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PubMed1980

Lateralization of functions in the vertebrate brain: a review.

That the human left and right cerebral hemispheres perform different functions is widely accepted; but there is little evidence of whether or not similar functional asymmetries exist in non-human vertebrates. In this paper, neuro-anatomical similarities between human and other vertebrate brains are considered, and data concerning physical asymmetries reviewed. The defining features of human lateralization are taken to be right-handedness, as a skewed but continuous distribution of preferences, and a greater involvement of the left hemisphere in species-specific vocalization, with right-hemisphere superiority in spatial perception and emotionality less well-marked characteristics. Rodents, cats, at least one species of marsupial, and macaque monkeys have consistent hand preferences for food reaching. These may result from constitutional factors, but in every species studied the distribution of preferences is unskewed. Canaries appear to have left-hemisphere dominance of vocal production, and there is limited support for the conjecture that macaque monkeys have left-hemisphere dominance for reception of species-specific cries, and/or for short-term auditory memory. Left and right unilateral hemispheric damage may have appreciably different effects on emotionality in rats, sound localization in cats, and tactile discrimination in monkeys, although the available evidence is equivocal. It seems possible that asymmetries of cerebral function are widespread in vertebrates. In particular, left hemisphere dominance of species-specific communication might be common in birds and primates: left-hemisphere dominance of human speech may be an example of a general vertebrate tendency towards unilateral control of vocalization.

Animals↗
PubMed1969

Echo-location and evoked potentials of bats after ablation of inferior colliculus.

1. Echo-location and evoked potentials of blinded Yuma bats (Myotis yumanensis) were studied before and after ablation of the inferior colliculus (I.C.). A task of obstacle-avoidance was given to the bats: hits and misses of strands in the flight path were counted. Orientation sounds emitted by the bats during flight were recorded.2. Bilateral ablation of the dorso-medial region of I.C. including the internuclear cortex and commissure had no effect on obstacle-avoidance performance. The bats avoided even strands of 0.2 mm diameter with orientation sounds.3. Bilateral ablation of the dorsal half of I.C. including the external nucleus (lateral cortex) also had no effect on echo-location.4. Bilateral ablation of the ventral half of I.C. caused severe deficiency in ability to avoid obstacles. The main nucleus appeared to be very important for echo-location. When bilateral ablation including the main nucleus was moderate, the bats failed to avoid strands of less than 0.5 mm diameter in spite of detecting them, but avoided large obstacles such as 3.7 mm strands. With severe bilateral ablation including the main nucleus, the bats did not avoid even the 3.7 mm strands in spite of frequent emission of orientation sounds, but often avoided crashing into the wall.5. Severe unilateral ablation of I.C. including the main nucleus and a part of the lateral lemniscus had no effect on ability to avoid obstacles. Since sound localization by such bats are not explained by Van Bergeijk's model based on Békésy's, a modification of Van Bergeijk's model has to be considered.6. Of the positive evoked potentials recorded with an active electrode placed at the dorsal surface of I.C., the slow component with a 7-9 msec peak latency reflected activity of inferior collicular neurones, while the fast component (N(4)) with a 3 msec peak latency represented activity of ascending lateral lemniscal fibres.

Animals↗
PubMed1969

Echo-location of bats after ablation of auditory cortex.

1. Echo-location of blinded Yuma bats (Myotis yumanensis) was studied after ablation of the auditory cortex (A.C.). A task of obstacle-avoidance was given to the bats. Hits and misses of strands were counted, and orientation sounds emitted by the bats during flight were recorded.2. After bilateral ablation of A.C., two bats out of six failed to avoid even large obstacles such as 3.7 mm strands and wall. These bats emitted orientation sounds at a rate of 10-15/sec during flight, but did not change that rate before crossing the obstacles and crashed into them. In these bats, other cortical areas in addition to A.C. were probably ablated.3. In three bats out of six, obstacle-avoidance performance was quite normal. These bats avoided even 0.2 mm strands with orientation sounds, the repetition rate of which was systematically increased before crossing the obstacles. In two of them, the dorsal half of the inferior colliculus (I.C.) was bilaterally ablated in addition to A.C. But ability to avoid the obstacles was not impaired at all. Their cerebral cortices did not show the normal positive-negative diphasic potential change in response to tonal stimuli, although the normal diphasic potential change was retained in A.C. of bats which could not echo-locate as a result of bilateral ablation of the main nucleus of I.C. A.C. appeared to be not essential for echo-location.4. Unilateral ablation of A.C. and the internal capsule had no effect on echo-location, but bilateral ablation of them usually resulted in death from operational trauma.5. It was suggested that A.C. was less important for sound localization in bats than in cats.

