ang Li

Top-down modulation of prepulse inhibition of the startle reflex in humans and rats

Prepulse inhibition (PPI) is the attenuation of the startle reflex when the sudden intense startling stimulus is shortly preceded by a weaker, non-startling sensory stimulus (prepulse). PPI reflects a protective function of reducing disruptive influences to the processing of prepulse signals and is recognized as a model of sensorimotor gating. In humans, PPI is modulated by both attentional and emotional responses to prepulse, indicating that this early-stage gating is top-down modulated by higher-order cognitive processes. Recent studies have confirmed top-down modulation of PPI in animals, because PPI in rats is enhanced by auditory fear conditioning and perceived separation between fear-conditioned prepulse and masker. This review summarizes recent studies of top-down modulation of PPI conducted in humans and those in rats. Since both baseline PPI and attentional modulation of PPI in patients with schizophrenia are impaired, and both baseline PPI and conditional modulation of PPI in rats with isolation rearing are impaired, this review emphasizes that investigation of top-down modulation of PPI is critical for establishing new animal models for studying both cognitive features and neural bases of schizophrenia. Deficits in either baseline PPI or attentional modulation of PPI in either patients with attention-deficit/hyperactivity disorder (ADHD) or ADHD-modeling rats are also discussed.

The effect of perceived spatial separation on informational masking of Chinese speech

The effect of perceived spatial separation, induced by the precedence effect, on release from noise or speech masking was investigated. Listeners were asked to orally repeat Chinese nonsense sentences, which were spoken by a female talker and presented by both the left (-45 degrees) and right (+45 degrees) loudspeakers, when maskers, which were either speech-spectrum noise sounds or Chinese nonsense sentences spoken by two other female talkers, were presented by the same two loudspeakers. Delays between identical sounds presented over the two loudspeakers were used to control the perceived locations of the target (right only) and masker (right, center, or left). The results show that perceived 45 degrees and 90 degrees separations of target speech from masking speech led to equivalently marked improvement in speech recognition, even though the degree of improvement was smaller than that reported in [J. Acoust. Soc. Am. 106 (1999) 3578 (using English nonsense speech)]. When the masker was noise, however, perceived separation only marginally improved speech recognition. These results indicate that release from informational masking, due to perceived target/masker spatial separation induced by the precedence effect, also occurs for tonal Chinese speech. Compared to the 45 degrees perceived within-hemifield separation, the 90 degrees perceived cross-hemifield separation does not produce further unmasking.

The effect of voice cuing on releasing Chinese speech from informational masking

In a cocktail-party environment, human listeners are able to use perceptual-level and cognitive-level cues to segregate the attended target speech from other background conversations. At the cognitive level, priming the listener with part of the target speech in quiet can markedly improve the recognition of the remaining parts when the target speech and competing speech are presented at the same time. Hence, knowledge of content (content cuing) improves speech recognition when other people are talking. In addition, familiarity or knowledge of the voice characteristics of the target talker could also help the listener attend to the target talker when other talkers are present. The present study investigated the extent to which a cognitive-level cue (content cuing) and a perceptual-level cue (voice cuing) can improve word identification for speech masked by noise or by other speech in Chinese listeners. Specifically, listeners were primed with part of a sentence in quiet before a sentence was repeated in the presence of either noise or speech. The priming sentence was always in the same voice as the target sentence. Two kinds of primes were investigated: same-sentence primes, and different-sentence primes. Under speech-masking conditions, each of the two prime types significantly improved recognition of the last key word in the full-length target sentence. Under noise-masking conditions, same-sentence primes had a weak but significant releasing effect, but different-sentence primes had only a negligible releasing effect. These results suggest that in addition to content cues, voice cues can be used by Chinese listeners to release speech from masking by other talkers.

