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PubMedDate not supplied

Toward a unified model of vertebrate rod phototransduction.

Recently, we introduced a phototransduction model that was able to account for the reproducibility of vertebrate rod single-photon responses (SPRs) (Hamer et al., 2003). The model was able to reproduce SPR statistics by means of stochastic activation and inactivation of rhodopsin (R*), transducin (G alpha ), and phosphodiesterase (PDE). The features needed to capture the SPR statistics were (1) multiple steps of R* inactivation by means of multiple phosphorylations (followed by arrestin capping) and (2) phosphorylation dependence of the affinity between R* and the three molecules competing to bind with R* (G alpha, arrestin, and rhodopsin kinase). The model was also able to account for several other rod response features in the dim-flash regime, including SPRs obtained from rods in which various elements of the cascade have been genetically disabled or disrupted. However, the model was not tested under high light-level conditions. We sought to evaluate the extent to which the multiple phosphorylation model could simultaneously account for single-photon response behavior, as well as responses to high light levels causing complete response saturation and/or significant light adaptation (LA). To date no single model, with one set of parameters, has been able to do this. Dim-flash responses and statistics were simulated using a hybrid stochastic/deterministic model and Monte-Carlo methods as in Hamer et al. (2003). A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca2+-feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al., 2003); (2) a full dark-adapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al., 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods ( Forti et al., 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al. (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility. The model promises to be useful in testing hypotheses regarding both normal and abnormal photoreceptor function, and is a good starting point for development of a full-range model of cone phototransduction. Informative limitations of the model are also discussed.

Adaptation, Ocular↗
PubMed2003

Quantitative analysis of small pharmaceutical drugs using a high repetition rate laser matrix-assisted laser/desorption ionization source.

In this work, a high repetition rate laser matrix-assisted laser desorption/ionization (MALDI) source is studied on a quadrupole-time-of-flight (QqTOF) and a triple quadrupole (QqQ) mass spectrometer for rapid quantification of small pharmaceutical drugs. The high repetition rate laser allows an up to 100-fold higher pulse frequency as compared with regular MALDI lasers, resulting in much larger sample throughput and number of accumulated spectra. This increases the reproducibility of signal intensities considerably, with average values being around 5% relative standard deviation after taking into account the area ratio of the analyte to an internal standard. Experiments were conducted in MS/MS mode to circumvent the large chemical background due to MALDI matrix ions in the low mass range. The dynamic range of calibration curves on the QqTOF mass spectrometer extended over at least two orders of magnitude, whereas on the QqQ it extended over at least three orders of magnitude. Detection limits ranged from 60-400 pg/microL on the QqTOF and from 6-70 pg/microL on the QqQ for a series of benzodiazepines. The benzodiazepine content of commercial pill formulations was quantified, and less than 5% error was obtained between the present method and the manufacturer's certified values. Furthermore, a high sample throughput was achieved with this method, so that a single MALDI spot could be quantitatively scanned in as little as 15 s, and an entire 96-well MALDI plate in 24 min.

Benzodiazepines↗
PubMed1988

Adaptation in chemoreceptor cells. I. Self-adapting backgrounds determine threshold and cause parallel shift of response function.

1. The self-adapting effects of chemical backgrounds on the response of primary chemoreceptor cells to superimposed stimuli were studied using lobster (Homarus americanus) NH4 receptor cells. 2. These receptors responded for several seconds to the onset of the backgrounds, and then returned to their initial level of spontaneous activity (usually zero). The strongest response always occurred only during the steepest concentration change; the response then decayed back to zero or to the earlier spontaneous firing level, while the background concentration was still rising, and remained silent during the entire time that the background was maintained constant (20-30 min) 3. Exposure to constant self-adapting backgrounds eliminated the response of NH4 receptor cells to stimuli of concentration lower than the background, and reduced the responses to all higher stimulus concentrations tested by a nearly equal amount. This resulted in a parallel shift of the stimulus-response function to the right along the abscissa. 4. Since the response threshold was completely re-set by adaptation to backgrounds, NH4 receptors seem to function mostly as detectors of relative rather than absolute stimulus intensity across their entire dynamic range: the response to a given stimulus-to-background ratio remained the same over 3 log step increases of background concentration. 5. As in other sensory modalities, a parallel shift of response functions appears to be an important property of chemoreceptor cells, allowing for this sensory system to function over a wider stimulus intensity range than the instantaneous dynamic range of individual receptor cells.

