Temporal onset-order discrimination through the tactual sense

Tactual temporal-onset order thresholds were measured for two sinusoidal vibrations of different frequencies delivered to two separate locations (thumb and index finger) of a multi-finger tactual stimulating device. The frequency delivered to the thumb was fixed at 50 Hz and that to the index finger at 250 Hz. The amplitude and duration of each of the two sinusoidal vibrations were roved independently from trial to trial in a 1-interval, 2AFC procedure. Performance, measured as a function of stimulus-onset asynchrony (SOA), indicated that the temporal-onset-order threshold averaged 34 ms across four subjects. The data were further classified into subsets according to both the amplitude and duration of the two stimuli in each trial of the roving-discrimination paradigm. The results indicated that the amplitude differences of the two stimuli in each trial had a substantial effect on onset-order discrimination, while duration differences generally had little effect. The effects of amplitude differences are explained qualitatively in terms of amplitude latency relationships and stimulus interactions such as temporal masking. Overall, the results not only contribute to an enhanced understanding of the temporal sensitivity of the tactual system but also provide guidelines for the design of tactual aids for hearing-impaired persons.     

The effect of frequency change on discrimination of pulse bursts in an electrocutaneous tactual vocoder

Tactual vocoders (artificial hearing systems) transduce the acoustic energy of speech into patterns of stimulation that are presented to the skin. In an electrocutaneous tactual vocoder, energy within an acoustic-frequency band is generally represented at a particular skin locus by the rate or frequency of brief electrical pulse bursts. At present little is known about tactual sensitivity for changes in the frequency of brief, bipolar electrical pulses used in several current electrocutaneous vocoder designs. Accordingly, discrimination of frequency change of electrocutaneous bipolar signals was evaluated for standard frequencies of 48, 100, 148, 200, and 248 Hz at signal durations of 50, 100, and 250 ms. Bipolar pulses (height = 10 mA, width = 13 microseconds) were presented on a single electrode placed slightly above and 8 cm to either the left or right of the navel. In a same-different task, three practiced subjects judged pairs of stimuli separated by an interstimulus interval (ISI) of 300 ms. For standard frequencies of 48 and 100 Hz, psychometric functions were similar for all subjects and all signal durations. For these frequencies, delta f/f was constant at approximately 0.3. By contrast, for standard stimuli greater than 100 Hz, the Weber fraction was found to increase dramatically as a function of both standard frequency and signal duration. In a second, similar experiment the Weber fraction for a 248-Hz standard increased as ISI was decreased below 300 ms. By contrast, ISI had little effect on sensitivity for standard stimuli of 48 and 100 Hz. Overall, these results suggest guidelines for possible intensity coding schemes for future electrocutaneous vocoder designs.     

Temporal resolution and temporal masking properties of transient stimuli: data and an auditory model.

Temporal resolution is often measured using the detection of temporal gaps or signals in temporal gaps embedded in long-duration stimuli. In this study, psychoacoustical paradigms are developed for measuring the temporal encoding of transient stimuli. The stimuli consisted of very short pips which, in two experiments, contained a steady state portion. The carrier was high-pass filtered, dynamically compressed noise, refreshed for every stimulus presentation. The first experiment shows that, with these very short stimuli, gap detection thresholds are about the same as obtained in previous investigations. Experiments II and III show that, using the same stimuli, temporal-separation thresholds and duration-discrimination thresholds are better than gap-detection thresholds. Experiment IV investigates the significance of residual spectral cues for the listeners' performance. In experiment V, temporal separation thresholds were measured as a function of the signal-pip sensation level (SL) in both forward- and backward-masking conditions. The separation thresholds show a strong temporal asymmetry with good separation thresholds independent of signal-pip SL in backward-masking conditions and increasing separation thresholds with decreasing signal-pip SL in forward-masking conditions. A model of the auditory periphery is used to stimulate the gap-detection and temporal-separation thresholds quantitatively. By varying parameters like auditory-filter width and transduction time constants, the model provides some insight into how the peripheral auditory system may cope with temporal processing tasks and thus represents a more physiology-related complement to current models of temporal processing.     