Journal Article↗
PubMed2003

Evaluation of the limiting acuity of coincidence detection in nucleus laminaris of the chicken.

The localization of sounds requires the detection of very brief inter-aural time differences (ITDs). In birds, ITDs are first encoded in neurons of the nucleus laminaris (NL) through the precise coincidence of binaural synaptic inputs. We examined the effects of temperature on acuity of coincidence detection in chick NL, by utilizing whole-cell and cell-attached recording techniques in brain slices while applying electrical stimuli bilaterally to axonal projections from the nucleus magnocellularis to NL. The precision of coincidence detection was measured as a time window, corresponding to the time interval that gave the half-maximum spiking probability. Acuity improved with the elevation of recording temperature, and at 40 degrees C, the avian body temperature, the time window was 0.38 ms. Although all synaptic events were briefer at higher temperature, the duration of EPSPs were equivalent to or faster than that of EPSCs at 40 degrees C. Activation of low-threshold K+ currents by a slight membrane depolarization during an EPSP was responsible for this EPSP acceleration. EPSPs were prolonged following inhibition of low-threshold K+ currents by dendrotoxin (40 nM) or hyperpolarization-activated cation currents by Cs+ (3 mM). The EPSP time course had a strong positive correlation with the sharpness of coincidence detection. The limiting value of the time window (0.16 ms), calculated from the estimated EPSP time course, was narrow enough to explain the acuity of ITD detection at NL in vivo.

Animals↗
PubMed2005

Time and intensity coding at the hair cell's ribbon synapse.

The activity of individual afferent neurones in the mammalian cochlea can be driven by neurotransmitter released from a single synaptic ribbon in a single inner hair cell. Thus, a ribbon synapse must be able to transmit all the information on sound frequency, intensity and timing carried centrally. This task is made still more demanding by the process of binaural sound localization that utilizes separate computations of time and intensity, with temporal resolution as fine as 10 micros in central nuclei. These computations may rely in part on the fact that the response phase (at the characteristic frequency) of individual afferent neurones is invariant with intensity. Somehow, the ribbon synapse can provide stronger synaptic drive to signal varying intensity, without accompanying changes in transmission time that ordinarily occur during chemical neurotransmission. Recent ultrastructural and functional studies suggest features of the ribbon that may underlie these capabilities.

Action Potentials↗
PubMed2005

Topographic organization in the auditory brainstem of juvenile mice is disrupted in congenital deafness.

There is an orderly topographic arrangement of neurones within auditory brainstem nuclei based on sound frequency. Previous immunolabelling studies in the medial nucleus of the trapezoid body (MNTB) have suggested that there may be gradients of voltage-gated currents underlying this tonotopic arrangement. Here, our electrophysiological and immunolabelling results demonstrate that underlying the tonotopic organization of the MNTB is a combination of medio-lateral gradients of low-and high-threshold potassium currents and hyperpolarization-activated cation currents. Our results also show that the intrinsic membrane properties of MNTB neurones produce a topographic gradient of time delays, which may be relevant to sound localization, following previous demonstrations of the importance of the timing of inhibitory input from the MNTB to the medial superior olive (MSO). Most importantly, we demonstrate that, in the MNTB of congenitally deaf mice, which exhibit no spontaneous auditory nerve activity, the normal tonotopic gradients of neuronal properties are absent. Our results suggest an underlying mechanism for the observed topographic gradient of neuronal firing properties in the MNTB, show that an intrinsic neuronal mechanism is responsible for generating a topographic gradient of time-delays, and provide direct evidence that these gradients rely on spontaneous auditory nerve activity during development.

Action Potentials↗
PubMed2000

Monaural and binaural detection of sinusoidal phase modulation of a 500-Hz tone.

The detectability of phase modulation was measured for three subjects in two-alternative temporal forced-choice experiments. In experiment 1, the detectability of sinusoidal phase modulation in a 1500-ms burst of an 80-dB (SPL), 500-Hz sinusoidal carrier presented to the left ear (monaural condition) was measured. The experiment was repeated with an 80-dB, 500-Hz static (unmodulated) tone at the right ear (dichotic condition). At a modulation rate of 1 Hz, subjects were an order of magnitude more sensitive to phase modulation in the dichotic condition than in the monaural condition. The dichotic advantage decreased monotonically with increasing modulation rate. Subjects ceased to detect movement in the dichotic stimulus above 10 Hz, but a dichotic advantage remained up to a modulation rate of 40 Hz. Thus, although sound movement detection is sluggish, detection of internal phase modulation is not. In experiment 2, thresholds for detecting 2-Hz phase modulation were measured in the dichotic condition as a function of the level of the pure tone in the right ear. The dichotic advantage persisted even when the level of the pure tone was reduced by 50 dB or more. The findings demonstrate a large dichotic advantage which persists to high modulation rates and which depends very little on interaural level differences.

