Biomedical subjects

A Bieser

Publications and source records attributed to A Bieser.

8 recordsLinked to original sources

PubMed1998

Processing of twitter-call fundamental frequencies in insula and auditory cortex of squirrel monkeys.

Amplitude-modulated (AM) and frequency-modulated (FM) elements are prominent periodic sound features of squirrel monkeys' twitter calls. To investigate how the periodic FM elements are represented in the spike activity of cortical neurons, single units in the insula, primary auditory field (AI) and rostral auditory field (R) were recorded. In five monkeys, 566 units (insula, n = 181; AI, n = 221; R, n = 164) were exposed to synthesized fundamental frequencies and one natural twitter call. Neuronal encoding of periodic FM elements takes place by phase-locking to either the up- or the down-directed FM sweeps. The phase-locking was strongly influenced by the FM-period repetition rate. The ability of neurons in both auditory fields and the insula to encode all periodic FM elements showed a marked reduction at 16 Hz FM-period repetition rate. The neurons' best frequency (BF) influenced the quality of periodicity encoding, but neurons with BFs outside the frequency range of the fundamentals also responded with periodic discharge rates. Even neurons in AI (6.8%) and the insula (22.6%) that did not respond to pure tones showed clear periodic FM encoding. The percentage of neurons able to encode all periodic FM elements within the twitter fundamental was significantly higher in field R than in AI and the insula. From 58 simultaneously recorded pairs of units in AI and the insula that had positive cross-correlation coefficients of spontaneous activity, the influence of the FM-period repetition rate on neuronal correlation was investigated. Correlated firing of AI and insula neurons seems limited to low-period repetition rates. The cross-correlation coefficients obtained for spontaneous activity and six different periodic FM sounds showed a band-pass characteristic. The natural twitter call evoked stronger neuronal responses in all fields than the synthesized fundamental frequencies with corresponding bi-directional FM sweeps. The better encoding of the transient features in the natural call can be attributed to the amplitude modulation added to the FM elements in the natural call. These amplitude modulations divide the FM elements of twitter calls into syllable-like sound elements. It is probable that encoding the complex pattern in the time and frequency domains of a call must undergo some integration at a cortical level. Additionally, these data provide the first evidence that insula neurons contribute to the encoding of complex FM signals.

Acoustic Stimulation↗

PubMed1997

Salivary cortisol: a non-invasive measure of hypothalamo-pituitary-adrenocortical activity in the squirrel monkey, Saimiri sciureus.

Salivary cortisol is a non-invasive and easy-to-assess measure of the activity of the hypothalamo-pituitary-adrenocortical (HPA) system. Here we report that salivary cortisol determination can be used in squirrel monkeys (Saimiri sciureus) to monitor variations in HPA system activity induced by both housing and experimental conditions. Saliva cortisol assessment has several advantages over blood cortisol analysis such as stress-free frequent sampling, laboratory independence and lower costs. Therefore, this non-invasive measure can be the method of choice in primatological research projects and routine programmes related to the well-being of these laboratory animals.

Animals↗

PubMed1996

Auditory responsive cortex in the squirrel monkey: neural responses to amplitude-modulated sounds.

