Three-dimensional organization of vestibular related eye movements to rotational motion in pigeons.
(41/2370)
During rotational motions, compensatory eye movement adjustments must continually occur in order to maintain objects of visual interest as stable images on the retina. In the present study, the three-dimensional organization of the vestibulo-ocular reflex in pigeons was quantitatively examined. Rotations about different head axes produced horizontal, vertical, and torsional eye movements, whose component magnitude was dependent upon the cosine of the stimulus axis relative to the animal's visual axis. Thus, the three-dimensional organization of the VOR in pigeons appears to be compensatory for any direction of head rotation. Frequency responses of the horizontal, vertical, and torsional slow phase components exhibited high pass filter properties with dominant time constants of approximately 3 s. (+info)
Apparent position governs contour-element binding by the visual system.
(42/2370)
An assumption inherent in many models of visual space is that the spatial coordinates of retinal cells implicitly give rise to the perceptual code for position. The results of the experiments reported here, in which it is shown that retinally non-veridical locations of contour elements are used by the visual system for contour-element binding, lend support to a different view. The visual system does not implicitly code position with reference to the labelled locations of retinal cells, but dynamically extracts spatial position from the aggregate result of local computations. These computations may include local spatial relationships between retinal cells, but are not confined to them; other computations, including position derived from local velocity cues, are combined to code the position of objects in the visual world. (+info)
Similarity in the response of smooth pursuit and manual tracking to a change in the direction of target motion.
(43/2370)
Subjects were asked to track, with their eyes or their hand, the movement of a target that maintained a constant speed and made a single, abrupt change in direction. The tracking speed and direction of motion after the step change in target direction were compared for the eyes and the hand. After removal of the saccades from the eye movement records, it was found that in both cases, there was a slow rotation from the initial direction to the new direction. For the eyes and the hand, it was found that this change in direction of movement occurred at a similar rate that was proportional to the magnitude of the abrupt change in target direction. This was further described by comparing the direction of pursuit tracking with the response of a second-order system to a step input. In addition, it was found that the speed of manual and pursuit tracking was modulated in a similar manner, with a reduction in tracking speed occurring before the change in tracking direction. This reduction in speed following the change in the direction of target motion was very similar for the hand and the eye, despite the large difference in the inertias of the two systems. Taken together, these data suggest that the neural mechanisms for smooth pursuit and manual tracking have common functional elements and that musculoskeletal dynamics do not appear to be a rate-limiting factor. (+info)
Conformation and backbone dynamics of bacteriorhodopsin revealed by (13)C-NMR.
(44/2370)
It is demonstrated here how the secondary structure and dynamics of transmembrane helices, as well as surface residues, such as interhelical loops and N- or C-terminus of bacteriorhodopsin (bR) in purple membrane, can be determined at ambient temperature based on very simple (13)C-NMR measurements, together with a brief experimental background. In contrast to the static picture of bR, currently available from X-ray diffraction or cryo-electron microscopy, the structure consists of dynamically heterogeneous domains which undergo various types of local fluctuations with a frequency range of 10(2)--10 (8) Hz. The significance of this picture is discussed in relation to the biological function of this protein. (+info)
False cerebral activation on BOLD functional MR images: study of low-amplitude motion weakly correlated to stimulus.
(45/2370)
BACKGROUND AND PURPOSE: Movements of the participant during blood oxygen level-dependent (BOLD) functional MR imaging cerebral activation studies are known to produce occasionally regions of false activation, especially when these movements are relatively large (>3 mm) and highly correlated with the stimulus. We investigated whether minimal (<1 mm), weakly correlated movements in a controlled functional MR imaging model could produce false activation artifacts that could potentially mimic regions of true activation in size, location, and statistical significance. METHODS: A life-size brain phantom was constructed by embedding vials of a dilute carboxylic acid solution within a gadolinium-doped gelatin mold. Imaging was performed at 1.5 T using a 2D spiral sequence (3,000/5 [TR/TE]; flip angle, 88 degrees; matrix, 64 x 64; field of view, 24 cm; section thickness, 5 mm). Controlled, in-plane, submillimeter movements of the phantom were generated using a pneumatic system and were made to correlate with a hypothetical "boxcar" stimulus over the range 0.31 < r < 0.96. Regions of false activation were sought using standard statistical methods (SPM96) that excluded phantom edges and accounted for spatial extent (regions tested at P < .05, corrected for multiple comparisons). A similar experiment was performed on a resting volunteer. RESULTS: The pneumatic system provided motion control with average in-plane displacements and rotations of 0.74 mm and 0.47 degrees, respectively, in the 18 data sets analyzed. No areas of false activation in the phantom were identified for poorly correlated motions (r < 0.52). Above this level, false activations occurred with increasing frequency, scaling in size and number with the degree of motion correlation. For motions with r > 0.67, areas of false activation were seen in every experiment. For a statistical threshold of P = .001, the median number of falsely activated regions was 3.5, with a mean size of 71.7 voxels (approximately 5 cc). Areas of possibly false activation of average size 72.5 voxels resulting from passive motion of the resting human participant were observed in two of four experiments. CONCLUSION: Participant movements of 1 mm or less that are only modestly correlated with a blocked stimulus paradigm can produce appreciable false activation artifacts on BOLD functional MR imaging studies, even when strict image realignment methods are used to prevent them. (+info)
Active hair bundle motion linked to fast transducer adaptation in auditory hair cells.
