Search PubMed⌕ Search

Biomedical subjects

N A Schellart

Publications and source records attributed to N A Schellart.

At least 19 recordsLinked to original sources
PubMed2001

Transient and maintained changes of the spontaneous occipital EEG during acute systemic hypoxia.

BACKGROUND: It has been reported that during systemic hypoxia the spectral power of spontaneous EEG (closed eyes) increases, except for the power in the alpha band, which decreases. HYPOTHESIS: It is surmised that after a sudden decrease in inspired oxygen, the EEG parameters and SaO2 show similar temporal characteristics. METHODS: Normobaric acute hypoxia in 14 healthy subjects was evoked by breathing a hypoxic gas. Frequency spectra were calculated from occipital recordings and quantified together with the parameters of the spectral alpha peak. RESULTS: During the first 20 min of exposure, SaO2 decreases exponentially. With eyes open, the frequency bands show a transient increase in power (110-160%). The peak-time of the transient is shorter for the alpha and beta1 bands (4.5 min) than for the delta2 and theta bands (7 min). After the transient, the bands reach sustained power levels ca. 65% higher than the control value. The frequency of the alpha peak decreases with 0.61 +/- 0.16 Hz (mean +/- SE). In contrast to the eyes-closed condition, the peak amplitude increases (41 +/- 12%). CONCLUSIONS: During systemic hypoxia many characteristics of the spontaneous EEG with open eyes differ from those under normoxia, and alpha activity deviates strongly from that during hypoxia with closed eyes. During the first 20 min of exposure changes in power are most prominent and do not follow the change of SaO2. Monitoring total EEG power with open eyes, SaO2 and respiration gives a good impression of the hypoxia-related neurophysiological state of the subject.

Adolescent↗
PubMed1999

Voluntary breath holding affects spontaneous brain activity measured by magnetoencephalography.

Spontaneous brain activity was measured by multichannel magnetoencephalography (MEG) during voluntary breath holds. Significant changes in the activity are limited to the alpha rhythm: 0.25 Hz frequency increase and narrowing of the peak. The area of alpha activity shifts slightly toward (fronto-) temporal. The topography of other rhythms is unaffected by breath holding. Electroencephalographic (EEG; 36 channels maximal) recordings generally made simultaneously with the MEG recordings show similar effects. However, EEG was inadequate to reveal the small topographic differences. Systemic hypercapnia caused by a long breath hold is unlikely to play an important role in producing the observed effects.

Adult↗
PubMed1997

Neuronal encoding of sound direction in the auditory midbrain of the rainbow trout.

Acoustical stimulation causes displacement of the sensory hair cells relative to the otoliths of the fish inner ear. The swimbladder, transforming the acoustical pressure component into displacement, also contributes to the displacement of the hair cells. Together, this (generally) yields elliptical displacement orbits. Alternative mechanisms of fish directional hearing are proposed by the phase model, which requires a temporal neuronal code, and by the orbit model, which requires a spike density code. We investigated whether the directional selective response of auditory neurons in the midbrain torus semicircularis (TS; homologous to the inferior colliculus) is based on spike density and/or temporal encoding. Rainbow trout were mounted on top of a vibrating table that was driven in the horizontal plane to simulate sound source direction. Rectilinear and elliptical (or circular) motion was applied at 172 Hz. Generally, responses to rectilinear and elliptical/circular stimuli (irrespective of direction of revolution) were the same. The response of auditory neurons was either directionally selective (DS units, n = 85) or not (non-DS units, n = 106). The average spontaneous discharge rate of DS units was less than that of non-DS units. Most DS units (70%) had spontaneous activities < 1 spike per second. Response latencies (mode at 18 ms) were similar for both types of units. The response of DS units is transient (19%), sustained (34%), or mixed (47%). The response of 75% of the DS units synchronized to stimulus frequency, whereas just 23% of the non-DS responses did. Synchronized responses were measured at stimulus amplitudes as low as 0.5 nm (at 172 Hz), which is much lower than for auditory neurons in the medulla of the trout, suggesting strong convergence of VIIIth nerve input. The instant of firing of 42% of the units was independent of stimulus direction (shift <15 degrees), but for the other units, a direction dependent phase shift was observed. In the medial TS spatial tuning of DS units is in the rostrocaudal direction, whereas in the lateral TS all preferred directions are present. On average, medial DS units have a broader directional selectivity range, are less often synchronized, and show a smaller shift of the instant of firing as a function of stimulus direction than lateral DS units. DS response characteristics are discussed in relation to different hypotheses. We conclude that the results are more in favor of the phase model.

