Injury- and use-related plasticity in the primary sensory cortex of adult mammals: possible relationship to perceptual learning

1. Restricted cochlear lesions in adult animals result in a reorganization of auditory cortex such that the cortical region deprived of its normal input by the lesion is occupied by expanded representations of adjacent cochlear loci (and thus of the frequencies represented at those loci). Analogous injury-induced reorganization is seen in somatosensory, visual and motor cortices of adult animals after restricted peripheral lesions. 2. Rather than constituting a central compensation for the peripheral loss, such reorganization appears to be an extreme form of changes in cortical organization that occur as a consequence of altered patterns of input such as arise from differential use of restricted regions of receptor surfaces ('use-related' reorganization). Thus, the frequency organization of auditory cortex is modified in animals trained to perform a frequency discrimination task and analogous changes in the frequency selectivity of cortical neurons are produced by classical conditioning procedures. 3. Recent evidence from the visual system suggests that changes similar to those involved in injury- and use-related cortical reorganization may underlie some forms of what has been called 'perceptual learning', the improvement in sensory/ perceptual discriminative performance with practice. Some forms of such learning are highly specific to the particular stimuli used in training (i.e. do not generalize to other stimuli), suggesting that the improved performance reflects a change in neural circuitry at a relatively early level of sensory processing. The limited available evidence supports the occurrence of such learning in the auditory system. 4. Recent studies using functional imaging and related techniques indicate that injury- and use-related reorganization occurs in human sensory and motor cortex.

Alkalinisation of lignocaine to reduce the pain of digital nerve blockade

OBJECTIVE

To see if the alkalinisation of lignocaine caused a reduction in the pain of injection for digital nerve blockade.

METHODS

The study was a prospective randomised double blind study with each patient acting as their own control. During the study period, all patients aged 16 years and over presenting to the accident and emergency department with a condition requiring digital nerve blockade were considered for inclusion in the study. Each patient received an injection of both alkalinised and non-alkalinised lignocaine. The pain of each injection was then assessed on a visual analogue scale.

RESULTS

98 patients were entered in the study. The mean difference in pain scores between the non-alkalinised and alkalinised injections was 0.739, P < 0.001, 95% confidence interval 0.47 to 1.01.

CONCLUSIONS

Alkalinisation of lignocaine reduces the pain of injection for digital nerve blockade.

Metabolic acidosis induced by cetrimide-chlorhexidine solution in hydatid cyst surgery

A patient with a large hydatid cyst of the left lobe of the liver developed metabolic acidosis following rather liberal use of cetrimide-chlorhexidine solution as a scolicidal agent. The progress and management of this complication are described in the patient is being reported.

Additivity of loud-sound--induced threshold losses in the cat under conditions of active or inactive cochlear efferent-mediated protection

1. An additivity model for the accretion of cochlear sensorineural hearing losses has been described from studies in the guinea pig cochlea. Among other aspects, the model allows determination of how residual hearing losses after an initial exposure (E1) affect hearing losses to be expected to a subsequent second exposure (E2). In the present study, the model was applied to temporary hearing losses produced in the cat cochlea by loud pure tones at a frequency from 3 to 15 kHz, affecting regions from 2 to 28 kHz. Successive identical exposures, generally with an interexposure interval of approximately equal to 35 min, were used to produce compound action potential (CAP) threshold losses. Total losses after E2 were compared with those predicted by the model. Testing was carried out under conditions where olivocochlear bundle (OCB)-mediated protection was or was not activated. (As shown elsewhere, OCB-mediated protection is activated by particular binaural exposures, but not monaural exposure, and reduces threshold losses in the binaural condition with intact OCB compared with losses in either the monaural condition, or the binaural condition where the OCB was cut before loud sound.) 2. The additivity model was a very good predictor of total losses under a variety of conditions; different exposure frequencies, monaural and binaural exposures, and with intact or cut OCB pathways. In these exposures, the model's application could be generalized so that as long as residual losses just pre-E2 were well specified in an animal, total losses could be as well predicted using normative data bases of a single exposure with the same parameters. 3. The model also allowed determination of whether OCB-mediated protection was exercised during E2 in dual identical exposures. Expression of protection for E2 depended on whether E1 elicited protection. When tested with monaural (at 7 or 15 kHz) or binaural exposures (at kHz) for which E1 did not elicit protection, neither did E2. However, when tested with a binaural E1 (at 7, 11, or 15 kHz), which activated protection, E2 also elicited protection. In the latter case, for 7- and 11-kHz exposures, the amount of E2 protection increased with total hearing loss, a relationship similar to that seen for single exposures in cat and guinea pig. For 15-kHz exposure, the amount of E2 protection was constant across test frequencies. 4. Finally, a critical observation with 11-kHz exposure was that a binaural E1 eliciting protection was able to "prime" the OCB so that protection could be elicited by a subsequent monaural E2, which, by itself as a singel exposure, does not evoke protection. This result has important implications in terms of the physiology of the protective OCB pathways and clinically in terms of the manner in which loud-sound-induced hearing loss accumulated.

