Growth rate of simultaneous masking in cat auditory-nerve fibers: relationship to the growth of basilar-membrane motion and the origin of two-tone suppression.

Although many aspects of the mechanisms by which low-frequency sounds exert their powerful masking on responses to high-frequency sounds are well documented and understood, there are few data on the growth of masking for signal frequencies near, but not necessarily at, auditory-nerve-fiber characteristic frequency (CF). Masking of responses to 6- or 8-kHz tones by a continuous 300-Hz band of noise centered at 500 Hz was measured in single auditory-nerve fibers with various CFs. The growth rate of maskings averaged approximately 2 dB/dB, was typically largest for tones about 10% above fiber CF, and decreased at higher and lower frequencies. This pattern of masking versus frequency relative to CF resembles the pattern of compression of the growth of basilar membrane motion versus frequency at a fixed cochlear place. This correspondence supports the hypothesis that the high growth rate of masking by low-frequency sounds is due to the same mechanisms which produce the compression in the growth of basilar membrane motion.

Effects of stapedius-muscle contractions on the masking of auditory-nerve responses.

There has been little exploration of the mechanisms by which stapedius muscle contractions reduce the masking of responses to high-frequency sounds by low-frequency sounds. To fill this gap in knowledge, controlled stapedius contractions were elicited with direct shocks in anesthetized cats, and measurements were made of the effects of these contractions on the masking of single auditory-nerve fibers and on the attenuation of middle-ear transmission. The results show that the stapedius-induced reductions of masking can be much larger than the attenuations of low-frequency sound. With a 300-Hz band of masking noise centered at 500 Hz, and signal tones at 6 or 8 kHz, unmasking effects over 40 dB were observed for sounds 100 dB SPL or less. The data suggest that much larger unmasking might occur. The observed unmasking can be explained completely by a linear stapedius-induced attenuation of sound transmission through the middle ear and a nonlinear growth rate of masking for auditory-nerve fibers. No central effects are required. It is argued that the reduction of the upward spread of masking is probably one of the most important functions of the stapedius muscle.

Manual discrimination of force using active finger motion.

In these experiments, two plates were grasped between the thumb and forefinger and squeezed together along a linear track. An electromechanical system presented a constant resistance force during the squeeze up to a predetermined location on the track, whereupon the force effectively went to infinity (simulating a wall) or to zero (simulating a cliff). The task of the subject was to discriminate between two alternative levels of the constant resistance force (a reference level and a reference-plus-increment level). Results of these experiments indicate a just noticeable difference of roughly 7% of the reference force using a one-interval paradigm with trial-by-trial feedback over the ranges 2.5 less than or equal to F0 less than or equal to 10.0 newtons, 5 less than or equal to D less than or equal to 30 mm, 45 less than or equal to S less than or equal to 125 mm, and 25 less than or equal to V less than or equal to 160 mm/sec, where F0 is the reference force, D is the distance squeezed, S is the initial fingerspan, and V is the mean velocity of the squeeze. These results, based on tests with 5 subjects, are consistent with a wide range of previous results, some of which are associated with other body surfaces and muscle systems and many of which were obtained with different psychophysical methods.

Visual evoked potential responses of the anesthetized cat to contrast modulation of grating patterns.

Contrast modulation affords independent control of static contrast (C) and changes in contrast (delta C). We found that in anesthetized, paralyzed cats, the visual evoked potential (VEP) was dependent only on magnitude of delta C at each pattern transition, and was independent of the starting or ending contrast level. Increasing modulation frequency to above 2 Hz reduced the VEP monotonically, implying that the time constant for differentiation by the VEP is of the order of 250 msec. The essentially perfect a.c. coupling suppresses standing contrast completely, permitting the full dynamic range of the VEP response system to be used for detection of contrast increments (which results in a decreasing Weber fraction). The difference between our results and those of behavioral studies using contrast modulation can be explained by eye movements present in the behavioral studies which refresh the retinal image of the static contrast in a way uncorrelated to temporal modulation of the stimulus, thus introducing a masking effect.

Interaural magnification.

In principle, magnification of interaural differences, as could be achieved by the use of an enlarged head, will improve auditory spatial perception. This note presents some preliminary thoughts and observations on a signal-processing scheme for interaural magnification.

Recent progress in x-ray stereoscopes.

An improved reconstructed stereoscopic x-ray image display system, based on the principle of binocular parallax, is described. Pulse-synchronized operation of two triode x-ray tubes and one plumbicon TV camera is employed to separate the left and right images for presentation to the two eyes, respectively. The principle of binocular perception, quantitative characteristics of the stereoscopic depth, the overcoming of the inertia problem of the single-camera scheme, and a way to achieve low x-ray dosage and enhanced contrast of image are presented. Experimental results and applications are discussed.