Best Frequency Research Articles

Level dependence of cochlear nucleus onset unit responses and facilitation by second tones or broadband noise.

1. The responses of onset units in the cochlear nucleus of the anesthetized guinea pig have been measured to single tones, two-tone complexes, and broadband noise (BBN; 20-kHz bandwidth). The onset units were subdivided into three groups, onset-I (OnI), onset-L (OnL), and onset-C (OnC), on the basis of a decision tree using their peristimulus time histogram (PSTH) shape and discharge rate in response to suprathreshold best-frequency (BF) tone bursts. 2. PSTHs were constructed from responses either to single tones at a unit's BF or to BBN as a function of level. When sufficient sustained activity could be elicited from the unit, arbitrarily defined as > 100 spikes/s, a coefficient of variation (CV) was calculated; the majority were characterized by a CV that was similar to transient chopper units (0.35 < CV < 0.5). First spike latency decreased monotonically with increasing sound level. For the majority of onset units, the first spike timing was very precise. 3. BF rate-level functions recorded from OnL and OnC units did not show any signs of discharge rate saturation at the highest sound levels we have used (100-115 dB SPL). No systematic relationship was observed between the threshold at BF and the shape of the rate-level function. BBN rate-level functions were typically characterized by higher discharge rates than in response to BF tones. However, for OnI units and a minority of other onset units, there was little difference in the shape of their rate-level functions in response to BF tones or BBN. 4. The threshold of most onset units to BBN was similar to the threshold to a BF tone that had similar overall root-mean-square (RMS) energy. The BBN threshold was, on average, 5.5 dB greater than the BF threshold. This result contrasts with that found in auditory-nerve fibers recorded in the same species, with the use of an identical sound system, where the threshold to BBN was, on average, 19.4 dB higher. The mean threshold difference between BBN and BF tones for a population of chopper units recorded in the same series of experiments was 17.7 dB. The relative thresholds to BBN and BF tones indicated that the bandwidths near the onset units' BF threshold were broader than could be estimated with the use of single tones. Ten units were characterized by bimodal response areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural modeling of the dorsal cochlear nucleus: cross-correlation analysis of short-duration tone-burst responses.

A conceptual model of a portion of dorsal cochlear nucleus (DCN) neural circuitry has emerged over the past two decades. This model suggests that the response properties of the DCN's major projection neurons, called type IV units, are due, in part, to the behavior of local circuit inhibitory interneurons called type II units (Young and Brownell 1976). Cross-correlation studies of simultaneously recorded pairs of DCN units in decerebrate cat derived from 50-s best frequency (BF) stimuli are consistent with and have extended this conceptual model (Voigt and Young 1980, 1985, 1988, 1990). Interestingly, Gochin et al. (1989) found no signs of inhibition in the anesthetized rat DCN in cross-correlograms derived from 55-ms short-duration BF tone bursts. This seemingly contradictory result has motivated this study. Computer simulations were run using our network model of the intrinsic DCN neural circuitry. This model has previously been shown to reproduce the major features of both type II and type IV rate-level curves and the inhibitory trough (IT) observed in cross-correlograms derived from long-duration stimuli (Voigt and Davis 1994). The goal was to study the stimulus-duration-dependent strength of ITs in the cross-correlograms derived from short-duration BF tone-burst stimuli. The results suggest that ITs may not be detectable when the stimulus duration is 50 ms but may be detectable when the stimulus duration is 200 ms or greater. Furthermore, when the ITs are detected in cross-correlograms derived from 200-ms data sets, the strength of the IT, as measured by effectiveness, is comparable to the strength of ITs measured when the stimulus duration is 50 s.