Epidermal growth factor (EGF) promotes chemomigration of a human prostate tumor cell line, and EGF immunoreactive proteins are present at sites of metastasis in the stroma of lymph nodes and medullary bone

Prostate tumor cells preferentially metastasize to bony sites and lymph nodes at a frequency in excess of that which would be predicted by random tumor cell dissemination. In order to determine whether chemoattractants in these organs promote organ-specific metastasis, we utilized human cell lines derived from and/or related to these organs as sources of potential chemoattractants. Secretory proteins derived from the cell lines MG-63 (osteosarcoma), SK-ES-1 (Ewing's sarcoma), and KG-1 (leukemia) stimulated chemomigration of the TSU-pr1 prostate tumor cells in a dose-dependent manner in Boyden chambers. In addition, secretory proteins from a human prostatic stromal cell line (hPS) and from the TSU-Pr1 prostate tumor cell line were also able to stimulate chemomigration of the TSU-pr1 cells through Boyden chambers. Since lymph nodes and bony sites represent organs of hematopoietic/lymphoid proliferation and activation, we undertook identification of specific cytokines present at these sites which may promote the chemomigration of prostate tumor cells. In this context, the cytokines interleukin-1 alpha, interleukin-2, interleukin-6, tumor necrosis factor-beta, transforming growth factor-beta, interferon alpha 2-a, and granulocyte-macrophage colony-stimulating factor did not stimulate chemomigration of the TSU-pr1 prostate tumor cell line. In contrast, the cytokine epidermal growth factor (EGF) stimulated chemomigration of the TSU-pr1 prostate tumor cells through the Boyden chambers in a dose-dependent manner. Western blot analysis of secretory proteins from the cell lines KG-1, SK-ES-1, MG-63, hPS, and TSU-pr1 identified EGF-immunoreactive proteins in all cases. In addition, EGF immunoreactivity was localized to the stroma of the human prostate, the osteogenic stroma of pelvic medullary bone, and the stroma within the capsule and trabeculae of pelvic lymph nodes. Hence, these results demonstrate that the cytokine EGF promotes the chemomigration of the TSU-pr1 prostate tumor cell line, and that EGF within the stroma of pelvic lymph nodes and medullary bone may act as a chemoattractant for prostate tumor cells, thereby facilitating the preferential formation of metastatic foci within these organs.

Sensitivity to interaural intensity differences of neurons in primary auditory cortex of the cat. I. types of sensitivity and effects of variations in sound pressure level

