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At least 595 records · Page 33Linked to original sources
PubMed1995

An emergent model of orientation selectivity in cat visual cortical simple cells.

It is well known that visual cortical neurons respond vigorously to a limited range of stimulus orientations, while their primary afferent inputs, neurons in the lateral geniculate nucleus (LGN), respond well to all orientations. Mechanisms based on intracortical inhibition and/or converging thalamocortical afferents have previously been suggested to underlie the generation of cortical orientation selectivity; however, these models conflict with experimental data. Here, a 1:4 scale model of a 1700 microns by 200 microms region of layer IV of cat primary visual cortex (area 17) is presented to demonstrate that local intracortical excitation may provide the dominant source of orientation-selective input. In agreement with experiment, model cortical cells exhibit sharp orientation selectivity despite receiving strong iso-orientation inhibition, weak cross-orientation inhibition, no shunting inhibition, and weakly tuned thalamocortical excitation. Sharp tuning is provided by recurrent cortical excitation. As this tuning signal arises from the same pool of neurons that it excites, orientation selectivity in the model is shown to be an emergent property of the cortical feedback circuitry. In the model, as in experiment, sharpness of orientation tuning is independent of stimulus contrast and persists with silencing of ON-type subfields. The model also provides a unified account of intracellular and extracellular inhibitory blockade experiments that had previously appeared to conflict over the role of inhibition. It is suggested that intracortical inhibition acts nonspecifically and indirectly to maintain the selectivity of individual neurons by balancing strong intracortical excitation at the columnar level.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1996

Fluoroscopic functional evaluation of bileaflet prostheses: effect of different intraoperative valve orientation.

Fluoroscopy is a reliable, easy, and readily available technique to follow-up prosthesis functioning after heart valve surgery. The different orientation given to the prosthesis may represent a limitation of the technique accounting for unsatisfactory results in 10% to 40% of the cases. The aim of the study was to evaluate whether and to what extent different intraoperative valve orientation influence feasibility and accuracy of postoperative fluoroscopic evaluation of bileaflet prostheses. We prospectively evaluated 90 patients who had aortic, mitral, and/or tricuspid valve replacement with Sorin Bicarbon or CarboMedics bileaflet prostheses. Fifty percent of the patients in each group were randomly assigned to receive prostheses oriented in a perpendicular or a parallel position with respect to the ventricular septum. Fluoroscopic evaluation was considered appropriate when the prosthesis' "tilting disk" projection was obtained. The valve's hemodynamic performance was investigated through Doppler study. A proper fluoroscopic evaluation was rapidly (15 +/- 5 seconds) achieved in all patients with the former orientation, whereas it was impossible to obtain it in 8 of 20 (40%), 19 of 20 (95%), and 4 of 5 (80%) of patients with the latter orientation. In the remaining patients extremely angulated, uneasy projection was often required to get a correct fluoroscopic image. The Doppler study showed a similarly favorable hemodynamic performance regardless of valve orientation. Prosthesis orientation crucially affects the rate of success of the fluoroscopic evaluation. The orientation perpendicular to the ventricular septum greatly facilitates the postoperative feasibility and accuracy of fluoroscopy, and it is not detrimental to the valve's hemodynamic performance. This valve orientation may provide a better fluoroscopic window whenever a valve dysfunction is suspected.

Aortic Valve↗
PubMed1998

Mutual orientation of tRNAs and interactions between the codon-anticodon duplexes within the ribosome: a stereochemical analysis.

Analysis of the available data demonstrated that a codon in the i(th) codon-anticodon duplex should interact with the wobble pair of the i - 1(th) duplex. This interduplex interaction should take place throughout the ribosomal elongation cycle in order to prevent unprogrammed frameshifting. An experimentally observed flexibility of tRNA allows to conserve the interduplex interaction at different mutual orientations of tRNAs, including conventional R and S. Moreover, the tRNA flexibility allows novel mutual orientations of tRNAs in which tRNA molecules, as in conventional R and S orientations, also form the codon-anticodon duplexes, and the CCA-ends are located adjacently. The R and S orientations do not offer any advantages over the novel orientations. Therefore, besides the conventional R and S orientations, the novel orientations should also be considered, i.e. the interpretations of the available experimental data on the mutual orientations of tRNAs should be reconsidered. All mutual orientations of tRNAs that are compatible with the available experimental data are given.

Anticodon↗
PubMed1999

Is the development of orientation selectivity instructed by activity?

