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PubMed2006

The influences of tube voltage and scan direction on combined tube current modulation: a phantom study.

BACKGROUND: For the optimum application of tube current modulation in paediatric CT it is important to know the influences of different imaging parameters on it. OBJECTIVE: This phantom study was performed to evaluate the influences of tube voltage and scan direction on combined tube current modulation, which is a combination of angular and z-axis modulation. MATERIALS AND METHODS: Sixteen-slice spiral CT with combined tube current modulation was performed using four different phantoms (one adult-size anthropomorphic phantom and three cone-shaped acryl phantoms) at three different tube voltages (80, 100 and 120 kVp) and two scan directions (craniocaudal and caudocranial). Effective tube current-time product (mAs) was chosen to maintain a consistent CTDIvol for each phantom. Other parameters, including detector collimation, slice width, pitch, reconstruction algorithm, scan field of view, and scan range were identical for each phantom. Changes in CTDIvol and mAs resulting from the modulation were calculated and compared. RESULTS: Changes in CTDIvol and mAs resulting from the modulation were different among the three tube voltages. In larger phantoms the greatest reductions (-6.1+/-3.9% in CTDIvol and -12.5+/-4.0% in mAs) were obtained with 120 kVp, whereas in smaller phantoms they were obtained with 80 kVp (-2.8+/-0.9% in CTDIvol and -2.5+/-4.9% in mAs). Smaller CTDIvol (2.4+/-0.9 mGy vs. 2.5+/-1.0 mGy, P=0.017) and mAs (57.2+/-40.4 mAs vs. 61.4+/-43.2 mAs, P=0.002) were used in caudocranial scans than in craniocaudal scans with the modulation. CONCLUSION: Combined tube current modulation is influenced by tube voltage and scan direction depending on phantom profile.

Adult↗
PubMed2006

Perceptual interaction between carrier periodicity and amplitude-modulation in the gerbil (Meriones unguiculatus).

Due to its extended low-frequency hearing, the Mongolian gerbil (Meriones unguiculatus) has become a well-established animal model for human auditory processing. Here, two experiments are presented which quantify the gerbil's sensitivity to amplitude modulation (AM) and carrier periodicity (CP) in broad-band stimuli. Two additional experiments investigate a possible interaction of the two types of periodicity. The results show that overall sensitivity to AM and CP is considerably less than in humans (by at least 10 dB). The gerbil's amplitude-modulation sensitivity is almost independent of modulation frequency up to a modulation frequency of 1 kHz. Above, amplitude-modulation sensitivity deteriorates dramatically. On the basis of individual animals, carrier-periodicity detection may improve with increasing fundamental frequency up to about 500 Hz or may be independent of fundamental frequency. Amplitude-modulation thresholds are consistent with the hypothesis that intensity difference limens in the gerbil may be considerably worse than in humans, leading to the relative insensitivity for low modulation frequencies. Unlike in humans, inner-ear filtering appears not to limit amplitude-modulation sensitivity in the gerbil. Carrier-periodicity sensitivity changes with fundamental frequency similar to humans. Unlike in humans, there is no systematic interaction between AM and CP in the gerbil. This points to a relatively independent processing of the perceptual cues associated with AM and CP.

Acoustic Stimulation↗
PubMed1997

Discrimination of sinusoidally frequency-modulated sound signals mimicking species-specific communication calls in the FM-bat Phyllostomus discolor.

In the lesser spear-nosed bat, Phyllostomus discolor, maternal directive calls are characterized by an individual type of sinusoidal frequency modulation (= SFM) pattern. Beside modulation frequency, modulation depth, carrier frequency, and number of modulation cycles per call contribute to the mother's vocal signature. Since juvenile P. discolor learn to adapt their isolation calls to the corresponding call characteristics of the own mother or even to playback of a computer-stored directive call, if hand-reared in the absence of conspecifics, the bats' auditory system ought to be able to resolve interindividual differences in communication call structure. However, quantitative psychoacoustic data on the discrimination of SFM signals in this species are not available. Thus, in the present study, lesser spear-nosed bats were trained in a two-alternative forced-choice procedure to discriminate between two alternatingly presented SFM sound signals differing in modulation frequency. Other characteristics of acoustic stimuli were identical and designed to mimick the fundamental of species-specific calls. By gradually reducing the difference in modulation frequency between both stimuli within the behavioural relevant range until the animals' performance dropped below the 75%-correct level, a considerable auditory spectro-temporal resolution has been revealed. Particularly in comparison to the overall interindividual variation of this call parameter (minimal modulation frequency = 49 Hz, maximum = 100 Hz), the determined average difference limen for modulation frequency of 2.42 +/- 0.29 Hz seems substantial and sufficient for labelling individuals.