1. Interaural intensity differences (IIDs) provide the major cue to the azimuthal location of high-frequency narrowband sounds. In recent studies of the azimuthal sensitivity of high-frequency neurons in the primary auditory cortex (field AI) of the cat, a number of different types of azimuthal sensitivity have been described and the azimuthal sensitivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity dependence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sensitivity from a large sample of neurons with a characteristic frequency (CF) > 5.5 kHz in AI of anesthetized cats. IID sensitivity functions were classified in a manner that facilitated comparison with previously obtained data on azimuthal sensitivity, and the effects of changes in the base intensity at which IIDs were introduced were examined. 2. IID sensitivity functions for CF tonal stimuli were obtained at one or more intensities for a total of 294 neurons, in most cases by a method of generating IIDs that kept the average binaural intensity (ABI) of the stimuli at the two ears constant. In the standard ABI range at which a function was obtained for each unit, five types of IID sensitivity were distinguished. Contra-max neurons (50% of the sample) had maximum response (a peak or a plateau) at IIDs corresponding to contralateral azimuths, whereas ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maximum response (peak) in the range of +/- 10 dB IID, whereas a small group of tough neurons (2%) had a restricted range of minimal responsiveness with near-maximal responses at IIDs on either side. A final 18% of AI neurons were classified as insensitive to IIDs. The proportions of neurons exhibiting the various types of sensitivity corresponded closely to the proportions found to exhibit corresponding types of azimuthal sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form of its IID sensitivity function. Thus, neurons excited by monaural stimulation of only one ear but with either inhibitory, facilitatory, or mixed facilitatory-inhibitory effects of stimulation of the other ear had predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral monaural stimulation was excitatory). Neurons driven weakly or not at all by monaural stimulation but facilitated binaurally almost all exhibited near-zero-max IID sensitivity. The exception to this tight association between binaural input and IID sensitivity was provided by neurons excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to IIDs, our data were in agreement with two recent reports indicating that they can exhibit various forms of IID sensitivity: only 23 of 75 EE neurons were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more intensities (3-5 in most cases) for 84 neurons. The form of the IID sensitivity function (defined in terms of both shape and position along the IID axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (approximately 15% if a strict criterion of invariance was employed), and for many of these neurons the spike counts associated with a given IID varied with ABI, particularly at near-threshold levels. When the patterns of variation in the form of IID sensitivity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimuthal sensitivity, there was a close correspondence between the effects of intensity on corresponding types of azimuthal and IID sensitivity

Involvement of cochlear efferent pathways in protective effects elicited with binaural loud sound exposure in cats

1. Studies in guinea pigs have shown that the crossed olivocochlear efferent [crossed olivocochlear bundle (OCB)] pathways can reduce the cochlear neural desensitization caused by loud sounds. In one experimental paradigm, binaural loud sound exposure produced less damage than did monaural exposure, and various tests confirmed that this could be attributed to the OCB. In contrast, a study in cats has shown no such protection from the OCB for binaural exposures. There are some methodological differences between the cat and guinea pig studies that could account for the difference. The present study was carried out to determine whether two factors, namely anesthetic or exposure frequency, could account for the difference. Experiments were carried out in cats using barbiturate anesthesia, as in the guinea pig experiments. 2. Using a unilateral middle ear muscle (MEM) tenotomy, it was confirmed that under Nembutal anesthesia the MEM did not affect the threshold losses to monaural or binaural exposure. However, comparing results for monaural versus binaural 11-kHz exposures, there were significantly less threshold losses to the binaural exposure, independent of the presence of the MEM. No such difference between monaural and binaural exposures was seen for 3-kHz exposures. 3. By employing unilateral surgical strategies such as MEM tenotomy and/or various brain stem incisions, it was confirmed that the protection with binaural compared with monaural 11-kHz exposure was due specifically to the OCB. In unilaterally deefferented animals, binaural 11-kHz exposure always produced lower threshold losses on the OCB-intact side than on the OCB-cut side, regardless of the status of the MEM. Brain stem cuts that affected other rostrally or laterally located structures but not the OCB produced similar threshold losses bilaterally after binaural exposure, and the losses were comparable with the protected levels seen in other cases with intact OCB and binaural exposure. These data confirmed that when using an exposure frequency very similar to that used in the guinea pig experiments, a protective OCB effect could be demonstrated in cats, as previously seen in guinea pigs. The MEMs appeared to be inactive in barbiturate-anesthetized animals and were not activated by 3-, 7-, or 11-kHz exposures at 100 dB SPL for 10 min. The companion manuscript demonstrates this same effect across a wider range of exposures.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency and loss dependence of the protective effects of the olivocochlear pathways in cats

