Effect of masker level on infants’ detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from ?5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants’ performance was more adultlike for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N₀ at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants’ thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants’ thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Effect of masker level on infants' detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from -5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants' performance was more adult-like for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N0 at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants' thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants' thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Auditory temporal summation in infants and adults: Effects of stimulus bandwidth and masking noise

A visually reinforced operant procedure was employed to determine the behavioral thresholds of 6- to 7-month-old infants and adults for stimuli of various bandwidths and durations. Experiment 1 compared absolute thresholds for broadband and 1/3-octavefiltered clicks and 300-msec noise bursts. For adult subjects, the difference in threshold for clicks and noise bursts was -quite comparable in the two bandwidth conditions, but infants’ click-noise threshold differences were significantly larger for broadband than for 1/3-octave stimuli. In Experiment 2, 2-point threshold-duration functions were compared for 4-kHz tones and octave-band noise bursts presented in backgrounds of quiet and continuous noise. Infants’ threshold-duration function for octave-band noise bursts was significantly steeper than the comparable adult function in quiet, but not in masking noise. These results suggest that young infants may have particular difficulty detecting low intensity broadband sounds when durations are very short.     

Is there low frequency vibrotactile temporal summation?

Reaction times of subjects to threshold vibratory stimuli of various durations and frequencies were measured simultaneously with the thresholds in order to test whether more time is needed for the detection of stimuli of long than short duration at threshold intensities. Vibratory stimuli of 20 or 150 Hz frequency and 50, 150, or 300 ms duration were applied to the back of the hand. Lower detection thresholds and longer reaction times were obtained for both 20 and 150 Hz vibration with increase of stimulus duration. The results suggest that temporal summation of high frequencies is due to energy integration , whereas at low frequencies probability summation explains better the threshold decrement.     

A comparison of thresholds for 1/3-octave filtered clicks and noise bursts in infants and adults

Behavioral thresholds of 6-month-old infants and adults were determined for 1/3-octave filtered clicks and 300-msec noise bursts with center frequencies ranging from .5 to 8 kHz. For noise bursts, differences between infant and adult thresholds were largest at low frequencies and smallest at 8 kHz. For clicks, infants’ thresholds were most like adults’ at 4 kHz, and age differences increased at both lower and higher frequencies. Differences between click and noise thresholds were significantly larger for infants than for adults at .5, 1, and 8 kHz, but not at 2 and 4 kHz. These results suggest that improvements in threshold for long-duration stimuli during infancy may not be accompanied by comparable changes in threshold at short durations. The delayed development of sensitivity to low- and high-frequency clicks appears consistent with maturational trends recently described for the auditory brainstem response.     

Across-channel masking of changes in modulation depth for amplitude- and frequency-modulated signals

This study examines a form of masking that can take place when the signal and masker are widely separated in frequency and cannot be explained in terms of the traditional concept of the auditory filter or critical band. We refer to this as across-channel masking. The task of the subject was to detect an increment in modulation depth of a 1000-Hz sinusoidal carrier. The carrier could either be sinusoidally amplitude modulated or sinusoidally frequency modulated at a 10-Hz rate. Modulation increment thresholds of this “target” signal were measured for the target alone, and in the presence of two interfering sounds with carrier frequencies of 230 and 3300 Hz. When the interfering sounds were unmodulated, they had no effect on modulation increment thresholds. When the interfering sounds were either amplitude or frequency modulated, thresholds increased. Amplitude modulation (AM) increment thresholds were affected by both amplitude-modulated and frequency-modulated interference. Similarly, frequency modulation (FM) increment thresholds were affected by both amplitude-modulated and frequency-modulated interference. For both types of signal, the interference was tuned for modulation rate; across-channel masking was greatest when the interfering sounds were modulated at rates close to 10 Hz, and declined for higher or lower rates. However, the tuning was rather broad. When the target and interfering sounds were modulated at the same rate, there was no effect of the relative phase of the modulators. Two possible explanations for the results are discussed. One is based on the idea that carriers that are modulated in a similar way tend to be perceptually “grouped”. The other is based on the idea that there are “channels” in the auditory system tuned for AM and FM rate. Neither explanation appears completely satisfactory.     

Monaural and binaural temporal integration of noise bursts

Summary A comparison was made between monaural and binaural temporal integration of noise bursts at threshold. The data indicate partial integration, with approximately a 6 dB decrease in threshold per decade increase in noise burst duration for both conditions of stimulation (i.e., parallel functions) for durations ranging from 4 to 256 msec. When thresholds in dB are plotted as a function of log duration, the linear component accounts for 99% of the data indicating no essential change in the partial integration functions up to at least 256 msec. The intercept difference between the monaural and binaural integration functions is 2.5 dB.     