Action Potentials↗
PubMed2006

Response of electrically coupled spiking neurons: a cellular automaton approach.

Experimental data suggest that some classes of spiking neurons in the first layers of sensory systems are electrically coupled via gap junctions or ephaptic interactions. When the electrical coupling is removed, the response function (firing rate vs. stimulus intensity) of the uncoupled neurons typically shows a decrease in dynamic range and sensitivity. In order to assess the effect of electrical coupling in the sensory periphery, we calculate the response to a Poisson stimulus of a chain of excitable neurons modeled by n-state Greenberg-Hastings cellular automata in two approximation levels. The single-site mean field approximation is shown to give poor results, failing to predict the absorbing state of the lattice, while the results for the pair approximation are in good agreement with computer simulations in the whole stimulus range. In particular, the dynamic range is substantially enlarged due to the propagation of excitable waves, which suggests a functional role for lateral electrical coupling. For probabilistic spike propagation the Hill exponent of the response function is alpha=1, while for deterministic spike propagation we obtain alpha=1/2, which is close to the experimental values of the psychophysical Stevens exponents for odor and light intensities. Our calculations are in qualitative agreement with experimental response functions of ganglion cells in the mammalian retina.

Journal Article↗
PubMed1998

Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor intensity.

Response properties of the receptor potential at steady state were analyzed in a biophysical model of an olfactory sensory neuron embedded in a multicell environment. The neuron structure was described as a set of several identical dendrites (or cilia) bearing the transduction mechanisms, joined to a nonsensory part--dendritic knob, soma, and axon. The different ionic compositions of the media surrounding the neuron sensory and nonsensory parts and the extraneuronal voltage sources, which both result from the presence of auxiliary cells, were also taken into account. Analytical solutions were found to describe how the receptor potential at the nonsensory part responds to a uniform change in the odorant-dependent conductance resulting from odorant stimulation of the sensory dendrites. We investigated the influence of various geometrical and electrical parameters on the receptor-potential response in the classical model neuron within a homogeneous environment and in the model neuron surrounded with auxiliary cells. First, it was found that the maximum amplitude of the receptor potential is independent of the neuron structure in the absence of auxiliary cells but not in their presence. In the latter case, the amplitude decreases with the length and number of sensory dendrites and with the input resistance of the nonsensory part. Second, the sensitivity (as measured by the increase in membrane conductance at half-maximum response) of the neuron model in the absence of auxiliary cells is higher, but its dynamic range is narrower than in their presence. The dynamic range is wide and the sensitivity low when the input resistance of the nonsensory part is small and the sensory dendrite is unbranched. Both sensitivity and dynamic range are higher for a longer dendrite. These results help understand the morphology of insect olfactory sensilla and can be generalized to other neuron types.

Animals↗
PubMed2002

Auditory cortical images of cochlear-implant stimuli: coding of stimulus channel and current level.