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: Effects of changing the azimuthal location of an unmasked pure-tone stimulus

Azimuth functions (discharge rates evoked by tone bursts as a function of stimulus azimuth) were obtained from neurons in the inferior colliculus (IC) of C57 mice aged 2, 7 and 12 months. Because of a gene that affects the cochlea, C57 mice exhibit high-frequency sensorineural hearing loss at 7 and 12 months. Azimuth functions were examined for differences that might be related to the decline in localization acuity that accompanies hearing loss in this strain. Irrespective of age group, nearly all neurons in the central area of the IC were sensitive to the azimuth of a best frequency (BF) stimulus, as revealed by azimuth functions in which firing rates varied by more than 50% from maximum to minimum at one or more intensities. The age groups were similar in many respects (e.g., there were no significant differences in the proportion of functions meeting the criterion for direction sensitivity, the proportion of neurons with direction sensitive functions over a range of intensities, azimuth function shapes, the locations or stability of ‘borders’ separating angles evoking high versus low discharge rates). However, in 7- and 12-month-olds: the proportion of IC neurons in which the strongest excitatory driving was evoked by ipsilateral stimulation was significantly larger; azimuth function borders were more likely to be ‘reversed’ (i.e., the high rates being evoked by the more ipsilateral angle); and a greater proportion of azimuth functions met the criterion for direction sensitivity only minimally. The findings suggest that binaural excitatory-inhibitory interactions are altered in IC neurons of hearing-impaired mice.

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: Effects of changing the azimuthal location of a continuous noise masker on responses to contralateral tones

Extracellular recordings were obtained from inferior colliculus neurons of young adult (2-month-old) C57 mice with normal hearing and middle-aged (6-month-old) C57 mice with sensorineural hearing loss as they responded to best frequency (BF) tones (signal) in the presence of a continuous background noise (masker). Rate/level functions were obtained for the signal alone, noise bursts alone, and the signal in continuous noise as a function of masker location. For both groups of mice, thresholds for BF tones were significantly elevated in the presence of noise at all three noise locations. Separating the signal and masker sources significantly improved masked tone thresholds of 2-month-old mice but not hearing-impaired mice. The decreased ability of middle-aged mice to benefit from separation of the signal and masker sources may reflect alterations in binaural processing as a result of sensorineural hearing loss.

Two separate inhibitory mechanisms shape the responses of dorsal cochlear nucleus type IV units to narrowband and wideband stimuli

1. The principal cells of the dorsal cochlear nucleus (DCN) are mostly inhibited by best frequency (BF) tones but are mostly excited by broadband noise (BBN), producing the so-called type IV response characteristic. The narrowband inhibitory responses can be explained by the inhibitory influence of interneurons with type II response characteristics. However, it is not clear that all the details of the type IV responses can be accounted for by this neural circuit. In particular, many type IV units are inhibited by band-reject noise (notch noise); type II units tend to be only weakly excited by these stimuli, if at all. In this paper we study the relationships between the narrowband, inhibitory and the wideband, excitatory regimens of the type IV responses and present the case for the existence of a second inhibitory source in DCN, called the wideband inhibitor (WBI) below. 2. Type IV units were studied using pure tones, noise bands arithmetically centered on BF, notch noise centered on BF, and BBN. We measured the rate-level function (response rate as function of stimulus level) for each stimulus. This paper is based on the responses of 28 type IV units. 3. Evidence for low-threshold inhibitory input to type IV units is derived from analysis of rate-level functions at sound levels just above threshold. Notch noise stimuli of the appropriate notch width produce inhibition at threshold in this regime. When BBN is presented, this inhibition appears to summate with excitation produced by energy in the band of noise centered on BF, resulting in BBN rate-level functions with decreased slope and maximum firing rate. A range of slopes and maximal firing rates is observed, but these variables are strongly correlated and they are negatively correlated with the strength of the inhibition produced by notch noise; this result supports the conclusion that a single inhibitory source is responsible for these effects. 4. By contrast, there is a weak (nonsignificant) positive correlation between the strength of the inhibitory effect of notch noise and the slope/maximal firing rate in response to narrowband stimuli, including BF tones. The contrast between this positive nonsignificant correlation and the significant negative correlation mentioned above suggests that more than one inhibitory effect operates: specifically, the type II input is responsible for inhibition by narrowband stimuli and a different inhibitory source, the WBI, produces inhibition by notch stimuli. 5. Several lines of evidence are given to show that type II units cannot produce the inhibition seen with notch noise stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Units in the primary auditory cortex of the Japanese monkey can demonstrate a conversion of temporal and place pitch in the central auditory system

