Frequency-dependent self-induced bias of the basilar membrane and its potential for controlling sensitivity and tuning in the mammalian cochlea

A displacement-sensitive capacitive probe technique was used in the first turn of guinea pig cochleas to examine whether the motion of the basilar membrane includes a displacement component analogous to the dc receptor potentials of the hair cells. Such a "dc" component apparently exists. At a given location on the basilar membrane, its direction toward scala vestibuli (SV) or scala tympani (ST) varies systematically with frequency of the acoustic stimulus. Furthermore, it appears to consist of two parts: a small asymmetric offset response to each gated tone burst plus a progressive shift of the basilar membrane from its previous position. The mean position shift is cumulative, increasing with successive tone bursts. The amplitude of the immediate offset response, when plotted as a function of frequency, appears to exhibit a trimodal pattern. This displacement offset is toward SV at the characteristic frequency (CF) of the location of the probe, while at frequencies either above or below the CF the offset is relatively larger, and toward ST. The mechanical motion of the basilar membrane therefore appears to contain the basis for lateral suppression. The cumulative mean position shift, however, appears to peak toward ST at the apical end of the traveling wave envelope and appears to be associated with a resonance, not of the basilar membrane motion directly, but coupled to it. The summating potential, measured concurrently at the round window, shows a more broadly tuned peak just above the CF of the position of the probe. This seems to correspond to the peak at the CF of the mechanical bias. As the preparation deteriorates, the best frequency of the vibratory displacement response decreases to about a half-octave below the original CF. There is a corresponding decrease in the frequency of the peaks of the trimodal pattern of the asymmetric responses to tone bursts. The trimodal pattern also broadens. In previous experiments the basilar membrane has been forced to move in response to a low-frequency biasing tone. The sensitivity to high-frequency stimuli varies in phase with the biasing tone. The amplitudes of slow movement in these earlier experiments and in the present experiments are of the same order of magnitude. This suggests strongly that the cumulative shift toward ST to a high-frequency acoustic stimulus constitutes a substantial controlling bias on the sensitivity of the cochlea in that same high-frequency region. Its effect will be to reduce the slope of neural rate-level functions on the high-frequency side of CF.     

Comodulation detection differences using noise-band signals

In a variant of the standard paradigm employed to study comodulation masking release (CMR), a narrow noise band was used as a signal in the presence of "cue" bands which had either the same or different temporal envelopes. The number of cue bands present ranged from zero to four; when there were two or four cue bands, they were either all presented at the same overall level or the spectral profile was "scrambled" in a haphazard manner. Different noise samples were presented within and across trials. The result was in the opposite direction from the standard CMR outcome; that is, better performance was obtained when the envelopes of the cue band(s) were uncorrelated with those of the signal band than when they were correlated. These comodulation detection differences (CDDs) ranged from a decibel or two up to 10-12 dB in different conditions, and were generally larger the more cue bands present. Standard CMR conditions, which were run as controls, revealed that the detectability of a tonal signal does not increase as the number of cue bands is increased from one to four-an outcome which differs from those obtained in profile analysis experiments. The data taken with the equal-level and the scrambled-level cues differed little in both the CDD and the CMR conditions. All noise bands were 100 Hz wide, and approximately 250 ms in duration. The signal band in CDD and the masker band in CMR were centered at 2500 Hz. The psychophysical procedure was two-interval forced choice.     

Vibrational modes and infrared absorption of interstitial oxygen in silicon

A quasi-linear Si₂O molecule model (QLMM) is suggested from an analysis of the configuration and the interactions of an isolated oxygen atom with its neighbor silicon atoms. The vibrational modes are assigned and the infrared absorption spectra are calculated in detail with the model. The theoretical results are in reasonably good agreement with reported experimental values. This agreement shows that for the analysis of the vibrational modes of the interstitial oxygen atom in silicon crystals it is not necessary to consider the coupling of the molecule with the rest of the lattice. The interaction of the oxygen atom with its six second-nearest silicon atoms only causes the level separation of the ₂ mode and the formation of the fine structure.     

Structure and dynamics of fractal aerogels

Silica aerogels form mutually self-similar fractal structures for a broad range of densities and preparation conditions. This allows to scale dynamical results obtained on longitudinal acoustic waves up to the onset of the phonon to fracton crossover. An effective value of the fracton dimension is found, and various possible interpretations of that value are discussed.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Baryons as rotating excitations of the chiral soliton

We show that the usual ansatz used for quantizing the collective coordinates corresponding to a rotation of chiral soliton does not satisfy the equations of motion. We suggest a more general ansatz that admits the separation of rotational degrees of freedom and does not contradict the equations of motion.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Elliptic semiplanes and group divisible designs with orthogonal resolutions

In this paper we prove that there exists an elliptic semiplaneS(v, k, m) withk –m 2 if and only if there exists a group divisible design GDD _k ((k –m)(k – 1);k –m; 0, 1) withm pairwise orthogonal resolutions. As an example of this theorem, we construct an elliptic semiplaneW(45, 7, 3) and show thatW is isomorphic to the elliptic semiplaneS(45, 7, 3) given by R. D. Baker.     

On the multiplicative group generated by a dense sequence of integers

The subgroup of the positive rationals, generated by a sequence of upper density , has index essentially not exceeding ^–1.In honor of the seventieth birthday of Professor E. HlawkaPartially supported by N.S.F. Contract No. DMS-8500949.