Infants’ detection of increments in low- and high-frequency noise

A visually reinforced operant paradigm was employed to examine the relationship between the difference limen (DL) for intensity and level of the standard during infancy. In Experiment 1,7-month-old infants and adults detected increments in continuous noise presented via headphones at each of four levels ranging from 28 to 58 dB SPL. Noise stimuli were 2-octave bands centered at either 400 or 4000 Hz, and increments were 10 and 100 msec in duration. Infants’ DLs were significantly larger than those of adult subjects and significantly larger for low- than for high-frequency stimuli. For the high-frequency noise band, infants’ DLs were generally consistent with Weber’s law,remaining essentially constant for standards higher than 28 dB SPL (3 dB SL) for 100-msec increments and 38 dB SPL (13 dB SL) for 10-msec increments. For low-frequency noise, infants’ absolute thresholds were exceptionally high, and sensation levels of the standards were too low to adequately describe the relationship. In Ex-periment 2, 7-month-old infants detected 10- and 100-msec increments in 400-Hz noise stimuli presented in sound field. Infants’ low-frequency DLs were large at low intensities and decreased with increases in level of the standard up to at least 30 dB SL. For both low- and high-frequency noise, the difference between DLs for 10- and 100-msec increments tended to be large at low levels of the standard and to decrease at higher levels. These results suggest that the relationship between the DL and level of the standard varies with both stimulus frequency and duration during infancy. However, stimulus-dependent immaturities in increment detection may be most evident at levels within approximately 30 dB of absolute threshold.     

Temporal integration of monocular images separated in time: stereopsis, stereoacuity, and binocular luster

We evaluated stereopsis and binocular luster using electronically controlled shutter glasses with alternating monocular stimulation. In Experiment 1, we used the standard method for testing stereoacuity to obtain a gradual measure of stereopsis. Stereo thresholds decreased with increasing alternating frequency of two monocular half-images without a delay between them. Increasing delays led to increasing thresholds. In Experiment 2, we compared stereopsis resulting from two monocular half-images of a random-dot stereogram and binocular luster with respect to the minimum alternating frequency of the two half-images and the maximum interocular delay that were tolerated without a breakdown of the impression. Below 3 Hz, no stereopsis occurred. Binocular luster was observed only above 10 Hz. The mean threshold of interocular delay for detecting the global figure in a random-dot stereogram was about 51 msec, but for binocular luster it was about 20 msec. Overall, temporal integration was better for stereopsis than for binocular luster.     

A temporary threshold shift for self-motion detection following sustained,oscillating linear acceleration

Thresholds for detecting linear motion (self-motion) increased following exposure of human observers to sustained linear oscillation (fatiguing stimulus) at 0.26 Hz and approximately 0.5 G peak-to-peak for durations up to 30 min. Recovery to preexposure levels took place over a period of 10 to 15 rain following the sustained oscillation termination. Differences in threshold shift magnitudes following sustained oscillation in various orientations support the interpretation that the observed threshold shifts resulted from fatigue of the utricular otolith receptors. Threshold shifts were not obtained following exposure of the human observers to sustained, unidirectional linear acceleration (2 G) for durations up to 10 rain with a centrifuge. The results of this study suggest a previously unobserved response property of the otolith organs, namely, that these organs can be fatigued in a manner analogous to other receptor systems.     

Reflex inhibition in humans: Sensitivity to brief silent periods in white noise

A white noise (60 dB SPL) was always present except for brief silent periods (“gaps”) which occurred just before an eyelid reflex was elicited in human volunteers by a brief innocuous shock to the forehead. In Experiment 1 (n=8), 10-msec gaps (“S1”) were given 40, 80, 120, 160, or 200 msec before the shock (“S2”). Compared with S2-alone trials, the reflex was inhibited by about 50% at intervals of 80 msec and beyond. Experiment 2 (n=12) first provided detection thresholds for gaps using a simple version of the method of limits: on average a gap of 5.4 msec duration was just detected. Then gaps of 0, 2, 4, 6, 8, and 10 msec were given in random order, each 100 msec before S2. The 4-msec stimulus was an effective inhibitor of the reflex, and inhibition further increased on to 6- and then to 8-msec durations. A comparison of the values obtained on reflex inhibition with the 5.4-msec threshold obtained with the conventional psycho-physical test reveals that in humans reflex inhibition provides an objective index of stimulus detection that is at least of sufficient sensitivity to warrant its clinical application. The steady increase in reflex inhibition as gap duration increased from 2 to 8 msec may be of significance for tracing the rate of decay of afferent stimulation following noise offset, as it presumably reflects the growing sensitivity to the resumption of the noise as the duration of the silent period is increased.     

