Single Noise Burst Research Articles

Responses of Auditory Cortex to Complex Stimuli: Functional Organization Revealed Using Intrinsic Optical Signals

We used optical imaging of intrinsic signals to study the large-scale organization of ferret auditory cortex in response to complex sounds. Cortical responses were collected during continuous stimulation by sequences of sounds with varying frequency, period, or interaural level differences. We used a set of stimuli that differ in spectral structure, but have the same periodicity and therefore evoke the same pitch percept (click trains, sinusoidally amplitude modulated tones, and iterated ripple noise). These stimuli failed to reveal a consistent periodotopic map across the auditory fields imaged. Rather, gradients of period sensitivity differed for the different types of periodic stimuli. Binaural interactions were studied both with single contralateral, ipsilateral, and diotic broadband noise bursts and with sequences of broadband noise bursts with varying level presented contralaterally, ipsilaterally, or in opposite phase to both ears. Contralateral responses were generally largest and ipsilateral responses were smallest when using single noise bursts, but the extent of the activated area was large and comparable in all three aural configurations. Modulating the amplitude in counter phase to the two ears generally produced weaker modulation of the optical signals than the modulation produced by the monaural stimuli. These results suggest that binaural interactions seen in cortex are most likely predominantly due to subcortical processing. Thus our optical imaging data do not support the theory that the primary or nonprimary cortical fields imaged are topographically organized to form consistent maps of systematically varying sensitivity either to stimulus pitch or to simple binaural properties of the acoustic stimuli.

Spectral loudness summation for sequences of short noise bursts

Recent loudness data of single noise bursts indicate that spectral loudness summation depends on signal duration. To gain insight into the mechanisms underlying this duration effect, loudness was measured as a function of signal bandwidth centered around 2 kHz for sequences of 10-ms noise bursts at various repetition rates and, for comparison, for single noise bursts of either 10- or 1000-ms duration. The test-signal bandwidth was varied from 200 to 6400 Hz. For the repeated noise bursts, the reference signal had a bandwidth of 400 Hz. For the single noise bursts, data were obtained for two reference bandwidths: 400 and 3200 Hz. In agreement with previous results, the magnitude of spectral loudness summation was larger for the 10-ms than for the 1000-ms noise bursts. The reference bandwidth had no significant effect on the results for the single noise bursts. Up to repetition rates of 50 Hz, the magnitude of spectral loudness summation for the sequences of noise bursts was the same as for the single short noise burst. The data indicate that the mechanism underlying the duration effect in spectral loudness is considerably faster than the time constant of about 100 ms commonly associated with the temporal integration of loudness.

The precedence effect for lateralization for the mild sensory neural hearing impaired

Using dichotic signals presented by headphone, stimulus onset dominance (the precedence effect for lateralization) was investigated for six sensorineural hearing-impaired subjects. Stimuli were based on 2400-Hz lowpass-filtered 5-ms noise bursts. We used the paradigm, as described by Goverts et al. (2000, Hear. Res. 148, 88–94), in which a single noise burst is divided into leading and lagging parts, with opposite lateralization cues (viz. an interaural time delay of 0.2 ms). The occurrence of onset dominance was investigated by measuring the lateral perception of the stimulus (‘left’ or ‘right’) with fixed, equal durations of leading and lagging parts, while decreasing the absolute signal level or adding a filtered white noise. The dominance of the leading part was quantified by measuring the lateral perception of the stimulus as a function of the relative duration of the leading (and thus the lagging) part. This was done at about 40 dB sensation level (SL) in quiet and in filtered white noise, at a signal-to-noise ratio resulting in a SL of about 6 dB. Results are compared to normal hearing reference data at various SLs. Hearing-impaired data show a large variance and overall a decreased precedence effect in terms of both occurrence and quantification, which cannot be explained on basis of reduced audibility. Mean performance of the hearing-impaired subjects at 40 dB in quiet was similar to normal hearing performance in masking noise at a signal-to-noise ratio of 0 dB.

Kainic acid lesions of the superior olivary complex: effects on sound localization by the albino rat.

