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951.
952.
Ben Hamed S Duhamel JR Bremmer F Graf W 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2001,140(2):127-144
The representation of the visual field in the primate lateral intraparietal area (LIP) was examined, using a rapid, computer-driven
receptive field (RF) mapping procedure. RF characteristics of single LIP neurons could thus be measured repeatedly under different
behavioral conditions. Here we report data obtained using a standard ocular fixation task during which the animals were required
to monitor small changes in color of the fixated target. In a first step, statistical analyses were conducted in order to
establish the experimental limits of the mapping procedure on 171 LIP neurons recorded from three hemispheres of two macaque
monkeys. The characteristics of the receptive fields of LIP neurons were analyzed at the single cell and at the population
level. Although for many neurons the assumption of a simple two-dimensional gaussian profile with a central area of maximal
excitability at the center and progressively decreasing response strength at the periphery can represent relatively accurately
the spatial structure of the RF, about 19% of the cells had a markedly asymmetrical shape. At the population level, we observed,
in agreement with prior studies, a systematic relation between RF size and eccentricity. However, we also found a more accentuated
overrepresentation of the central visual field than had been previously reported and no marked differences between the upper
and lower visual representation of space. This observation correlates with an extension of the definition of LIP from the
posterior third of the lateral intraparietal sulcus to most of the middle and posterior thirds. Detailed histological analyses
of the recorded hemispheres suggest that there exists, in this newly defined unitary functional cortical area, a coarse but
systematic topographical organization in area LIP that supports the distinction between its dorsal and ventral regions, LIPd
and LIPv, respectively. Paralleling the physiological data, the central visual field is mostly represented in the middle dorsal
region and the visual periphery more ventral and posterior. An anteroposterior gradient from the lower to the upper visual
field representations can also be identified. In conclusion, this study provides the basis for a reliable mapping method in
awake monkeys and a reference for the organization of the properties of the visual space representation in an area LIP extended
with respect to the previously described LIP and showing a relative emphasis of central visual field.
Electronic Publication 相似文献
953.
After preliminary threshold determination, each of 25 Ss was given 120 trials (24 blank, 96 signal) in a visual signal detection task, with signal presentation being recorded with S's EKG. There was no relationship between hit rate and phase of cardiac cycle whether assessed over four measured phases, or as the difference in hit rate for signals presented either in the P-wave or the QRS complex. Implications for theory are discussed. 相似文献
954.
Dr. B. Grubeck-Loebenstein H. Vierhapper W. Waldhäusl A. Korn M. Graf S. Panzer 《Journal of molecular medicine (Berlin, Germany)》1982,60(16):823-828
Summary Changes in blood pressure (BP) and plasma norepinephrine (NE) following various stimuli of the sympathetic nervous system were studied in six healthy subjects and in 17 diabetic patients. The latter were subdivided in three groups: (1) six patients with neither peripheral neuropathy nor autonomic dysregulation, (2) six patients with severe peripheral neuropathy without autonomic dysregulation, and (3) five patients with autonomic dysregulation, three of whom suffered also from peripheral neuropathy. The following procedures were performed: (1) cold pressor test (2 min), (2) mechanical irritation of the skin by suction (0.75 kg/cm2, 10 min), (3) orthostasis (10 min), and (4) i.v. infusion of NE (50, 100, 200 ng kg–1 min–1 for 15 min each). Both the stimulated endogenous plasma NE levels and BP response to exogenous NE were the same in normal subjects, in diabetic controls and in diabetics with peripheral neuropathy without autonomic dysregulation. In contrast, diabetics with postural hypotension showed a less pronounced release of NE to standing (P<0.05), but not to cold pressor test and mechanical skin irritation. Furthermore, they showed increased vasoreactivity to the highest dose (P<0.05), but not to the lower doses of exogenous NE. Thus NE release and adrenergic BP regulation seem to be altered only in diabetics with clinical signs of autonomic dysregulation. These alterations can only be evaluated when patients are exposed to stimuli of higher intensity, such as orthostasis or infusion of a high NE dose. 相似文献
955.
956.
Effect of knee joint laxity on long-loop postural reflexes: evidence for a human capsular-hamstring reflex 总被引:3,自引:0,他引:3
R. P. Di Fabio B. Graf M. B. Badke A. Breunig K. Jensen 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》1992,90(1):189-200
Summary The onset latency and discharge amplitude of preprogrammed postural responses were evaluated in order to determine if the structure of synergistic activation could be altered by ligamentous laxity at the knee joint. Twelve subjects with unilateral and one subject with bilateral anterior cruciate ligament (ACL) insufficiency were tested while standing on a moveable platform. External balance perturbations (6 cm anterior or posterior horizontal displacements of the platform) were presented at velocities ranging from 15 to 35 cm/s. Perturbations were presented under the following experimental conditions: unilateral and bilateral stance, knees fully straight or flexed, and with ankle motion restricted or free. These stance, knee position, and ankle motion conditions were introduced to alter the stress transmitted to the knee joint during movement of the support surface. The automatic postural response was recorded from the tibialis anterior (T), quadriceps (Q), and medial hamstrings muscles (H) bilaterally. The normal response to an externally induced backward sway involved the automatic activation of T and Q at latencies of 80 ms and 90 ms respectively. Activation of the hamstrings in the non-injured extremity was not coupled with the postural response. Hamstrings are not typically involved in the correction posterior sway because H activation would tend to pull the center of mass further backwards. However, when the response in the ACL-deficient extremity was compared to the non-injured limb: (1) the automatic postural response in the ACL-deficient extremity was restructured to include hamstrings activation (100 ms latency), (2) H activation time was faster and less variable in the ACL-deficient limb, and (3) the ratio of H/Q discharge amplitude integrated over 100 ms and 200 ms from the onset of EMG activation showed a dominance of hamstring activity during unilateral stance on the lax limb. In addition, H/Q ratios integrated over 200 ms showed dominant hamstring activity in the ACL-deficient limb during bilateral stance. (4) Crosslimb comparisons showed greater normalized IEMG amplitudes for T, H, and Q during unilateral stance on the lax limb. These results suggest that a capsular-hamstring reflex is integrated into the existing structure of a preprogrammed postural synergy in order to compensate for ligamentous laxity. Furthermore, the generalized increase of response gain observed during perturbations of unilateral stance on the lax limb indicates that joint afference can modulate central programming to control localized joint hypermobility. A concept of postural control is discussed with respect to the capsular reflex, joint loading and displacement of the center of gravity. 相似文献
957.