Effects of the menstrual cycle on vibrotactile sensitivity

Psychophysical thresholds for the detection of vibration delivered to the thenar eminence of the right hands of young males and females were measured every other day for 30–34 days. The frequency of the vibratory stimulus was either 15 or 250 Hz. The sinusoidal stimuli applied through a 3.0-cm² contactor were 700 msec in duration and had rise-fall times of 25 msec. Stimulus amplitude was measured with a calibrated accelerometer mounted on the moving element of the vibrator. Data were plotted as a function of successive days in the menstrual cycle. Thresholds for detecting the 15-Hz stimulus did not change significantly, whereas thresholds for detecting the 250-Hz stimulus varied significantly over the menstrual cycle. The 250-Hz threshold became progressively lower as subjects approached the onset of menstruation. After the onset of menstruation, the 250-Hz threshold gradually increased, reaching a maximum value approximately 12 or 13 days later. Soon after the 12th or 13th day of the cycle, the threshold again began to decline and continued to decline until the onset of the next menstrual cycle. The thresholds of women taking birth control pills did not change systematically over the testing period.     

Effects of time-order, interstimulus interval, and feedback in duration discrimination of noise bursts in the 50- and 1000-ms ranges

Participants compared durations of paired white-noise bursts, with interstimulus interval (ISI) 100, 300, 900, or 2700 ms, presented in the order standard (St)-comparison (Co) or Co-St. St was 50 or 1000 ms, and 75% difference thresholds for "longer" and "shorter" judgments were estimated. In Experiment 1 feedback was given; in Experiment 2, with ISIs 900 and 2700 ms, there was no feedback. For St=1000 ms, the just noticeable difference (JND) in noise duration was generally smaller with the order St-Co than with Co-St; for St=50 ms, the JND relation was the opposite. JNDs increased with shorter ISIs. Time-order errors were positive for St=50 ms and negative for St=1000 ms, and approached zero for longer ISIs. Using Hellstr?m's sensation-weighting (SW) model, the ratio of the stimulus weights for the first and second burst was estimated; this ratio was generally >1 for St=50 ms and <1 for St=1000 ms. JNDs were smaller with feedback than without; the greatest reduction was found for St=1000 ms and an ISI of 2700 ms with the order Co-St, possibly because feedback increased participants' attention to the first stimulus. These results demonstrate the sensitivity of discrimination measures for long as well as brief durations to the factors of ISI, presentation order, and feedback. They also suggest different modes of stimulus processing for short and long durations.     

Frequency discrimination between and within line gratings by dynamic touch

Line gratings were used to investigate the tactual discrimination thresholds for line frequency. In Experiment 1, participants were asked to discriminate between two gratings, each with a different line frequency. We used four standard frequencies in the eightfold range from 0.5 to 4 lines/cm. Thresholds were found to be constant at about 10.6%. In this experiment, we also measured hand speed and contact force. Hand speed was roughly in the range between 0.12 and 0.44 m/sec; contact force ranged from 0.62 to 2.76 N. In Experiment 2, we determined discrimination thresholds for line frequency transitions within a single grating. We used two frequencies and three transition lengths. The transition length had no effect on the threshold. In a third experiment, line frequency was modulated periodically. Varying the standard frequency and the size of the modulation period was found to have no effect on the discrimination thresholds. We conclude three things. First, Weber fractions for line frequency discrimination decrease as a function of line frequency within the experimental range. Second, discrimination thresholds are not altered by the length of the transition between two adjacent gratings with different line frequencies. And finally, the size of a modulation period in periodically modulated gratings is of no influence on the modulation detection threshold.     

Spectral, intensive, and temporal factors influencing overshoot

Threshold was measured for a 10-msec, 4.0-kHz signal presented near the onset or in the temporal centre of a 400-msec noise masker. Overshoot, the difference (in dB) between these two thresholds, was seen only for masker bandwidths wider than a critical band. The threshold near masker onset, and hence overshoot, could be reduced by the presence of an additional noise that was presented continuously or gated on and off prior to masker onset. The spectral, intensive, and temporal properties of this effect were studied. When the additional noise was continuous and either bandpass filtered with a variable bandwidth or notch filtered with a variable notchwidth, the results indicated that energy both near and remote from the signal frequency contributed to the reduction in overshoot. The effect of this additional noise was highly dependent upon its relative level. When the additional noise was 400 msec in duration and the delay between its offset and the onset of the masker was varied, overshoot “recovered” to its maximum value within about 50 msec. Finally, as the duration of the additional noise was varied from 3 to 400 msec while the time between its offset and masker onset was fixed, the reduction in overshoot was virtually complete for durations of about 25-50 msec. The results are consistent with the notion that overshoot at least partly reflects peripheral adaptation, and that this adaptation is not restricted to the signal frequency channel but, rather, extends in both directions over several channels.     