Minimum audible angle thresholds for broadband noise as a function of the delay between the onset of the lead and lag signals

Minimum audible angle (MAA) thresholds were determined for six experienced subjects using a two-alternative, forced-choice adaptive paradigm. Broadband pink noise from a single generator was led to two identical speakers. The two sources were activated sequentially, each for a period of 10 ms. The subject's task was to indicate whether the second (lag) sound came from a source to the right or left of the first (lead) sound. The delay between the onset of the lead and the onset of the lag signal [interstimulus onset interval (ISOI)] was systematically varied from 1 ms (both 10-ms signals were concurrently active for 9 ms) to 200 ms. For a given ISOI, the spatial separation was varied adaptively to determine the MAA. A 450% improvement in auditory spatial resolution was evident as the ISOI increased from 1 to 150 ms. A further increase in the ISOI had no systematic effect on spatial resolution. These results indicate that there is a minimum integration period between 100-150 ms for the resolution of spatial information in the auditory modality.     

Temporal gap resolution in narrow-band noises with center frequencies from 6000-14000 Hz

Temporal resolution was examined in normal-hearing subjects using a broadband noise and five narrow-band noises with center frequencies (fc) spaced 2 kHz apart between 6 and 14 kHz. Bandwidths of the narrow-band signals were equal to 0.16 fc, and broadband noise maskers with spectral notches were used to restrict the listening bands. Subjects used a Békésy procedure to track the minimum signal level required to keep a periodic temporal gap of fixed duration at threshold. Gap durations from 25 ms to the smallest trackable value were tested with each signal to generate performance curves, which showed the relationship between gap resolution and signal level in the low-to-moderate intensity range. Results showed that gap resolution improved progressively with increased signal level to about 35 dB SL, where minimum gap thresholds of about 3 ms were observed for all signals. These results, when combined with previous low-frequency data, indicate that gap threshold decreases systematically with increased signal frequency to about 5 kHz, and asymptotes at 2-3 ms for higher frequencies. In the context of functional models, the frequency effect is qualitatively consistent with the notion that both the auditory filter and a sensory integrator operate in series to govern temporal resolution in audition.     

Tactual display of consonant voicing as a supplement to lipreading

This research is concerned with the development and evaluation of a tactual display of consonant voicing to supplement the information available through lipreading for persons with profound hearing impairment. The voicing cue selected is based on the envelope onset asynchrony derived from two different filtered bands (a low-pass band and a high-pass band) of speech. The amplitude envelope of each of the two bands was used to modulate a different carrier frequency which in turn was delivered to one of the two fingers of a tactual stimulating device. Perceptual evaluations of speech reception through this tactual display included the pairwise discrimination of consonants contrasting voicing and identification of a set of 16 consonants under conditions of the tactual cue alone (T), lipreading alone (L), and the combined condition (L + T). The tactual display was highly effective for discriminating voicing at the segmental level and provided a substantial benefit to lipreading on the consonant-identification task. No such benefits of the tactual cue were observed, however, for lipreading of words in sentences due perhaps to difficulties in integrating the tactual and visual cues and to insufficient training on the more difficult task of connected-speech reception.     

Method of implementing frequency encoded multiplexer and demultiplexer systems using nonlinear semiconductor optical amplifiers