The neural response to amplitude-modulated sinus sounds (AM sound) was investigated in the auditory cortex and insula of the awake squirrel monkey. It was found that 78.1% of all acoustically driven neurons encoded the envelope of the AM sound; the remaining 21.9% displayed simple On, On/Off or Off responses at the beginning or the end of the stimulus sound. Those neurons with AM coding were able to encode the AM sound frequency in two different ways: (1) the spikes followed the amplitude modulation envelopes in a phase locked manner; (2) the spike rate changed significantly with changing modulation frequencies. As reported in other species, the modulation transfer functions for rate showed higher modulation frequencies than the phase-locked response. Both AM codings exhibited a filter characteristic for AM sound. Whereas 46.6% of all neurons had the same filter characteristic for both the spike discharge and the phase-locked response, the remaining neurons displayed combinations of different filter types. The discharge pattern of a neuron to simple tone or noise bursts suggests the behaviour of this neuron when AM sound is used as the stimulus. Neurons with strong onset responses to tone/noise bursts tended to have higher phase-locked AM responses than neurons with weak onset responses. The spike rate maxima for AM sound showed no relation to the tone/noise burst discharge patterns. Varying modulation depth was encoded by the neuron's ability to follow the envelope cycles and not by the non-phase-locked spike rate frequency. The organization of the squirrel monkey's auditory cortex has previously been established by an anatomical study. We have added two new fields using physiological parameters. All fields investigated showed a clear functional separation for time-critical information processing. The best temporal resolution was shown by the primary auditory field (AI), the first-temporal field (T1) and the parainsular auditory field (Pi). The neural data in these fields and the amplitude modulation frequency range of squirrel monkey calls suggest a similar correlation between vocalization and perception as in human psychophysical data for speech and hearing sensation. The anterior fields in particular failed to follow the AM envelopes. For the first time in a primate, the insula was tested with different sound parameters ranging from simple tone bursts to AM sound. It is suggested that this cortical region plays a role in time-critical aspects of acoustic information processing. The observed best frequencies covered the same spectrum as AI. As in the auditory fields, most neurons in the insula encoded AM sound with different filter types. The high proportion of neurons unable to encode AM sound (40.6%) and the low mean best modulation frequency (9.9 Hz) do not support a prominent role of the insula in temporal information processing.

Acoustic Stimulation↗

PubMed1994

Neural encoding of amplitude modulation within the auditory midbrain of squirrel monkeys.

The neuronal responses to amplitude modulated (AM) sounds were investigated in the auditory midbrain of the squirrel monkey. Sinusoidally modulated tones and noise served as acoustic stimuli. In order to describe the response properties of collicular neurons, Fast-Fourier-Transformation (FFT), a cross-correlation algorithm and spike-rate counts were applied to translate the neuronal reactions into modulation transfer functions. FFT and cross-correlation defined a measure for synchronicity of the neuronal discharges with the modulation cycles. All neurons (542) responded selectively to AM-sounds insofar as all displayed a best modulation frequency (BMF). Most of them furthermore had a band-pass-like modulation transfer function, whose center frequencies were mainly between 8 and 128 Hz. Transfer functions obtained by spike-rate showed less selectivity: a relatively great number of neurons did not change their spike rate as a function of modulation frequency. The results show that encoding of amplitude-modulated sounds occurs to a greater extent via phase locking of discharges than via changes in spike number. In the same way, changing modulation depth is processed: whereas spike rate on average remains constant between 100% and 0% modulation, there is a drastic reduction in synchronicity. No clear relationship was found between a unit's characteristic frequency and BMF; the same applied to BMF and recording place. The results furthermore show that amplitude modulations are encoded selectively in a band pass function in a non-human primate. The midbrain thereby occupies an intermediate position within the pathway from the periphery to the cortex. This form of temporal resolution probably underlies mechanisms caused by the increasing synaptic activity in the course of the pathway. This may indicate adaptation since those modulation frequencies embedded in this species' vocal repertoire fit quite well with the system's tuning properties for amplitude modulation.

Acoustic Stimulation↗

PubMed1984

Light and electron microscopic identification of nerve terminal sprouting and retraction in normal adult frog muscle.