Air Sacs↗
PubMed1997

Effect of dysoxia and moderate air-hyperbarism on red-green color sensitivity.

The effect of acute air-hyperbarism (maximal 520 kPa), normobaric low and high FIO2 levels (minimal 0.1, maximal 1.0) and hyperbaric oxygen (HBO2) on the red-green sensitivity ratio (rgSR) and on color discrimination for foveal vision were studied. Effects were quantified by measuring the red-green flicker (16 Hz) fusion point for normals with the Oscar tester (Am J Optom Physiol Opt 1983; 60:892-901). Color discrimination was examined with the Lanthony's Desaturated 15 Hue test After 15-20 min of exposure rgSR is enhanced 4% (relatively increased red sensitivity) by normobaric acute hypoxia, and reduced 4% by normobaric hyperoxia (FIO2 = 1.0), but HBO2 gives a smaller reduction, and air-hyperbarism (FIO2 = 0.21) has no effect. Hypercapnia (increased FICO2), normobaric hypoxia (reduced FIO2), and HBO2 increase the duration of the Lanthony's test about 20-40%, but the number of errors were practically unchanged. The reduced effect during HBO2 upon rgSR is attributed to an opposing effect of hypercapnia. The absence of an effect during air-hyperbarism is probably due to a suppression by nitrogen of the effect of high PO2. In general, during moderate air-hyperbarism and HBO2 color vision seems to be normal, but evaluation of the colored scene is probably less stable and slightly slower.

Adolescent↗
PubMed1995

Mapping of sound direction in the trout lower midbrain.

In the trout lower midbrain 35% of the auditory neurons are directionally selective (DS). Most of these neurons have a higher directional selectivity than the sensory hair cells. DS units and non-DS units occur in vertical clusters, with the former more dorsally. The direction of preference is topographically mapped. Apparently, auditory space mapping is a common feature in the midbrain of vertebrates.

Animals↗
PubMed1992

Velocity- and acceleration-sensitive units in the trunk lateral line of the trout.

1. The two main types of lateral line organs of lower vertebrates are the superficial neuromasts (SN), with a cupula that protrudes in the surrounding water, and the canal neuromasts (CN), located in the lateral line canal. The scales of the trunk lateral line canal of fish contain SNs as well as CNs. In this study, we examine whether there exist two functional classes of afferent fibers in the trunk lateral line nerve of the rainbow trout that can be attributed to the SNs and CNs. 2. The response properties of the afferent fibers in the trunk lateral line nerve have been determined during stimulation with sinusoidally varying water motion generated by a small vibrating sphere. Linear frequency response analysis revealed the presence of two distinct populations of afferent fibers in the lateral line nerve. The fibers belonging to the two populations showed significant differences in the frequency at which the sensitivity was maximal, the low-frequency response slope and the low-frequency asymptotic phase angle. 3. One population of fibers has a maximum sensitivity at 36 +/- 13 (SD) Hz (n = 22) and responds up to this frequency to water velocity. The low-frequency slope of the frequency response of these fibers was 20 +/- 3 (SD) dB/decade and the low-frequency phase lead was 121 +/- 11 degrees (mean +/- SD), both with respect to sphere displacement. The fibers of the other population have a maximum sensitivity at 93 +/- 14 (SD) Hz (n = 12) and respond up to this frequency to water acceleration. The low-frequency slope of these fibers was 35 +/- 5 (SD) dB/decade, and the low-frequency phase lead was 188 +/- 13 degrees (mean +/- SD). 4. Analysis of the stochastic properties of the spontaneous activity of both types of fibers revealed that the mean firing rate of the fibers responding to water velocity (26 +/- 12 spikes/s, mean +/- SD; n = 22) was significantly higher than that of the fibers responding to acceleration (36 +/- 11 spikes/s, mean +/- SD; n = 12). The other statistical properties of the spontaneous activity were found to be indistinguishable. 5. From comparison of the results with the available quantitative data on frequency responses of lateral line organs in other species, it has been concluded that the fibers responding (< or = 40 Hz) to water velocity innervate SNs and that the fibers responding (< or = 90 Hz) to water acceleration innervate CNs.(ABSTRACT TRUNCATED AT 400 WORDS)

