Temporal binding of auditory spatial information across dynamic binaural events

Simultaneity judgments were used to measure temporal binding windows (TBW) for brief binaural events (changes in interaural time and/or level differences [ITD and ILD]) and test the hypothesis that ITD and ILD contribute to perception via separate sensory dimensions subject to binding via slow (100+ ms)—presumably cortical—mechanisms as in multisensory TBW. Stimuli were continuous low-frequency noises that included two brief shifts of either type (ITD or ILD), both of which are heard as lateral position changes. TBW for judgments within a single cue dimension were narrower for ITD (mean = 444 ms) than ILD (807 ms). TBW for judgments across cue dimensions (i.e., one ITD shift and one ILD shift) were similar to within-cue ILD (778 ms). The results contradict the original hypothesis, in that cross-cue comparisons were no slower than within-cue ILD comparisons. Rather, the wide TBW values—consistent with previous estimates of multisensory TBW—suggest slow integrative processing for both types of judgments. Narrower TBW for ITD than ILD judgments suggests important cue-specific differences in the neural mechanisms or the perceptual correlates of integration across binaural-cue dimensions.     

Effects of the build-up and resetting of auditory stream segregation on temporal discrimination

The tendency to hear a tone sequence as 2 or more streams (segregated) builds up, but a sudden change in properties can reset the percept to 1 stream (integrated). This effect has not hitherto been explored using an objective measure of streaming. Stimuli comprised a 2.0-s fixed-frequency inducer followed by a 0.6-s test sequence of alternating pure tones (3 low [L]-high [H] cycles). Listeners compared intervals for which the test sequence was either isochronous or the H tones were slightly delayed. Resetting of segregation should make identifying the anisochronous interval easier. The HL frequency separation was varied (0-12 semitones), and properties of the inducer and test sequence were set to the same or different values. Inducer properties manipulated were frequency, number of onsets (several short bursts vs. one continuous tone), tone:silence ratio (short vs. extended bursts), level, and lateralization. All differences between the inducer and the L tones reduced temporal discrimination thresholds toward those for the no-inducer case, including properties shown previously not to affect segregation greatly. Overall, it is concluded that abrupt changes in a sequence cause resetting and improve subsequent temporal discrimination.     

Auditory motion segregation: a limited analogy with vision

Interaural time delay (ITD) is the dominant cue to sound-source location. When one component of a complex sound changes rapidly but smoothly in ITD, it perceptually segregates from the complex. When different components were changed in ITD in succession, a recognizable melody was heard. Each note was more detectable from the transition than from a distinct ITD with respect to the complex. However, a transition relative to a coherently changing complex produced no segregation, whereas cyclic modulation of ITD, too rapid to be perceived as movement, did produce segregation. These two results suggest that the relevant cue is not movement per se but rather the lack of a well-defined ITD during the transition. Segregation largely disappeared when the transition in ITD was replaced by a temporal gap in the complex of the order of 100 ms. The effect seems similar to visual motion segregation but is best explained by the mechanism of binaural unmasking.     

Some characteristics of auditory spatial attention revealed using rhythmic masking release

The tuning of auditory spatial attention with respect to interaural level and time difference cues (ILDs and ITDs) was explored using a rhythmic masking release (RMR) procedure. Listeners heard tone sequences defining one of two simple target rhythms, interleaved with arhythmic masking tones, presented over headphones. There were two conditions, which differed only in the ILD of the tones defining the target rhythm: For one condition, ILD was 0 dB and the perceived lateral position was central, and for the other, ILD was 4 dB and the perceived lateral position was to the right; target tone ITD was always zero. For the masking tones, ILD was fixed at 0 dB and ITDs were varied, giving rise to a range of lateral positions determined by ITD. The listeners' task was to attend to and identify the target rhythm. The data showed that target rhythm identification accuracy was low, indicating that maskers were effective, when target and masker shared spatial position, but not when they shared only ITD. A clear implication is that at least within the constraints of the RMR paradigm, overall spatial position, and not ITD, is the substrate for auditory spatial attention.     

The effect of tone duration on auditory stream formation

In a study in which the effect of tone duration on the formation of auditory streams was investigated, subjects were presented with 15-sec alternating pure-tone sequences (ABAB …) and were asked to orient their attention over the duration of the sequence toward hearing either a temporally coherent or a segregated percept. At stimulus offset, the subjects indicated whether their percept at the end of the stimulus had been that of a temporally coherent ABAB trill or that of segregated A and B streams. The experimental results indicated that the occurrence of stream segregation increases as (1) the duration of the A and B tones increases in unison and (2) the difference in duration between the A and B tones increases, with the duration differences between the tones producing the strongest segregation effects. A comparison of these experimental results with those of other studies strongly suggests that the time interval between the offset and onset of consecutive tones in the same frequency range is the most important temporal factor affecting auditory stream formation. Furthermore, a simulation of the experimental results by the Beauvois and Meddis (1996) stream segregation model suggests that both the tone duration effects reported here and Gestalt auditory grouping on the basis of temporal proximity can be understood in terms of low-level neurophysiological processes and peripheral-channeling factors.     