Adult↗
PubMed2001

The influence of interaural stimulus uncertainty on binaural signal detection.

This paper investigated the influence of stimulus uncertainty in binaural detection experiments and the predictions of several binaural models for such conditions. Masked thresholds of a 500-Hz sinusoid were measured in an NrhoSpi condition for both running and frozen-noise maskers using a three interval, forced-choice (3IFC) procedure. The nominal masker correlation varied between 0.64 and 1, and the bandwidth of the masker was either 10, 100, or 1,000 Hz. The running-noise thresholds were expected to be higher than the frozen-noise thresholds because of stimulus uncertainty in the running-noise conditions. For an interaural correlation close to +1, no difference between frozen-noise and running-noise thresholds was expected for all values of the masker bandwidth. These expectations were supported by the experimental data: for interaural correlations less than 1.0, substantial differences between frozen and running-noise conditions were observed for bandwidths of 10 and 100 Hz. Two additional conditions were tested to further investigate the influence of stimulus uncertainty. In the first condition a different masker sample was chosen on each trial, but the correlation of the masker was forced to a fixed value. In the second condition one of two independent frozen-noise maskers was randomly chosen on each trial. Results from these experiments emphasized the influence of stimulus uncertainty in binaural detection tasks: if the degree of uncertainty in binaural cues was reduced, thresholds decreased towards thresholds in the conditions without any stimulus uncertainty. In the analysis of the data, stimulus uncertainty was expressed in terms of three theories of binaural processing: the interaural correlation, the EC theory, and a model based on the processing of interaural intensity differences (IIDs) and interaural time differences (ITDs). This analysis revealed that none of the theories tested could quantitatively account for the observed thresholds. In addition, it was found that, in conditions with stimulus uncertainty, predictions based on correlation differ from those based on the EC theory.

Attention↗
PubMed2001

Manipulating the "straightness" and "curvature" of patterns of interaural cross correlation affects listeners' sensitivity to changes in interaural delay.

The purpose of this study was to test the hypothesis that stimuli characterized by "straight" trajectories of their patterns of cross correlation foster greater sensitivity to changes in interaural temporal disparities (ITDs) than do stimuli characterized by more "curved" trajectories of their patterns of cross correlation. To do so, sensitivity to changes in ITD was measured, as a function of duration, using a set of "reference" stimuli that yielded differing relative amounts of straightness within their patterns of cross correlation while keeping the dominant trajectory at or near midline. The relative amounts of straightness were manipulated by employing specific combinations of bandwidth, ITD, and interaural phase disparity (IPD) of Gaussian noises centered at 500 Hz. The results were consistent with expectations in that the patterning of the threshold ITDs revealed increasingly poorer sensitivity as greater and greater curvature was imposed on the dominant, "midline," trajectory. The variations in threshold ITD across the stimulus conditions can be accounted for quite well quantitatively by assuming either that the listeners based their judgments on changes in the position of the most central peak of the cross-correlation function or that they based their judgments on changes in the centroid of a second-level cross-correlation function. In a second experiment, binaural detection was measured using a subset of the reference stimuli as maskers. As expected, sensitivity was poorest with the maskers characterized by the greatest curvature, which were also those having the lowest interaural correlation.

Adult↗
PubMed2002

Rhythmic masking release: contribution of cues for perceptual organization to the cross-spectral fusion of concurrent narrow-band noises.

The contribution of temporal asynchrony, spatial separation, and frequency separation to the cross-spectral fusion of temporally contiguous brief narrow-band noise bursts was studied using the Rhythmic Masking Release paradigm (RMR). RMR involves the discrimination of one of two possible rhythms, despite perceptual masking of the rhythm by an irregular sequence of sounds identical to the rhythmic bursts, interleaved among them. The release of the rhythm from masking can be induced by causing the fusion of the irregular interfering sounds with concurrent "flanking" sounds situated in different frequency regions. The accuracy and the rated clarity of the identified rhythm in a 2-AFC procedure were employed to estimate the degree of fusion of the interferring sounds with flanking sounds. The results suggest that while synchrony fully fuses short-duration noise bursts across frequency and across space (i.e., across ears and loudspeakers), an asynchrony of 20-40 ms produces no fusion. Intermediate asynchronies of 10-20 ms produce partial fusion, where the presence of other cues is critical for unambiguous grouping. Though frequency and spatial separation reduced fusion, neither of these manipulations was sufficient to abolish it. For the parameters varied in this study, stimulus onset asynchrony was the dominant cue determining fusion, but there were additive effects of the other cues. Temporal synchrony appears to be critical in determining whether brief sounds with abrupt onsets and offsets are heard as one event or more than one.