This study quantified the accuracy with which populations of neurons in the auditory cortex can represent aspects of electrical cochlear stimuli presented through a cochlear implant. We tested the accuracy of coding of the place of stimulation (i.e., identification of the active stimulation channel) and of the stimulus current level. Physiological data came from the companion study, which recorded spike activity of neurons simultaneously from 16 sites along the tonotopic axis of the guinea pig's auditory cortex. In that study, cochlear electrical stimuli were presented to acutely deafened animals through a 6-electrode animal version of the 22-electrode Nucleus banded electrode array (Cochlear). Cochlear electrode configurations consisted of monopolar (MP), bipolar (BP + N) with N inactive electrodes between the active and return electrodes (0 < or = N < or = 3), tripolar (TP) with one active electrode and two flanking return electrodes, and common ground (CG) with one active electrode and as many as five return electrodes. In the present analysis, an artificial neural network was trained to recognize spatiotemporal patterns of cortical activity in response to single presentations of particular stimuli and, thereby, to identify those stimuli. The accuracy of pair-wise discrimination of stimulation channels or of current levels was represented by the discrimination index, d', where d' = 1 was taken as threshold. In many cases, the threshold for discrimination of place of cochlear stimulation was < 0.75 mm, and the threshold for discrimination of current levels was < 1 dB. Cochlear electrode configurations varied in the accuracy with which they signaled to the auditory cortex the place of cochlear stimulation. The BP + N and TP configurations provided considerably greater sensitivity to place of stimulation than did the MP configuration. The TP configuration maintained accurate signaling of place of stimulation up to the highest current levels, whereas sensitivity was degraded at high current levels in BP + N configurations. Electrode configurations also varied in the dynamic range over which they signaled stimulus current level. Dynamic ranges were widest for the BP + 0 configuration and narrowest for the TP configuration. That is, the configuration that showed the most accurate signaling of cochlear place of stimulation (TP) showed the most restricted dynamic range for signaling of current level. These results suggest that the choice of the optimal electrode configuration for use by human cochlear-prosthesis users would depend on the particular demands of the speech-processing strategy that is to be employed.

Action Potentials↗
PubMedDate not supplied

Using response models to study coding strategies in monkey visual cortex.

Usually the conditional probabilities needed to calculate transmitted information are estimated directly from empirically measured distributions. Here we show that an explicit model of the relation between response strength (here, spike count) and its variability allows accurate estimates of transmitted information. This method of estimating information is reliable for data sets with nine or more trials per stimulus. We assume that the model characterizes all response distributions, whether observed in a given experiment or not. All stimuli eliciting the same response are considered equivalent. This allows us to calculate the channel capacity, the maximum information that a neuron can transmit given the variability with which it sends signals. Channel capacity is uniquely defined, thus avoiding the difficulty of knowing whether the 'right' stimulus set has been chosen in a particular experiment. Channel capacity increases with increasing dynamic range and decreases as the variance of the signal (noise) increases. Neurons in V1 send more variable signals in a wide dynamic range of spike counts, while neurons in IT send less variable signals in a narrower dynamic range. Nonetheless, neurons in the two areas have similar channel capacities. This suggests that variance is being traded off against dynamic range in coding.

Action Potentials↗
PubMed1985

Single-unit responses in the inferior colliculus: different consequences of contralateral and ipsilateral auditory stimulation.

Monaural excitatory responses of 181 single units in the central nucleus of the inferior colliculus of 15 anesthetized gerbils (Meriones unguiculatus) were examined quantitatively. Pure-tone stimuli were presented monaurally through sealed, calibrated sound-delivery systems. Most units were excited only by contralateral stimulation (EO); 23% were bilaterally excitable (EE). The threshold frequency tuning curves for contralateral stimulation of EE units were significantly broader than those produced by ipsilateral stimulation of EE units and those produced by contralateral stimulation of EO units. The frequency at which threshold was lowest (best frequency), or BF) was very similar for ipsilateral and contralateral stimulation of individual EE units; however, ipsilateral BFs were slightly but significantly lower than contralateral BFs. For EE units, ipsilateral BF thresholds (mean: 29.2 dB SPL) were significantly higher than contralateral BF thresholds (mean: 14.9 dB SPL). Monotonic and nonmonotonic relationships between discharge rate and stimulus intensity at BF were observed in responses evoked both by contralateral and ipsilateral stimulation. Interestingly, for individual EE units it was not uncommon for the rate/intensity function for one monaural condition to be monotonic although the relationship for stimulation of the other ear was markedly nonmonotonic. There was no qualitative difference between rate/intensity functions evoked by contralateral stimulation in EO and EE units. Ipsilateral discharge rates were characteristically much lower than contralateral rates for a given stimulus intensity. For 50 BF tones of 100 ms duration, the median peak numbers of discharges for contralateral stimulation of EO and EE units were 361 and 339, respectively; the median for ipsilateral stimulation of EE units was 102. The dynamic range of each rate/intensity function was calculated by measuring the intensity range associated with an increase in spike count from 10 to 90% of the peak rate. No differences were detected between the distributions of dynamic range for contralateral stimulation in EO or EE units, or between contralateral and ipsilateral dynamic ranges within individual EE units. For all response types the distributions of dynamic range were approximately normal, with means near 20 dB. The minimum mean latency to the first spike at BF was generally longer for ipsilateral than for contralateral responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1997