Acoustic Stimulation↗
PubMed1998

Possible involvement of the ampullary electroreceptor system in detection of frequency-modulated electrocommunication signals in Eigenmannia.

Behavioral and electrophysiological experiments were conducted to examine whether frequency-modulated electrocommunication signals are detected by the ampullary electroreceptor system in Eigenmannia. First, frequency-modulated electric organ discharges were found to contain a low-frequency component that could be detected by the ampullary system. Second, fish were successfully trained to distinguish a frequency-modulated signal, which contained a low-frequency component as in natural signals, from an artificial signal in which the low-frequency component was eliminated but still modulated in frequency. Subsequently, the trained fish responded without reinforcement to a low-frequency sinusoidal signal which mimicked the low-frequency component in the frequency-modulated signal, suggesting that the fish used the ampullary system to detect frequency modulation. Finally, physiological recording from ampullary afferent fibers demonstrated that they respond to frequency-modulated signals as predicted from the signal's low-frequency component. Electrophysiological study also showed that detection of frequency modulation by the ampullary system is immune to the presence of other constant electric organ discharges.

Action Potentials↗
PubMed1993

The outer mitochondrial membrane channel, VDAC, is modulated by a protein localized in the intermembrane space.

The mitochondrial outer membrane channel, VDAC, provides a pathway for the flux of metabolites between the cytoplasm and mitochondrion. VDAC is voltage-dependent and occupies states of differing conductivity and ion selectivity that are dependent on transmembrane potential. A protein, derived from preparations of mitochondria, has been shown to increase the voltage dependence of VDAC and is called the VDAC modulator. Both VDAC and the VDAC modulator have been extensively characterized by reconstitution into planar lipid bilayers. In order for the VDAC modulator to have physiological significance it must have physical access to VDAC in the cell. This constraint dictates that the modulator be an extrinsic outer mitochondrial membrane protein, occupy the mitochondrial intermembrane space, or be a cytoplasmic constituent. To address the question of subcellular localization, purified mitochondria were selectively lysed with digitonin or treated with trypsin while resuspended in hypo-osmotic or iso-osmotic medium. Marker enzymes and modulator activity were monitored during the various treatments. Results indicate that the integrity of the outer membrane was necessary to prevent modulator release or protection from trypsin digestion. Outer membrane lysis, under conditions where the inner membrane remained intact, resulted in modulator release or inactivation by trypsin. These results suggest an intermembrane space location for the VDAC modulator in the mitochondrion.

Digitonin↗
PubMed1986

Centrifugal modulation of the rat tail flick reflex evoked by graded noxious heating of the tail.

Centrifugal modulation from the midbrain, pons and medulla of the spinal nociceptive tail flick (TF) reflex evoked by graded noxious heating of the tail was studied in lightly pentobarbital-anaesthetized rats. In initial experiments, the relationship between the intensity of the noxious thermal stimulus and the TF latency was characterized. The thermal stimulus was provided by a lamp focused on the ventral surface of a rat's tail. Five different rates of heating of the tail were varied systematically by altering the voltage supplied to the lamp and were characterized using a thermocouple to measure the temperature of tissue exposed to the radiant heat. A linear stimulus-response function relating the inverse of the latency of the TF to the rate of heating of the tail was established. However, the mean cutaneous tissue temperature of the exposed tail at the time of the TF was found to be invariant and independent of the rate of heating. Focal electrical stimulation in the midbrain, pons and medulla modulated the TF reflex in two different ways, analogous to modulations of stimulus-response functions of single-cell recordings of spinal dorsal horn neurons. A Type I modulation, analogous to the parallel shift in response threshold seen in spinal dorsal horn neurons, was an absolute increase in the thermal threshold of the TF reflex. A Type II modulation, analogous to a change in slope or gain seen in spinal dorsal horn neurons, was a linear increase in the thermal threshold of the TF reflex as a function of the rate of heating. Type I modulations were produced by electrical stimulation in the ventromedial medulla (n. raphe magnus and n. reticularis gigantocellularis) and lateral periaqueductal gray of the midbrain. Type II modulations were produced by electrical stimulation in the dorsolateral pons, locus coeruleus-subcoeruleus and in the medial periaqueductal gray. This experimental approach has shown itself to be useful in the characterization of descending inhibition of nociception. Much simpler and less invasive than analogous spinal dorsal horn single cell electrophysiologic studies, it can be used to study the mechanisms of centrifugal modulation of nociceptive flexion reflexes and further establishes the utility of the lightly anaesthetized rat preparation for studies of nociception-antinociception.

