Neuronal mechanisms of auditory backward recognition masking in macaque auditory cortex

The sensation of a single sound event can be altered by subsequent sounds. This study searched for neural mechanisms of such retroactive effects in macaque auditory cortex by comparing neural responses to single tones with responses to two consecutive tones. Retroactive influences were found to affect late parts of the response to a tone, which comprised 53/134 of the recordings of action potentials and 88/131 of the recordings of field potentials performed in primary, caudal, and medial auditory fields. If before or during the occurrence of the late response to the first tone a second tone was presented the late response was suppressed. Suppression of late cortical responses parallels perceptual phenomena like backward recognition masking, suggesting that suppression of late responses provides a neural correlate of auditory backward effects.     

Time course of simultaneous masking in the starling's auditory forebrain

Simultaneous masking of pure tones was studied in the primary auditory forebrain of a songbird species, the European starling (Sturnus vulgaris). The responses of 32 multi-unit clusters in the input layer of the auditory neostriatum (field L2a) were recorded via radiotelemetry from freely moving birds. The probe was a 10-ms tone burst at the units' characteristic-frequency (CF) presented 20 dB above the threshold. The masker was an 80-ms tone burst presented either at the units' CF (excitatory masker) or at a frequency located in inhibitory side-bands (inhibitory masker) of the units' tuning curves. The probe was presented either 3 ms or 63 ms after masker onset. Probes presented at a 3-ms delay were influenced at significantly lower levels of an excitatory masker than probes presented at a 63-ms delay. The mean difference in masker level at the detection thresholds for both probe delays was 8 dB. No difference in masker level was observed for inhibitory-frequency maskers. The observed neural masking effects may be explained by at least four mechanisms: (1) swamping of the probe response by the response to the masker, (2) a reduction of the probe response during neural adaptation of the response to the masker, (3) a reduction of the probe response during side-band inhibition in the central nervous system, and (4) suppression originating in the cochlea.     

Use of contralateral masking in the measurement of the auditory brainstem response

In the first portion of this study, the effects of two levels of contralateral masking on the auditory brainstem response (ABR) were investigated in 10 normal-hearing subjects. No significant changes were observed in the mean latency-intensity functions or the mean amplitude-intensity functions of this group of subjects when noise of various levels was added to the nontest ear. In the second portion of this study, ABRs were also recorded from the poorer ear of four subjects with a profound unilateral sensorineural hearing loss. Results from the latter group revealed a crossed-over wave V in all cases when the stimulus was delivered to the poorer ear and the nontest (better) ear was not masked. Contralateral masking obliterated this "crossed ABR" in all four unilaterally impaired subjects. These results provide support for the use of contralateral masking when recording from the poorer ear of subjects having asymmetrical hearing loss.     

Nonsimultaneous auditory masking in the budgerigar (Melopsittacus undulatus).

Compared the frequency resolving power of 3 male budgerigar birds and 3 humans on several nonsimultaneous masking procedures in which one pure tone was used to mask another. Similar patterns of frequency selectivity were found for all 3 masking procedures (forward, backward, and combined forward/backward) for both species. Budgerigars showed considerably greater frequency resolving power on all 3 procedures than humans. Budgerigars also showed differences in frequency resolving power across masking conditions, but human Ss did not. Results indicate that the budgerigar auditory system may be even more highly tuned than was previously thought and suggest fundamental differences between the mechanisms of frequency selectivity of birds and humans. (19 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Time course of forward masking tuning curves in cat primary auditory cortex