The present study has examined whether the temporal pitch known as the «missing fundamental» is co-place-coded with the place pitch in the auditory cortex, using adult Japanese monkeys (Macaca fuscata) prepared for chronic recordings. The stimuli were white noise bursts, tone bursts and bursts of a combination of higher harmonics of a low frequency. Unit recordings were made from the contralateral (left) primary auditory cortex (Al). During recording sessions, the animal was anesthetized with a mixture of nitrous oxide and oxygen, supplemented by ketamine and xylazine injections. The data have confirmed a tonotopicity, low frequency anteriorly while high frequency posteriorly, in Al when the recording time window was short (<20 msec) with stimulus at threshold levels. For longer time windows and/or suprathreshold stimulus levels, determining the best frequency (BF) for a neuron was often difficult. In the low frequency area (≤500 Hz), the same neuron responded to BF as well as a combination of successive higher harmonics of BF without BF. However, they did not or little responded to the harmonics themselves. The findings imply that the temporally-coded pitch at the periphery appears to be already place-coded together with the place pitch by the same neuron at Al. This evidence agrees well with previous psychoacoustical findings

Processing of frequency-modulated sounds in the cat's anterior auditory field

1. Single-neuron activity was recorded from the anterior auditory field (AAF) in the cortex of gas-anesthetized cats. 2. Tone bursts and broad-band complex sounds were used for auditory stimulation. Responses to frequency-modulated (FM) sounds, in particular, were studied systematically. 3. Linear FM sweeps were centered around the best frequency (BF) of a neuron and had an excursion large enough to cover its whole frequency tuning range. Rate and direction of change of the FM sweeps were varied. 4. In 69% of the FM responses, a peak was found at an instantaneous frequency that corresponded to the BF in the pure-tone response. Thirty-three percent of the units had multiple maxima in their FM response. These secondary maxima were not always reflected in the pure-tone response of the same neurons. 5. The vast majority of AAF neurons showed one of two types of selectivity for FM rate. Depending on the criterion, almost half of the cells (46%) preferred fast changes of > 200 Hz/ms (high-pass) in both FM directions. Forty-eight percent of all neurons showed band-pass behavior with a clear preference in the middle range of FM rates in one or both directions. Low-pass or all-pass neurons made up only a small proportion (4 and 1%, respectively) of AAF neurons. 6. When both directions of an FM sweep (low-to-high and high-to-low-frequency) were tested, 66% of the neurons clearly were selective for one direction. This selectivity was not present necessarily at the preferred FM rate. In general, FM direction selectivity was most pronounced at slower FM rates. 7. The selectivity of AAF neurons for the rate and direction of FM sounds makes these neurons suitable for the detection and analysis of communication sounds, which often contain FM components with a particular sweep rate and direction.

Processing of amplitude-modulated signals that mimic echoes from fluttering targets in the inferior colliculus of the little brown bat, Myotis lucifugus