Animals↗
PubMed1992

Comparison of the Ca2+ binding properties of the gamma-carboxyglutamic acid-containing module of protein Z in the intact protein and in N-terminal fragments.

Protein Z is a vitamin K-dependent plasma protein of unknown function. Its modular structure is identical with those of factors VII, IX, X, and protein C. These proteins have an N-terminal gamma-carboxyglutamic acid (Gla)-containing module which binds six to ten Ca2+. In factors IX, X, and protein C, the adjacent epidermal growth factor (EGF)-like module binds one Ca2+ whereas the EGF-like module in protein Z does not. We have compared the Ca2+ binding properties of a fragment of protein Z comprising the Gla and N-terminal EGF-like modules (pZ-GlaEGFN) with those of intact protein Z and the isolated Gla module by measuring the Ca(2+)-induced quenching of the intrinsic protein fluorescence. The similar Ca2+ affinities of pZ-GlaEGFN and protein Z indicate that pZ-GlaEGFN has a native conformation and normal Ca2+ binding properties. A comparison of the Ca2+ binding to pZ-GlaEGFN with those to the corresponding fragments of factors IX, X, and protein C indicate that Ca2+ binding to the N-terminal EGF-like modules in the latter proteins does not influence the folding and Ca2+ binding properties of their Gla modules. Furthermore, the Ca(2+)-induced fluorescence enhancements of GlaEGF fragments from factors IX, X, and protein C appear to be caused by Ca2+ binding to the site in the EGF-like modules since it is not observed for pZ-GlaEGFN.

1-Carboxyglutamic Acid↗
PubMed1994

Crystal structure of the tenth type III cell adhesion module of human fibronectin.

The crystal structure of the cell adhesion module of fibronectin (FNIII10) has been determined at 1.8 A resolution. A recombinant fragment corresponding to the tenth type III module of human fibronectin was crystallized in space group P2(1) with a = 30.7, b = 35.1 and c = 37.7 A and beta = 107 degrees. The structure was determined by molecular replacement and refined by least squares methods. The crystallographic R-factor for the final model of the 91 amino acid module plus 56 solvent atoms is 0.18 for 10 to 1.8 A data. The module consists of two layers of beta-sheet, one with three antiparallel strands and the other with four antiparallel strands. The beta-sheets enclose a hydrophobic core of 24 amino acid side-chains. The module contains the RGD cell recognition sequence in a flexible loop connecting two beta-strands. The tertiary structure of the FNIII10 module has been used to develop a structure-based sequence alignment of 17 type III modules in fibronectin based on the striking conservation of homologous hydrophobic residues. A similar pattern of homologous alternating hydrophobic residues is also evident in a comparison of type III modules in proteins unrelated to fibronectin such as cytokine receptors and muscle proteins.

Amino Acid Sequence↗
PubMed1982

Detectability of amplitude- and frequency-modulation of suprathreshold sine-wave gratings.

The detectability of amplitude- and frequency-modulation of sine-wave carrier gratings was measured for a number of carrier frequencies, modulation frequencies and carrier contrasts. The results show that for low modulation frequencies (spatial frequency components close together; separation less than 0.5 x carrier frequency) modulation thresholds are determined by both the amplitude and phase spectrum of the stimulus; for high modulation frequencies (frequency components more widely separated) they are determined by the amplitude spectrum only. The dependence of modulation thresholds on carrier contrast was found to be very similar, irrespective of the modulation frequency, carrier frequency and kind of modulation selected, and was in agreement with known results of contrast-difference threshold measurements and of masking experiments.

