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Biomedical subjects

R Rajan

Publications and source records attributed to R Rajan.

At least 73 records · Page 4Linked to original sources
PubMed1992

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.

Acoustic Stimulation↗
PubMed1990

Electrical stimulation of the inferior colliculus at low rates protects the cochlea from auditory desensitization.

The effects of inferior collicular (IC) stimulation on cochlear responses were tested with pulsed electrical trains and with 1 min long continuous bursts. Pulsed trains did not cause any effects at the contralateral cochlea. However, a 1 min burst, containing pulses at low rates, was able to significantly reduce temporary threshold shifts (TTS) in cochlear sensitivity caused by a loud sound exposure. Intracochlear perfusion of hexamethonium blocked this effect. The time course of the hexamethonium blocking action paralleled its blocking action on the cochlear effects of electrical stimulation at the brainstem of an auditory efferent pathway, the crossed olivocochlear bundle (COCB). The protective IC effects were persistent and TTS reductions could be obtained even with a 5 min delay between IC stimulus and the loud sound. However, these persistent protective effects did not appear to occur at the cochlea. Finally, electrical stimulation at the IC ipsilateral to a cochlea exposed to loud sound also reduced TTS, but only by smaller amounts and at higher stimulation rates. Thus the IC appears to provide a strong descending influence that modulates the excitability levels of the olivocochlear nuclei in the brainstem. Both crossed and uncrossed OCB appear to be involved and able to reduce TTS. It is proposed that the protective effects may be due solely to the medial olivocochlear system and possibly only those fibres originating from one of the nuclei of the medial system.

Acoustic Stimulation↗
PubMed1990

Infantile hemangiopericytoma of nasal cavity.

Hemangiopericytoma is an uncommon vascular tumour. Of the 26 intranasal hemangiopericytomas reported in the English literature only two paediatric cases have been noted. A benign course is more common in the nose and paranasal sinuses. A pre-operative diagnosis being not possible, definitive treatment cannot be preplanned. Wide surgical excision by a lateral rhinotomy approach is still the treatment of choice. The appropriate investigations and treatment line followed are described. A lifelong followup is mandatory.

Female↗
PubMed1990

Absence of tonic activity of the crossed olivocochlear bundle in determining compound action potential thresholds, amplitudes and masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

In Nembutal- or Urethane-anaesthetised guinea pigs N1 audiograms and N1 input-output functions were measured as were compound action potential (CAP) tuning curves under forward masking and simultaneous masking conditions. Then the crossed olivocochlear bundle was lesioned at the floor of the fourth ventricle and the cochlear responses were re-measured. There were never any changes in the N1 audiograms, input-output functions, or the CAP tuning curves. Thus, the crossed efferent pathways do not appear to play any tonic role in determining cochlear threshold sensitivities, selectivities or masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

Action Potentials↗
PubMed1990

Does electrical stimulation of the crossed olivo-cochlear bundle produce movement of the organ of Corti?

Low-frequency microphonic waveforms have been recorded in the basal turn of the guinea pig cochlea with and without electrical stimulation of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle. Stimulation of the COCB increased the amplitude of the microphonic waveforms as described previously, but did not alter the shape of the waveforms markedly. The changes observed with COCB stimulation are consistent with a reduction in the impedance of the basolateral wall of the outer hair cells by about 50%, and possibly a 20% increase in the vibration of the organ of Corti at low frequencies, but suggest little or no change in the operating point on the transfer curve relating deflection of the hair bundles to the receptor current through the hair cells. It therefore seems that if slow contraction of the outer hair cells occurs during acute efferent stimulation in vivo, then it produces only a small deflection of the outer hair cell stereocilia, equivalent to a transverse displacement of the organ of Corti of less than 1.5 nm.

Action Potentials↗
PubMed1990

The effect of upper pontine transections on normal cochlear responses and on the protective effects of contralateral acoustic stimulation in barbiturate-anaesthetized normal-hearing guinea pigs.