The role of binaural and fundamental frequency difference cues in the identification of concurrently presented vowels

The relative importance of voice pitch and interaural difference cues in facilitating the recognition of both of two concurrently presented synthetic vowels was measured. The interaural difference cues used were an interaural time difference (400 μsec ITD), two magnitudes of interaural level difference (15 dB and infinite ILD), and a combination of ITD and ILD (400 μsec plus 15 dB). The results are analysed separately for those cases where both vowels are identical and those where they are different. When the two vowels are different, a voice pitch difference of one semitone is found to improve the percentage of correct reports of both vowels by 35.8% on average. However, the use of interaural difference cues results in an improvement of 11.5% on average when there is a voice pitch difference of one semitone, but only a non-significant 0.1% when there is no voice pitch difference. When the two vowels are identical, imposition of either a voice pitch difference or binaural difference reduces performance, in a subtractive manner. It is argued that the smaller size of the interaural difference effect is not due to a “ceiling effect” but is characteristic of the relative importance of the two kinds of cues in this type of experiment. The possibility that the improvement due to interaural difference cues may in fact be due to monaural processing is discussed. A control experiment is reported for the ITD condition, which suggests binaural processing does occur for this condition. However, it is not certain whether the improvement in the ILD condition is due to binaural processing or use of the improvement in signal-to-noise ratio for a single vowel at each ear.     

A comparison of the effects of differences in temporal gating and ear of presentation on profile discrimination

The aim of the study was to examine the effects of differences in temporal gating and ear of presentation (both separately and in combination) on listeners' ability to detect an increment in the level of a 1 kHz component (the target) relative to that of four spectrally flanking components. The flanking components were always presented to the listeners' right ear, while the target component was either presented to the same ear (monaural presentation) or to the left ear (dichotic presentation). Similarly, the target and flanking components were either gated on and off at the same time (synchronous presentation), or else the target component began 100 ms before and terminated 100 ms after the four flanking components (asynchronous presentation). On average, thresholds were lowest in the synchronous, monaural condition, and highest in the two asynchronous conditions. Ear differences alone did result in elevated thresholds for most listeners. However, combining differences in gating and ear of presentation produced thresholds that were indistinguishable from those obtained when gating differences alone were employed. These results are consistent with the suggestion that differences in temporal gating lead to more complete segregation of concurrent frequency components than differences in spatial location.     

Hemispheric differences in the effects of context on vowel perception

Listeners perceive speech sounds relative to context. Contextual influences might differ over hemispheres if different types of auditory processing are lateralized. Hemispheric differences in contextual influences on vowel perception were investigated by presenting speech targets and both speech and non-speech contexts to listeners’ right or left ears (contexts and targets either to the same or to opposite ears). Listeners performed a discrimination task. Vowel perception was influenced by acoustic properties of the context signals. The strength of this influence depended on laterality of target presentation, and on the speech/non-speech status of the context signal. We conclude that contrastive contextual influences on vowel perception are stronger when targets are processed predominately by the right hemisphere. In the left hemisphere, contrastive effects are smaller and largely restricted to speech contexts.     

The relation between gap discrimination and auditory stream segregation

Gap discrimination and stream segregation were examined using sequences of 2, 4, 8, or 16 tones. The frequency differences between tones ranged from 1/24 to 2 1/2 octaves. Judgments of stream segregation show large intersubject variability, whereas gap thresholds are comparatively stable. Gap thresholds and streaming judgments are both affected by the frequency separation between tones. However, only streaming judgments are affected by presentation rate. Gap-threshold functions show no discontinuities or plateaus with increasing frequency differences and faster presentation rates. These results suggest that stream segregation is not a primary factor limiting gap-discrimination performance in tonal sequences.     

