Spatial processing in the primate auditory cortex.

Spatial localization of auditory stimuli is dependent on the cerebral cortex, yet it remains unclear how cortical activity gives rise to spatial percepts. It has recently been proposed that spatial information is processed serially within the primate auditory cortex, initially in the primary auditory cortex (AI) through the auditory areas caudal to AI, particularly the caudomedial (CM) and caudolateral fields, and onward to the parietal lobe. The activity of single neurons in AI and CM supports this hypothesis, where a greater percentage of CM neurons are sensitive to the spatial location of acoustic stimuli than AI neurons, and the spatial sensitivity of CM neurons extends across a broader range of the stimulus spectrum compared to AI neurons. Further, populations of CM neurons are better able to predict sound localization ability than are populations of AI neurons. We have recently explored the effects of stimulus intensity on both sound localization performance and the spatial sensitivity of auditory cortical neurons. The preliminary results of these experiments again indicate that spatial information is serially processed between AI and the caudal fields. The effects of visual stimulation on auditory localization have also been investigated. Under the appropriate circumstances, visual stimuli can "capture" the spatial location of auditory stimuli in both humans and monkeys. This perceptual illusion suggests that there is a plastic shift in auditory spatial perception. Where the representation of this shift resides is unknown, although two likely candidates are the multimodal regions of the parietal lobe and the superior temporal sulcus.     

Spectral features control temporal plasticity in auditory cortex.

Cortical responses are adjusted and optimized throughout life to meet changing behavioral demands and to compensate for peripheral damage. The cholinergic nucleus basalis (NB) gates cortical plasticity and focuses learning on behaviorally meaningful stimuli. By systematically varying the acoustic parameters of the sound paired with NB activation, we have previously shown that tone frequency and amplitude modulation rate alter the topography and selectivity of frequency tuning in primary auditory cortex. This result suggests that network-level rules operate in the cortex to guide reorganization based on specific features of the sensory input associated with NB activity. This report summarizes recent evidence that temporal response properties of cortical neurons are influenced by the spectral characteristics of sounds associated with cholinergic modulation. For example, repeated pairing of a spectrally complex (ripple) stimulus decreased the minimum response latency for the ripple, but lengthened the minimum latency for tones. Pairing a rapid train of tones with NB activation only increased the maximum following rate of cortical neurons when the carrier frequency of each train was randomly varied. These results suggest that spectral and temporal parameters of acoustic experiences interact to shape spectrotemporal selectivity in the cortex. Additional experiments with more complex stimuli are needed to clarify how the cortex learns natural sounds such as speech.     

Profile of auditory temporal processing in older listeners.

This investigation examined age-related performance differences on a range of speech and nonspeech measures involving temporal manipulation of acoustic signals and variation of stimulus complexity. The goal was to identify a subset of temporally mediated measures that effectively distinguishes the performance patterns of younger and older listeners, with and without hearing loss. The nonspeech measures included duration discrimination for simple tones and gaps, duration discrimination for tones and gaps embedded within complex sequences, and discrimination of temporal order. The speech measures were undistorted speech, time-compressed speech, reverberant speech, and combined time-compressed + reverberant speech. All speech measures were presented both in quiet and in noise. Strong age effects were observed for the nonspeech measures, particularly in the more complex stimulus conditions. Additionally, age effects were observed for all time-compressed speech conditions and some reverberant speech conditions, in both quiet and noise. Effects of hearing loss were observed also for the speech measures only. Discriminant function analysis derived a formula, based on a subset of these measures, for classifying individuals according to temporal performance consistent with age and hearing loss categories. The most important measures to accomplish this goal involved conditions featuring temporal manipulations of complex speech and nonspeech signals.     