Form Perception↗
PubMed1995

Sensitivity to orientation modulation in micropattern-based textures.

We have measured the sensitivity of the human visual system to sinusoidal modulations of orientation in micropattern-based textured stimuli. The result is the orientation modulation function, or OMF, which describes this sensitivity as a function of the spatial frequency of orientation modulation. We found that the OMF was bandpass with peak sensitivity at spatial frequencies ranging between 0.06 and 0.2 c/deg, depending on the size of the micropatterns. The OMF was found to be scale invariant, that is its position on the spatial frequency axis did not change with viewing distance when spatial frequency was measured in object rather than retinal units. This scale invariance was shown to result from the visual system taking into account the scale rather than the density of the micropatterns as viewing distance was changed. It has been argued by Bergen [(1991) Vision and visual dysfunction (Vol. 10B) New York: Macmillan] that scale invariance in textures is a consequence of the coupling of mechanisms which detect textural features with those which detect local luminance contrasts. We reasoned that Gabor micropattern textures might therefore show narrower OMFs compared to line micropattern textures. However we found no difference in OMF bandwidth between the Gabor and line micropattern textures, suggesting that the line micropatterns were acting as selectively as the Gabor micropatterns for the spatial scale of the mechanisms which detected the orientation modulation. Evidence is presented which suggests that the mechanisms which detected the orientation modulation in our stimuli are non-linear. Finally we showed similar OMFs for sine-wave and square-wave modulations of micropattern orientation, and similar OMFs for modulations of micropattern with orientation about the horizontal and about the vertical, the direction of modulation in both cases being horizontal. The implications of these findings for the mechanisms involved in orientation-defined texture processing is discussed.

Contrast Sensitivity↗
PubMedDate not supplied

Human VEP contrast modulation sensitivity: separation of magno- and parvocellular components.

Human cortical visual evoked potentials (VEPs) were retrieved in real time (without averaging). The stimuli were sinusoidal gratings whose contrast was temporally modulated about some mean value. Electrophysiologically determined contrast modulation thresholds were measured at standing contrast over the range from 2.5% to 50%, defining an increment threshold function. Increment threshold functions were obtained under two different spatio-temporal stimulus conditions identified as "sustained" (4 c/deg grating modulated at 1.5 Hz) and "transient" (1 c/deg grating modulated at 20 Hz). Under each of these conditions, threshold responses were retrieved at both the fundamental and second harmonic of the contrast modulation frequency. Under "sustained" conditions log increment threshold responses and the fundamental the second harmonic of the modulation frequency were similar to those at the fundamental except for a saturation effect (i.e., above a mean contrast of 25% there was little further reduction in modulation sensitivity). There was no contrast gain control under "transient" stimulus conditions. In other words, the same absolute amount of contrast change produced threshold responses for all mean levels up to 25%. This was true at both the fundamental and second harmonic of the modulation frequency. Stimulus differences produce striking differences in the electrophysiologically inferred increment threshold function for grating contrast, but fundamental and second harmonic evoked responses reflect processes with similar increment threshold behavior.

Contrast Sensitivity↗
PubMed1988

Representation of amplitude modulation in the auditory cortex of the cat. II. Comparison between cortical fields.

