Reverse correlation study of cochlear filtering in normal and pathological guinea pig ears

The filtering properties of single cochlear fibres have been determined in normal and kanamycin-treated guinea pigs using the reverse correlation technique. This method allows investigation of filtering over a wide dynamic range. For normal guinea pig fibres, the near threshold filter functions obtained with this method correspond to the tone derived frequency threshold curves ((FTCs). The 10 dB bandwidth of the filter functions increased monotonically with increasing noise levels above threshold. Thus with noise levels at approximately 50 dB above threshold, the 10 dB bandwidth had increased by a factor of 1.3--3. The changes in 3 dB bandwidth with increasing levels were, for some fibres, different from those of the 10 dB bandwidths. For the pathological fibres, the derived filter functions corresponded to their tone determined FTCs, and were therefore comparatively broadly tuned. Their tuning (Q10dB) approximated to those of normal fibres when the latter were measured 60 dB or more above threshold (i.e., at similar levels of stimulus), and did not increase further with increase in level. The findings in the normal guinea pig are consistent with those obtained by others in rodents, but are not consistent with those from the cat, where normal filtering is more robust to high levels of stimulus noise.

Rate-versus-intensity functions and related AP responses in normal and pathological guinea pig and human cochleas

Cochlear fiber discharge rate-versus-intensity functions, across frequency, have been measured in pathological guinea pig cochleas (ototoxic antibiotic poisoning) and compared with the normal animal. The frequency dependence of the slopes of these functions is reduced in cochlear pathology which results in minimum threshold elevations of more than 50 dB, i.e., there is a reduction in this frequency dependent nonlinearity. The rate functions at characteristic frequency (CF) become abnormally steep (e.g., 4--5 spikes/s/dB compared with a normal 1--2 spikes/s/dB), and comparable to those of the low-frequency tail region of normal cochlear fibers. The CF dynamic range is reduced from 30--40 to 10--15 dB in cochlear pathology. The fiber study has been confirmed by (and has confirmed) a method of indirect measurement of rate functions from AP suppression-versus-intensity functions using a pure-tone forward masking paradigm [Abbas and Gorga, J. Acoust. Soc. Am. 69, 492--499 (1981)]. This method has been used in normal and pathological guinea pigs, and the results parallel the single fiber study. In addition, AP suppression functions, across frequency, have been obtained in human subjects with (near) normal hearing thresholds, and in patients with sensorineural hearing loss of cochlear origin, during transtympanic electrocochleography. The AP suppression curves in pathology indicate, as for the animal studies, a loss of the frequency dependency of the rate function slopes, and predict steep rate functions at CF (and thus reduced dynamic range) in cochlear deafness. The findings are related to loudness recruitment.

AP tuning curves from normal and pathological human and guinea pig cochleas

Measures of cochlear selectivity can be obtained from compound responses using tone-on-tone masking procedures [Dallos and Cheatham, J. Acoust. Soc. Am. 59, 591--597 (1976)]. For the normal guinea pig, cochlear fiber tuning is sharper by a factor of 1.8 than AP tuning curves using simultaneous masking (threshold criterion = 25% N1 amplitude reduction). Anesthesia does not appear to affect AP tuning. In pathological cochleas, AP tuning is broadened by a factor of 2--3, and differences between forward and simultaneous masking curves are reduced. Tuning changes can sometimes occur without threshold elevation. AP tuning curves were obtained from humans during transtympanic electrocochleography. For subjects with near normal thresholds, Q10dB values (simultaneous masking) are approximately 2.3 at 2 kHz, 3.6 at 4 kHz, and 4.7 at 8 kHz. Using the relationship between cochlear fiber tuning and AP tuning in the guinea pig, estimates of human cochlear fiber tuning are 4.2 at 2 kHz, 6.5 at 4 kHz, and 8.5 at 8 kHz. Patients with threshold elevations of more than 30 dB resulting from cochlear deafness have AP tuning curves less sharply tuned by a factor of 2--3.

The frequency selectivity of the normal and pathological human cochlea

Human AP tuning curves (tone on tone simultaneous and forward masking curves) were measured during transtympanic electrocochleography. For subjects with near normal thresholds, average Q10 dB values (simultaneous masking) were 2.3 at 2 kHz, 3.6 at 4 kHz, and 4.7 at 8 kHz. Patients with threshold elevations of more than 40 dB, resulting from sensorineural hearing loss of cochlear origin, had tuning curves less sharply tuned by a factor of 2-3, with Q10 dB values of 1-2 at 2 kHz and at 4 kHz, and 1-2.3 at 8 kHz. AP tuning curves and single fibre tuning curves (frequency threshold curves) were measured in normal guinea pigs; cochlear fibre tuning is sharper than AP tuning (simultaneous masking) by a factor of 1.8 (2-20 kHz). Assuming that this factor can be applied to the human cochlea, estimates of normal human cochlear fibre Q10 dB values are 4.2 at 2 kHz, 6.5 at 4 kHz, and 8.5 at 8 kHz.

Some aspects of temporal coding by single cochlear fibres from regions of cochlear hair cell degeneration in the guinea pig

Some temporal coding properties of cochlear nerve fibers are investigated in kanamycin-treated guinea pigs (GPs) with various degrees of outer hair cell (OHC) degeneration. In particular, the phase locking ability of fibres from pathological cochleas, and also their adaptation properties are compared with the properties of normal cochlear fibres. No systematic effects of OHC loss on these properties have so far been found. These preliminary results therefore suggest (in so far as these animals can be regarded as models of sensorineural hearing loss of cochlear origin in man) that little deterioration should be expected in functions purely dependent upon faithful temporal coding of the stimulus waveform.

Psychosocial adjustment to and control of diabetes mellitus: differences by disease type and treatment

The relationship of diabetic patients' psychosocial adjustment to disease type, treatment mode, and indexes of control was examined in a representative community population. The psychometric properties of the Diabetes Educational Profile (DEP) and its application to psychosocial research in diabetes were also investigated. Findings support the reliability and validity of the DEP. Various measures of psychosocial adjustment were related to diabetes control but the specific relationships depended on the particular aspect of adjustment, the specific measure of diabetes control, and the type of disease and treatment. The findings suggest that analyses that aggregate dissimilar patient groups are subject to misinterpretation due to ecological masking and supressor effects. The results indicate that clinicians and researchers must recognize that patients with different disease types and treatment modes have different norms for psychological adjustment and diabetes control. Further, the particular aspects of psychosocial adjustment associated with diabetes control differ across patient groups.

Some otological differences between pigmented and albino-type guinea pigs

Cochlear action potential thresholds across frequency (AP audiograms) and AP tuning curves were compared in albino (Dunkin-Hartley) guinea pigs and normal pigmented animals. AP audiograms were very similar; on average the albinos had AP thresholds 2-3 dB lower at 2 kHz and below compared with the pigmented guinea pig. AP tuning curves at 16 kHz were, on average, significantly less sharply tuned in the albino, but similar at 8, 4, and 2 kHz. Although we find only small differences in these measures of normal cochlear function, other evidence in the literature suggests that in auditory research the use of "albino" guinea pigs is best avoided.