Acceleration↗
PubMed1992

The pattern of trunk lateral line afferents and efferents in the rainbow trout (Salmo gairdneri).

The primary projections of the mechanosensory posterior lateral line nerve of the rainbow trout Salmo gairdneri, a teleost without lateral line specializations, were studied by applying horseradish peroxidase (HRP). The afferents project nearly exclusively to the nucleus medialis and a small nucleus caudalis in the hindbrain, and to the eminentia granularis. In addition, there is a sparse projection to the cerebellum, but a projection to the magnocellular nucleus is lacking. The afferent projection to the lateral part of the eminentia granularis is more dense than the projection to the medial nucleus, as shown by image processing of the HRP labeling. The efferent fibers originate from a bilateral pair of large octavolateral nuclei in the hindbrain. Characteristic for these nuclei are their large, fusiform and bipolar neurons. Another specific feature is the oblique orientation of the somata, with the main dendritic shaft coursing ventrolaterally and the axonal shaft coursing dorsomedially. The axons of the efferent neurons bifurcate. The shape, orientation and size of the efferent somata are the same in the entire efferent nucleus. In contrast to other teleosts, the occurrence of efferent somata found contralaterally (21%) is substantial. The part of the octavolateral efferent nucleus innervating the neuromasts of the trunk is estimated to contain at most 150 cell bodies unilaterally.

Afferent Pathways↗
PubMed1992

Contrast sensitivity of air-breathing nonprofessional scuba divers at a depth of 40 meters.

Photopic contrast sensitivity of air-breathing scuba divers was measured with a translucent test pattern at depths up to 40 m. The pattern was composed of sine wave gratings with spatial frequency and contrast changing logarithmically. The spatial transfer characteristics were measured at various depths under controlled optical conditions in seawater and in fresh water. Analysis indicates that the visual contrast sensitivity, and therefore probably also acuity, of sport divers is not affected up to depths of 40 m. This holds under ideal as well as poor diving conditions.

Adult↗
PubMed1987

A model for the nearfield acoustics of the fish swimbladder and its relevance for directional hearing.

It is a well-known fact that fish can determine the direction of an underwater sound source. For bony fish without Weberian ossicles the mechanism proposed here is the analysis of the elliptical movement of the otolith macula relative to the otolith in case a pure tone is emitted. This movement results from the superposition of the direct displacement wave and the displacement wave scattered by the swimbladder acting as a damped harmonic oscillator. A derivation of a closed analytical expression of the scattered wave is given for a prolate spheroidal air bubble (representing the swimbladder) in water. The elliptical displacement orbits of the surrounding medium were calculated. A general numerical approach is given for irregular gas bodies.

Acoustics↗
PubMed1987

A model for directional and distance hearing in swimbladder-bearing fish based on the displacement orbits of the hair cells.