Is the development of orientation selectivity in visual cortex instructed by the patterns of neural activity of input neurons? We review evidence as to the role of activity, review models of activity-instructed development, and discuss how these models can be tested. The models can explain the normal development of simple cells with binocularly matched orientation preferences, the effects of monocular deprivation and reverse suture on the orientation map, and the development of a full intracortical circuit sufficient to explain mature response properties including the contrast-invariance of orientation tuning. Existing experiments are consistent with the models, in that (a) selective blockade of ON-center ganglion cells, which will degrade or eliminate the information predicted to drive development of orientation selectivity, in fact prevents development of orientation selectivity; and (b) the spontaneous activities of inputs serving the two eyes are correlated in the lateral geniculate nucleus at appropriate developmental times, as was predicted to be required to achieve binocular matching of preferred orientations. However, definitive tests remain to be done to firmly establish the instructive rather than simply permissive role of activity and determine whether the retinotopically and center type-specific patterns of activity predicted by the models actually exist. We conclude by critically examining alternative scenarios for the development of orientation selectivity and maps, including the idea that maps are genetically prespecified.

Animals↗
PubMed2002

The alpha-factor receptor C-terminus is important for mating projection formation and orientation in Saccharomyces cerevisiae.

Successful mating of MATa Saccharomyces cerevisiae cells is dependent on Ste2p, the alpha-factor receptor. Besides receiving the pheromone signal and transducing it through the G-protein coupled MAP kinase pathway, Ste2p is active in the establishment and orientation of the mating projection. We investigated the role of the carboxyl terminus of the receptor in mating projection formation and orientation using a spatial gradient assay. Cells carrying the ste2-T326 mutation, truncating 105 of the 135 amino acids in the receptor tail including a motif necessary for its ligand-mediated internalization, display slow onset of projection formation, abnormal shmoo morphology, and reduced ability to orient the mating projection toward a pheromone source. This reduction was due to the increased loss of mating projection orientation in a pheromone gradient. Cells with a mutated endocytosis motif were defective in reorientation in a pheromone gradient. ste2-Delta296 cells, which carry a complete truncation of the Ste2p tail, exhibit a severe defect in projection formation, and those projections that do form are unable to orient in a pheromone gradient. These results suggest a complex role for the Ste2p carboxy-terminal tail in the formation, orientation, and directional adjustment of the mating projection, and that endocytosis of the receptor is important for this process. In addition, mutations in RSR1/BUD1 and SPA2, genes necessary for budding polarity, exhibited little or no defect in formation or orientation of mating projections. We conclude that mating projection orientation depends upon the carboxyl terminus of the pheromone receptor and not the directional machinery used in budding.

Cell Differentiation↗
PubMed1983

Structural basis of orientation sensitivity of cat retinal ganglion cells.

We investigated the structural basis of the physiological orientation sensitivity of retinal ganglion cells (Levick and Thibos, '82). The dendritic fields of 840 retinal ganglion cells labeled by injections of horseradish peroxidase into the dorsal lateral geniculate nucleus (LGNd) or optic tracts of normal cats. Siamese cats, and cat deprived of patterned visual experience from birth by monocular lid-suture (MD) were studied. Mathematical techniques designed to analyze direction were used to find the dendritic field orientation of each cell. Statistical techniques designed for angular data were used to determine the relationship between dendritic field orientation and angular position on the retina (polar angle). Our results indicate that 88% of retinal ganglion cells have oriented dendritic fields and that dendritic field orientation is related systematically to retinal position. In all regions of retina more that 0.5 mm from the area centralis the dendritic fields of retinal ganglion cells are oriented radially, i.e., like the spokes of a wheel having the area centralis at its hub. This relationship was present in all animals and cell types studied and was strongest for cells located close to the horizontal meridian (visual streak) of the retina. Retinal ganglion cells appear to be sensitive to stimulus orientation because they have oriented dendritic fields.

Animals↗
PubMed1993

Orientation sensitive elements in the corticofugal influence on centre-surround interactions in the dorsal lateral geniculate nucleus.

In a previous study, we have shown that the corticofugal projection to the dLGN enhances inhibitory mechanisms underlying length tuning. This suggests that the inhibitory influences deriving from the corticofugal feedback should exhibit characteristics that reflect the response properties of orientation-tuned layer VI cells. Here we report data obtained from experiments using a bipartite visual stimulus, with an inner section over the dLGN cell receptive field centre and an outer section extending beyond it. For both X and Y cells there was a modulation of the strength of the surround antagonism of centre responses that was dependent on the orientation alignment of contours in the two components of the stimulus. Layer VI cells showed maximal responses when the two components were aligned to the same orientation; dLGN cells showed a minimal response. Varying the orientation alignment of the inner and outer components of the stimulus in a randomised, interleaved fashion showed that bringing the stimulus into alignment resulted in a 24.28% increase in the surround antagonism of the centre response. Blocking cortical activity showed this effect of alignment to be strongly dependent on corticofugal feedback. This effect of orientation alignment appears to apply for any absolute orientation of the alignment condition and supports the view that an entire subset of cortical orientation columns generate the feedback influencing any given dLGN cell. This mechanism makes dLGN cells sensitive to the orientation domain discontinuities in elongated contours moving across their receptive field.

