Masking Period Patterns Research Articles

Masking period patterns: presentation of a model consistent with an enlarged set of experimental results

Masking period patterns (MPP) have been proposed by Zwieker as psychoaoustic equivalent of period histograms (PSTH) measured in auditory neurons. A large set of results have been obtained and represented on a model for predicting MPP [E. Zwicker, Biol. Cybern. 23, 49–60 (1976)]. We have already used this technique for studying PSTH when the basilar membrane is stimulated by trapezoidal patterns of excitation [P. Escudier et al., J. Acoust. Soc. Am. Suppl. 1 67, S102 (1980)]. Our results cannot be completely interpreted on the basis of Zwicker's model. We present a new model, using a peripheral auditory system simulation previously elaborated in our group [J. M. Dolmazon et al., Speech Commun. 1, 52–73 (1982)] and some simple additional hypotheses. This model is shown to be able to predict the whole set of available MPP results.

Delayed evoked oto-acoustic emissions and their suppression by Gaussian-shaped pressure impulses.

The sound pressure of delayed evoked oto-acoustic emissions was measured as a function of the sound pressure of the stimulating sound impulse. They are directly proportional for sensation levels of the stimuli lower than about 20 dB; above that level, the emission saturates. Spontaneous emissions lying in the same frequency range as evoked emissions seem to influence this simple relation. Within the linear range, delayed emissions superimpose linearly even throughout long lasting delayed emissions. Short test tone bursts were used as test sound, to produce masking-period patterns, and as stimulus, to produce suppression-period patterns, respectively, while low-frequency Gaussian-shaped pressure impulses served as masker and as suppressor. The very close relation of the two patterns is indicated by their mirrored forms. This is considered directly relevant to the phenomenon of masking.

Temporal resolution in hard-of-hearing patients.

Temporal resolution is, next to frequency resolution, the most important ability of the ear for perceiving speech. A simplified method for measuring temporal resolution clinically is described. It is based on masking period patterns, but measures only their maximum and minimum through the use of continuous and square-wave amplitude-modulated masking noises. Test tones and filtered masking noises are presented at 500, 1 500, and 4 000 Hz. Temporal resolution data of a normally hearing group and of seven pathological groups are given: Patients with conductive hearing loss show normal resolution; those with noise-induced, age-induced, and sudden hearing loss show normal resolution at low and middle frequencies, but reduced resolution at 4 000 Hz; patients with retrocochlear hearing loss and Menière's disorder show a tendency for reduced resolution at all frequencies, but reduction at 4 000 Hz, while ototoxic hearing loss shows the greatest reduction at 1 500 Hz. The method used also gives an indication for reduced frequency selectivity.

Masking-period patterns and cochlear acoustical responses

A ‘suppression-period pattern’ for acoustical responses has been measured in a similar way to the masking-period pattern. A triggered low-frequency masker decreases the acoustical response to a short test signal differently at different presentations within the period of the masker. The two patterns show such close relationships that a masking-period pattern can be developed out of a set of suppression-period patterns: masked threshold is reached with the same parameter conditions under which the acoustic responses diminish. The rules elaborated for masking-period patterns seem to hold for suppression-period patterns as well.

Acoustical responses and suppression-period patterns in guinea pigs

Acoustical responses to short sound stimuli have been measured in the external ear canal of guinea pigs. Unequivocal responses had frequencies between 1.8 and 3.2 kHz, latencies (to their peak pressures) of 2.8 to 4.5 ms and sound pressures up to −10 dB SPL. A set of criteria was developed to clearly distinguish acoustical responses from after-resonances of the stimuli. Added phase-locked, verylow-frequency tones were able under some conditions to totally suppress the acoustical responses. The suppression-period pattern thus produced strongly resemble those obtained for man and the masking-period patterns obtained with human subjects.

Cochlear models: Evidence in support of mechanical nonlinearity and a second filter (a review)

We have studied properties of a nonlinear one-dimensional model for motion of the basilar membrane. The model is able to reproduce a variety of physiological and psychophysical effects, and in some instances predictions of the model have been subsequently confirmed. We regard the agreement between model and nature as evidence that the major features of the model are qualitatively correct. Areas of agreement include generation and propagation of two-tone distortion products, two-tone suppression, and Zwicker's psychophysical masking period patterns. In order to reproduce all of the above effects, the model must contain the following major features: 1. (1) A mechanical nonlinearity. This nonlinearity is asymmetric, with membrane damping increasing during the same phase of membrane motion that elicits response activity in primary autidory fibers. 2. (2) An additional stage of sharpening, a ‘second filter’, between the physical variable controlling mechanical nonlinearity and the physical variable controlling neural excitation.

Model study of Zwicker's "masking period patterns".

Zwicker has demonstrated that the threshold for a high-frequency test-tone burst in the presence of a continuous low-frequency masking tone is a complicated function of the frequency and intensity of the masker and the phase of the masker at which the test tone is presented. "Masking period patterns" measured for these stimuli show nonlinear effects in that a high masker levels the threshold of test-tone bursts reaches local maxima at two different phases of the masker. We have investigated the implications of these psychophysical data on a computational model for motion of the basilar membrane. The model consists of a nonlinear mechanical system followed by an additional stage of frequency selectivity ("second filter"). The output of the model is applied as input to a threshold-level detector. With this model it is possible to reproduce the effects Zwicker observed. Masking period patterns are interpreted as a manifestation of two-tone suppression. On the basis of our computer simulation of Zwicker's psychophysical data, we make the following specific predictions concerning the nature of mechanical to neural transduction at the auditory periphery: (1) Membrane motion in one direction produces a nonlinear increase in the loss term, while membrane motion in the other direction does not. (2) The direction of membrane motion that produces increased loss is also the direction of motion that produces neural excitation. (3) There is a stage of sharpening, a "second filter," between membrane motion and the neural excitatory signal.

Masking period patterns of harmonic complex tones

Masking period patterns, i.e., the SPL of just audible triggered sequences of short high-frequency test tone bursts as a function of their temporal spacing throughout the period of a masker, were measured using the following maskers: low-frequency two-tone and three-tone complexes, Gaussian-shaped impluses and differentiated Gaussian impulses with both condensation and rarefaction peaks. The similarity between masking period patterns and neurophysiologically measured period histograms was confirmed with respect to the temporal structure. However, distinct differences appear in the amplitude structure of both patterns, indicating a strong nonlinearity involved in the transformation of the masker’s time function into the masking period pattern. Starting a few decibels above threshold in quiet, the peaks of the patterns are elevated by 10 dB for each 5-dB increment of masker level. This fact points towards a square law for high-frequency test signals and low-frequency maskers. The steepness of slope of the masking period pattern seems to reach a maximal value of 10 dB/msec. Relations between neurophysiological period histograms and corresponding masking period patterns as well as differences between the patterns produced by condensation and rarefaction peak maskers, respectively, are discussed. Subject Classification: [43]65.58, [43]65.35, [43]65.68.

A model for predicting masking-period patterns.

Based on the available data for masking period patterns (MPP) produced by sinusoidal maskers of different frequencies and levels, a model is proposed which not only allows the peculiar shapes of the MPP of complex maskers to be understood, but also allows the MPP to tbe calculated. Several approximations have been made to describe the linear and nonlinear behaviour of the networks in the model. The agreement between the measured and calculated data is not exact, but in view of the incertainties of the physical parameters of the masker it is quite satisfactory.