The ability of rats to localize sounds in space was determined before and after kainic acid lesions of the superior olivary complex (SOC). Animals were tested with a 45-ms noise burst delivered from loudspeakers on the right or left of midline. Anatomical data showed that the lesions destroyed neurons in SOC while preserving fibers of passage in the trapezoid body and other decussating pathways of the auditory brainstem. Animals with either unilateral or bilateral SOC lesions were impaired in their ability to localize a single noise burst postoperatively. Deficits were also found after unilateral lesions restricted primarily to the lateral superior olive. SOC lesions resulted in an elevation in minimum audible angles for sound localization.

Sound localization after kainic acid lesions of the dorsal nucleus of the lateral lemniscus in the albino rat.

The ability of rats to localize sounds in space was determined before and after kainic acid lesions of the dorsal nucleus of the lateral lemniscus (DNLL). The rats were trained to approach a 45-ms noise burst delivered from loudspeakers on the right or left of midline. Lesions were made by local injection of kainic acid into the DNLL. Rats with unilateral lesions of DNLL were impaired in their postoperative ability to localize a single noise burst. Rats with bilateral lesions also had deficits in postoperative performance, but the severity of the impairment was not substantially greater than that expected from a unilateral lesion. The mean pre- and postoperative minimum audible angles were 14.8 degrees and 40.4 degrees for rats with complete unilateral lesions and 13.5 degrees and 36.0 degrees for rats with bilateral lesions.

Sound localization precision under conditions of the precedence effect: Effects of azimuth and standard stimuli

Minimum audible angles (MAAs) were estimated for single noise bursts, and for burst pairs that satisfied the conditions of the precedence effect (that is, produced fused images). In one burst-pair condition, the bursts to be discriminated differed in lead location; in the other, they differed in lag location. Sounds were presented over loudspeakers. MAAs were lowest for single bursts, slightly higher for lead discrimination, and much higher for lag discrimination. Presence of a standard reference burst had no reliable effect on performance. The data are interpreted using a model of Shinn-Cunningham et al. [J. Acoust. Soc. Am. 93, 2923-2932 (1993)] in which discrimination of precedence-effect burst pairs is based on the lateral position of the auditory image, which is a weighted average of the positions of the leading and lagging bursts.

Sound localization after unilateral lesions of inferior colliculus in the ferret (Mustela putorius)

1. The ability of ferrets to localize sounds in space was determined before and after large unilateral lesions of the inferior colliculus. Three animals sustained restricted lesions that destroyed the inferior colliculus caudally but spared some tissue rostrally. These restricted lesions did not extend substantially beyond the cytoarchitectonic boundaries of the inferior colliculus. Four animals received more extensive lesions that destroyed all of the inferior colliculus, including the rostral pole. These lesions also involved to various degrees other midbrain structures, including the dorsal nucleus of the lateral lemniscus and the lateral tegmentum. Psychophysical curves and minimum audible angles were obtained for midline and left and right lateral fields. Comparisons were made of deficits ipsilateral and contralateral to the lesion. 2. The animals were tested in a semicircular apparatus with loudspeakers located at various positions around the periphery. Separate sound localization tests were conducted for midline, left, and right field positions. In each test the animals were trained first to discriminate between speakers separated by 60 degrees. They were then tested with progressively smaller angles of speaker separation to obtain their psychophysical threshold (minimum audible angle). All critical tests of sound localization were based on the ability of animals to identify the location of a 45-ms noise burst presented at the beginning of each trial. Correct responses were rewarded by delivery of a small quantity of water from spouts located around the perimeter of the apparatus. 3. Unilateral lesions restricted to the inferior colliculus had relatively little effect on sound localization. All three animals with restricted lesions were still capable of localizing a single noise burst and minimum audible angles were similar before and after surgery for midline, left, and right field tests. There was no difference in postoperative performance ipsilateral and contralateral to the lesion site. 4. More extensive unilateral lesions that included the dorsal nucleus of the lateral lemniscus and the lateral tegmentum as well as the inferior colliculus resulted in severe deficits in sound localization. Each of the four animals with large unilateral lesions had a substantial impairment in sound localization in the field contralateral to the lesion site. Postoperative performance in the contralateral field was too low to obtain a minimum audible angle for these animals. Midline sound localization was also affected in some cases but ipsilateral sound localization was only slightly affected.