Subcortical neural coding mechanisms for auditory temporal processing

Biologically relevant sounds such as speech, animal vocalizations and music have distinguishing temporal features that are utilized for effective auditory perception. Common temporal features include sound envelope fluctuations, often modeled in the laboratory by amplitude modulation (AM), and starts and stops in ongoing sounds, which are frequently approximated by hearing researchers as gaps between two sounds or are investigated in forward masking experiments. The auditory system has evolved many neural processing mechanisms for encoding important temporal features of sound. Due to rapid progress made in the field of auditory neuroscience in the past three decades, it is not possible to review all progress in this field in a single article. The goal of the present report is to focus on single-unit mechanisms in the mammalian brainstem auditory system for encoding AM and gaps as illustrative examples of how the system encodes key temporal features of sound. This report, following a systems analysis approach, starts with findings in the auditory nerve and proceeds centrally through the cochlear nucleus, superior olivary complex and inferior colliculus. Some general principles can be seen when reviewing this entire field. For example, as one ascends the central auditory system, a neural encoding shift occurs. An emphasis on synchronous responses for temporal coding exists in the auditory periphery, and more reliance on rate coding occurs as one moves centrally. In addition, for AM, modulation transfer functions become more bandpass as the sound level of the signal is raised, but become more lowpass in shape as background noise is added. In many cases, AM coding can actually increase in the presence of background noise. For gap processing or forward masking, coding for gaps changes from a decrease in spike firing rate for neurons of the peripheral auditory system that have sustained response patterns, to an increase in firing rate for more central neurons with transient responses. Lastly, for gaps and forward masking, as one ascends the auditory system, some suppression effects become quite long (echo suppression), and in some stimulus configurations enhancement to a second sound can take place.     

Maturation of visual and auditory temporal processing in school-aged children

PURPOSE: To examine development of sensitivity to auditory and visual temporal processes in children and the association with standardized measures of auditory processing and communication. Methods Normative data on tests of visual and auditory processing were collected on 18 adults and 98 children aged 6-10 years of age. Auditory processes included detection of pitch from temporal cues using iterated rippled noise and frequency modulation detection at 2 Hz, 40 Hz, and 240 Hz. Visual processes were coherent form and coherent motion detection. Test-retest data were gathered on 21 children. RESULTS: Performance on perceptual tasks improved with age, except for fine temporal processing (iterated rippled noise) and coherent form perception, both of which were relatively stable over the age range. Within-subject variability (as assessed by track width) did not account for age-related change. There was no evidence for a common temporal processing factor, and there were no significant associations between perceptual task performance and communication level (Children's Communication Checklist, 2nd ed.; D. V. M. Bishop, 2003) or speech-based auditory processing (SCAN-C; R. W. Keith, 2000). CONCLUSIONS: The auditory tasks had different developmental trajectories despite a common procedure, indicating that age-related change was not solely due to responsiveness to task demands. The 2-Hz frequency modulation detection task, previously used in dyslexia research, and the visual tasks had low reliability compared to other measures.     

Neonatal nicotine exposure impairs development of auditory temporal processing

Accurate temporal processing of sound is essential for detecting word structures in speech. Maternal smoking affects speech processing in newborns and may influence child language development; however, it is unclear how neonatal exposure to nicotine, present in cigarettes, affects the normal development of temporal processing. The present study used the gap-induced prepulse inhibition (gap-PPI) of the acoustic startle response to investigate the effects of neonatal nicotine exposure on the normal development of gap detection, a behavioral testing procedure of auditory temporal resolution. Neonatal rats were injected twice per day with saline (control), 1mg/kg nicotine (N-1 mg) or 5 mg/kg nicotine (N-5 mg) from postnatal day 8-12 (P8-P12). During the first month after birth, rats showed poor gap-PPI in all three groups. At P45 and P60, gap-PPI in control rats improved significantly, whereas rats exposed to nicotine exhibited less improvement. At P60, the gap-detection threshold in the N-5 mg group was significantly higher than in the control group, suggesting that neonatal nicotine exposure affects the normal development of gap-detection acuity. Additionally, 1h after receiving an acute nicotine injection (1 mg/kg), gap-PPI recorded in adult rats from the N-5 mg group showed a temporary significant improvement. These results suggest that neonatal nicotine exposure reduces gap-PPI implying an impairment of the normal development of auditory temporal processing by inducing changes in cholinergic systems.     