Auditory gating processes and binaural inhibition in the inferior colliculus

Physiological/behavioral/perceptual responses to an auditory stimulus can be inhibited by another leading auditory stimulus at certain stimulus intervals, and have been considered useful models of auditory gating processes. Two typical examples of auditory gating are prepulse inhibition of the startle reflex and the precedence effect (echo suppression). This review summarizes studies of these two auditory gating processes with regard to their biological significance, cognitive modulation, binaural properties, and underlying neural mechanisms. Both prepulse inhibition and the precedence effect have gating functions of reducing the disruptive influence of the lagging sound, but prepulse inhibition has a much longer temporal window than the precedence effect. Attentional processes can modulate prepulse inhibition, and the listeners previous experience can modulate the precedence effect. Compared to monaural hearing, binaural hearing reduces prepulse inhibition but enhances the precedence effect. The inferior colliculus, the major structure of the auditory midbrain, plays an important role in mediating these two auditory gating processes, and inhibitory neural transmissions within the inferior colliculus may account for binaural inhibition observed in prepulse inhibition and lag suppression recorded in the inferior colliculus. The neural mechanisms underlying binaural inhibition in the inferior colliculus are also discussed.

Perceptual integration between target speech and target-speech reflection reduces masking for target-speech recognition in younger adults and older adults

This study evaluated unmasking functions of perceptual integration of target speech and simulated target-speech reflection, which were presented by two spatially separated loudspeakers. In both younger-adult listeners with normal hearing and older-adult listeners in the early stages of presbycusis, reducing the time interval between target speech and target-reflection simulation (inter-target interval, ITI) from 64 to 0ms not only progressively enhanced perceptual integration of target-speech signals, but also progressively released target speech from either speech masking or noise masking. When the signal-to-noise ratio was low, the release from speech masking was significantly larger than the release from noise masking in both younger listeners and older listeners, but the longest ITI at which a significant release from speech masking occurred was significantly shorter in older listeners than in younger listeners. These results suggest that in reverberant environments with multi-talker speech, perceptual integration between the direct sound wave and correlated reflections, which facilitates perceptual segregation of various sources, is critical for unmasking attended speech. The age-related reduction of the ITI range for releasing speech from speech masking may be one of the causes for the speech-recognition difficulties experienced by older listeners in such adverse environments.

Attribute capture in the precedence effect for long-duration noise sounds.

Listeners perceptually fuse the direct wave from a sound source with its reflections off nearby surfaces into a single sound image, located at or near the sound source (the precedence effect). This study investigated how a brief gap presented in the middle of either a direct wave or simulated reflection is incorporated into the fused image. For short (<9.5 ms) delays between the direct (leading) and reflected (lagging) waves, no sound was perceived from the direction of the lagging wave. For delays between 10 and 15 ms, both sounds were perceived, but the gap was heard only on the leading side. When the gap was only in the correlated lagging sound at short delays, it also was perceived as occurring on the leading side. Moreover, gap detection thresholds were the same for gaps in the leading and lagging sounds, suggesting that the perception of the gap was not suppressed, but rather incorporated into the leading sound. Finally, scalp event-related potentials were not associated with the precedence effect until the gap occurred. This suggests that cortical mechanisms are engaged to maintain fusion when attributes in direct or reflected waves change.