1. Neurophysiological recordings were undertaken to determine how neurons in the central nucleus of the inferior colliculus (ICc) of the little brown bat, Myotis lucifugus, extract amplitude modulations that span across a series of tone pulses (i.e., signals that simulate echoes from fluttering targets). Two types of stimuli were presented to the bats. The first served as a control and consisted of an unmodulated train of tone pulses having different repetition rates (TPu, 5-400 pulses per second). The second was a train of tone pulses that were sinusoidally amplitude modulated (TPm, 5-110 Hz) across sequential pulses. The modulated trains of pulses were presented at five different repetition rates (25, 50, 100, 200, and 400 pulses per second) encompassing the range of biosonar emission rates in these bats at different stages of target-directed flight. 2. One hundred fifty-two single neurons were isolated in the ICc of M. lucifugus; their basic response properties and temporal firing patterns were characterized. The best frequencies (BFs) ranged from 10 to 80 kHz and the minimum thresholds at BF were distributed widely (10-95 dB SPL). The frequency tuning selectivity ranged widely, from very broadly tuned (Q10dB = 1.3) to narrowly tuned (Q10dB = 89). Units with very narrow frequency tuning (Q values > 20) were restricted to BFs of 30-50 kHz. The temporal firing pattern of ICc units could be categorized into primary-like (PL), chopper (C), onset-immediate (OI), and onset-late (OL). 3. In response to TPu ICc units exhibited varying degrees of response selectivities as evidenced by their count-based response functions (using the spike count as a measure) versus repetition rate. The count-based response functions of ICc units exhibited five filtering characteristics including band-pass, low-pass, high-pass, band-suppression, and all-pass characteristics. The temporal firing pattern of a unit showed certain correlations with its count-based response function. For example, the majority of OI and OL units, and about half of the C units, showed tuned band-pass response functions. The remaining C and onset types showed mostly low-pass response functions. In contrast, PL neurons showed mostly high-pass response functions, but one third displayed band-pass response functions. 4. The ability of ICc neurons to time-lock their discharges to the individual pulses in a train was characterized by using the synchronization coefficient (SC) as a measure. The SC was plotted against the repetition rate to construct units' synchronization-based response function.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative analysis of spectro-temporal receptive fields, reverse correlation functions, and frequency tuning curves of auditory-nerve fibers.

The tuning properties of single auditory-nerve fibers (ANFs) are characterized with spectro-temporal receptive fields (STRFs), reverse correlation functions (revcors), and frequency tuning curves (FTCs). Measures of tuning and latency from the STRFs and revcors are largely comparable to the traditional measures of tuning from FTCs and measures of latency from peristimulus time histograms (PSTHs), but several important differences are found. As is well known, revcors can only characterize low (< 6 kHz) best frequency (BF) units, whereas STRFs are able to characterize all units studied (BFs ranging from 0.26-23 kHz), except for a few very low-BF examples. Whereas tuning bandwidth derived from revcor exceeds that measured from FTCs at all BFs and increases with sound level. STRF bandwidth is comparable to FTC bandwidth, except at low BFs, and is stable with sound level. The STRF may reflect nonlinear properties of auditory-nerve fibers such as refractoriness and two-tone suppression that are absent in the FTC and revcor characterizations. The principal drawback of the STRF is its narrow dynamic range.

Effects of interleukin 4 on production of IgE monoclonal antibodies directed against glucose oxidase.

Interleukin 4 is a cytokine that promotes the growth of B cells and the synthesis of IgE subclass antibody. Here we report the results of our study that was designed to determine if recombinant murine interleukin 4 (rmIL-4) can increase the yield of hybridomas secreting IgE subclass monoclonal antibodies directed against glucose oxidase. Three BALB/c mice (test group) were primed i.p. with 50 micrograms of glucose oxidase and then boosted i.v. 2 weeks later with 5 micrograms of glucose oxidase plus 200 ng of rmIL-4. A standard hybridoma fusion was performed 3 days later, and the fusion products were then cultured in the presence of 20 ng of rmIL-4 per milliliter. Another group of three mice underwent an identical immunization and fusion procedure, except that the rmIL-4 was omitted (control group). At 21 days after fusion, the test group had a significantly greater frequency of wells with hybridomas (average frequency = 191 per 1000 wells in test group versus 58 in control group; chi 2 = 1803; p = 0.0001) and wells containing IgG anti-glucose oxidase (mean of 23 per 1000 wells in test group versus 3 in control group; chi 2 = 506, p = 0.0001). Moreover, the IL-4-treated group had a significantly higher frequency of wells containing IgE antibodies (mean of 44 versus 11 in control group; chi 2 = 288, p = 0.0001) and IgE antibodies directed against glucose oxidase (mean of 5 per 1000 wells versus 2 in control group; chi 2 = 21, p = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Dorsal cochlear nucleus single neurons can enhance temporal processing capabilities in background noise.