Nonsimultaneous two-tone interactions were studied in the primary auditory cortex of anesthetized cats. Poststimulatory effects of pure tone bursts (masker) on the evoked activity of a fixed tone burst (probe) were investigated. The temporal interval from masker onset to probe onset (stimulus onset asynchrony), masker frequency, and intensity were parametrically varied. For all of the 53 single units and 58 multiple-unit clusters, the neural activity of the probe signal was either inhibited, facilitated, and/or delayed by a limited set of masker stimuli. The stimulus range from which forward inhibition of the probe was induced typically was centered at and had approximately the size of the neuron's excitatory receptive field. This "masking tuning curve" was usually V shaped, i.e., the frequency range of inhibiting masker stimuli increased with the masker intensity. Forward inhibition was induced at the shortest stimulus onset asynchrony between masker and probe. With longer stimulus onset asynchronies, the frequency range of inhibiting maskers gradually became smaller. Recovery from forward inhibition occurred first at the lower- and higher-frequency borders of the masking tuning curve and lasted the longest for frequencies close to the neuron's characteristic frequency. The maximal duration of forward inhibition was measured as the longest period over which reduction of probe responses was observed. It was in the range of 53-430 ms, with an average of 143 +/- 71 (SD) ms. Amount, duration and type of forward inhibition were weakly but significantly correlated with "static" neural receptive field properties like characteristic frequency, bandwidth, and latency. For the majority of neurons, the minimal inhibitory masker intensity increased when the stimulus onset asynchrony became longer. In most cases the highest masker intensities induced the longest forward inhibition. A significant number of neurons, however, exhibited longest periods of inhibition after maskers of intermediate intensity. The results show that the ability of cortical cells to respond with an excitatory activity depends on the temporal stimulus context. Neurons can follow higher repetition rates of stimulus sequences when successive stimuli differ in their spectral content. The differential sensitivity to temporal sound sequences within the receptive field of cortical cells as well as across different cells could contribute to the neural processing of temporally structured stimuli like speech and animal vocalizations.     

Directional solidification and phase equilibria in the Ni-Al system

A method for the optimization of a phase diagram on the basis of results from directional solidification experiments is proposed. The experimental microstructure selection map is compared with a calculated map and the input parameters are varied until a satisfactory agreement is obtained. The calculation of the microstructure selection map is based on the maximum temperature criterion and on analytical models for the growth of plane front, dendritic, and eutectic structures. This method is applied to the Ni-Al system where the phase equilibria close to the melting point of the γ′-Ni₃Al phase have been subject to discussion for over 50 years. A new version of this part of the phase diagram is proposed, which is coherent with the results from directional solidification experiments.     

Enhancing the reading of dyslexic children by reading acceleration and auditory masking.

This research attempted to improve the reading performance of dyslexic children through two different methods: reading acceleration and auditory masking. Participants were 52 dyslexic children and 52 reading-level matched normal, novice readers. Results indicated that whereas acceleration improved reading performance in both groups, auditory masking was beneficial to dyslexic children only. Furthermore, a combined condition of both acceleration and masking was the most effective in enhancing dyslexic children's comprehension. It is argued that because normal readers use the phonological route quite effectively, its masking is detrimental to performance. On the other hand, auditory masking reduces the impact of the presumed phonological impairment of dyslexic children. Analysis of decoding mistakes suggested that both manipulations might have resulted in a more effective utilization of orthographic information and enhanced top-down context effects for dyslexic and novice readers. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Release from masking due to spatial separation of sources in the identification of nonspeech auditory patterns

A nonspeech pattern identification task was used to study the role of spatial separation of sources on auditory masking in multisource listening environments. The six frequency patterns forming the signal set were comprised of sequences of eight 60-ms tone bursts. Bursts of masking sounds were played synchronously with the signals. The main variables in the study were (1) the difference in spatial separation in the horizontal plane between signals and maskers and (2) the nature of the masking produced by the maskers. Spatial separation of signal and masker ranged from 0-180 degrees. The maskers were of two types: (1) a sequence of eight 60-ms bursts of Gaussian noise intended to produce predominantly peripherally based "energetic masking" and (2) a sequence of eight 60-ms bursts of eight-tone complexes intended to produce primarily centrally based "informational masking." The results indicated that identification performance improved with increasing separation of signal and masker. The amount of improvement depended upon the type of masker and the center frequency of the signal patterns. Much larger improvements were found for spatial separation of the signal and informational masker than for the signal and energetic masker. This was particularly apparent when the acoustical advantage of the signal-to-noise ratio in the more favorable of the two ears (the ear nearest the signal) was taken into account. The results were interpreted as evidence for an important role of binaural hearing in reducing sound source or message uncertainty and may contribute toward solving the "cocktail party problem."     