Marginal shell of the anteroventral cochlear nucleus: single-unit response properties in the unanesthetized decerebrate cat.

The marginal shell of the anteroventral cochlear nucleus (AVCN) is anatomically different from its central core. We investigated 38 single units in the shells of 10 cats and contrasted them with 62 single units in the cores of 15 cats. The sites of all shell units were localized with the use of reconstructed electrode tracks. The shell units were divided into acoustically well-driven (68%) and weakly/not-driven (32%) subgroups. The shell units mostly exhibited low spontaneous rates (SRs). Among the well-driven shell units, a large majority (68%) exhibited wide dynamic ranges (> or = 50 dB) to tones, noise, or both, with some ranges as wide as 89 dB. In contrast, a large majority (80%) of the core units exhibited narrow dynamic ranges (< 50 dB) to tones and noise. The poststimulus time histograms (PSTHs) of the well-driven shell units included pause-build (29%), onset (24%), and unusual (33%) types, whereas those of the core units included mainly primary-like (47%) and chopper (29%) types. The excitatory-inhibitory areas (EIAs) of the well-driven shell units included types I/III (47%), III (22%), IV (13%), and II (9%), whereas those of the core units included mainly types III (52%) and I/III (32%). On the basis of Fisher's exact tests, we conclude that the shell and core neural groups of the AVCN are significantly different regarding all of the following physiological characteristics: SR, maximum driven rate, threshold and dynamic range to tones and noise, frequency response area, PSTH type, latency, and EIA type. Wide dynamic ranges of the well-driven shell units suggest that they may play a role in encoding absolute intensity of acoustic stimulus.

Acoustic Stimulation↗
PubMed1979

Responses of primate spinothalamic neurons to graded and to repeated noxious heat stimuli.

1. The responses of primate spinothalamic tract cells innervating the glabrous skin of the foot to noxious thermal stimuli have been examined. 2. Of the 41 cells studied, 98% responded to noxious thermal stimuli. Heating the cutaneous receptive field with a series of stimuli from 35 to 43, 47, and 50 degrees C produced a graded increase in discharge rate. The responses were characterized by an onset, which occurred after the temperature change had either slowed or stopped, an acceleration in the discharge up to a peak, and then an adaptation to a new base-line level. The time constants of adaptation were faster than those reported for C polymodal nociceptors. 3. No systematic differences were found in the responses to noxious thermal stimuli of cells with wide dynamic range receptive fields and of cells with narrow dynamic range, high-threshold receptive fields. There were also no differences in the responses of cells located in the marginal zone and of cells located in the neck of the dorsal horn. 4. The relationship between peak frequency and final skin temperature with a 30 s stimulus duration can best be described by a power function with an exponent of 2.1. An increase in the stimulus duration to 120 s resulted in an increase in the exponent of the power function to 3.2. 5. Repetition of the series of 30-s heat stimuli resulted in an increase in peak frequency, total impulse count, and background activity. Repetition of stimuli having a duration of 120 s produced an increase in the peak frequency at 43 and 45 degrees C, a smaller increase at 47 degrees C, and a decrease at 50 degrees C. Background activity was increased by the lower temperature stimuli, but was decreased following higher temperature stimuli. 6. In six additional cells, the skin was heated with three consecutive presentations at each temperature level (43, 45, 47, and 50 degrees C) for 30 s. No change was observed in the peak frequencies of the responses to successive stimuli of the same intensity. However, the exponent of the power function relating the average peak frequency for the six cells to changes in skin temperature was 3.9. This exponent was larger than that seen when two series of graded heat stimuli of 120 s duration were used, indicating more sensitization despite the fact the total time of exposure to noxious heat was less. 7. A role for both high-threshold and wide dynamic range spinothalamic cells in transmitting nociceptive information to the diencephalon is postulated.