Envelope temporal fluctuations are critical for effective processing of biologically relevant sounds including speech, animal vocalizations, sound-source location and pitch. Amplitude modulation (AM) of sound envelopes can be encoded in quiet with high fidelity by some auditory neurons, including those of the cochlear nucleus. From both neurophysiological and clinical perspectives, it is important to understand the effects of background noise on the processing of AM. To further this goal, single-unit recordings were made from dorsal cochlear nucleus (DCN) units in urethane-anesthetized chinchillas. All units of this study were classified as pauser/buildup or On-s units according to PSTH response patterns, first spike latencies, and shape of best-frequency (BF) rate-intensity functions. BF pure-tone and AM (10-500 Hz) tone bursts were presented at several sound levels, in quiet and in the presence of a continuous wideband masker. The following was found: (1) DCN units can enhance their AM coding relative to quiet in the presence of loud noise (+14 or +19 dB S/N) and at high signal levels (e.g. 75 dB SPL); (2) for the sample of units of the present study, this is usually achieved by lowering the average firing rate and increasing the synchronous (fundamental frequency) response; (3) for some units, the AM coding stays the same or declines in the background noise. The nature of these findings suggests that part of a DCN unit's abilities to preserve or enhance AM coding with masking noise results from peripheral operating range shifts, whereas part comes from intrinsic circuitry (inhibitory inputs) or cellular mechanisms (dendritic filtering of sound temporal features) within the DCN.

Tonotopic and functional organization in the auditory cortex of the big brown bat, Eptesicus fuscus.

1. In Eptesicus the auditory cortex, as defined by electrical activity recorded from microelectrodes in response to tone bursts, FM sweeps, and combinations of FM sweeps, encompasses an average cortical surface area of 5.7 mm2. This area is large with respect to the total cortical surface area and reflects the importance of auditory processing to this species of bat. 2. The predominant pattern of organization in response to tone bursts observed in each cortex is tonotopic, with three discernible divisions revealed by our data. However, although cortical best-frequency (BF) maps from most of the individual bats are similar, no two maps are identical. The largest division contains an average of 84% of the auditory cortical surface area, with BF tonotopically mapped from high to low along the anteroposterior axis and is part of the primary auditory cortex. The medium division encompasses an average of 13% of the auditory cortical surface area, with highly variable BF organization across bats. The third region is the smallest, with an average of only 3% of auditory cortical surface area and is located at the anterolateral edge of the cortex. This region is marked by a reversal of the tonotopic axis and a restriction in the range of BFs as compared with the larger, tonotopically organized division. 3. A population of cortical neurons was found (n = 39) in which each neuron exhibited two BF threshold minima (BF1 and BF2) in response to tone bursts. These neurons thus have multipeaked frequency threshold tuning curves. In Eptesicus the majority of multipeaked frequency-tuned neurons (n = 27) have threshold minima at frequencies that correspond to a harmonic ratio of three-to-one. In contrast, the majority of multipeaked neurons in cats have threshold minima at frequencies in a ratio of three-to-two. A three-to-one harmonic ratio corresponds to the "spectral notches" produced by interference between overlapping echoes from multiple reflective surfaces in complex sonar targets. Behavioral experiments have demonstrated the ability of Eptesicus to use spectral interference notches for perceiving target shape, and this subpopulation of multipeaked frequency-tuned neurons may be involved in coding of spectral notches. 4. The auditory cortex contains delay-tuned neurons that encode target range (n = 99). Most delay-tuned neurons respond poorly to tones or individual FM sweeps and require combinations of FM sweeps. They are combination sensitive and delay tuned.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural encoding of single-formant stimuli in the cat. I. Responses of auditory nerve fibers.