It is known that teleosts, without Weberian ossicles but with a swimbladder, can detect the direction of and, under appropriate conditions, the distance to a sound source [e.g., Schuijf and Hawkins, Nature 302, 143-144 (1983)]. It is hypothesized here that the underlying mechanism is the analysis of the parameters of the elliptical movement of the hair cells with respect to the otoliths. This movement results from the displacement wave impinging directly upon the labyrinth and the response displacement wave reradiated by the swimbladder. For a given swimbladder geometry, given the positions of the maculae of both labyrinths with respect to the swimbladder and the damping of the swimbladder, the displacement orbits of the maculae can be calculated [de Munck and Schellart, J. Acoust. Soc. Am. 81, 556-560 (1987)]. These calculations were made for the cod and the trout with the frequency, direction, and distance between the fish and the sound source as parameters with the source within the same horizontal plane as the fish. The orbit model predicts that the utriculus has the most strategic position to detect direction and distance of such a sound source. Moreover, the model predicts that this could basically be done monaurally. A hypothesis is proposed to describe how the utricular system analyzes the orbit parameters. The model is evaluated in relation to the results of behavioral experiments described in the literature.

Animals↗
PubMed1986

Acousticolateral processing in the torus semicircularis of the trout Salmo gairdneri.

Characteristics of single units in the trout torus semicircularis to acousticolateral stimulation were investigated with tone bursts. Auditory (A) units (33%, N = 67), exclusively sensitive to frequencies higher than 125 Hz, and low frequency (L, 12%) units responding to tones up to 125 Hz were found next to the third type, the broadband (B,55%) unit, sensitive to both low and high frequencies. For many B units sensitivity to intermediate frequencies is absent. The sensitivity range of 85% of the units does not exceed 450 Hz. Most units (70%) show an increase in spike firing during the tone burst irrespective of frequency (ON units), but for a minority (9%) firing is maximal in the silent intervals (OFF units). OFF units are mostly found among A units. All L units are ON units. The remaining ON/OFF (ON behaviour for some and OFF behaviour for other frequencies) or ON + OFF (ON and OFF behaviour for all frequencies) units are all B units. Tonic ON units are most common. Some units show habituation and only sensitive ones may exhibit phase-locking. The different behaviour of some B units for low versus high frequencies, as expressed in their ON and OFF behaviour and latencies, presumably reflects convergence of inputs from acousticolateral subsystems with distinct properties.

Acoustic Stimulation↗
PubMed1986

Computerized pattern recognition used for grain counting in high resolution autoradiographs with low grain densities.

Using a video-image system coupled to a minicomputer with commercial image handling software, autoradiographic grains displayed in dark-field are counted with a fast (ca. 3.5 min for 120,000 microns 2) and reliable (false scores less than 5%) grain-recognizing FORTRAN program executed in the users memory. The grain counts are printed in a raster of adjustable size overlying a bright-field image, so that the counts can be related directly to the underlying histological structures.

Autoradiography↗
PubMed1984

Influence of GABA on the spectral and spatial coding of goldfish retinal ganglion cells.

The responses of center red ON and center red OFF ganglion cells of the isolated goldfish retina to a flickering stimulus light diminish by GABA. Without stimulation GABA blocks the maintained activity of the OFF cells. With a green flickering spot in the receptive field center, the green ON-response of the OFF cells becomes an OFF-response during GABA administration. It is concluded that, besides the common antagonistic green input, the receptive field center of these cells also has a synergistic green input, which is mediated by a pathway other than the center opponent red and green inputs.

Animals↗
PubMed1983

Acousticolateral and visual processing and their interaction in the torus semicircularis of the trout, Salmo gairdneri.

In the torus semicircularis of the trout, visual, auditory, visual-auditory, acousticolateral broadband, lateral line, visual-acousticolateral broadband and visual-lateral line units have been found, listed in decreasing frequency of occurrence. In contrast to the acousticolateral characteristics, the visual characteristics of toral units are poor; only sudden changes in the visual field seem to be of importance. When bimodal units are stimulated simultaneously with light and sound the responses of most units behave in a basically additive fashion. The activity of some units which responded to only one of the modalities could be modulated by the other modality.

Animals↗