Integrated versus segregated accounting and the magnitude effect in temporal discounting

Temporal discounting rates in humans generally decrease as the amount of reward increases, a phenomenon known as themagnitude effect. In the present study, we examined whether temporal discounting and the magnitude effect are related to segregation of choices in terms of gains or losses for waiting for or expediting receipt of a reward. Subjects (N = 24) responded to a series of hypothetical choices about amounts of money available either immediately or after a delay. The immediate and delayed amounts either were presented as integrated amounts in the baseline condition or were segregated as differential gains or losses for choosing delayed or expedited consumption (delay and speedup conditions, respectively). Temporal discounting rates decreased in the segregated conditions, in accord with the standard discounted utility model but contrary to the hypothesis that the subjects were choosing on the basis of reward differentials in the baseline condition. The size of the magnitude effect was comparable in the baseline and the delay conditions but decreased in the speed-up condition. These results challenge explanations of the magnitude effect in terms of an increasing proportional sensitivity property of the utility function (Loewenstein & Prelec, 1992) and the hypothesis that subjects choose on the basis of differentials even when the rewards are presented as integrated amounts.     

Rhyming and the right hemisphere

Subjects determined whether two successively presented and orthographically different words rhymed with each other. The first word was presented at fixation and the second was presented either to the left or to the right of fixation, either alone (unilateral presentation) or accompanied by a distractor word in the other visual hemifield (bilateral presentation). Subjects were more accurate when the words did not rhyme, when presentation was unilateral, and when the target was flashed to the right visual hemifield. It was predicted that bilateral presentation would produce interference when information from both visual fields was processed by one hemisphere (callosal relay), but not when each of the two hemispheres performed a task independently (direct access). That is, callosal relay tasks should show greater laterality effects with bilateral presentations, whereas direct access tasks should show similar laterality effects with both bilateral and unilateral presentations. Greater laterality effects were observed for bilaterally presented rhyming words, but nonrhyming words showed similar laterality effects for both bilateral and unilateral presentations. These results suggest that judgment of nonrhyming words can be performed by either hemisphere, but that judgment of rhyming words requires callosal relay to the left hemisphere. The absence of a visual field difference with nonrhyming word pairs suggests further that judgment of nonrhyming word pairs may be accomplished by the right hemisphere when presentation is to the left visual field.     

Spatial position versus ear of entry as determinant of the auditory laterality effects: a stereophonic test.

The study concerned discriminating between ear of entry and apparent spatial position as possible determinants of lateral asymmetries in the recall of simultaneous speech messages. Apparent localization to the left or right of the median plane was created either through a time difference (.7 msec), through intensity differences between presentations of the same verbal message at the two ears, or through dichotic presentations. Right-side advantage was observed with the three types of presentation (Experiments 1, 2, and 3). The finding of right-side advantage with stereophony based on a time difference only, in the absence of intensity difference, cannot be accounted for in terms of an ear advantage and shows that apparent spatial separation of the sources can by itself produce a laterality effect. Differences in the degree of lateral asymmetry between the various conditions were also observed. The findings of Experiments 4 and 5 suggest that these differences are better explained in terms of different impressions of localization of the sound sources than in terms of relative intensity at the "privileged" ear.     

Hemispheric asymmetries for the temporal resolution of brief auditory stimuli

This study investigates a left-hemisphere (LH) advantage for the discrimination of fine temporal events within the auditory modality in 24 normal adults. Brief noise bursts were delivered monaurally to the left or right ears, half of which contained a gap lasting between 2 and 8 msec. Subjects were required to indicate whether or not the noise burst contained a gap. Research within this area has yielded conflicting results. It is possible that variations in gap position or the properties of the noise burst may have contributed to this conflict. The effect of gap position was investigated by systematically varying the position of the gap during half of the trials. White noise bursts were used to avoid the use of spectral cues. Reaction time, response bias, and response error measures revealed a right-ear (and hence LH) advantage for gap detection. Stable gap positions were recognized more accurately than variable gap positions. Gap position had no effect on the observed LH advantage. These results suggest that the auditory properties of noise burst, rather than gap position, play an important role in the discrepancies observed in this area.     

Language after temporal or frontal lobe surgery in children with epilepsy

The purpose of this study was to compare language function in children undergoing temporal or frontal lobe surgery to control intractable epilepsy. Language measures (expressive vocabulary, receptive vocabulary, comprehension, reading, spelling, phonemic fluency, and category fluency) were administered to 9 children with frontal lobe epilepsy (mean age: 10.8 +/- 2.7 years) and 10 children with temporal lobe epilepsy (11.5 +/- 2.6 years). The results indicate that no differences exist, in language function, before and after surgery between children with frontal and temporal lobe epilepsy (all p > or = .05). Children with left hemisphere lesions had significantly lower scores than those with right on category fluency and comprehension, but laterality effects were not seen on the other measures. In both groups, language function was not significantly affected by surgery.     