1. In the previous manuscript I suggested a frequency dependency to olivocochlear bundle (OCB)-mediated protection from loud sound by showing protection for binaural compared with monaural 11-kHz exposures but not 3-kHz exposures of the same intensity and duration. To determine whether this was the case, experiments were carried out in barbiturate-anesthetized cats using the paradigm of a unilateral brain stem incision to deefferent one cochlea in each animal before presentation of a binaural loud sound exposure. With equal-intensity, equal-duration binaural exposures, in different groups protection in OCB-intact compared with OCB-cut ears was seen only for exposures at 11, 15, or 20 kHz, but not at 3 or 7 kHz, suggesting that OCB-mediated protection was found only for higher-frequency exposures. This would be consistent with the OCB-mediated protection in guinea pig studies where 10-kHz exposures were used and its absence in a study in cats where 6-kHz exposures were used. However, this conclusion had to be qualified by the fact that the lower-frequency exposures resulted in smaller threshold losses than did the higher-frequency exposures. 2. To determine whether OCB-mediated protection could be obtained for lower-frequency exposures that were made as damaging as or more damaging than the high-frequency exposures, longer-duration, lower-frequency exposures were used. OCB-mediated protection could then be obtained for exposure at 7 kHz, 100 dB SPL for 15 min but not at 3 or 5 kHz, 100 dB SPL for 20 min or at 3 kHz, 100 dB SPL for 40 min or 106 dB SPL for 20 min. Finally, when large threshold losses were produced with exposure at 3 kHz, 106 dB SPL for 40 min, OCB-mediated protection could be obtained for this low-frequency exposure too. These effects suggested that there were different "activation threshold" for OCB-mediated protection as a function of exposure frequency. To determine whether this also applied for the higher-frequency exposures (11, 15, and 20 kHz), all of which had elicited OCB-mediated protection when presented at 100 dB SPL for 10 min, these exposure frequencies were presented at 100 dB SPL for 7 min to produce low threshold losses. Now protection was found for the 11- and 15-kHz exposures but not for the 20-kHz exposure. 3. Thus the activation threshold for OCB-mediated protection varied in a frequency-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Very low-dose warfarin prophylaxis to prevent thromboembolism in women with metastatic breast cancer receiving chemotherapy: an economic evaluation

PURPOSE

A recent double-blind, randomized trial demonstrated that very low-dose warfarin (VLDW) reduced the incidence of venous thromboembolism (VTE) without increasing the rate of bleeding in women with metastatic breast cancer receiving chemotherapy. We have evaluated the economic impact on the health care system of using VLDW in such patients.

METHODS

The records of patients entered onto the trial and a simultaneous, fully allocated, costing model for a tertiary care hospital in Hamilton, Canada were used to determine the difference in costs associated with the care of patients with and without VLDW.

RESULTS

The cost of providing VLDW was $ 21,854 (Canadian dollars) per 100 patients. This therapy led to a reduction in costs of $ 24,297 per 100 patients, thus saving the health care system $ 2,443 per 100 patients. In the sensitivity analysis, VLDW prophylaxis still did not increase health care costs unless the cost of VLDW was greatly increased, the cost of treating thromboembolic episodes was markedly reduced, or the incidence of either VTE or bleeding with VLDW was increased above the rates observed in the trial.

CONCLUSION

We conclude that for women receiving chemotherapy for metastatic breast cancer, the benefits of VLDW can be realized without increased health care costs.