Graf IM Seemann G Weiss DL Dössel O 《Medical & biological engineering & computing》2005,43(6):783-792
The application of strong electrical stimuli is a common method used for terminating irregular cardiac behaviour. The study
presents the influence of electrophysiological heterogeneity on the response of human hearts to electrical stimulation. The
human electrophysiology was simulated using the ten Tusscher-Noble-Noble-Panfilov cell model. The anisotropic propagation
of depolarisation in three-dimensional virtual myocardial preparations was calculated using bidomain equations. The research
was carried out on different types of virtual cardiac wedge. The selection of the modelling parameters emphasises the influence
of cellular electrophysiology on the response of the human myocardium to electrical stimulation. The simulations were initially
performed on a virtual cardiac control model characterised by electrophysiological homogeneity. The second preparation incorporated
the transmural electrophysiological heterogeneity characteristic of the healthy human heart. In the third model type, the
normal electrophysiological heterogeneity was modified by the conditions of heart failure. The main currents responsible for
repolarisation (Ito, IKs and IKl) were reduced by 25%. Successively, [Na+]i was increased by the regulation of the Na+−Ca2+ exchange function, and fibrosis was represented by decreasing electrical conductivity. Various electrical stimulation configurations
were used to investigate the differences in the responses of the three different models. Monophasic and biphasic electrical
stimuli were applied through rectangular paddles and needle electrodes. A whole systolic period was simulated. The distribution
of the transmembrane voltage indicated that the modification of electrophysiological heterogeneity induced drastic changes
during the repolarisation phase. The results illustrated that each of the heart failure conditions amplifies the modification
of the response of the myocardium to electrical stimulation. Therefore a theoretical model of the failing human heart must
incorporate all the characteristic features. 相似文献
958.
The flatfish species constitute a natural paradigm for investigating adaptive changes in the vertebrate central nervous system. During metamorphosis all species of flatfish experience a 90 degree change in orientation between their vestibular and extraocular coordinate axes. As a result, the optic axes of both eyes maintain their orientation with respect to earth horizontal, but the horizontal semicircular canals become oriented vertically. Since the flatfish propels its body with the same swimming movements when referenced to the body as a normal fish, the horizontal canals are exposed to identical accelerations, but in the flatfish these accelerations occur in a vertical plane. The appropriate compensatory eye movements are simultaneous rotations of both eyes forward or backward (i.e., parallel), in contrast to the symmetric eye movements in upright fish (i.e., one eye moves forward, the other backward). Therefore, changes in the extraocular muscle arrangement and/or the neuronal connectivity are required. This study describes the peripheral and central oculomotor organization in the adult winter flounder, Pseudopleuronectes americanus. At the level of the peripheral oculomotor apparatus, the sizes of the horizontal extraocular muscles (lateral and medial rectus) were considerably smaller than those of the vertical eye muscles, as quantified by fiber counts and area measurements of cross sections of individual muscles. However, the spatial orientations and the kinematic characteristics of all six extraocular muscles were not different from those described in comparable lateral-eyed animals. There were no detectable asymmetries between the left and the right eye. Central oculomotor organization was investigated by extracellular horseradish peroxidase injections into individual eye muscles. Commonly described distributions of extraocular motor neurons in the oculomotor, trochlear, and abducens nuclei were found. These motor neuron pools consisted of two contralateral (superior rectus and superior oblique) and four ipsilateral populations (inferior oblique, inferior rectus, medial rectus, and lateral rectus). The labeled cells formed distinct motor neuron populations, which overlapped little. As expected, the numbers of labeled motoneurons differed in horizontal and vertical eye movers. The numerical difference was especially prominent in comparing the abducens nucleus with one of the vertical recti subdivisions. Nevertheless, there was bilateral symmetry between the motoneurons projecting to the left and right eyes.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
959.
O. Graf 《Pflügers Archiv : European journal of physiology》1957,266(1):101
Ohne Zusammenfassung 相似文献
960.
Inductively coupled solenoid coils fitting to objects in the size of mice or rats were developed to adapt modem whole-body MR scanners featuring sufficient gradient strength for animal examinations with high spatial resolution. Homogenous receiver characteristics is achievable over almost the whole inner region of the solenoid coils. The SNR can be increased by a factor 2 to 6 with the adapting coils for examinations using the head coil as connected receiver. Standard sequences on clinical 1.5 T scanners can be applied with adapted transmitter voltages. For example, a SNR value of about 30 is achievable in a mouse liver after 10 minutes measuring time using a 2-D spin echo imaging sequence and a size of 0.3 x 0.3 x 0.8 mm3 for the picture elements. 相似文献