Human auditory response to low-flying aircraft noise.

Noise from low-flying aircraft may constitute a potential risk of noise-induced hearing loss. In this study, hearing threshold levels were measured following human exposures to audio recordings at ground level of noise from low-flying aircraft. The noise was characterized by sudden onset, durations of about 10 s, maximum A-weighted sound pressure levels in the frequency region of 1200–2400 Hz, and A-weighted peak levels up to 130 dB. The primary independent variables were A-weighted peak levels and successive repetitions of the noise. In phase I, subjects experienced single noise bursts at a maximum level of 130 dB with no observable changes in hearing, as reported at the ASA meeting in Houston. In phase II, a single noise burst was repeated at the same sound level up to eight successive repetitions or until a TTS was observed. Hearing threshold levels were measured at 4 and 6 kHz immediately following each individual noise burst. The results of phase II and the overall study will be discussed in terms of noise-induced hearing loss relative to damage risk criteria and ISO 1999.

Human auditory response to low flying aircraft noise.

Noise from low flying aircraft may constitute a potential risk of noise induced hearing loss. In this study, hearing threshold levels were measured following human exposures to audio recordings at ground level of noise from low flying aircraft. The noise was characterized by sudden onset, durations of about 10 s, maximum A-weighted sound pressure levels in the frequency region of 1200–2400 Hz, and A-weighted peak levels up to 130 dB. The primary independent variables were A-weighted peak levels and successive repetitions of the noise. In phase I, subjects experienced single noise bursts with the first at a level of 115 dB and each succeeding noise increased an additional 5 dB until a temporary threshold shift (TTS) was observed or the maximum level of 130 dB was completed. In phase II, a single noise burst was repeated at the same sound level up to eight successive repetitions or until a TTS was observed. Hearing threshold levels were measured at 4 and 6 kHz immediately following each individual noise burst. Results will be discussed in terms of noise induced hearing loss relative to damage risk criteria and ISO 1999.

Temporal Masking of Speech

Temporal effects of 50-msec wideband and narrow-band noise bursts on the perception of three voiceless stop consonants in CV syllables, /pa/, /ta/, /pu/, /tu/, and /ku/, were investigated. In each trial a single noise burst was presented in a specified temporal relation with a syllable. The time intervals between the beginning of the syllable and the end of the noise burst were studied in a range between −600 msec (burst before the syllable) and +150 msec (burst completely within the syllable). The listening tests were conducted at 88 and 96 dB noise burst SPL and seven S/N ratios ranging between −22 and −50 dB. The results demonstrated appreciable masking within ±150-msec interval. Maximum masking occurred when the noise burst overlapped a part of the frictional noise of the stop consonant. Forward masking, measured as decrease in intelligibility, decreased linearly as the logarithmetic function of interval. Consistent relations were found between the noise spectra and the amount of masking and the type of substitution errors. Differences as a function of talkers and vowels were also observed.

Unit responses of the inferior colliculus in cats to lateralization of repetitive acoustic stimuli

1. Unity activity of the inferior colliculus during a series of short noise bursts (duration of bursts 2 msec, frequency 20–30 Hz, duration of series 600–700 msec) was investigated in cats anesthetized with chloralose and urethane. The following properties of unit activity were established during repetitive stimulation: a prolonged discharge throughout the series; lowering of the threshold of the response compared with that to a single noise burst of duration 2 msec and a continuous noise of duration 600–700 msec; a sharp increase in latency of the response and its dispersion at the threshold. These properties were characteristic of neurons of both phasic and tonic type. 2. During lateralization of a series of clicks of the same frequency and duration of the series, only two-thirds of the neurons respond by changes in spike activity to the first stimuli in the series and to its subsequent part. In one-third of the neurons, only the response to the subsequent part of the series is changed, the response to the first stimuli remaining unchanged. 3. The results suggest the existence of neurons capable of detecting the direction of movement of a source of sound.