Topographic representation of tone intensity along the isofrequency axis of cat primary auditory cortex

The sound pressure level (SPL), henceforth termed intensity, of acoustic signals is encoded in the central auditory system by neurons with different forms of intensity sensitivity. However, knowledge about the topographic organization of neurons with these different properties and hence about the spatial representation of intensity, especially at higher levels of the auditory pathway, is limited. Here we show that in the tonotopically organized primary auditory cortex (AI) of the cat there are orderly topographic organizations, along the isofrequency axis, of several neuronal properties related to the coding of the intensity of tones, viz. minimum threshold, dynamic range, best SPL, and non-monotonicity of spike count--intensity functions to tones of characteristic frequency (CF). Minimum threshold, dynamic range, and best SPL are correlated and alter periodically along isofrequency strips. The steepness of the high-intensity descending slope of spike count--intensity functions also varies systematically, with steepest slopes occurring in the regions along an isofrequency strip where low thresholds, narrow dynamic ranges and low best SPLs are found. As a consequence, CF-tones of various intensities are represented by orderly and, for most intensities, periodic, spatial patterns of distributed neuronal activity along an isofrequency strip. For low--to--moderate intensities, the mean relative activity along the entire isofrequency strip increases rapidly with intensity, with the spatial pattern of activity remaining quite constant along the strip. At higher intensities, however, the mean relative activity along the strip remains fairly constant with changes in intensity, but the spatial patterns change markedly. As a consequence of these effects, low- and high-intensity tones are represented by complementary distributions of activity alternating along an isofrequency strip. We conclude that in AI tone intensity is represented by two complementary modes, viz. discharge rate and place. Furthermore, the magnitude of the overall changes in the representation of tone intensity in AI appears to be closely related to psychophysical measures of loudness and of intensity discrimination.

Effect of unilateral partial cochlear lesions in adult cats on the representation of lesioned and unlesioned cochleas in primary auditory cortex

We examined the effect of unilateral restricted cochlear lesions in adult cats on the topographic representations ("maps") of the lesioned and unlesioned cochleas in the primary auditory cortex (AI) contralateral to the lesioned cochlea. Frequency (tonotopic) maps were derived by conventional multineuron mapping procedures in anesthetized animals. In confirmation of a study in adult guinea pigs (Robertson and Irvine [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months after the unilateral cochlear lesion the map of the lesioned cochlea in the contralateral AI was altered so that the AI region in which frequencies with lesion-induced elevations in cochlear neural sensitivity would have been represented was occupied by an enlarged representation of lesion-edge frequencies (i.e., frequencies adjacent to those with elevated cochlear neural sensitivity). Along the tonotopic axis of AI the total representation of lesion-edge frequencies could extend up to approximately 2.6 mm rostal to the area of normal representation of these frequencies. There was no topographic order within this enlarged representation. Examination of threshold sensitivity at the characteristic frequency (CF, frequency to which the neurons were most sensitive) in the reorganized regions of the map of the lesioned cochlea established that the changes in the map reflected a plastic reorganization rather than simply reflecting the residue of prelesion input. In contrast to the change in the map of the lesioned contralateral cochlea, the map of the unlesioned ipsilateral cochlea did not differ from those in normal animals. Thus, in contrast to the normal very good congruency between ipsilateral and contralateral AI maps, in the lesioned animals ipsilateral and contralateral maps differed in the region of AI in which there had been a reorganization of the map of the lesioned cochlea. Outside the region of contralateral map reorganization, ipsilateral and contralateral AI maps remained congruent within normal limits. The difference between the two maps in the region of contralateral map reorganization suggested, in light of the physiology of binaural interactions in the auditory pathway, that the cortical reorganization reflected subcortical changes. Finally, response properties of neuronal clusters within the reorganized map of the lesioned cochlea were compared to normative data with respect to threshold sensitivity at CF, the size of frequency "response areas," and response latencies. In the majority of cases, CF thresholds were similar to normative data. The frequency "response areas" were slightly less sharply tuned than normal, but not significantly. Response latencies were significantly shorter than normal in three animals and significantly longer in one animal.

Myxoid lipoma of the tongue: a case report

Myxoid lipoma is a rare variety of lipoma and its occurrence in the oral cavity is a clinically curiosity. One such case is reported.

Gastric pull-up by eversion stripping of oesophagus

Gastric pull-up is a reliable method of one-stage reconstruction following total laryngopharyngoesophagectomy (TLPE). However, the technique of blunt finger dissection for extrapleural extraction of the oesophagus is liable to produce chest complications like pneumothorax. We report a series of 45 patients who underwent gastric pull-up using the technique of oesophageal extraction by stripping which produced virtually no thoracic complications. This simple technique has greatly reduced morbidity associated with gastric pull-up.