(46/2370)
During transduction in auditory hair cells, hair bundle deflection opens mechanotransducer channels that subsequently reclose or adapt to maintained stimuli, a major component of the adaptation occurring on a submillisecond time scale. Using a photodiode imaging technique, we measured hair bundle motion in voltage-clamped turtle hair cells to search for a mechanical correlate of fast adaptation. Excitatory force steps imposed by a flexible glass fiber attached to the bundle caused an initial movement toward the kinocilium, followed by a fast recoil equivalent to bundle stiffening. The recoil had a time course identical to adaptation of the transducer current, and like adaptation, was most prominent for small stimuli, was slowed by reducing extracellular calcium, and varied with hair cell resonant frequency. In free-standing hair bundles, depolarizations positive to 0 mV evoked an outward current attributable to opening of transducer channels, which was accompanied by a sustained bundle deflection toward the kinocilium. Both processes were sensitive to external calcium concentration and were abolished by blocking the transducer channels with dihydrostreptomycin. The similarity in properties of fast adaptation and the associated bundle motion indicates the operation of a rapid calcium-sensitive force generator linked to the gating of the transducer channels. This force generator may permit stimulus amplification during transduction in auditory hair cells. (+info)
Molecular dynamics simulation of human prion protein including both N-linked oligosaccharides and the GPI anchor.
(47/2370)
Although glycosylation appears to protect prion protein (PrP(C)) from the conformational transition to the disease-associated scrapie form (PrP(Sc)), available NMR structures are for non-glycosylated PrP(C), only. To investigate the influence of both the two N-linked glycans, Asn181 and Asn197, and of the GPI anchor attached to Ser230, on the structural, dynamical and electrostatic behavior of PrP, we have undertaken molecular dynamics simulations on the C-terminal region of human prion protein HU:PrP(90-230), with and without the three glycans. The simulations used the AMBER94 force field in a periodic box model with explicit water molecules, considering all long-range electrostatic interactions. The results suggest the structured part of the protein, HU:PrP(127-227) is stabilized overall from addition of the glycans, specifically by extensions of Helix-B and Helix-C and reduced flexibility of the linking turn containing Asn197, although some regions such as residues in the turn (165-170) between Strand-B and Helix-B have increased flexibility. The stabilization appears indirect, by reducing the mobility of the surrounding water molecules, and not from specific interactions such as H bonds or ion pairs. The results are consistent with glycosylation at Asn197 having a stabilizing role, while that at Asn181, in a region with already stable secondary structure, having a more functional role, in agreement with literature suggestions. Due to three negatively charged SiaLe(x) groups per N-glycan, the surface electrostatic properties change to a negative electrostatic field covering most of the C-terminal part, including the surface of Helix-B and Helix-C, while the positively charged N-terminal part PrP(90-126) of undefined structure creates a positive potential. The unusual hydrophilic Helix-A (144-152) is not covered by either of these dominant electrostatic fields, and modeling shows it could readily dimerize in anti parallel fashion. In combination with separate simulations of the GPI anchor in a membrane model, the results show the GPI anchor is highly flexible and would maintain the protein at a distance between 9 and 13 A from the membrane surface, with little influence on its structure or orientational freedom. (+info)
Tigerinins: novel antimicrobial peptides from the Indian frog Rana tigerina.
(48/2370)
Four broad-spectrum, 11 and 12 residue, novel antimicrobial peptides have been isolated from the adrenaline-stimulated skin secretions of the Indian frog Rana tigerina. Sequences of these peptides have been determined by automated Edman degradation, by mass spectral analysis and confirmed by chemical synthesis. These peptides, which we have named as tigerinins, are characterized by an intramolecular disulfide bridge between two cysteine residues forming a nonapeptide ring. This feature is not found in other amphibian peptides. Conformational analysis indicate that the peptides tend to form beta-turn structures. The peptides are cationic and exert their activity by permeabilizing bacterial membranes. Tigerinins represent the smallest, nonhelical, cationic antimicrobial peptides from amphibians. (+info)