In barbiturate-anaesthetized guinea pigs with normal cochlear neural sensitivities, upper pontine transections were made to totally isolate the cell bodies of the olivocochlear neurons in the lower brainstem from all higher centres. The effects of this procedure were examined at the cochlea on normal compound action potential (CAP) thresholds and amplitudes, on the temporary threshold shifts (TTS) in CAP sensitivity caused by monaural loud sound exposures, and on the protective effects of low-level contralateral acoustic stimulation (Cody and Johnstone, 1982; Rajan and Johnstone, 1983a, 1988). The transection had no effects on any of these responses. These results suggest that centres above the metencephalon do not exert any tonic effects on the cell bodies of the olivocochlear pathways that result in tonic effects at the cochlea. Further, these results also suggest that the protective effects of contralateral acoustic stimulation are exercised solely through lower brainstem pathways.

Acoustic Stimulation↗
PubMed1990

Azimuthal sensitivity of neurons in primary auditory cortex of cats. I. Types of sensitivity and the effects of variations in stimulus parameters.

1. Preliminary to studying the organization of azimuthal sensitivity of neurons along frequency-band strips in the primary auditory cortex (AI) of cat (see companion paper), this study examined the sensitivity of 251 units in cat AI to variations in the azimuthal location of sound sources in the frontal hemifield. Most units (231) were tested with tones at the characteristic frequency (CF; frequency to which the unit had the lowest threshold). Unit CFs ranged from 5 to 36 kHz. A large number of units (91) were tested with broadband noise stimuli, and a few units were also tested at other frequencies within the cell's tuning response area. 2. When tested at stimulus intensities 20-30 dB above CF or noise threshold, the different forms of azimuthal sensitivity exhibited by AI neurons could be divided into (1) contra-field azimuth functions; (2) ipsi-field functions; (3) central-field functions; (4) omnidirectional functions, and (5) multipeaked functions. Contra-field azimuth functions were the most prevalent, with 45.9% of units tested with CF tones and 42.9% of units tested with noise exhibiting this type of azimuthal sensitivity. Ipsi-field azimuthal sensitivity was found in 16.9% of units tested with CF tones and 19.8% of units tested with noise. Central-field azimuthal sensitivity was seen in 10.8% of units tested with CF tones and 17.6% of units tested with noise. Omnidirectional azimuthal sensitivity was seen in 19.9% of units tested with CF tones and 17.6% of units tested with noise, whereas multipeaked azimuthal sensitivity was found in 6.5% of units tested with CF tones and 5.5% of units tested with noise. 3. The effects of increasing stimulus intensity on azimuthal sensitivity were examined in 185 units tested with CF tones and 67 units tested with noise. For four major classes of azimuthal sensitivity (contra-field, ipsi-field, central-field and omnidirectional), the most common effect (approximately 60% of each class) was for the azimuth function to remain constant in form by the defining criteria for these classes. The next most common effect for all classes except omnidirectional azimuth functions was for an expansion of the azimuthal range eliciting responses. (The definition of omnidirectionality precluded any expansion of the response range in this class of azimuth function). A smaller number of units in some classes showed a compression of the azimuth function to a smaller response range, and others showed more complex expansive and compressive effects with increasing stimulus intensity.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1990

Azimuthal sensitivity of neurons in primary auditory cortex of cats. II. Organization along frequency-band strips.

