals are given on the right. ‘x’ indicates that no threshold could be determined because the threshold criterion was not reached. Thresholds in brackets were derived from cumulative normal distribution functions that yielded R²<0.875 with the raw data and not used in further analyses; such cumulative normal distribution functions are not shown.</p

Methods and procedures.

<p><b>(A)</b> Schematic of the experimental setup. Free-field stimuli were presented from a subset of an array of 15 loudspeakers distributed between -90° and 90° (gray and orange). Virtual-headphone stimuli were presented from the loudspeakers positioned at ±18° (orange). Animals moved on a Y-shaped platform and initiated trials by disrupting a light-barrier in a half-ring-shaped poke-hole (inset) with their nose. Animals' movements and responses were monitored by further light-barriers. Correct responses were rewarded by dispensing food rewards into food bowls from dispensers (not shown) fixed on the ceiling of the sound-attenuated booth. <b>(B)</b> Sketch illustrating the generation of virtual-headphone stimuli using cross-talk cancellation. The undesired signal paths (pink) between the loudspeakers and the respective contralateral ears are eliminated by destructive interference in the ears resulting in only the direct signal paths (blue) remaining present at the ears. <b>(C)</b> The free-field performance measured as probability of an approach to the right was transformed to free-field interaural level difference (ILD) and free-field interaural time difference (ITD) performance by extracting ITD and ILD values from the directional transfer functions (DTFs) obtained for the angular positions tested. Thresholds were then calculated by fitting a cumulative normal distribution function (green and red lines) to the raw data (circles) and determining the difference in ITD or ILD at the inflection point of the function (IP, dotted line) and 0.26 above the inflection point (IP+0.26, dashed line), thus corresponding to a d’-value of 1.</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

Schematic drawing of the five masker types used in the experiment.

<p>Schematic drawing of the five masker types (grey) in the two frequency regions used in the experiment as indicated on the ordinate. “Harm” = harmonic complex, “Mistuned” = mistuned compared to target, “Inh/Sess” = inharmonic, frequencies varied per session, “Inh/Pres” = inharmonic, frequencies varied per stimulus presentation, “Noise” = bandpass noise. See text for further explanations of the maskers. Target signals were 1 or 8 kHz pure tones, respectively (black).</p

The percentage of a 2-stream percept in relation to the modulation frequency of the B SAM tone.

<p>The percentage of a 2-stream percept depicted in relation to the modulation frequency of the B SAM tone in octaves (with reference to 1 Hz). Each of the four blocks represent data for a specific modulation depth (%) and presentation time (s). The two columns within each block represent data for different carrier frequencies (Hz). A vertical black line indicates the position of the reference modulation frequency <i>f_modA</i>, which is also indicated on the right hand side of each row of panels. The error bars denote the standard error and the coloured symbols indicate data obtained for the three different tone patterns (shortly denoted as TD/SOA_across).</p