Adaptation, Physiological↗
PubMed1991

Stimulus presentation level and speech perception through a single-channel cochlear implant.

This study evaluated the effects of stimulus presentation level on 12 adult 3M/House single-channel cochlear implant users' speech perception performance. Dynamic ranges and loudness growth functions were measured for meaningful speech, and performance-intensity functions were plotted for VCV and CVC nonsense stimuli to determine the presentation level(s) that produced maximum speech perception performance for each subject. Considerable variability was found in the subjects' dynamic ranges. Generally, loudness growth functions were steep for subjects having restricted dynamic ranges and more gradual for those having wide dynamic ranges. No single optimal presentation level was determined; instead, a range of levels produced maximum performance for each subject. Mean levels producing peak scores in equivalent dB SPL were 80 for VCVs and 72 for CVCs. Presentation levels producing optimal performance varied with type of speech stimulus.

Acoustic Stimulation↗
PubMed1986

Encoder response of isolated frog muscle spindle elicited by pseudorandom noise stimuli.

The present experiments investigated the signal transfer in the isolated frog muscle spindle by using pseudorandom noise (PRN) as the analytical probe. In order to guarantee that the random stimulus covered the entire dynamic range of the receptor, PRN stimuli of different intensities were applied around a constant mean length, or PRN stimuli of the same intensity were used while varying the mean length of the spindle. Subthreshold receptor potentials, local responses, and propagated action potentials were recorded simultaneously from the first Ranvier node of the afferent stem fiber, thus providing detailed insight into the spike-initiating process within a sensory receptor. Relevant features of the PRN stimulus were evaluated by a preresponse averaging technique. Up to tau = 2 ms before each action potential the encoder selected a small set of steeply rising stretch transients. A second component of the preresponse stimulus ensemble (tau = 2-5 ms) opposed the overall stretch bias. Since each steeply rising stretch transient evoked a steeply rising receptor potential that guaranteed the critical slope condition of the encoding site, this stimulus profile was most effective in initiating action potentials. The dynamic range of the muscle spindle receptor extended from resting length, L0, to about L0 + 100 microns. At the lower limit (L0) the encoding membrane was depolarized to its firing level and discharged action potentials spontaneously. When random stretches larger than the upper region of the dynamic range were applied, the spindle discharged at the maximum impulse rate and displayed no depolarization block or "overstretch" phenomenon. Random stretches applied within the dynamic range evoked regular discharge patterns that were firmly coupled to the PRN. The afferent discharge rate increased, and the precision of phase-locking improved when the intensity of the PRN stimulus was increased around a constant mean stretch; or the mean prestretch level was raised to higher values while the intensity of the PRN stimulus was kept constant. In the case when the PRN stimulus covered the entire dynamic range, the temporal pattern of the afferent discharge remained constant for at least 10 consecutive sequences of PRN. A spectral analysis of the discharge patterns averaged over several sequences of PRN was employed. At the same stimulus intensity the response spectra displayed low-pass filter characteristics with a 10-dB bandwidth of 300 Hz and a high-frequency slope of -12 dB/oct. Increasing the mean intensity of the PRN stimulus or raising the prestretch level increased the response power.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1992

[Correlation between electrically-induced stapedius reflex and discomfort threshold in cochlear implant patients].