1. We have studied auditory responses to a set of speech-related narrowband sounds, single-formant stimuli (SFSs), in populations of auditory nerve fibers (ANFs). An analytic method was developed to extract the envelope of temporal discharge patterns of the ANF responses to nonsinusoidally modulated stimuli, whose spectra have multiple clusters of components. Such responses are often encountered in the auditory system when complex stimuli are used and have traditionally been studied by analyzing the fundamental component of the responses. 2. The envelope modulation in the SFSs is shown to be represented by the response patterns of ANFs. When the whole ANF population is considered, the information on modulation in stimulus envelope does not disappear at the highest sound level tested at all best frequencies (BFs) we studied (1-10 kHz). The representation is the best at medium sound levels and degrades at high sound levels. Low/medium-spontaneous rate (SR) ANFs showed greater envelope modulation in their responses at high sound levels than do high-SR ANFs. The quality of the representation at high sound levels is, on average, proportional to BF threshold of an ANF. On the basis of populations of ANFs with all SRs, the envelope modulation in the SFSs is represented over a wide range of sound levels. 3. We found that low-BF ANFs differ from high-BF ANFs in representing envelope modulation in the SFSs. For ANFs with BFs less than approximately 6 kHz, information on stimulus envelope is not only contained in spectral components near direct current but also in components at the vicinities of frequencies equal to BF and its multiples. In fact, for ANFs with BFs < 3 kHz, the contribution from spectral components centered at BF to overall response modulation is greater than that from spectral components near direct current. These findings indicate that, by using measures solely based on the fundamental component, the amount of modulation in the responses to narrowband stimuli is underestimated for low-BF ANFs. 4. Off-BF stimulation of ANFs with SFSs was found to result in increased envelope modulation in responses at high sound levels. The further away the stimulus is centered relative to unit BF, the greater the modulation it induces, provided that the stimulus is capable of exciting the unit. An SFS centered as close as 15% off unit BF can produce a significant increase in the modulation of responses at very high sound levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional Organization of Auditory Cortex in the Mongolian Gerbil (Meriones unguiculatus). I. Electrophysiological Mapping of Frequency Representation and Distinction of Fields

The frequency representation within the auditory cortex of the anaesthetized Mongolian gerbil (Meriones unguiculatus) was studied using standard microelectrode (essentially multiunit) mapping techniques. A large tonotopically organized primary auditory field (AI) was identified. High best frequencies (BFs) were represented rostrally and low BFs caudally along roughly dorsoventrally oriented isofrequency contours. Additional tonotopic representations were found adjacent to AI. Rostral to AI was a smaller field with a complete tonotopic gradient reversed with respect to that in AI (mirror image representation) and was termed the anterior auditory field (AAF). BFs in the range from 0.1 to 43 kHz, apparently covering the hearing range of the Mongolian gerbil, were found in AI and AAF. Units in these two core fields responded to narrow frequency ranges with short latencies. Ventral to the common high-frequency border to AAF and AI, a rapid transition to very low BFs suggested the presence of a ventral field (V). Caudal to AI two small tonotopically organized fields were identified, a dorsoposterior field (DP) and a ventroposterior field (VP). The VP showed a tonotopic organization mirror imaged to that of AI, i.e. low frequencies were represented rostrally near the caudal border of AI, and high frequencies caudally. The DP showed a concentric frequency organization with high BFs located in the centre. Units in DP and VP fired less strongly, with considerably longer latencies, and responded to a broader range of frequencies than units in AI and AAF. Dorsocaudal to AI a dorsal field (D) was identified, harbouring units that responded to very broad ranges of frequencies. A tonotopic organization of field D could not be discerned. In the border region of AI and D, low-frequency responses were similar to those found in parts of AI and AAF, but without a clear-cut tonotopic organization. This region was termed Ald. The two core fields AI and AAF appeared to be located within the koniocortex, while the remaining fields lay outside. Our data show that the organization of the gerbil auditory cortex is highly elaborate, with parcellation into fields as complex as in cat or primates.