Multiplexing and demultiplexing are the essential parts of any communication network. In case of optical multiplexing and demultiplexing the coding of the data as well as the coding of control signals are most important issues. Many encoding/decoding mechanisms have already been developed in optical communication technology. Recently frequency encoding technique has drawn some special interest to the scientific communities. The advantage of frequency encoding technique over any other techniques is that as the frequency is fundamental character of any signal so it remains unaltered in reflection, refraction, absorption, etc. during transmission of the signal and therefore the system will execute the operation with reliability. On the other hand, the switching speed of semiconductor optical amplifiers (SOA) is sufficiently high with property of best on/off contrast ratio. In our present communication we propose a method of implementing a ‘4-to-1’ multiplexer (MUX) and a ‘1-to-4’ demultiplexer (DEMUX) exploiting the switching character of nonlinear SOA with the use of frequency encoded control signals. To implement the ‘4-to-1’ MUX and ‘1-to-4’ DEMUX system, the frequency selection by multiquantum well (MQW)-grating filter-based SOA has been used for frequency routing purpose. At the same time, the polarization rotation character of SOA has also been exploited to get the desired purpose. Here the fast switching action of SOA with reliable frequency encoded control input signals, it is possible to achieve a faithful MUX/DEMUX service at tera-Hz operational speed.     

Auditory brainstem response recovery in the dolphin as revealed by double sound pulses of different frequencies.

Recovery of auditory brainstem responses (ABR) in a bottlenose dolphin was studied in conditions of double-pip stimulation when two stimuli in a pair differed in frequency and intensity. When the conditioning and test stimuli were of equal frequencies, the test response was markedly suppressed at short interstimulus intervals; complete recovery appeared at intervals from about 2 ms (when two stimuli were of equal intensity) to 10-20 ms (when the conditioning stimulus exceeded the test by up to 40 dB). When the two stimuli were of different frequencies, the suppression diminished and was almost absent at a half-octave difference even if the conditioning stimulus exceeded the test one by 40 dB. Frequency-dependence curves (ABR amplitude dependence on frequency difference between the two stimuli) had equivalent rectangular bandwidth from +/-0.2 oct at test stimuli of 20 dB above threshold to +/-0.5 oct at test stimuli of 50 dB above threshold.     

Temporal resolution and temporal integration of short pulses at the auditory periphery of echolocating animals

To explain the temporal integration and temporal resolution abilities revealed in echolocating animals by behavioral and electrophysiological experiments, the peripheral coding of sounds in the high-frequency auditory system of these animals is modeled. The stimuli are paired pulses similar to the echolocating signals of the animals. Their duration is comparable with or smaller than the time constants of the following processes: formation of the firing rate of the basilar membrane, formation of the receptor potentials of internal hair cells, and recovery of the excitability of spiral ganglion neurons. The models of auditory nerve fibers differ in spontaneous firing rate, response thresholds, and abilities to reproduce small variations of the stimulus level. The formation of the response to the second pulse of a pair of pulses in the multitude of synchronously excited high-frequency auditory nerve fibers may occur in only two ways. The first way defined as the stochastic mechanism implies the formation of the response to the second pulse as a result of the responses of the fibers that did not respond to the first pulse. This mechanism is based on the stochastic nature of the responses of auditory nerve fibers associated with the spontaneous firing rate. The second way, defined as the repeatition mechanism, implies the appearance of repeated responses in fibers that already responded to the first pulse but suffered a decrease in their response threshold after the first spike generation. This mechanism is based on the deterministic nature of the responses of fibers associated with refractoriness. The temporal resolution of pairs of short pulses, which, according to the data of behavioral experiments, is about 0.1–0.2 ms, is explained by the formation of the response to the second pulse through the stochastic mechanism. A complete recovery of the response to the second pulse, which, according to the data of electrophysiological studies of short-latency evoked brainstem potentials in dolphins, occurs within 5 ms, is explained by the formation of the response to the second pulse through the repetition mechanism. The time constant of temporal integration, which, according to the behavioral experiments at threshold levels of pulses, is about 0.2–0.3 ms, is explained by the integrating properties of internal hair cells, etc. It is shown that, at the high-frequency auditory periphery, the temporal integration imposes no limitations on the temporal resolution, because both integration and resolution are different characteristics of the same multiple response of synchronously excited fibers.     

A binaural analog of gap detection.