The responses of neuronal clusters to amplitude-modulated tones were studied in five auditory cortical fields of the anesthetized cat: the primary auditory field (AI), second auditory field (AII), anterior auditory field (AAF), posterior auditory field (PAF) and the ventro-posterior auditory field (VPAF). Modulation transfer functions (MTFs) for amplitude-modulated tones were obtained at 172 cortical locations. MTFs were constructed by measuring firing rate (rate-MTFs) and response synchronization (synchronization-MTFs) to sinusoidal and rectangular waveform modulation of CF-tones. The MTFs were characterized by their 'best-modulation frequency' (BMF) and a measure of their quality of 'sharpness' (Q2dB). These characteristics were compared for the five fields. Rate and synchronization MTFs for sinusoidal and rectangular modulation produced similar estimates of BMF and Q2dB. Comparison of averaged BMFs between the cortical fields revealed relatively high BMFs in AAF (mean: 31.1 Hz for synchronization to sinusoidal AM) and moderately high BMFs in AI (14.2 Hz) whereas BMFs encountered in AII, VPAF and PAF were generally low (7.0, 5.2, and 6.8 Hz). The MTFs were relatively broadly tuned (low Q2dB) in AAF and sharper in a low modulation group containing AII, PAF and VPAF. The ventro-posterior field was the most sensitive to changes in the modulation waveform. We conclude that there are significant differences between auditory cortical fields with respect to their temporal response characteristics and that the assessment of these response characteristics reveals important aspects of the functional significance of auditory cortical fields for the coding and representation of complex sounds.

Animals↗
PubMed1989

Spectral characteristics of the responses of primary auditory-nerve fibers to amplitude-modulated signals.

The spectral responses of cat single primary auditory nerve fibers to sinusoidal amplitude-modulated (AM) and double-sideband (DSB) acoustic signals applied to the ear were examined. DSB is an amplitude-modulated signal with a suppressed carrier. Period histograms were compiled from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. For DSB signals, spectral components were found to be present at the frequencies of the stimulus as well as at certain combination frequencies. For AM signals, several clusters of spectral components were present. The lowest-frequency cluster consisted of components at DC, at the modulation frequency, and at its harmonics. A higher frequency cluster occurs around a component with the frequency of the carrier. The components of cluster are separated from the carrier by the modulation frequency and its harmonics. Yet higher-frequency clusters appear around multiples of the carrier frequency with components at frequencies separated from these multiples by the modulation frequency and its harmonics. The magnitudes of these spectral components were determined for carrier frequencies located below, at, and above the characteristic frequency of the units, and for different stimulus levels, modulation frequencies, and modulation depths. The low-frequency components present in the neural spike train appear to be the result of demodulation taking place in the inner ear. The demodulated components are strong and are present over a wide range of sound levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. This demodulation may be significant for speech recognition.

Acoustic Stimulation↗
PubMed2005

Cardiac contractility modulation by non-excitatory currents: studies in isolated cardiac muscle.

BACKGROUND: Myocardial contractility can be altered using voltage clamp techniques by modulating amplitude and duration of the action potential resulting in enhanced calcium entry in the cell of isolated muscle strips (Non-Excitatory Currents; NEC). Extracellular electrical stimuli delivered during the absolute refractory period (Cardiac Contractility Modulation; CCM) have recently been shown to produce inotropic effects in-vivo. AIM: Understanding the cellular mechanism, underlying the CCM effect, is essential for evaluating its clinical potential. We tested the hypothesis that NEC and CCM modulate contractility via similar cellular mechanisms. METHODS: Square wave electric currents were applied in the organ bath to isometrically contracting rabbit RV papillary muscle and human failing trabecular muscle during the absolute refractory period (ARP). RESULTS: These currents, which did not initiate new action potentials or contractions, modulated action potential duration (shortened or lengthened) and contractility (enhanced or depressed) in a manner that depended upon their amplitude, duration and delay from the pacing stimulus. The contractility modulation effect in the rabbit RV papillary muscle was markedly blunted after exposure to ryanodine, indicating that the sarcoplasmic reticulum plays an important role in the contractility modulation. CONCLUSION: Like voltage clamping, extracellular currents applied during the ARP can similarly modulate action potential duration in-vitro and modulate myocardial contractility by similar intracellular mechanisms. This concept provides the potential of a therapeutic strategy in patients with heart failure to enhance contractility.

Animals↗
PubMed2006

Emotional modulation of spinal nociception and pain: the impact of predictable noxious stimulation.