Neurons in the inferior colliculus, auditory cortex and pontine nuclei of the FM bat, Eptesicus fuscus respond to pulse repetition rate differently

Single-neuron responses to pulse repetition rate in the inferior colliculus, auditory cortex and pontine nuclei of the FM bat, Eptesicus fuscus were studied under free-field stimulation conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were first determined with a 4 ms pulse broadcast from a specific point (response center) of the bat's frontal auditory space at which the neuron had maximal spatial sensitivity. The neuron's intensity-rate function was then studied with a 4 ms BF pulse delivered at 10 dB increments above its MT in order to determine the best intensity to which the neuron discharged maximally. The neuron's discharge pattern and number of impulses to 32 trials of 300 ms train stimuli, which consisted of different number of 4 ms BF and best intensity pulses (1, 2, 3, 8, 12, 19, 24, 29 pulses/train) and delivered at an interpulse interval of 1000, 250, 170, 100, 40, 25, 15, 12 and 10 ms (i.e. at a pulse repetition rate of 1, 4, 6, 10, 25, 40, 67, 83, 100 pulses/s), were sequentially recorded. All neurons recorded from the inferior colliculus, auditory cortex and pontine nuclei discharged phasically (1–3 impulses) but they responded to the pulse repetition rate in different manners. More than 63% of 38 inferior collicular and 65 pontine neurons studied discharged impulses to each pulse within a train stimulus when the pulse repetition rate was up to 40 pulses/s. Although the number of neurons showing such a one-to-one following response decreased with increasing pulse repetition rate, at least 20% of neurons still maintained such a response pattern even at a pulse repetition rate of 100 pulses/s. In contrast, responses of 60% of 36 cortical neurons followed the pulse repetition rate only up to 10 pps beyond which all but one discharged impulses to the first pulse of a train stimulus. We suggest that the different discharge pattern to pulse repetition rate among individual neurons from these three brain centers is primarily due to their different recovery cycles.

Spectral representation of envelope beat frequency in the discharge of cochlear nucleus units in the chinchilla

Nonlinear processing of stimulus envelope was investigated in the discharge of cochlear nucleus (CN) neurons. A standard method of investigating nonlinearities is to examine the output of a system for distortion products when the input consists of two tones. For a tone that is amplitude modulated by a two-tone complex, there is no spectral energy at the envelope beat frequency for either the stimulus or the half-wave rectified version of the stimulus. Post-stimulus time histograms (PSTHs) were generated for CN units recorded from barbiturate anesthetized chincillas to amplitude modulated best frequency (BF) tones modulated by a one- or two-tone complex. Modulation frequencies were between 4–64 Hz. Fourier analysis of the PSTHs shows spectral peaks corresponding to envelope beat frequencies. The magnitudes of the spectral peaks at the beat frequencies are less than the magnitudes of the spectral peaks at the primary modulator frequencies. Spectral peaks in neural spectra at envelope beat frequencies presumably arise from a compressive nonlinearity. Psychophysically, low-frequency envelope beating can be an effective masker of AM detection at the beat frequency (Sheft and Yost, in review). The preliminary results suggest that a spectral representation of the envelope beat frequency does exist at the level of the CN. [Work supported by a PPG grant from NIDCD.]

Purified Murine Hematopoietic Stem Cells Function Longer on Nonirradiated W41/Wv Than on + / + Irradiated Stroma

AbstractMouse bone marrow (BM) was separated into low-density, lineage- negative, wheat germ agglutinin-positive (WGA+), Rhodamine-123 bright (Rhbright) or dim (Rhdim) cells to obtain populations that were highly enriched for committed progenitors (Rhbright cells) or for more primitive stem cells (Rhdim). When 2,500 Rhbright or Rhdim cells were seeded onto 6-week-old irradiated (20 Gy) long-term BM cultures (LTBMC), the nonadherent cell production from Rhbright cells was transient and ended after 5 weeks. Production from Rhdim cells did not begin until week 3, peaked at week 5, and ended at week 8, when the irradiated stroma seemed to fail. Termination of cell production from Rhdim cells did not occur in nonirradiated LTBMC from W41/Wv mice. During peak nonadherent cell production, 25% to 30% of the cells in the nonirradiated LTBMC from W41/Wv mice had donor cell markers. Two approaches were tested to try to enhance the proportion or number of donor cells. Addition of Origen-HGF at the time of seeding Rhdim cells caused a nonspecific increase in both host and donor cell production, but a specific increase in production of donor cells was obtained by seeding the cultures at 2 weeks rather than 6 weeks. Limiting dilution of Rhdim cells gave the same frequency of wells producing cells on both irradiated +/+ and nonirradiated W41/Wv or W/Wv cultures.