Detection of moving local density differences in dynamic random patterns by human observers

The way in which movement enhances target visibility has been investigated by measuring the detectability of the direction of motion of a dot pattern added to a background of dynamic visual noise. When the positions of all the dots were changed randomly from frame to frame, so that there was no dot configuration to define the target area (experiments 1 and 2), the threshold density difference necessary was for direction of motion detection less than 3 dots/frame (between 20% and 50% density difference). The spatial displacement (S) at which optimal detection occurs increased when a target elongated in the direction of motion was used. If S was either larger or smaller than its optimal value, thresholds rose progressively. The rise in threshold when S was smaller than 0.25 deg (the width of the target area) decreased when the target dots had a fixed spatial arrangement (experiment 3). It is suggested that in both fixed and random target configurations there is a grouping of dots with similar trajectories via a global directionally-selective process. The strength of the overall motion signal is greater in the fixed-dot configuration because each target dot has associated with it a vector precisely aligned in the direction of the target motion.     

Duration discrimination by musicians and nonmusicians

This study investigated the effects of stimulus modality, standard duration, sex, and laterality in duration discrimination by musicians and nonmusicians. Seventeen musicians (M age = 24.1 yr.) and 22 nonmusicians (M age = 26.8 yr.) participated. Auditory (1,000 Hz) and tactile (250 Hz) sinusoidal suprathreshold stimuli with varying durations were used. The standard durations tested were 0.5 and 3.0 sec. Participants discriminated comparison stimuli which had durations slightly longer and shorter than the standard durations. Difference limens were found by the method of limits and converted to Weber fractions based on the standard durations. Musicians had lower, i.e., better, Weber fractions than nonmusicians in the auditory modality, but there was no significant difference between musicians and nonmusicians in the tactile modality. Auditory discrimination was better than tactile discrimination. Discrimination improved when the standard duration was increased both for musicians and nonmusicians. These results support previous findings of superior auditory processing by musicians. Significant differences between discrimination in the millisecond and second ranges may be due to a deviation from Weber's law and the discontinuity of timing in different duration ranges reported in the literature.     

Effect of simultaneous auditory stimulation on vibrotactile thresholds

Vibrotactile thresholds were determined at 250 and 400 Hz in the presence of (1) the sounds emitted by the vibrator, (2) continuous tonal or narrow-band masking noise, or (3) a pulsed tone synchronized with the vibrator signal. The measure of a cross-modality effect was the threshold shift occurring between each condition and the control condition, in which earmuff silencers eliminated the vibrator sounds. Continuous tones or noise had no effect upon vibrotactile thresholds. However, auditory signals synchronized with the vibrator signals did significantly elevate vibrotactile thresholds.     

Tactile sensitivity of children: effects of frequency, masking, and the non-Pacinian I psychophysical channel

Tactile perception depends on the contributions of four psychophysical tactile channels mediated by four corresponding receptor systems. The sensitivity of the tactile channels is determined by detection thresholds that vary as a function of the stimulus frequency. It has been widely reported that tactile thresholds increase (i.e., sensitivity decreases) as a function of age. However, there is controversial evidence with regard to the progressive loss of sensitivity starting from childhood. In this study, the tactile thresholds of children (n=9, ages 7-11 years) were measured and compared with the thresholds of young adults (n=11, ages 21-27 years). The stimuli consisted of sinusoidal bursts of mechanical displacements, which were applied to the left index fingertips of the participants by using a cylindrical probe (base area=0.126 cm2) without a contactor surround. Absolute thresholds were measured at frequencies of 2, 10, 40, 100, 250, and 500 Hz without masking. The absolute thresholds decreased at high frequencies and were similar to data from the literature except for some discrepancy because of methodological differences. In addition, the threshold of the non-Pacinian I channel was measured at 40 Hz by elevating the thresholds of the Pacinian channel by forward masking. The effects of forward masking in children were similar to results in young adults. In conclusion, there were no significant differences between the tactile thresholds of children and those of young adults at key frequencies: 40 Hz for the Pacinian and non-Pacinian I channels and 250 Hz for the Pacinian channel. These findings contradict the hypothesis that there is gradual loss of tactile sensitivity starting from childhood to early adulthood. The loss of sensitivity due to aging probably is more abrupt and occurs at a later age.     

Vibrotactile thresholds measured at the finger

Absolute vibrotactile thresholds were determined over the distal pad of the middle finger and thenar eminence of the right hands of five Ss. Measurements were made using eight frequencies between 25 and 700 Hz and seven contactor sizes between .0008 and 1.3 cm². When plotted as a function of frequency, the threshold curve measured at the fingerpad with a .005-cm² contactor is U shaped, with a maximum sensitivity in the region of 250 Hz. When plotted as a function of contactor size, the threshold decreases at a rate of 3 dB per doubling of the area.