Cortical responses in rats predict perceptual sensitivities to complex sounds

The common assumption that perceptual sensitivities are related to neural representations of sensory stimuli has seldom been directly demonstrated. The authors analyzed the similarity of spike trains evoked by complex sounds in the rat auditory cortex and related cortical responses to performance in an auditory task. Rats initially learned to identify 2 highly different periodic, frequency-modulated sounds and then were tested with increasingly similar sounds. Rats correctly classified most novel sounds; their accuracy was negatively correlated with acoustic similarity. Rats discriminated novel sounds with slower modulation more accurately than sounds with faster modulation. This asymmetry was consistent with similarities in cortical representations of the sounds, demonstrating that perceptual sensitivities to complex sounds can be predicted from the cortical responses they evoke.     

Occupation as a risk factor for uveal melanoma in Germany

OBJECTIVE: This study explored occupational risks linked to uveal melanoma. METHODS: The analysis pooled data from two case-referent studies (hospital- and population-based) conducted in Germany between 1995 and 1998, with incident cases matched with several referents by age, gender, and region of residence. The subjects were contacted through personal or telephone interviews. Their exposure status was based on their occupational history. Dichotomous coding for the main task and categorization into different occupational classification systems was performed. Altogether of 118 cases and 475 referents were included. Adjusted odds ratios were calculated by conditional logistic regression. RESULTS: Relevant occupations included food, beverage, and tobacco processors [odds ratio (OR) 4.7, 95% confidence interval (95% CI) 0.99-22.0] and miners (OR 2.3, 95% CI 0.92-5.99) among the men and station, engine and heavy equipment operators and freight handlers (OR 2.5, 95% CI 0.94-6.58) and medical, dental, pharmaceutical and veterinary workers (OR 2.1, 95% CI 0.71-6.02) among the women according to the International Standard Classification of Occupations, whereas, according to the European Industrial Classification, the relevant occupations were the food industry (OR 3.4, 95% CI 1.08-10.5) and the chemical and pharmaceutical industry (OR 2.8, 95% CI 1.01-7.78) among the men and machine production (OR 3.2, 95% CI 0.96-10.7) and health and veterinary sector (OR 2.4, 95% CI 0.97-5.71) among the women. CONCLUSIONS: These analyses support the potential role of occupational exposure as a risk factor for uveal melanoma. The findings must be interpreted carefully since the exposure was assessed indirectly.     

Estimating exposures in the asphalt industry for an international epidemiological cohort study of cancer risk

BACKGROUND: An exposure matrix (EM) for known and suspected carcinogens was required for a multicenter international cohort study of cancer risk and bitumen among asphalt workers. METHODS: Production characteristics in companies enrolled in the study were ascertained through use of a company questionnaire (CQ). Exposures to coal tar, bitumen fume, organic vapor, polycyclic aromatic hydrocarbons, diesel fume, silica, and asbestos were assessed semi-quantitatively using information from CQs, expert judgment, and statistical models. Exposures of road paving workers to bitumen fume, organic vapor, and benzo(a)pyrene were estimated quantitatively by applying regression models, based on monitoring data, to exposure scenarios identified by the CQs. RESULTS: Exposures estimates were derived for 217 companies enrolled in the cohort, plus the Swedish asphalt paving industry in general. Most companies were engaged in road paving and asphalt mixing, but some also participated in general construction and roofing. Coal tar use was most common in Denmark and The Netherlands, but the practice is now obsolete. Quantitative estimates of exposure to bitumen fume, organic vapor, and benzo(a)pyrene for pavers, and semi-quantitative estimates of exposure to these agents among all subjects were strongly correlated. Semi-quantitative estimates of exposure to bitumen fume and coal tar exposures were only moderately correlated. EM assessed non-monotonic historical decrease in exposures to all agents assessed except silica and diesel exhaust. CONCLUSIONS: We produced a data-driven EM using methodology that can be adapted for other multicenter studies.     

Representation of the cochlea within the inferior colliculus of the cat

The representation of the cochlear partition within the 3 dimensions of the inferior colliculus of the cat was studied by determining the best frequency of neurons isolated in series of parallel electrode penetrations. Cats were anesthetized with sodium pentobarbital. There was a systematic representation of the cochlea within the pericentral nucleus of the inferior colliculus. The boundary between the pericentral nucleus and underlying central nucleus could be sharply drawn on physiological bases. There was a highly ordered representation of the cochlea across the 3 dimensions of the central nucleus. Neurons within the dorsomedial nucleus and external nucleus of the inferior colliculus were not driven by simple tonal stimulation. The change in best frequency as a function of depth in the colliculus was the same for parallel penetrations entering over almost the entire dorsal surface of the central nucleus. This indicated that a given frequency-band was represented across a laminar figure completely sectioning the nucleus; and that laminae were flat except along the extreme margins of the nucleus. Definition of ‘isofrequency contours’ from reconstruction of rows of parallel penetrations confirmed these conclusions. Frequency-band laminae were tilted down rostrally, especially in the middle and lateral aspects of the central nucleus. They were also tilted down laterally. Orientation of isofrequency contours paralleled that of laminae described within anatomical studies of the central nucleus²⁹. The disk-shaped regions representing lowest frequency octaves were not as thick nor as extensive as those representing middle and high frequency octaves. However, when best frequency values were converted to corresponding cochlear place, cochlear place was observed to be a nearly linear function of colliculus depth for the representation of the apical half of the cochlear partition. There was a somewhat larger representation, across colliculus depth, of the basal half of the cochlea. The change in best frequency with change of depth in the central nucleus was stepwise. This suggested that small sectors of the basilar membrane of approximately equal length were represented across individual anatomically defined cellular laminae within the nucleus.     