The temporal resolution of the binaural auditory system was measured using a binaural analog of gap detection. A binaural "gap" was defined as a burst of interaurally uncorrelated noise (Nu) placed between two bursts of interaurally correlated noise (N0). The Nu burst creates a dip in the output of a binaural temporal window integrating interaural correlation, analogous to the dip created by a silent gap in the output of a monaural temporal window integrating intensity. The equivalent rectangular duration (ERD) of the binaural window was used as an index of binaural temporal resolution. In order to derive the ERD, both the shortest-detectable binaural gap and the jnd for a reduction in interaural correlation from unity were measured. In experiment 1, binaural-gap thresholds were measured using narrow-band noise carriers as a function of center frequency from 250 to 2000 Hz (fixed 100-Hz bandwidth) and a function of lower-cutoff frequency from 100 to 400 Hz (fixed 500-Hz upper-cutoff frequency). Binaural-gap thresholds (1) increased significantly with increasing frequency in both tasks, and (2) at frequencies below 500 Hz, were shorter than corresponding silent-gap thresholds measured with the same N0 noises. In experiment 2, interaural-correlation jnd's were measured for the same conditions. The jnd's also increased significantly with increasing frequency. The results were analyzed using a temporal window integrating the output of a computational model of binaural processing. The ERD of the window varied widely across listeners, with a mean value of 140 ms, and did not significantly depend on frequency. This duration is about an order of magnitude longer than the ERD of the monaural temporal window and is, therefore, consistent with "binaural sluggishness."     

Temporal order discrimination of tonal sequences by younger and older adults: the role of duration and rate

The effect of tone duration and presentation rate on the discrimination of the temporal order of the middle two tones of a four-tone sequence was investigated in young normal-hearing (YNH) and older hearing-impaired (OHI) listeners. The frequencies and presentation level of the tone sequences were selected to minimize the effect of hearing loss on the performance of the OHI listeners. Tone durations varied from 20 to 400 ms and presentation rates from 2.5 to 25 toness. Two experiments were conducted with anisochronous (nonuniform duration and rate across entire sequence) and isochronous (uniform rate and duration) sequences, respectively. For the YNH listeners, performance for both isochronous and anisochronous sequences was determined primarily by presentation rate such that performance decreased at rates faster than 5 toness. For anisochronous tone sequences alone, the effects of rate were more pronounced at short tone durations. For the OHI listeners, both presentation rate and tone duration had an impact on performance for both isochronous and anisochronous sequences such that performance decreased as rate increased above 5 toness or duration decreased below 40 ms. Temporal masking was offered as an explanation for the interaction of short durations and fast rates on temporal order discrimination for the anisochronous sequences.     

Temporal window shape as a function of frequency and level

In an earlier article [Moore et al., J. Acoust. Soc. Am. 83, 1102-1116 (1988)], preliminary work on the temporal-window model of temporal resolution in the auditory system was described. The temporal window is conceived of as a temporal integrator that slides in time and that is implemented as an intensity-weighting function. The shape of the temporal window was estimated by measuring the threshold for a brief sinusoidal signal presented in a temporal gap between two bursts of noise as a function of the duration of the gap and the position of the signal within the gap. In this paper, a much more thorough examination of the effects of level and frequency on the shape of the window is presented, using the same basic technique. Temporal window shapes were measured at four different frequencies (300, 900, 2700, and 8100 Hz) and at three different masker levels covering a 20-dB range at each frequency. The shape of the temporal window was well described by modeling each side as the sum of two rounded-exponential (roex) functions. The equivalent rectangular duration (ERD) of the window decreased from about 13 to 9 ms as the center frequency increased from 300 to 900 Hz, but decreased only slightly, to 7 ms, as the center frequency increased to 8100. The greater ERD at 300 Hz does not seem to be explicable in terms of "ringing" in the auditory filter. The ERD decreased somewhat with increasing level, for example, having a value of about 10 ms at 2700 Hz with a 20-dB masker spectrum level and about 7 ms with a 40-dB masker spectrum level.     

Functional neuroimaging of speech perception in six normal and two aphasic subjects.