Purified murine hematopoietic stem cells function longer on nonirradiated W41/Wv than on +/+ irradiated stroma

Mouse bone marrow (BM) was separated into low-density, lineage- negative, wheat germ agglutinin-positive (WGA+), Rhodamine-123 bright (Rhbright) or dim (Rhdim) cells to obtain populations that were highly enriched for committed progenitors (Rhbright cells) or for more primitive stem cells (Rhdim). When 2,500 Rhbright or Rhdim cells were seeded onto 6-week-old irradiated (20 Gy) long-term BM cultures (LTBMC), the nonadherent cell production from Rhbright cells was transient and ended after 5 weeks. Production from Rhdim cells did not begin until week 3, peaked at week 5, and ended at week 8, when the irradiated stroma seemed to fail. Termination of cell production from Rhdim cells did not occur in nonirradiated LTBMC from W41/Wv mice. During peak nonadherent cell production, 25% to 30% of the cells in the nonirradiated LTBMC from W41/Wv mice had donor cell markers. Two approaches were tested to try to enhance the proportion or number of donor cells. Addition of Origen-HGF at the time of seeding Rhdim cells caused a nonspecific increase in both host and donor cell production, but a specific increase in production of donor cells was obtained by seeding the cultures at 2 weeks rather than 6 weeks. Limiting dilution of Rhdim cells gave the same frequency of wells producing cells on both irradiated +/+ and nonirradiated W41/Wv or W/Wv cultures.

Central auditory neurophysiology of a sound-producing fish: the mesencephalon of Pollimyrus isidori (Mormyridae)

This paper describes the auditory neurophysiology of the mesencephalon of P. isidori, a sound-producing mormyrid fish. Mormyrids have a specialized pressure-sensitive auditory periphery, and anatomical studies indicate that acoustic information is relayed to the mesencephalic nucleus MD. Fish were stimulated with tone bursts and clicks, and responses of MD neurons were recorded extracellularly. Auditory neurons had best frequencies (BF) and best sensitivities (BS) that fell within the range of frequencies and levels of the natural communication sounds of these fish. BSs were in the range of 0 to -35 dB (re. 1.0 dyne/cm2). Many of the neurons were tuned (Q10 dB: 2-6), and had BFs in the range of 100-300 Hz where the animal's sounds have their peak energy. A range of different physiological cell types were encountered, including phasic, sustained, and complex neurons. Some of the sustained neurons showed strong phase-locking to tones. Many neurons exhibited non-monotonic rate-level functions. Frequencies flanking the BF often caused a reduction in spontaneous activity suggesting inhibition. Many neurons showed excellent representation of click-trains, and some showed a temporal representation of inter-click-intervals with errors less than 1 ms.

Organization of response areas in ferret primary auditory cortex.

1. We studied the topographic organization of the response areas obtained from single- and multiunit recordings along the isofrequency planes of the primary auditory cortex in the barbiturate-anesthetized ferret. 2. Using a two-tone stimulus, we determined the excitatory and inhibitory portions of the response areas and then parameterized them in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). 3. The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above the BF preferred upward sweeps, and those from frequencies below the BF preferred downward sweeps. 4. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas. For instance, cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF, i.e., stimuli with the opposite asymmetry. 5. Columnar organization of the response area types was demonstrated in 66 single units from 16 penetrations. Consistent with this finding, it was also shown that response area asymmetry measured from recordings of a cluster of cells corresponded closely with those measured from its single-unit constituents. Thus, in a local region, most cells exhibited similar response area types and other response features, e.g., FM directional sensitivity. 6. The distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high-frequency inhibition (relative to the BF) becoming more effective caudally and low-frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic axis. 7. Response features that correlated with the response area types were also mapped along the isofrequency planes. Thus, in four animals, a map of FM directional sensitivity was shown to be superimposed on the response area map. Similarly, it was demonstrated in six animals that the spectral gradient of the most effective noise stimulus varied systematically along the isofrequency planes. 8. One functional implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks and edges and the spectral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)