Representation of cochlea within primary auditory cortex in the cat.

The representation of sound frequency (and of the cochlear partition) within primary auditory cortex has been investigated with use of microelectrode-mapping techniques in a series of 25 anesthetized cats. Among the results were the following: 1) Within vertical penetrations into AI, best frequency and remarkably constant for successively studied neurons across the active middle and deep cortical layers. 2) There is an orderly representation of frequency (and of represented cochlear place) within AI. Frequency is rerepresented across the mediolateral dimension of the field. On an axis perpendicular to this plane of rerepresentation, best-frequency (represented cochlear place) changes as a simple function of cortical location. 3) Any given frequency band (or sector of the cochlear partition) is represented across a belt of cortex of nearly constant width that runs on a nearly straight axis across AI. 4) There is a disproportionately large cortical surface representation of the highest-frequency octaves (basal cochlea) within AI. 5) The primary and secondary field locations were somewhat variable, when referenced to cortical surface landmarks. 6) Data from long penetrations passing down the rostral bank of the posterior ectosylvian sulcus were consistent with the existence of a vertical unit of organization in AI, akin to cortical columns described in primary visual and somatosensory cortex. 7) Responses to tonal stimuli were encountered in fields dorsocaudal, caudal, ventral, and rostral to AI. There is an orderly representation of the cochlea within the field rostal to AI, with a reversal in best frequencies across its border with AI. 8) Physiological definitions of AI boundaries are consistent with their cytoarchitectonic definition. Some of the implications of these findings are discussed.     

Receptive-field changes induced by peripheral nerve stimulation in SI of adult cats

1. Receptive fields (RFs) of neurons in the primary somatosensory (SI) cortex were defined before, during, and after electrical stimulation of myelinated fibers in the dorsal cutaneous branch of the ulnar nerve in adult pentobarbital sodium-anesthetized cats. 2. This stimulation resulted in an approximately threefold increase of SI multiunit RF sizes. Substantial changes were first recorded within 1-2 h of stimulation. RFs typically enlarged continuously over a several-hour stimulation period, then stabilized. 3. RF-area increases were observed within both the forepaw and hindpaw representational zones in the SI cortex contralateral to the stimulated forepaw nerve. RF sizes did not increase in the ipsilateral SI body surface representation or in sham-stimulation control animals. 4. Preliminary studies indicate that stimulation-induced changes can be halted and often reversed by the intravenous administration of the opiate antagonist naloxone. 5. These observations suggest a global naloxone-sensitive modulatory system that operates on large-diameter afferent inputs in the cat somatosensory system. The increases in RF size occur under nerve-stimulation conditions similar to those that result in the generation of widespread analgesia (Chung et al. 1984a,b; Gamble and Milne 1986; Toda and Ichioka 1978).     

Origins of the scalp recorded frequency-following response in the cat.

The frequency-following response (FFR) is a short-latency scalp-recorded evoked potential elicited by the presentation of low-frequency acoustic stimuli. It is thought to be the result of the synchronous electrical activity in brain stemauditory nuclei to each wave in the acoustic signal. The present investigation constitutes an attempt to determine the generators of the FFR in the cat by analysis of the response and by section of brain stem auditory nuclei and tracts. Among the results were the following: (1) the cochlear nuclei contribute approximately 50% of the amplitude of the scalp-recordedFFR in the cat. (2) The cochlea also makes a significant contribution, accounting for an average of nearly 25% of the response amplitude. (3) The superior olivary nuclei (and/or the nuclei of the lateral lemnisci) account for about 20% of the response amplitude. (4) The contributions from the inferior colliculi (contrary to earlier studies) were found to be relatively insignificant. (5) As a consequence of the existence of multiple generators, the FFR ample area of hair cell excitation for each stimulus frequency involved in the mediation of the FFR, suggestion that scalp-recorded FFRs could be used to ascertain low-frequency hearingsensitivity in uncooperative human subjects.     

Neuronal discharge rate is unsuitable for encoding sound intensity at the inferior-colliculus level

Rate-intensity functions from single neurons in the central nucleus of the inferior colliculus (ICC) of the cat in response to tone bursts and continuous noise were recorded. Only 6% of 64 quantitatively studied neurons had monotonic functions in response to tone bursts, 12.5% in response to continuous noise. The other neurons had functions with a single peak which could occur at any super-threshold level tested (3-80 dB), or with multiple peaks. In 78% of the neurons the rate-intensity functions in response to tones and noise were of different shape. We interpret this stimulus dependency of the shape of rate-intensity functions of most neurons as evidence against sound-intensity coding only on the basis of the shape of rate-intensity functions at the ICC level. Rate-intensity functions averaged from neurons with similar characteristic frequencies or from all neurons of our sample indicate a constant average discharge over at least 60 dB sound intensity. This excludes sound-intensity coding by means of an average neuronal discharge rate in the ICC.