Antioxidant potential of specific estrogens on lipid peroxidation

Coronary heart disease (CHD) is the leading cause of death in postmenopause. Estrogen administration in postmenopause lowers the risk of CHD by 50%. A variety of estrogen preparations are currently used in postmenopausal hormone replacement therapy. It is unknown, however, if structural differences in the estrogen molecule influence the cardioprotective effects of estrogens. In this communication we have shown that equine estrogens (especially equilin) exhibit higher antioxidant potency (as measured by fatty acids and sterols oxidation) when compared to estrone and estradiol-17 beta.

Rapid changes in the frequency tuning of neurons in cat auditory cortex resulting from pure-tone-induced temporary threshold shift

The response areas (frequency by intensity) of single neurons in primary auditory cortex of anesthetized cats were studied before and after temporary threshold shifts in cochlear sensitivity induced by an intense pure tone. Cochlear temporary threshold shift was monitored through the threshold of the gross auditory nerve compound action potential and in most cases involved a notch-like loss centered at the characteristic frequency of the unit under study. Only two neurons showed changes in response area that mirrored the changes at the auditory periphery. Most neurons (14) showed more complex changes involving both expansion and contraction of response areas. Expansion of response areas was indicated by lower thresholds at some frequencies and by the emergence of sensitivity to previously ineffective frequencies. A change was classified as contraction when the response area after the intense-tone exposure was smaller than would be expected by applying the profile of the temporary threshold shift to the initial response area. Contraction of both upper (high intensity) and lower boundaries of response areas was found; in the most extreme cases, neurons were totally unresponsive after the intense-tone exposure. The complexity of effects of temporary threshold shifts on the response areas of cortical neurons is likely to be related to mechanisms that normally determine the frequency response limits of these neurons. The response areas of cortical neurons are more complex than those of auditory nerve fibers, and are thought to reflect the integration of excitatory and inhibitory inputs. The variety of effects observed in this study are consistent with the excitatory and inhibitory components of the response area of a given neuron being differentially affected by the temporary threshold shift.

Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. II: Organization of response properties along the 'isofrequency' dimension

The spatial distribution of neuronal responses to tones and frequency-modulated (FM) stimuli was mapped along the 'isofrequency' dimension of the primary auditory cortex (AI) of barbiturate-anesthetized cats. In each cat, electrode penetrations roughly orthogonal to the cortical surface were closely spaced (average separation approximately 130 microns) along the dorsoventral extent of a single 'isofrequency' strip in high frequency parts of AI (> 15 kHz). Characteristic frequency (CF), minimum threshold, sharpness of frequency tuning (Q10 and Q20), the dynamic range of the spike count-intensity function at CF, sensitivity to the rate of change of frequency (RCF) and to the direction of frequency-modulation (DS) were determined for contralaterally-presented tone and FM stimuli. Sharpness of tuning attained maximum values at central loci along the dorsoventral 'isofrequency' axis and values declined towards more dorsal and more ventral locations. Minimum threshold and dynamic range varied between high and low values in a similar and correlated periodic fashion. Their combined organization yielded an orderly spatial representation of response strength, relative to maximum, as a function of stimulus amplitude. The distributions of the most common forms of FM rate sensitivity (RCF response categories) and best RCF along 'isofrequency' strips were significantly non-random although there was a considerable degree of variability between cats. FM directional preference and sensitivity appeared to be randomly distributed. Sharpness of tuning may be related to the analysis of the spectral content of an acoustic stimulus, both minimum threshold and dynamic range are related to the encoding of stimulus intensity, and measures of FM rate and directional sensitivity assess the coding of temporal changes of stimulus spectra. The independent, or for minimum threshold and dynamic range dependent, topographic organizations of these neuronal parameters therefore suggest parallel and independent processing of these aspects of acoustic signals in AI.

Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. I: Effects of variation of stimulus parameters

In the primary auditory cortex (AI) of barbiturate-anesthetized cats multi-unit responses to tones and to frequency-modulated (FM) tonal stimuli were analyzed. Characteristic frequency (CF), sharpness of tuning, minimum threshold, and dynamic range of spike count--intensity functions were determined. Minimum threshold and dynamic range were positively correlated. The response functions to unidirectional FM sweeps of varying linear rate of change of frequency (RCF) that traversed the excitatory frequency response areas (FRAs) displayed a variety of shapes. Preferences for fast RCFs (> 1000 kHz/s) were most common. Best RCF was not correlated with measures of sharpness of tuning. Directional preference and sensitivity were quantified by a DS index which varied with RCF. About two-thirds of the multi-unit responses showed a preference for downward sweeps. Directional sensitivity was independent of CF and independent of best RCF. Measurements of latencies of phasic responses to unidirectional FM sweeps of different RCF demonstrated that the discharges of a given multi-unit over its effective RCF range were initiated at the same instantaneous frequency (effective Fi), independent of RCF. Effective Fis fell within the excitatory FRA of a given multi-unit. The relationships of effective Fis to CF show that responses were evoked only when the frequency of the signal was modulated towards CF and not when modulated away from it, and that responses were initiated before the modulation reached CF. Changes in the range and depth of modulation had only minor, if any, effects on RCF response characteristics, FM directional sensitivity, and effective Fis, as long as the beginning and ending frequencies of FM sweeps fell outside a multi-unit's FRA. Stimulus intensity also had only moderate effects on RCF response characteristics and DS. However, effective Fis were influenced in systematic fashions; with increases in intensity, effective Fis to upward and downward sweeps decreased and increased, respectively. Thus, for higher intensities FM responses were initiated at instantaneous frequencies occurring earlier in the signal. The results are compared with previous data on tone and FM sensitivity of auditory neurons in cortical and subcortical structures, and mechanisms of FM rate and directional sensitivity are discussed. The topographic representations of these neuronal properties in AI are reported in the companion report.

Additivity of threshold losses produced by acute acoustic trauma

We have previously [Patuzzi and Rajan, Hear. Res. 60, 165-177, 1992] formulated a model to describe how the threshold elevations produced by a variety of independent, short-term cochlear manipulations add when the manipulations are combined. The manipulations were presumed to affect only the 'active process' in the cochlea. The present report applied this model to the effects observed after acute acoustic trauma in normal-hearing guinea pigs and in guinea pigs with idiopathic threshold losses. Successive loud pure-tone exposures were presented to the normal-hearing guinea pigs, while only a single exposure was presented to the guinea pigs with idiopathic hearing losses. Various parameters of exposure and inter-exposure delays were used to create a variety of threshold elevations, and the total hearing losses observed in the various groups were compared to the total hearing losses predicted by the model. In most cases a statistically-valid 1:1 relationship was obtained between the predicted values and the observed values. In cases where the model's predictions were found not to fit the data, this appeared to be due to inclusion of data previously defined to be outside the scope of the model. When such data were excluded, there was good agreement between the model's predictions and the observed data. The model was further tested by comparing its predictions with data obtained in studies of acute noise trauma in chinchillas and humans by other researchers. The model's predictions were found to agree with these data as well. Thus, across a number of different types and conditions of exposures, the model appears to provide a very good description of the additivity of threshold losses produced by acute acoustic trauma. The generality of and constraints on the model are discussed.

Additivity of threshold elevations produced by disruption of outer hair cell function

We present a simple model describing the additivity of hearing loss in the mammalian cochlea produced by disruption of the outer hair cell transduction processes. The validity of this model has been tested experimentally in the guinea-pig by inducing threshold elevations using two simultaneous cochlear manipulations, including acoustic overstimulation, two-tone suppression, low-frequency acoustic biasing of the cochlear partition and electrical stimulation of the medial olivo-cochlear system of efferent fibres. The results of these experiments suggest that the model presented is an adequate description, within the measurement error of our experiments, of the hearing losses produced.