等调制深度时间调制转换曲线评估豚鼠下丘和听皮层的时间分辨率

目的探讨以听觉系统对调幅信号响应幅值随调制频率的改变计算等调制深度时间调制转换函数（temporal modulation transfer function，TMTF）来客观评估听觉系统时间分辨率的可行性。方法豚鼠下丘和听皮层分别埋植慢性电极，记录正弦调幅纯音（载频为8kHz，调制深度固定为100％）诱发电位，反应幅值经快速傅立叶变换（fast Fourier transform，F丌）得出相对反应幅值，并以相对反应幅值和调制频率绘制出等调制深度TMTF。记录正弦调幅纯音每个调制频率的调制深度从100％至10％的诱发电位，得出与传统的调制深度阈值TMTF方法相当的等幅值TMTF，与等调制深度TMTF相比较，判断等调制深度TMTF方法的有效性。结果豚鼠下丘和听皮层的等调制深度TMTF与等幅值TMTF都分别表现为带通和低通特性；等调制深度TMTF的截止频率与等幅值TMTF的截止频率差异无统计学意义（P值均〉0．05）。豚鼠听皮层等调制深度TMTF的截止频率与传统的行为学结果基本一致。结论以100％调制深度的正弦调幅纯音诱发反应幅值与调制频率绘制等调制深度TMTF是一种有效的客观评估听觉系统时间分辨率的方法，其中豚鼠听皮层的等调制深度TMTF可用于行为学预测。     

The auditory basis of language impairments: temporal processing versus processing efficiency hypotheses

Claims have been made that language-impaired children have deficits processing rapidly presented or brief sensory information. These claims, known as the 'temporal processing hypothesis', are supported by demonstrations that language-impaired children have excess backward masking (BM). One explanation for these results is that BM is developmentally delayed in these children. However, little was known about how BM normally develops. Recently, we assessed BM in normally developing 6- and 8-year-old children and adults. Results showed that BM thresholds continue to improve over a comparatively protracted period (>10 years old). We also analysed reported deficits in BM in language-impaired and younger children, in terms of a model of temporal resolution. This analysis suggests that poor processing efficiency, rather than deficits in temporal resolution, can account for these results. This 'processing efficiency hypothesis' was recently tested in our laboratory. This experiment measured BM as a function of delays between the tone and the noise in children and adults. Results supported the processing efficiency hypothesis, and suggested that reduced processing efficiency alone could account for differences between adults and children. These findings provide a new perspective on the mechanisms underlying communication disorders, and imply that remediation strategies should be directed towards improving processing efficiency, not temporal resolution.     

Complex tone processing and critical band in the human auditory cortex

Psychophysical experiments in humans have indicated that the auditory system has a well-defined bandwidth for resolution of complex stimuli. This bandwidth is known as the critical bandwidth (CBW). Physiological correlates of the CBW were examined in the human auditory cortex. Two- and three-tone complexes were used as the sound stimuli with all signals presented at 55 dB sound pressure level (SPL). The duration of stimulation was 500 ms, with rise and fall ramps of 10 ms. Ten normal-hearing subjects took part in the study. Auditory-evoked fields were recorded using a 122-channel whole-head magnetometer in a magnetically shielded room. The latencies, source strengths, and coordinates of the N1m waves, which were found above the left and right temporal lobes approximately 100 ms after the onset of stimulation, were analyzed. The results indicated that N1m amplitudes were approximately constant when the frequency separation of a two-tone complex or the total bandwidth of a three-tone complex was less than the CBW; however, the N1m amplitudes increased with increasing frequency separation or total bandwidth when these were greater than the CBW. These findings indicate critical band-like behavior in the human auditory cortex. The N1m amplitudes in the right hemisphere were significantly greater than those in the left hemisphere, which may reflect a right-hemispheric dominance in the processing of tonal stimuli.     

Aging and the processing of sound duration in human auditory cortex

Age-related declines in coding the fine temporal structure of acoustic signals is proposed to play a critical role in the speech perception difficulties commonly observed in older individuals. This hypothesis was tested by measuring auditory evoked potentials elicited by sounds of various durations in young, middle-aged and older adults. All stimuli generated N1 and P2 waves that peaked at about 104 and 200 ms post-stimulus onset. The N1 amplitude increased linearly with increases in the tonal duration in young, middle-aged, and older adults. The P2 amplitude also increased linearly with signal duration, but only in young and middle-aged adults. The results demonstrate that the N1 and P2 waves can resolve duration differences as short as 2-4 ms and that normal aging decreases the temporal resolving power for processing small differences in sound duration.     