Attention effects on auditory scene analysis: insights from event-related brain potentials

Sounds emitted by different sources arrive at our ears as a mixture that must be disentangled before meaningful information can be retrieved. It is still a matter of debate whether this decomposition happens automatically or requires the listener’s attention. These opposite positions partly stem from different methodological approaches to the problem. We propose an integrative approach that combines the logic of previous measurements targeting either auditory stream segregation (interpreting a mixture as coming from two separate sources) or integration (interpreting a mixture as originating from only one source). By means of combined behavioral and event-related potential (ERP) measures, our paradigm has the potential to measure stream segregation and integration at the same time, providing the opportunity to obtain positive evidence of either one. This reduces the reliance on zero findings (i.e., the occurrence of stream integration in a given condition can be demonstrated directly, rather than indirectly based on the absence of empirical evidence for stream segregation, and vice versa). With this two-way approach, we systematically manipulate attention devoted to the auditory stimuli (by varying their task relevance) and to their underlying structure (by delivering perceptual tasks that require segregated or integrated percepts). ERP results based on the mismatch negativity (MMN) show no evidence for a modulation of stream integration by attention, while stream segregation results were less clear due to overlapping attention-related components in the MMN latency range. We suggest future studies combining the proposed two-way approach with some improvements in the ERP measurement of sequential stream segregation.     

Laterality in metaphor processing: lack of evidence from functional magnetic resonance imaging for the right hemisphere theory

We investigated processing of metaphoric sentences using event-related functional magnetic resonance imaging (fMRI). Seventeen healthy subjects (6 female, 11 male) read 60 novel short German sentence pairs with either metaphoric or literal meaning and performed two different tasks: judging the metaphoric content and judging whether the sentence has a positive or negative connotation. Laterality indices for 8 regions of interest were calculated: Inferior frontal gyrus (opercular part and triangular part), superior, middle, and inferior temporal gyrus, precuneus, temporal pole, and hippocampus. A left lateralised network was activated with no significant differences in laterality between the two tasks. The lowest degree of laterality was found in the temporal pole. Other factors than metaphoricity per se might trigger right hemisphere recruitment. Results are discussed in the context of lesion and hemifield studies.     

Does segregation by colour/luminance facilitate the detection of structure-from-motion in noise?

The aim in the experiments was to examine whether the detection of structure-from-motion (SFM) in noise was facilitated when target and noise were segregated by colour and/or luminance polarity. The SFM target was a rotating 'V-shape' structure simulated with limited-lifetime Gaussian micropatterns and embedded in random-motion noise. Threshold levels of V-shape slant were measured for stimuli in which target and noise were segregated or unsegregated by colour/luminance, and under two conditions, with and without static form cues to the SFM target. The presence or absence of static form cues to the SFM target was manipulated by varying the relative numbers of micropatterns in target and noise. In the absence of static form cues, segregation of target and noise by colour and/or luminance polarity did not facilitate target detection, even when subjects knew which micropatterns belonged to the target. On the other hand, when static form cues were present, segregation improved performance. These results imply that SFM processing is 'form-cue invariant' except when the target form is immediately identifiable in the static view of the stimulus. The significance of the results for understanding the role of colour vision in breaking camouflage and in 'grouping' is discussed.     

Laterality briefed: laterality modulates performance in a numerosity-congruity task

It is widely agreed that irrelevant numerical values are automatically activated. However, automatic and intentional activations may give rise to different numerical representations. We examined processing of symbolic and non-symbolic (i.e., numerosity) representations asking whether they differ in automatic and intentional processing. Participants were presented with two-dimensional displays containing repetitions of a digit and were asked to report, in different blocks, whether the digit or numerosity was smaller or larger than 5. Incongruent trials differed either in laterality between the relevant and irrelevant dimensions (i.e., the location of both dimensions in reference to the midpoint 5) or in numerical distance between dimensions. Congruency affected performance regardless of symbolic or non-symbolic presentation. For incongruent trials, laterality (not distance) affected performance, again regardless of presentation. This implies that automaticity does not mean similar processing of relevant and irrelevant dimensions. Specifically, the relevant dimension is processed elaborately whereas the irrelevant dimension is processed crudely.     

Asymmetry questionnaire outcomes correlate with several hemisphericity measures

The asymmetry questionnaire segregated subjects (n=143) into two groups. These were significantly correlated with similar groups separated, not only by three new biophysical hemisphericity protocols (Dichotic Deafness Test, Phased Mirror Tracing, Best Hand Test), but also by two preference-type measures (polarity questionnaire, preference questionnaire). Each of the 15 asymmetry questionnaire statements was significantly correlated with the outcomes of these five laterality measures. This is the third questionnaire whose outcomes correlate with those of the new biophysical measures of hemisphericity.