1. The organization of azimuthal sensitivity of units across the dorsoventral extent of primary auditory cortex (AI) was studied in electrode penetrations made along frequency-band strips of AI. Azimuthal sensitivity for each unit was represented by a mean azimuth function (MF) calculated from all azimuth functions obtained to characteristic frequency (CF) stimuli at intensities 20 dB or more greater than threshold. MFs were classified as contrafield, ipsi-field, central-field, omnidirectional, or multipeaked, according to the criteria established in the companion paper (Rajan et al. 1990). 2. The spatial distribution of three types of MFs was not random across frequency-band strips: for contra-field, ipsi-field, and central-field MFs there was a significant tendency for clustering of functions of the same type in sequentially encountered units. Occasionally, repeated clusters of a particular MF type could be found along a frequency-band strip. In contrast, the spatial distribution of omnidirectional MFs along frequency-band strips appeared to be random. 3. Apart from the clustering of MF types, there were also regions along a frequency-band strip in which there were rapid changes in the type of MF encountered in units isolated over short distances. Most often such changes took the form of irregular, rapid juxtapositions of MF types. Less frequently such changes appeared to show more systematic changes from one type of MF to another type. In contrast to these changes in azimuthal sensitivity seen in electrode penetrations oblique to the cortical surface, much less change in azimuthal sensitivity was seen in the form of azimuthal sensitivity displayed by successively isolated units in penetrations made normal to the cortical surface. 4. To determine whether some significant feature or features of azimuthal sensitivity shifted in a more continuous and/or systematic manner along frequency-band strips, azimuthal sensitivity was quantified in terms of the peak-response azimuth (PRA) of the MFs of successive units and of the azimuthal range over which the peaks occurred in the individual azimuth functions contributing to each MF (the peak-response range). In different experiments shifts in these measures of the peaks in successively isolated units along a frequency-band strip were found generally to fall into one of four categories: 1) shifts across the entire frontal hemifield; 2) clustering in the contralateral quadrant; 3) clustering in the ipsilateral quadrant; and 4) clustering about the midline. In two cases more than one of these four patterns were found along a frequency-band strip.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1989

Contralateral cochlear destruction mediates protection from monoaural loud sound exposures through the crossed olivocochlear bundle.

Destruction of the cochlea contralateral to one subsequently exposed to a high intensity acoustic exposure has been shown to reduce the threshold losses caused by the exposure (Rajan and Johnstone, 1983a). The present study tested this manipulation on a wide variety of exposures of varying intensity and duration and found that the amount by which ipsilateral threshold losses were reduced was related to the amount of threshold losses that would have occurred in the absence of the contralateral manipulation. This loss-related protection is also found when the COCB is electrically stimulated during loud sound exposures (Rajan, 1988b; Rajan and Johnstone, 1988b). When the COCB was lesioned at the floor of the fourth ventricle contralateral cochlea destruction no longer protected the test cochlea, confirming that the crossed cochlear protection was exercised through the COCB. The contralateral manipulation did not appear to directly activate the COCB but may have acted in a facilitatory manner on the COCB, allowing activation only when a sufficiently high level exposure was subsequently presented ipsilaterally: a variety of responses at the ipsilateral cochlea and at the brainstem, remeasured after contralateral cochlear destruction and prior to an ipsilateral loud sound exposure, were found to be unaltered, although the TTS to the subsequent exposure was significantly reduced.

Animals↗
PubMed1989

Tonic activity of the crossed olivocochlear bundle in guinea pigs with idiopathic losses in auditory sensitivity.

In animals with pre-existing N1 threshold losses of unknown etiology, transection of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle resulted in a marked improvement in thresholds in the region of the loss but not at adjacent frequencies with normal thresholds or increased threshold sensitivities. There was also an increase in N1 amplitudes. These effects were not obtained if, prior to COCB transection, the COCB were continuously stimulated for 1 min in these animals, nor were they obtained with COCB transection in animals with normal N1 thresholds and amplitudes. In the animals with idiopathic N1 threshold losses, there appeared to be a linear relationship between the amount of threshold sensitivity recovered after COCB transection and the amount of loss existing initially. Parallels between these results and the recently-demonstrated protective COCB effects on temporary threshold shifts in auditory sensitivity are discussed.

Animals↗
PubMed1988

Binaural acoustic stimulation exercises protective effects at the cochlea that mimic the effects of electrical stimulation of an auditory efferent pathway.