Encoding loudness by electric stimulation of the auditory nerve

Electric charge has long been hypothesized to be the effective stimulus variable that determines loudness evoked by directly stimulating the auditory nerve. This 'equal-charge, equal-loudness' hypothesis predicts that stimulus amplitude and duration can be traded linearly to produce equal loudness. Loudness sensations from threshold to maximum loudness were measured systematically as a function of stimulus amplitude and duration in cochlear implant listeners. The measured data do not support the equal-charge, equal-loudness hypothesis: an increment in stimulus amplitude produces a significantly louder sensation than the same change in stimulus duration. Instead of the linear equal-charge model, a power-function model successfully predicts the measured data and should be used to encode loudness in electric hearing.     

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.     

Implantation of the lateral cochlear wall for auditory nerve stimulation

Although cochlear implants now regularly achieve gratifying results, traditional intrascalar implants have certain limitations. Extraluminal implants may offset some of these problems by accessing neurons subserving a wider tonotopic range, avoiding intracochlear insertion trauma, and offering alternatives when cochlear obliteration is present. We have investigated the utility of a lateral cochlear wall implant in a normal-hearing cat model with implants at the middle and basal turns, and found successful activation of the auditory nerve at thresholds of 28.1 and 40.6 microA, respectively. No adventitial stimulation of the facial nerve was noted within the dynamic range. Maximum responsiveness was observed with implants of the middle turn of the cochlea, an area that is not reliably approached with current intrascalar implants. These observations support and extend prior observations of the feasibility of extraluminal stimulation of the auditory nerve.     

Guanylate cyclase activity in the inner ear and auditory nerve of the rat

Novel molecular mediation in the sound transductional process is implicitly suggested. We investigated the presence of the cGMP-synthesis enzyme, guanylate cyclase, in Corti's organ and auditory nerve of the rat. The soluble guanylate cyclase activity found was sensitive to changes of sound intensity in the different acoustic media used, suggesting a potential role for the system that involves this enzyme. The guanylate cyclase activity appeared to be inversely related (in the inner ear) with the sound intensity to which the animals were exposed; different behavior was observed for the auditory nerve. The enzymatic activity found in Corti's organ, a direct bio-receptor of sound, represents the first reported enzymatic activity of this type in this tissue, which apparently could be influenced by the intensity of a physical stimulus such as sound. Finally, an adequate ionic environment appears to play a potential role in the expression of the changes observed, indicating that it may function according to the requirements of the biological sensor.     

Frequency tuning of basilar membrane and auditory nerve fibers in the same cochleae

Responses to tones of a basilar membrane site and of auditory nerve fibers innervating neighboring inner hair cells were recorded in the same cochleae in chinchillas. At near-threshold stimulus levels, the frequency tuning of auditory nerve fibers closely paralleled that of basilar membrane displacement modified by high-pass filtering, indicating that only relatively minor signal transformations intervene between mechanical vibration and auditory nerve excitation. This finding establishes that cochlear frequency selectivity in chinchillas (and probably in mammals in general) is fully expressed in the vibrations of the basilar membrane and renders unnecessary additional ("second") filters, such as those present in the hair cells of the cochleae of reptiles.     

Effects of modulator phase for comodulation masking release and modulation detection interference

In an effort to evaluate the importance of across-frequency comparisons of envelope patterns in comodulation masking release (CMR) experiments and to compare joint effects of target-masker frequency separation for both CMR and modulation detection interference (MDI) tasks, thresholds were measured for three tasks. These tasks were: (a) the detection of sinusoidal amplitude modulation (SAM) of a tone, (b) the detection of a reduction in the modulation depth of a fully modulated SAM tone, and (c) the detection of a tone added to a narrow band of noise. Thresholds were obtained for the target alone and for the target presented with two maskers. For the detection of SAM, thresholds did not depend on whether the modulation patterns of the target and masker elements were the same or random. For the latter two tasks, modulator phase effects were apparent for target-masker frequency separations less than 1-2 oct. In contrast, past work has shown that observers can compare modulator envelope phases across frequency separations larger than 1-2 oct [Strickland et al., J. Acoust. Soc. Am. 86, 2160-2166 (1989); Yost and Sheft, J. Acoust. Soc. Am. 85, 848-857 (1989)]. In a second experiment, thresholds for the detection of SAM were obtained after prolonged exposure to a fully modulated SAM tone. For four of the five observers, modulation-rate specific adaptation was obtained for test/adapting carrier-frequency separations approaching 2 oct below and 1 oct above the adaptor.