Acoustic Stimulation↗
PubMed2002

The effects of spatial separation in distance on the informational and energetic masking of a nearby speech signal.

Although many studies have shown that intelligibility improves when a speech signal and an interfering sound source are spatially separated in azimuth, little is known about the effect that spatial separation in distance has on the perception of competing sound sources near the head. In this experiment, head-related transfer functions (HRTFs) were used to process stimuli in order to simulate a target talker and a masking sound located at different distances along the listener's interaural axis. One of the signals was always presented at a distance of 1 m, and the other signal was presented 1 m, 25 cm, or 12 cm from the center of the listener's head. The results show that distance separation has very different effects on speech segregation for different types of maskers. When speech-shaped noise was used as the masker, most of the intelligibility advantages of spatial separation could be accounted for by spectral differences in the target and masking signals at the ear with the higher signal-to-noise ratio (SNR). When a same-sex talker was used as the masker, the intelligibility advantages of spatial separation in distance were dominated by binaural effects that produced the same performance improvements as a 4-5-dB increase in the SNR of a diotic stimulus. These results suggest that distance-dependent changes in the interaural difference cues of nearby sources play a much larger role in the reduction of the informational masking produced by an interfering speech signal than in the reduction of the energetic masking produced by an interfering noise source.

Adult↗
PubMed2002

Enhancing sensitivity to interaural delays at high frequencies by using "transposed stimuli".

It is well-known that thresholds for ongoing interaural temporal disparities (ITDs) at high frequencies are larger than threshold ITDs obtained at low frequencies. These differences could reflect true differences in the binaural mechanisms that mediate performance. Alternatively, as suggested by Colburn and Esquissaud [J. Acoust. Soc. Am. Suppl. 1 59, S23 (1976)], they could reflect differences in the peripheral processing of the stimuli. In order to investigate this issue, threshold ITDs were measured using three types of stimuli: (1) low-frequency pure tones; (2) 100% sinusoidally amplitude-modulated (SAM) high-frequency tones, and (3) special, "transposed" high-frequency stimuli whose envelopes were designed to provide the high-frequency channels with information similar to that available in low-frequency channels. The data and their interpretation can be characterized by two general statements. First, threshold ITDs obtained with the transposed stimuli were generally smaller than those obtained with SAM tones and, at modulation frequencies of 128 and 64 Hz, were equal to or smaller than threshold ITDs obtained with their low-frequency pure-tone counterparts. Second, quantitative analyses revealed that the data could be well accounted for via a model based on normalized interaural correlations computed subsequent to known stages of peripheral auditory processing augmented by low-pass filtering of the envelopes within the high-frequency channels of each ear. The data and the results of the quantitative analyses appear to be consistent with the general ideas comprising Colburn and Esquissaud's hypothesis.

Adult↗
PubMed2003

Evaluation of array-processing algorithms for a headband hearing aid.

Several array-processing algorithms were implemented and evaluated with experienced hearing-aid users. The array consisted of four directional microphones mounted broadside on a headband worn on the top of the listener's head. The algorithms included two adaptive array-processing algorithms, one fixed array-processing algorithm, and a reference condition consisting of binaural directional microphones. The algorithms were evaluated under conditions with both one and three independent noise sources. Performance metrics included quantitative speech reception thresholds and qualitative subject preference ratings for ease-of-listening measured using a paired-comparison procedure. On average, the fixed algorithm improved speech reception thresholds by 2 dB, while the adaptive algorithms provided 7-9-dB improvement over the reference condition. Subjects judging ease-of-listening generally preferred all array-processing algorithms over the reference condition. The results suggest that these adaptive algorithms should be evaluated further in more realistic acoustic environments.

Aged↗
PubMed2003

Auditory spatial discrimination by barn owls in simulated echoic conditions.