This positron emission tomography study used a correlational design to investigate neural activity during speech perception in six normal subjects and two aphasic patients. The normal subjects listened either to speech or to signal-correlated noise equivalents; the latter were nonspeech stimuli, similar to speech in complexity but not perceived as speechlike. Regions common to the auditory processing of both types of stimuli were dissociated from those specific to spoken words. Increasing rates of presentation of both speech and nonspeech correlated with cerebral activity in bilateral transverse gyri and adjacent superior temporal cortex. Correlations specific to speech stimuli were located more anteriorly in both superior temporal sulci. The only asymmetry in normal subjects was a left lateralized response to speech in the posterior superior temporal sulcus, corresponding closely to structural asymmetry on the subjects' magnetic resonance images. Two patients, who had left temporal infarction but performed well on single word comprehension tasks, were also scanned while listening to speech. These cases showed right superior temporal activity correlating with increasing rates of hearing speech, but no significant left temporal activation. These findings together suggest that the dorsolateral temporal cortex of both hemispheres can be involved in prelexical processing of speech.     

Intensity and frequency resolution: masking of absolute identification and fixed and roving discrimination.

Auditory intensity and frequency resolution were studied in three paradigms under masking conditions. Absolute identifications of single stimuli (one-interval paradigm) and 2IFC judgments of fixed- and roving-level pairs of stimuli (two-interval paradigm) were obtained from the same experienced observers. Judgments were made under optimal (no mask) conditions, in the presence of a broadband noise mask (simultaneous mask), and when the stimulus(i) to be judged were either preceded (forward mask) or followed (backward mask) by a broadband noise mask. Substantial masking of intensity resolution was found in all mask conditions. Only a simultaneous mask affected frequency resolution. In the no mask condition, performance was best for fixed-level (or frequency) 2IFC discrimination, followed by roving-level (frequency) 2IFC, and finally absolute identification. These differences were maintained under masking for frequency resolution, but not for intensity resolution. The results are discussed in terms of the Braida and Durlach (1988) model of intensity resolution. A similar model is suggested for frequency resolution with differences suggested by the differences in neural coding of sound intensity and frequency.     

Hilbert-Huang变换分析THz脉冲信号的时频特性

提出一种基于Hilbert-Huang变换的THz时域光谱分析方法,将THz时域脉冲信号分解成有限数目的单分量信号之和,利用Hilbert变换求得瞬时频率来获得幅值的时频分布——Hilbert-Huang变换谱,实现了通过水蒸气的THz脉冲信号的时频分析,揭示了THz波与水蒸气相互作用的频谱时域分布特性,并与基于小波变换的时频图进行了对比分析。结果表明,该方法可以同时提高THz脉冲时频分布的时间分辨率和频率分辨率,具有局部化分析和自适应选择的特点,还能直观地表现出各频率成分之间的相对时间延迟。     

Temporal weighting functions for interaural time and level differences. II. The effect of binaurally synchronous temporal jitter

Recent work has demonstrated that sensitivity to interaural time differences (ITD) carried by high-rate cochlear implant pulse trains or analogous acoustic signals can be enhanced by imposing random temporal variation on the stimulus rate [see Goupell et al. (2009). J. Acoust. Soc. Am. 126, 2511-2521]. The present study characterized the effect of such "temporal jitter" on normal-hearing listeners' weighting of ITD and interaural level differences (ILD) applied to brief trains of Gabor clicks (4 kHz center frequency) presented at nominal interclick intervals (ICI) of 1.25 and 2.5 ms. Lateral discrimination judgments were evaluated on the basis of the ITD or ILD carried by individual clicks in each train. Random perturbation of the ICI significantly reduced listeners' weighting of onset cues for both ITD and ILD discrimination compared to corresponding isochronous conditions, consistent with enhanced sensitivity to post-onset binaural cues in jittered stimuli, although the reduction of onset weighting was not statistically significant at 1.25 ms ICI. An additional analysis suggested greater weighting of ITD or ILD presented following lengthened versus shortened ICI, although weights for such "gaps" and "squeezes" were comparable to other post-onset weights. Results are discussed in terms of binaural information available in jittered versus isochronous stimuli.     