Temporal envelope processing in the human auditory cortex: response and interconnections of auditory cortical areas

Temporal envelope processing in the human auditory cortex has an important role in language analysis. In this paper, depth recordings of local field potentials in response to amplitude modulated white noises were used to design maps of activation in primary, secondary and associative auditory areas and to study the propagation of the cortical activity between them. The comparison of activations between auditory areas was based on a signal-to-noise ratio associated with the response to amplitude modulation (AM). The functional connectivity between cortical areas was quantified by the directed coherence (DCOH) applied to auditory evoked potentials. This study shows the following reproducible results on twenty subjects: (1) the primary auditory cortex (PAC), the secondary cortices (secondary auditory cortex (SAC) and planum temporale (PT)), the insular gyrus, the Brodmann area (BA) 22 and the posterior part of T1 gyrus (T1Post) respond to AM in both hemispheres. (2) A stronger response to AM was observed in SAC and T1Post of the left hemisphere independent of the modulation frequency (MF), and in the left BA22 for MFs 8 and 16Hz, compared to those in the right. (3) The activation and propagation features emphasized at least four different types of temporal processing. (4) A sequential activation of PAC, SAC and BA22 areas was clearly visible at all MFs, while other auditory areas may be more involved in parallel processing upon a stream originating from primary auditory area, which thus acts as a distribution hub. These results suggest that different psychological information is carried by the temporal envelope of sounds relative to the rate of amplitude modulation.     

Cortical centres underlying auditory temporal processing in humans: a PET study

We have used positron emission tomography (PET) to test a specific hypothesis of a neural system subserving auditory temporal processing (acoustical stimulus duration discrimination). Maps of the cerebral blood flow distribution during specific stimulations were obtained from five normally-hearing and otherwise healthy subjects. The auditory stimuli consisted of sounds of varying duration and of auditorily presented words in which the duration of the initial phoneme was manipulated. All stimuli alternated with conditions of silence in a subtraction paradigm. The blood flow distribution was mapped with O-15-labelled water. The results demonstrated that stimuli requiring recognizing, memorizing, or attending to specific target sounds during temporal processing generally resulted in significant activation of both frontal lobes and the parietal lobe in the right hemisphere. Based on these results, we hypothesise that a network consisting of anterior and posterior auditory attention and short-term memory sites subserves acoustical stimulus duration perception and analysis (auditory temporal processing).     

Distributed representation of spectral and temporal information in rat primary auditory cortex.

Modulations of amplitude and frequency are common features of natural sounds, and are prominent in behaviorally important communication sounds. The mammalian auditory cortex is known to contain representations of these important stimulus parameters. This study describes the distributed representations of tone frequency and modulation rate in the rat primary auditory cortex (A1). Detailed maps of auditory cortex responses to single tones and tone trains were constructed from recordings from 50-60 microelectrode penetrations introduced into each hemisphere. Recorded data demonstrated that the cortex uses a distributed coding strategy to represent both spectral and temporal information in the rat, as in other species. Just as spectral information is encoded in the firing patterns of neurons tuned to different frequencies, temporal information appears to be encoded using a set of filters covering a range of behaviorally important repetition rates. Although the average A1 repetition rate transfer function (RRTF) was low-pass with a sharp drop-off in evoked spikes per tone above 9 pulses per second (pps), individual RRTFs exhibited significant structure between 4 and 10 pps, including substantial facilitation or depression to tones presented at specific rates. No organized topography of these temporal filters could be determined.     