Low-level acoustic stimulation of one (contralateral) ear reduced the neural desensitization caused by a simultaneous loud sound exposure in the other (ipsilateral) ear in a loss-related manner. Greatest reductions in the temporary threshold shifts (TTS) in the exposed ear were obtained when the exposure would have caused large amounts of TTS. Low-level exposures (reduced intensity or duration of exposure) which caused low levels of TTS, from which the cochlea could recover relatively quickly, were not affected by the contralateral stimulus. Intermediate levels of TTS showed intermediate levels of reduction for the same contralateral acoustic stimulus. These effects were similar to effects previously demonstrated with electrical stimulation of an efferent pathway to the cochlea, the crossed olivocochlear bundle (COCB); lesioning the COCB prevented the contralateral stimulus from having any effect on TTS due to an ipsilateral exposure. Like COCB stimulation, the contralateral acoustic stimulus had tonic effects, so that reductions in ipsilateral TTS could be obtained even when the contralateral stimulus was presented 5 min before the ipsilateral exposure. With 10 min delay no effect on TTS occurred. The contralateral stimulus did not appear to cause any changes in responses in the ipsilateral cochlea prior to the loud sound exposure. These results are discussed as indicating an interaction between the two inputs at a central locus, leading to activation of the COCB fibres to the cochlea exposed to the loud sound.

Acoustic Stimulation↗
PubMed1988

Electrical stimulation of cochlear efferents at the round window reduces auditory desensitization in guinea pigs. I. Dependence on electrical stimulation parameters.

Electrical stimulation at the round window with pulsed short trains has been shown to elicit classical efferent effects on N1 amplitudes at the cochlea. This report demonstrates that round window stimulation as a continuous burst can reduce temporary threshold shifts (TTS) caused by a simultaneous monaural loud sound exposure. This result is similar to recent reports that stimulation of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle can reduce TTS. Like COCB stimulation at the brainstem, the effect of round window stimulation could be abolished by strychnine, with a time course paralleling the blocking action of strychnine on the traditional COCB effects of pulsed short trains on N1 amplitudes. This report also established parameters for optimal effects of the round window stimulus and found them to be similar to the optimal parameters for the effects of brainstem stimulation on TTS. Tonic effects on TTS were also observed, with reductions in TTS being obtained as much as 7 min after a 1 min-long round window stimulus. Such tonic effects did not appear to be due to persistent effects at the cochlea but were suggested to be due to a long term resetting of some central site activated by antidromic stimulation from the round window.

Animals↗
PubMed1988

Electrical stimulation of cochlear efferents at the round window reduces auditory desensitization in guinea pigs. II. Dependence on level of temporary threshold shifts.

This report demonstrates that electrical stimulation of the efferents at the round window reduces temporary threshold shifts in a protective manner. For a standard set of stimulating parameters greatest reductions in TTS were found to exposures that caused the greatest amounts of TTS to occur. Low level exposures that caused low levels of TTS from which the cochlea could recover relatively quickly were not affected by the standard electrical stimulus. Intermediate reductions were obtained to intermediate levels of exposure, resulting in intermediate levels of TTS. Increasing current levels or duration of stimulation did not produce reductions in the low level TTS; a higher rate of stimulation was, however, able to reduce the low level TTS. Even with the higher rate of stimulation, greatest reductions in TTS occurred at the higher levels of exposure. These results are identical to the effects of COCB stimulation at the level of the brainstem and argue for viewing the COCB as a protective pathway.

Acoustic Stimulation↗
PubMed1988

Effect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. I. Dependence on electrical stimulation parameters.