In humans, directional hearing in reverberant conditions is characterized by a "precedence effect," whereby directional information conveyed by leading sounds dominates perceived location, and listeners are relatively insensitive to directional information conveyed by lagging sounds. Behavioral studies provide evidence of precedence phenomena in a wide range of species. The present study employs a discrimination paradigm, based on habituation and recovery of the pupillary dilation response, to provide quantitative measures of precedence phenomena in the barn owl. As in humans, the owl's ability to discriminate changes in the location of lagging sources is impaired relative to that for single sources. Spatial discrimination of lead sources is also impaired, but to a lesser extent than discrimination of lagging sources. Results of a control experiment indicate that sensitivity to monaural cues cannot account for discrimination of lag source location. Thus, impairment of discrimination ability in the two-source conditions most likely reflects a reduction in sensitivity to binaural directional information. These results demonstrate a similarity of precedence effect phenomena in barn owls and humans, and provide a basis for quantitative comparison with neuronal data from the same species.

Animals↗
PubMed2003

Enhancing interaural-delay-based extents of laterality at high frequencies by using "transposed stimuli".

An acoustic pointing task was used to determine whether interaural temporal disparities (ITDs) conveyed by high-frequency "transposed" stimuli would produce larger extents of laterality than ITDs conveyed by bands of high-frequency Gaussian noise. The envelopes of transposed stimuli are designed to provide high-frequency channels with information similar to that conveyed by the waveforms of low-frequency stimuli. Lateralization was measured for low-frequency Gaussian noises, the same noises transposed to 4 kHz, and high-frequency Gaussian bands of noise centered at 4 kHz. Extents of laterality obtained with the transposed stimuli were greater than those obtained with bands of Gaussian noise centered at 4 kHz and, in some cases, were equivalent to those obtained with low-frequency stimuli. In a second experiment, the general effects on lateral position produced by imposed combinations of bandwidth, ITD, and interaural phase disparities (IPDs) on low-frequency stimuli remained when those stimuli were transposed to 4 kHz. Overall, the data were fairly well accounted for by a model that computes the cross-correlation subsequent to known stages of peripheral auditory processing augmented by low-pass filtering of the envelopes within the high-frequency channels of each ear.

Adult↗
PubMed2003

Children's detection of pure-tone signals: informational masking with contralateral maskers.

When normal-hearing adults and children are required to detect a 1000-Hz tone in a random-frequency multitone masker, masking is often observed in excess of that predicted by traditional auditory filter models. The excess masking is called informational masking. Though individual differences in the effect are large, the amount of informational masking is typically much greater in young children than in adults [Oh et al., J. Acoust. Soc. Am. 109, 2888-2895 (2001)]. One factor that reduces informational masking in adults is spatial separation of the target tone and masker. The present study was undertaken to determine whether or not a similar effect of spatial separation is observed in children. An extreme case of spatial separation was used in which the target tone was presented to one ear and the random multitone masker to the other ear. This condition resulted in nearly complete elimination of masking in adults. In young children, however, presenting the masker to the nontarget ear typically produced only a slight decrease in overall masking and no change in informational masking. The results for children are interpreted in terms of a model that gives equal weight to the auditory filter outputs from each ear.

Adolescent↗
PubMed2003

Perceptual segregation of competing speech sounds: the role of spatial location.

Culling and Summerfield [J. Acoust Soc. Am. 92, 785-797 (1995)] showed that listeners could not use ongoing interaural time differences (ITDs) to achieve source segregation. The present experiments tested a free-field analog of their experiment. The stimuli consisted of narrow bands of noise, pairs of which represented the first and second formants of the whispered vowels "ar," "ee," "er," and "oo." A target noise-band pair (vowel) was presented at various angles on the listeners' left while a complementary distracter was presented on the listeners' right. Listeners correctly identified the target vowel in the free-field well above chance. Performance remained well above chance in headphone experiments that retained spatial cues but eliminated reverberations and head movements. The full range of cues that normally determine perceived spatial location provided sufficient information for segregation. Further experiments, which systematically evaluated the contribution of these cues in isolation and in combination, showed that some listeners, following training, exhibited the ability to segregate based on ongoing ITDs alone. Substantial individual differences were observed. The results show that listeners can use spatial cues to segregate simultaneous sound sources.

Adolescent↗
PubMed2003

Binaural models and the strength of dichotic pitches.

Modern physiologically based models of the binaural system incorporate internal delay lines in the pathways from left and right peripheries to central processing nuclei. Different binaural models for the formation of dichotic pitch employ these delay lines in different ways. Consequently, the different models make different predictions for the relative strengths of dichotic pitches made with particular phase conditions. The differences are magnified for dichotic pitches at low frequencies where especially long delay lines may be required. Data from four low-frequency pitch strength experiments on pure-tone-like dichotic pitches (two on Huggins pitch and two on binaural coherence edge pitch) are consistent with models of the equalization-cancellation type and not consistent with the central activity pattern model.

Adult↗