Intracortical microstimulation induced changes in spectral and temporal response properties in cat auditory cortex

Intracortical microstimulation (ICMS), consisting of a 40 ms burst (rate 300 Hz) of 10 microA pulses, repetitively administered once per second, for a total duration of 1 h, induced cortical reorganization in the primary auditory cortical field of the anesthetized cat. Multiple single-unit activity was simultaneously recorded from three to nine microelectrodes. Spiking activity was recorded from the same units prior to and following the application of ICMS in conjunction with tone pips at the characteristic frequency (CF) of the stimulus electrode. ICMS produced a significant increase in the mean firing rate, and in the occurrence of burst activity. There was an increase in the cross-correlation coefficient (R) for unit pairs recorded from sites distant from the ICMS site, and a decrease in R for unit pairs that were recorded at the stimulation site. ICMS induced a shift in the CF, dependent on the difference between the baseline CF and the ICMS-paired tone pip frequency. ICMS also resulted in broader tuning curves, increased driven peak firing rate and reduced response latency. This suggests a lasting reduction in inhibition in a small region surrounding the ICMS site that allows expansion of the frequency range normally represented in the vicinity of the stimulation electrode.     

Evaluation of temporal and suprasegmental auditory processing in patients with unilateral hearing loss

ObjectivesTo determine the temporal order, resolution, and perception of prosody skills in Single-Sided Deafness (SSD) compared to an age- and sex-matched normal hearing group's same side ear and both ears.MethodsThis was a Case-Control study including 30 subjects with SSD, and age- and sex-matched 30 subjects with bilateral normal hearing (total of 60 subjects- mean age: 38.7 ± 11.6 years). The Montreal Cognitive Assessment (MoCA), Frequency Pattern Test (FPT), Duration Pattern Test (DPT), Random Gap Detection Test (RGDT), Evaluation of Motor Response Time and Emotional Prosody Assessment were performed on the clinically normal ear in the SSD group, the same side ear in the normal hearing group, and both ears of the normal hearing group (the SSD, MNH, and BNH groups, respectively).ResultsIndividuals with SSD had worse results in DPT (p < .001), gap detection at 0.5 kHz (p < .001), gap detection at 4 kHz (p < .001), and composite score (p < .001) than the BNH group. For reaction time measurements, the SSD group had slower performance scores than the BNH group for DPT (p < .001) and FPT (p < .001).ConclusionsPoor temporal processing ability and reduced reaction times may help explain the difficulties in those with SSD in performing daily living activities such as speech understanding in noise and requires more processing efforts. However, there were no significant differences between the groups in frequency pattern performance and emotional prosody skills, supporting the claim that fundamental frequency is one of the most important measures of perception in emotional prosody. We demonstrated that unilateral hearing is adequate to analyze frequency patterns to aid in prosody perception. Analysis of reaction times in temporal processing and emotional prosody could provide different perspectives of auditory processing. Slower reaction time of SSD should be considered for habilitation purposes.     

Age-related changes in the auditory evoked brainstem

Summary The auditory evoked brainstem responses of guinea pigs in two age groups were recorded and examined for evidence of age-dependent changes at peripheral stations in the auditory pathway. Because pigmented guinea pigs have been found to be less sensitive to sounds than albinos, both groups were included in this study. Old and young animals did not differ in response latency or in the conduction times associated with the individual potentials. By contrast, the amplitudes of the brainstem responses to high-frequency stimuli were distinctly reduced in old guinea pigs, with no difference in the dynamic of the amplitude between the two age groups. Within each age group, albino and pigmented animals resembled one another in all parameters studied. The effect of the aging process at the peripheral stations of the auditory pathway is discussed in the light of these results.Supported by the Deutsche Forschungsgemeinschaft (DFG)     

Age-related morphological changes in auditory middle-latency response

Age-related changes in the waveforms of the middle latency response (MLR) were investigated in 9 adults and 28 children aged between 4 and 14 years. The children were classified into three groups according to their age. For obtaining characteristic configurations in the responses for each group, composite group averaging was performed by summating the individual recordings in each group. With high-pass digital filtering at 20 Hz, composite MLR for adults showed a well-defined Na-Pa-Nb-Pb complex with peak latencies at about 17, 30, 45 and 63 ms, respectively. The composite response for children aged 4-7 years was characterized by a broad positive deflection (Pa) followed by a negative peak (Nb) at about 40 and 60 ms after stimulus onset, respectively. The peak latency of Pa was close to the adult value in the composite MLR for subjects aged 8-11 years, while the complete adult pattern in the later part of the response was not reached even in the composite response for subjects aged 12-14 years.