1. This study examines the effect on auditory desensitization of electrically stimulating the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle. Auditory desensitization was induced by a loud high-frequency pure tone exposure and measured as temporary threshold shifts (TTS) in the sensitivity of the compound action potential recorded from the cochlea. COCB stimulation simultaneous with the loud sound exposure reduced the TTS. This reduction was contingent on the COCB stimulus being presented as a continuous burst for the entire duration (1 min) of the exposure. 2. The reduction in TTS could be abolished by prior administration of strychnine. The action of strychnine on these TTS effects of continuous COCB stimulation paralleled its action on the classical COCB effects elicited by pulsed short COCB trains. If the action of strychnine on the classical COCB effects was allowed to reverse, then continuous COCB stimulation reduced TTS as effectively as before. 3. The most effective COCB stimulus was found to be one that was presented at a high rate of stimulation simultaneous with the exposure. The COCB effect on TTS was also found to be a tonic one; smaller but significant reductions in TTS could still be obtained with the exposure presented 5 min after COCB stimulation though not when the delay was 10 min. The tonic reductions in TTS appeared to occur without any persisting changes at the cochlea. Normal cochlear responses remeasured in the delay between the stimulus and exposure were not altered. 4. It was hypothesized that the persisting effect responsible for TTS reductions did not occur at the cochlea but at some central site facilitated by antidromic action potentials along the COCB fibers. Subsequent exposure to loud sounds would activate the central site primed by the prior COCB stimulus. This hypothesis was tested by stimulating the COCB alone as before, but then lesioning the fibers before presenting the exposure. Persistent cochlear effects of the COCB stimulus should have still resulted in a reduction in TTS. However, if the persistent effect was at a more central location, lesioning the fibers would allow afferent input to act at the facilitated central location but would not allow subsequent expression of COCB effects at the cochlea. In this case, no reductions in TTS could be expected--precisely the results that were obtained in these experiments. Thus the COCB system appeared to have a "memory" component facilitated by prior stimulation and activated by a subsequent exposure.

Acoustic Stimulation↗
PubMed1988

Effect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. II. Dependence on the level of temporary threshold shifts.

1. This report presents a more detailed examination of crossed olivocochlear bundle (COCB) effects on temporary threshold shifts (TTS). The principal aim was to establish the details of the relationship between the effects of continuous COCB stimulation and the TTS caused by various exposures. Initially, in various groups, different intensities of a 1-min exposure were presented with or without COCB stimulation. Stimulation at any particular rate caused greatest reductions in TTS to occur for the higher intensity exposures than for the lower intensity ones. Thus COCB stimulation at 140 pulses/s reduced maximum threshold losses by the following amounts: 13.33 [106 dB sound pressure level (SPL) exposure], 13.29 (103 dB SPL exposure), 8.9 (101 dB SPL exposure), and 0.77 dB (97 dB SPL exposure). 2. The effect of COCB stimulation was also examined on a shorter duration (30 s) exposure causing TTS intermediate between that due to 1-min exposure at 97 dB SPL and that at 101 dB SPL. Reductions in TTS to the shorter duration exposure were not obtained with COCB stimulation at 140 pulses/s but only when the rate was increased to 260/s. 3. Thus COCB stimulation at any particular rate caused greatest reductions in TTS when the exposure would have caused a large amount of TTS, regardless of the intensity or duration of exposure. These two parameters of the exposure could be traded to provide a level of TTS that would be affected by a standard COCB stimulus. Low-level exposures that caused only small amounts of TTS were not affected by the standard COCB stimulus. As the level of the exposure, and therefore, the ensuing TTS, increased (with increased duration or intensity of exposure), the standard stimulus had greater effects. Increasing the rate of pulses in the COCB stimulus resulted in the low-level exposures now being affected. Even at the higher rate, greatest reductions in TTS were obtained when the exposure would have resulted in high levels of TTS. 4. In an attempt to determine the site of action of the continuous COCB stimulus the endocochlear potential (EP) and the summating potential (SP) were recorded from scala media of the basal turn while applying COCB stimuli similar to those used in the TTS experiments. Although the continuous COCB stimulus caused the traditional changes in the EP and SP associated with COCB stimulation with pulsed short electrical trains, these changes adapted quite rapidly and did not persist for the amount of time shown to be necessary for full reductions in TTS to be obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