Recent evidence suggests that emotional picture-viewing is a reliable method of engaging descending modulation of spinal nociception. The present study attempted to replicate these findings and determine the effect of noxious stimulus predictability. Participants viewed pictures from the International Affective Picture System (IAPS), during which pain and nociceptive flexion reflexes (NFR) were elicited by electric shocks delivered to the sural nerve. For half of the participants (n=25) shocks were preceded by a cue (predictable), whereas the other half received no cue (unpredictable). Results suggested emotion was successfully induced by pictures, but the effect of picture-viewing on the NFR was moderated by the predictability of the shocks. When shock was unpredictable, spinal nociception (NFR) and pain ratings were modulated in parallel. Specifically, pain and NFR magnitudes were lower during pleasant emotions and higher during unpleasant emotions. However, when shocks were predictable, only pain was modulated in this way. NFRs from predictable shocks were not altered by pictures. Further, exploratory analyses found that pain ratings, but not NFRs, were lower during predictable shocks. These data suggest emotional picture-viewing is a reliable method of engaging descending modulation of spinal nociception. However, descending modulation could not be detected in NFRs resulting from predictable noxious stimuli. Although preliminary, this study implies that separate mechanisms are responsible for emotional modulation of nociception at spinal vs. supraspinal levels, and that predictable noxious events may disengage modulation at the spinal level. The current paradigm could serve as a useful tool for studying descending modulation.

Adult↗
PubMed2000

Motion of contrast-modulated gratings is analysed by different mechanisms at low and at high contrasts.

We used a pedestal test [Lu & Sperling (1995a). Vision Research, 35, 2697-2722] to determine whether motion discrimination of contrast-modulated gratings has different properties at low contrast (4.5%) and at high contrast (45%). The amplitude-modulated gratings consisted of a 5 c/deg static carrier modulated by a moving 1 c/deg contrast envelope. We found that when contrast is low direction discrimination for contrast-modulated gratings is vulnerable to pedestals and becomes impossible at about 4 Hz. At high contrast contrast-modulated gratings are unaffected by pedestals and modulation sensitivity in a motion direction-discrimination task remains high up to 12 Hz. These results are consistent with the hypothesis that separate mechanisms analyse motion of contrast-modulated gratings at low and at high contrast; at low contrast motion analysis is based on feature tracking, whereas at high contrast, contrast-modulated gratings are analysed by spatio-temporal filters.

Contrast Sensitivity↗
PubMed2002

Spatiotemporal interactions in detection of texture orientation modulations.

Previous studies have revealed spatial and temporal characteristics of texture orientation modulation detection. This study examined spatiotemporal interactions. We measured threshold amplitudes for detecting orientation modulations in various waveforms. The orientation modulations were presented in a dynamic texture display in which the spatial arrangement and mean orientation of elements were randomly updated at a given frame duration (17-900 ms). The results of three experiments all indicated significant spatiotemporal interactions. As the frame duration was decreased, the detection sensitivity declined more steeply for the sinusoidal orientation modulations than for the square and missing-fundamental waveforms (Expt 1), declined more steeply for low spatial-frequency sinusoidal modulations than for high frequency ones (Expt 2), and declined more steeply for sparse textures than for dense textures (Expt 3). These results indicate that the visual system loses its sensitivity more profoundly for long-range orientation modulations than for short-range modulations as the rate of orientation change increases, suggesting that the mechanism for detecting orientation modulation reduces its effective spatial range for rapid input changes.

Contrast Sensitivity↗
PubMed1998

Contrast-modulation flicker: dynamics and spatial resolution of the light adaptation process.

We report a perceptual phenomenon that originates from a nonlinear operation during the visual process, and we use these observations to study the functional organization of the responsible nonlinearity; the regulation of visual sensitivity to light. When the contrast of a high frequency grating was modulated while its spatial and temporal average luminance was kept constant, observers saw brightness changes or desaturation in the field. If the contrast was modulated periodically between zero and a peak value, observers saw vivid flicker (contrast-modulation flicker), and this flicker could be seen even when the grating was too fine to be visually resolved as a pattern. This uniform-field flicker can be nulled by a modulation of space-average luminance at the contrast-modulation frequency, with appropriate phase and modulation depth. Contrast-modulation flicker is still measurable with gratings at 100 cycles/deg. The dynamics of contrast-modulation flicker suggest that it results from an early sensitivity-controlling mechanism, acting very rapidly (within about 20 msec). Its dependence on stimulus spatial frequency implies a strictly local luminance nonlinearity, one that either resides within individual photoreceptors or operates on signals from individual receptors.

Adaptation, Ocular↗