Functional MR imaging of visual and motor cortex stimulation at high temporal resolution using a flash technique on a standard 1.5 tesla scanner

Functional magnetic resonance imaging (fMRI) was performed on a conventional 1.5 T scanner by means of a modified FLASH-technique at temporal resolutions of 80 and 320 ms. The method's stability was assessed by phantom measurements and by investigation of three volunteers resulting in a low amplitude (3%) periodic (4 s) signal modulation for the in vivo measurements, which was not observable in the phantom experiments. fMRI activation studies of motor and visual cortices of four adjacent slices were carried out on 12 healthy right-handed volunteers. Stimulation was performed by a triggered single white light flash or single finger-to-thumb opposition movement, respectively. Event-related response of visual and motor activation was traced over 10.24 s with a temporal resolution of 320 ms for the four slice measurements. Brain activation maps were calculated by correlation of measured signal time courses with a time-shifted boxcar function. Activation was quantified by calculation of percentual signal change in relation to the baseline. Observed signal magnitudes were about 5–7% in visual and about 8–12% in primary motor cortex. While photic response was delayed by about 2 s, motor stimulation showed an instantaneous increase of the MR signal. MR signal responses for both stimuli had decayed completely after about 5 s. Our results show that event-related fMRI enables mapping of brain function at sufficient spatial resolution with a temporal resolution of up to 80 ms on a conventional scanner.     

Neural mechanisms of scanned and stationary touch

The neural mechanisms subserving the sense of touch set the limits for the acquisition of information regarding the spatial and temporal characteristics of stimuli impinging on the skin surface. The results of three different psychophysical experiments imply that the skin of the finger pad can resolve the elements of a stimulus separated by 0.9 mm when the stimulus is applied to the skin and held stationary. This resolution limit is only slightly improved (to about 0.7 mm) when movement between the stimulus and skin is allowed. Single-unit recordings from three classes of primary mechanoreceptive afferents in anesthetized monkeys shows that only one class, the slowly adapting afferents, resolve spatial detail of stationary stimuli near the resolution limit. In addition, slow adaptors appear to resolve moving stimuli (e.g., Braille-dot patterns) more effectively than do the other two classes. However, these observations do not explain the extraordinary capacity of the finger-pad skin for discriminating between fine textures. Neurophysiological evidence suggests that information about such textures (i.e., surfaces with spatial details below the resolution limit) may be conveyed by a code based on the relative engagement of the three receptor populations.     

The Effect of Exposure Duration on Stereopsis and Its Dependency on Spatial Frequency

To investigate the effect of exposure duration on stereopsis and its spatial frequency dependency, we measured disparity threshold for the depth discrimination varying stimulus exposure duration between 0.05 and 2 s for three spatial frequencies (0.23, 0.94 and 3.75 c/deg). The results showed that disparity threshold decreased with increase in exposure duration up to a certain duration, beyond which it was approximately constant (the duration is called critical duration). The critical duration was about 150 ms for gratings with low and middle spatial frequencies (0.23 and 0.94 c/deg) while the duration was about 750 ms for gratings with high spatial frequency (3.75 c/deg). This suggests that temporal integration property varies dependently on stimulus spatial frequency. We also attempted to relate the spatial frequency dependency of the temporal integration property to the differences in temporal frequency tuning to different spatial frequency stimuli.     

Masking of filtered noise bursts by synthetic vowels.

The present investigation assessed the simultaneous and temporal masking produced by computer-generated synthetic vowels. The durations (100 and 200 ms) of each of four vowel-like maskers were employed. The masker was presented at 70 dB SPL. The probe signals were three filtered noise bursts whose spectral distributions corresponded to regions of high spectral energy in three English stop consonants. Quiet and masked thresholds were determined using the method of adjustment. Data are reported for two experienced listeners who participated in all the listening conditions. The results were generally in accord with the results of masking experiments using nonspeech signals in that both the frequency specificity of masking and temporal masking effects were demonstrated.