Electrically elicited stapedius reflex thresholds are an objective criteria for the fitting of speech processors in very young children. Reflex thresholds generally fit well within the upper third of a subject's individual dynamic range, the difference between electrical threshold of hearing, (T) and maximum comfort (C) levels and can be used to predict ultimate behavioral maximum comfort levels. In acoustically elicited stapedius reflexes a saturation of impedance amplitude can be registered at approximately 110 dB, which is 90% of the dynamic range. Assuming a similar relationship for the electrically elicited stapedius reflex there would be two values within the dynamic range which could be used for extrapolation of the threshold level. In the present study, the electrically elicited stapedius reflex was examined in 16 deaf patients who had received 22-channel Clark/NUCLEUS cochlear implants. Using an apical, a medial and a basal electrode pair, different stimulation positions within the cochlea were tested. The contralateral reflexes could be elicited in 11 patients (69%). A saturation of the reflex amplitude was recordable in 10 subjects, at least in one of the electrode pairs. The reflex saturation in all cases was located close to the uncomfortable loudness level within the subjects' dynamic ranges (at 95% dynamic). This finding is comparable to acoustic matter. As a result, these data together with reflex threshold data suggest a means for predicting to predict the threshold levels.

Acoustic Impedance Tests↗
PubMed2000

Electrical stimulation of the auditory nerve. III. Response initiation sites and temporal fine structure.

Latency, temporal dispersion and input-output characteristics of auditory nerve fiber responses to electrical pulse trains in normal and chronically deafened cat ears were classified and tentatively associated with sites where activity is initiated. Spikes occurred in one or more of four discrete time ranges whose endpoints overlapped partially. A responses had latencies <0.44 ms, exhibited asymptotic temporal dispersion of 8-12 micros and possessed an average dynamic range of 1.2 dB for 200 pulses/s (pps) pulse trains. They likely originated from central processes of spiral ganglion cells. B(1) and B(2) responses (0.45-0.9 ms, 25-40 micros, 1.9 dB) likely stemmed from activity at myelinated and unmyelinated peripheral processes, respectively. C100 micros) likely originated from direct stimulation of inner hair cells, and D8 dB) arose from propagating traveling waves possibly caused by electrically induced motion of outer hair cells. C and D responses were recorded only in acoustically responsive ears. Mean latencies of spikes in all time ranges usually decreased with intensity, and activity at two or even three discrete latencies was often observed in the same spike train. Latency shifts from one discrete time range to another often occurred as intensity increased. Some shifts could be attributed to responses to the opposite-polarity phase of the biphasic pulse. In these cases, temporal dispersion and dynamic range were approximately equal for activity at each latency. A second type of latency shift was also often observed, in which responses at each latency exhibited dissimilar temporal dispersion and dynamic range. This behavior was attributed to a centralward shift in the spike initiation site and it occurred for monophasic as well as biphasic signals. Several fibers exhibited dual latency activity with a 40-90 micros time difference between response peaks. This may have stemmed from spike initiation at nodes on either side of the cell body. Increasing the stimulus pulse rate to 800-1000 pps produced small increases in temporal dispersion and proportionate increases in asymptotic discharge rate and dynamic range, but thresholds did not improve and slopes of rate-intensity functions (in spikes/s/dB) did not change. Responses to high-rate stimuli also exhibited discrete latency increases when discharge rates exceeded 300-400 spikes/s. Spike by spike latencies in these cases depended strongly on the discharge history. Implications for high-rate speech processing strategies are discussed.

Animals↗
PubMed2000

[A comparative study of analog and digital intraoral x-ray image detector systems].

UNLABELLED: The object of this investigation was to compare different intraoral, analog and digital X-ray image detector systems with respect to the diagnostic performance and to the relation of dose and image quality. METHODS: Three different intraoral film types and one digital system were compared. The same basic image quality-related technical parameters were measured, Contrast detail diagrams and images of pig teeth were captured and evaluated by visual inspection. RESULTS: The digital system has a speed that is at twice as high as least of the most sensitive analog system. Compared to the analog system, the digital system visualizes better low contrast structures such as carious defects, but shows problems in visualisation of high dynamic ranges such as crown margins or fillings: insufficient suitable dynamic range. Larger objects such as incisors could not be imaged in one exposure due to the small area of the digital detector (24.3 x 18.2 mm). Retakes may be required due to the small dynamic range and detector area. CONCLUSIONS: The complete imaging of a tooth with crown and apical region, as required in the "radiological guidelines" is in doubt with digital systems having small active areas. The image quality of digital systems differs significantly from that of analog systems, nevertheless, the achievable image quality fulfills the requirements of intraoral dental radiology.