A combined light and electron microscopic study was performed on neuromuscular junctions of normal adult frogs. In a previous investigation signs of new synapse formation, as well as abandoned former synaptic sites, have been observed in normal muscles (Wernig, Pécot-Dechavassine & Stöver, 1980a, b). Here we performed a detailed light and electron microscopic correlation to investigate those parts of junctions which, after staining for cholinesterase (ChE) and presynaptic axon terminals, were suspected either to be newly formed or sites abandoned by the presynaptic nerve and the Schwann cell. Thin presynaptic nerve branches, enclosed by Schwann cell sheaths along most of their length, formed synaptic contacts with the muscle fibre only at small circumscribed areas. In these regions post-synaptic secondary folds (invariably present at mature synapses) were either missing or were less well developed. At these small contacts, binding sites for fluorescein-labelled alpha-bungarotoxin were usually present. At other sites the ChE reaction product was present but an axon could not be detected in silver-stained preparations. Electron microscopic observation revealed post-synaptic secondary folds filled with ChE reaction product while the presynaptic axon and Schwann cell were missing. The sites with ChE remnants can thus be regarded as abandoned former synaptic contacts. No binding of fluorescein-labelled alpha-bungarotoxin could be detected at such sites. These findings confirm earlier suggestions that synaptic contacts in frog muscle are normally undergoing continual remodelling. The lack of binding sites for fluorescein-labelled alpha-bungarotoxin at abandoned synaptic sites suggests that a neural or Schwann cell factor is important for the maintainance of synaptic acetylcholine receptors.

Animals↗

PubMed1984

Different quantal responses within single frog neuromuscular junctions.

At frog neuromuscular junctions spontaneous miniature end-plate potentials (m.e.p.p.s) were recorded from several isolated spots within single synapses. This was done by consecutively placing an extracellular glass micro-electrode (focal electrode) at different recording sites, while the intracellular electrode remained in one place. After each set of recordings, muscles were stained to reveal both axon terminals and cholinesterase (ChE) such that the exact position of each recording site could be determined. In many nerve terminal branches a similar quantum size was found at several different spots. In other instances, however, mean quantum amplitudes varied by 10-60% at different spots along the same terminal branches. As a control, individual spots were recorded from repeatedly after repositioning the focal electrode. In these recordings mean m.e.p.p. amplitude varied by only 5-10%. It is concluded that quantum size within a single junction is similar at many spots, but deviates markedly at others. Correlation of this variation with the stained preparations suggested that spots where quanta significantly larger or smaller than normal were recorded were either at ChE rings or at the distal ends of nerve branches, respectively; at different nerve terminal branches within the same junction, quantum amplitudes were similar in many cases but deviated in others. The results are consistent with ultrastructural evidence that frog neuromuscular junctions are non-homogeneous structures which undergo continual remodelling.

Action Potentials↗

PubMed1981

Abandoned synaptic sites in muscles of normal adult frog.

In previous investigations light microscopic cholinesterase (ChE) deposits without any nerve were found adjacent to normally occupied parts of a frog neuromuscular synapse [4, 9]. After identifying one such site at the light microscopic level ultrathin sections were cut and viewed with an electron microscope. ChE reaction product-filled secondary, clefts were observed in the region studied but a nerve was invariably missing. From this it is concluded that these loci are former synaptic sites from which nerve and Schwann cell have retracted, When incubating muscles with fluorescence-labelled alpha-bungarotoxin, all 23 abandoned sites found in 6 muscles remained bare of visible amounts of label. This indicate that receptor molecules eventually disappear from the synaptic membrane after retraction of the nerve and Schwann cell. No information as to the underlying time schedule of nerve retraction, turnover of ChE and alpha-bungarotoxin binding sites was obtained. Taken together with the evidence for synapse new formation in untreated frog muscles obtained previously [9, 11] the present observations indicate some ongoing remodeling of frog neuromuscular junction.

Animals↗

PubMed1981

Light and electron microscopic identification of a nerve sprout in muscle of normal adult frog.

Recent evidence from this laboratory indicates that axonal sprouting (and regression) occurs in neuromuscular junctions of normal adult frogs. In the present investigation, the appearance of a single nerve branch, which from light microscopy was assumed to be a sprout, was studied in ultrathin serial sections. In confirming the light microscopic evidence small synaptic contacts were found, which showed characteristics of new synapse formation. Unexpectedly, the Schwann cell surrounding the axon extended several microns distally from the axon tip. It appears that nerve sprouting (and regression) is a physiological event in adult frog muscles.

Animals↗