Differentially Organized Top-Down Modulation of Prepulse Inhibition of Startle

Prepulse inhibition (PPI) of startle is the suppression of the startle reflex when a weaker sensory stimulus (the prepulse) shortly precedes the startling stimulus. PPI can be attentionally enhanced in both humans and laboratory animals. This study investigated whether the following three forebrain structures, which are critical for initial cortical processing of auditory signals, auditory fear conditioning/memories, and spatial attention, respectively, play a role in the top-down modulation of PPI in rats: the primary auditory cortex (A1), lateral nucleus of the amygdala (LA), and posterior parietal cortex (PPC). The results show that, under the noise-masking condition, PPI was enhanced by fear conditioning of the prepulse in a prepulse-specific manner, and the conditioning-induced PPI enhancement was further increased by perceptual separation between the conditioned prepulse and the noise masker. Reversibly blocking glutamate receptors in the A1 with 2 mm kynurenic acid eliminated both the conditioning-induced and perceptual separation-induced PPI enhancements. Blocking the LA eliminated the conditioning-induced but not the perceptual separation-induced PPI enhancement, and blocking the PPC specifically eliminated the perceptual separation-induced PPI enhancement. The two types of PPI enhancements were also eliminated by the extinction manipulation. Thus, the top-down modulation of PPI is differentially organized and depends on operations of various forebrain structures. Due to the fine-tuned modulation by higher-order cognitive processes, functions of PPI can be more flexible to complex environments. The top-down enhancements of PPI in rats are also useful for modeling some mental disorders, such as schizophrenia, attention deficit/hyperactivity disorder, and posttraumatic stress disorder.

Human Auditory Cortex Activity Shows Additive Effects of Spectral and Spatial Cues during Speech Segregation

In noisy social gatherings, listeners perceptually integrate sounds originating from one persons voice (e.g., fundamental frequency (f(0)) and harmonics) at a particular location and segregate these from concurrent sounds of other talkers. Though increasing the spectral or the spatial distance between talkers promotes speech segregation, synergetic effects of spatial and spectral distances are less well understood. We studied how spectral and/or spatial distances between 2 simultaneously presented steady-state vowels contribute to perception and activation in auditory cortex using magnetoencephalography. Participants were more accurate in identifying both vowels when they differed in f(0) and location than when they differed in a single cue only or when they shared the same f(0) and location. The combined effect of f(0) and location differences closely matched the sum of single effects. The improvement in concurrent vowel identification coincided with an object-related negativity that peaked at about 140 ms after vowel onset. The combined effect of f(0) and location closely matched the sum of the single effects even though vowels with different f(0), location, or both generated different time courses of neuromagnetic activity. We propose that during auditory scene analysis, acoustic differences among the various sources are combined linearly to increase the perceptual distance between the co-occurring sound objects.

The Effect of Voice Cuing on Releasing Speech From Informational Masking Disappears in Older Adults

Objective: To investigate whether older adults can use voice information to unmask speech. Design: Under a voice-priming condition, before a target-speech sentence was presented with a noise or speech masker, one or two voice-priming sentences were recited with the same voice reciting the target sentence. Eighteen younger adults and 12 older adults with clinically normal hearing were instructed to loudly repeat the target sentence. Results: Presenting the voice-priming sentence(s) improved target-speech identification only when the masker was speech in younger adults but not older adults. Conclusion: For older adults, the inability to use voice information to reduce informational masking contributes to their speech-recognition difficulties in “cocktail-party” environments.

Transient Auditory Storage of Acoustic Details Is Associated with Release of Speech from Informational Masking in Reverberant Conditions.

Perceptual integration of the sound directly emanating from the source with reflections needs both temporal storage and correlation computation of acoustic details. We examined whether the temporal storage is frequency dependent and associated with speech unmasking. In Experiment 1, a break in correlation (BIC) between interaurally correlated wideband or narrowband noises was detectable even when an interaural interval (IAI) was introduced. The longest IAI, which varied markedly across participants, could be up to about 20 ms for wideband noise and decreased as the center frequency was increased for narrowband noises. In Experiment 2, when the interval between target speech and its single-reflection simulation (intertarget interval [ITI]) was reduced from 64 to 0 ms, intelligibility of target speech was markedly improved under speech-masking but not noise-masking conditions. The longest effective ITI correlated with the longest IAI for detecting the BIC only in the low-frequency (<or=400 Hz) narrowband noise. Thus the ability to temporally store fine details contributes to perceptual integration of correlated leading and lagging sounds, which in turn, contributes to releasing speech from informational masking in noisy, reverberant environments.