Animals↗
PubMed1997

Error analysis in acoustic elastography. II. Strain estimation and SNR analysis.

Accurate displacement estimates are required to obtain high-quality strain estimates in elastography. In this paper the strain variance is derived from the statistical properties of the displacement field to define a point signal-to-noise ratio for elastography (SNR0). Displacements caused by compressional forces applied along the axis of the transducer beam are modeled by scaling and shifting the axial reflectivity profile of the tissue. The strain variance is given as a function of essential experimental parameters, such as the amount of tissue compression, echo waveform window length, and the amount of window overlap. SNR0 is defined in terms of applied compression and strain variance and normalized by the input signal-to-noise ratio (SNRi) for echo signals, to formulate the performance metric SNR0/SNRi. This quantity characterizes the noise properties, dynamic range, and sensitivity of strain images based on the spatial resolution requirements. The results indicate that low noise, high sensitivity, and limited dynamic range strain images are obtained for high-frequency bandpass signals when the applied strain is small. For large strains, however, one strategy for low-noise strain imaging employs base-band signals to obtain images with large dynamic range but limited peak sensitivity and noise figure. A better strategy includes companding, which eliminates the average strain in the echo signal before cross-correlation to reduce the dynamic range requirement and increase peak sensitivity for strain estimates.

Acoustics↗
PubMed1992

Ranges of dynamic motion of the wrist in healthy young and middle-aged men.

The difference in the range of dynamic motion of the wrist between young and middle-aged men was studied. To analyse wrist function easily, a new quantitative evaluation technique for wrist circumduction using a biaxial flexible electrogoniometer was introduced. A phase plane was made from two-channel signals of the electrogoniometer, and the 'ROM (range of motion) Index' for wrist circumduction was defined. Twelve healthy young men (age 19-31 years) and twelve healthy middle-aged men (age 45-63 years) were studied. Under forearm fixed conditions, there was no difference of the ROM Index between the two age groups. A significant decrease of the ROM Index in the middle-aged group was found in the non-dominant wrists without forearm fixation. The four quadrantal analysis of wrist circumduction clarified that this difference appeared especially in the radiodorsal direction. These results suggest that in the middle-aged men, the motor function involving the radiocarpal and midcarpal joints does not decrease, but that for the proximal and distal radioulnar joints in the non-dominant side drops.

Adult↗
PubMed1979

Inhibition of spinal cord interneurons by narcotic microinjection and focal electrical stimulation in the periaqueductal central gray matter.

Single cell evoked activity was recorded from spinal cord interneurons in rats prepared with microinjection cannulae or stimulating electrodes in the periaqueductal central gray matter (PAG). Morphine microinjections (4-16 microgram) inhibited the response evoked by a noxious stimulus in 55% of the wide dynamic range neurons tested. Microinjections of etorphine (0.25-0.5 microgram) inhibited 82% of the nociceptive neurons tested. Neither drug inhibited neurons which responded only to innocuous mechanical stimulation. The inhibition of wide dynamic range neurons produced by narcotic microinjection was antagonized by naloxone (1 mg/kg, i.p.) in 7 of 11 cases. Control experiments indicated that the effects obtained with microinjections could not be attributed to the drugs' diffusion to the spinal cord. Focal electrical stimulation of the PAG inhibited the responses to noxious stimuli of 60% of wide dynamic range neurons but was without effect on the responses of neurons that were activated only by innocuous stimuli. These experiments directly demonstrate that narcotic analgesics restricted to an intracerebral site of action activate a neural system which preferentially inhibits the responses of spinal cord wide dynamic range neurons to noxious stimuli. The system has a specificity for nociceptive input since non-nociceptive neurons were unaffected. Directly comparable results were produced by electrical stimulation of the PAG, supporting the concept that stimulation and narcotics modulate the transmission of nociceptive information by similar mechanisms.

Animals↗