Animals↗
PubMed2004

Is perception of upper body orientation based on the inertia tensor? Normogravity versus microgravity conditions.

During lateral leg raising, a synergistic inclination of the supporting leg and trunk in the opposite direction to the leg movement is performed in order to preserve equilibrium. As first hypothesized by Pagano and Turvey (J Exp Psychol Hum Percept Perform, 1995, 21:1070-1087), the perception of limb orientation could be based on the orientation of the limb's inertia tensor. The purpose of this study was thus to explore whether the final upper body orientation (trunk inclination relative to vertical) depends on changes in the trunk inertia tensor. We imposed a loading condition, with total mass of 4 kg added to the subject's trunk in either a symmetrical or asymmetrical configuration. This changed the orientation of the trunk inertia tensor while keeping the total trunk mass constant. In order to separate any effects of the inertia tensor from the effects of gravitational torque, the experiment was carried out in normo- and microgravity. The results indicated that in normogravity the same final upper body orientation was maintained irrespective of the loading condition. In microgravity, regardless of loading conditions the same (but different from the normogravity) orientation of the upper body was achieved through different joint organizations: two joints (the hip and ankle joints of the supporting leg) in the asymmetrical loading condition, and one (hip) in the symmetrical loading condition. In order to determine whether the different orientations of the inertia tensor were perceived during the movement, the interjoint coordination was quantified by performing a principal components analysis (PCA) on the supporting and moving hips and on the supporting ankle joints. It was expected that different loading conditions would modify the principal component of the PCA. In normogravity, asymmetrical loading decreased the coupling between joints, while in microgravity a strong coupling was preserved whatever the loading condition. It was concluded that the trunk inertia tensor did not play a role during the lateral leg raising task because in spite of the absence of gravitational torque the final upper body orientation and the interjoint coupling were not influenced.

Adult↗
PubMed2004

The effect of postural stability and spatial orientation of the upper limbs on interlimb coordination.

It has recently been reported that the spatial orientation of two moving limbs has a determining influence on the relative accuracy and stability of coordination patterns. The purpose of the present experiments was to test perceptual and neuromuscular explanations of these spatial orientation effects. Experiment 1 was an initial test of the hypotheses and an extension of a previous study [Lee et al. (2002) Exp Brain Res 146:205-212] that required participants to coordinate inphase and antiphase movement patterns in four spatial orientations: two symmetric orientations (90 degrees and 180 degrees separation between the limbs) and two asymmetric orientations (90 degrees and 135 degrees separation between the limbs). Results of Experiment 1 suggest that the symmetry of movement may be a key factor influencing spatial orientation effects observed during interlimb coordination. In Experiment 2, participants again performed inphase and antiphase movement patterns in symmetric and asymmetric spatial orientations. However, one-half of the participants in Experiment 2 were provided with mechanical constraints during the performance of the desired coordination patterns. The mechanical constraints provided postural support but did not influence the visual experience. Results showed that the addition of the postural support improved performance. These findings suggest that neuromuscular, and perhaps biomechanical, constraints contribute more to the influence of spatial orientation than visual-perceptual constraints.

Adult↗
PubMed2005

A role for terrain slope in orienting hippocampal place fields.

The three-dimensional topography of the environment is a potentially important source of orienting information for animals, but little is known about how such features affect either navigational behaviour or the neural representation of place. One component of the neural place representation comprises the hippocampal place cells, which show location-specific firing that can be oriented by directional cues in the environment. The present study investigated whether a simple topographical feature, terrain slope, could provide such orienting information to place cells. Place cells were recorded as rats explored a tilted (30 degrees) square box located in the centre of a dark, curtained and visually symmetrical circular enclosure. The orientation of the tilted surface was varied, first in conjunction with that of a visible cue card (to stabilise the system) and then in the absence of the cue card, when the slope of the box was the only remaining stable polarising cue in the environment. In the latter condition, place fields continued to be reliably oriented by the slope. Thus, terrain slope provides sufficient orienting information to set and probably maintain the orientation of the hippocampal place system. This may explain previous behavioural observations that spatial orientation is improved when slope information is available.

Analysis of Variance↗
PubMed2006

The influence of cortical feature maps on the encoding of the orientation of a short line.

The inhomogeneous distribution of the receptive fields of cortical neurons influences the cortical representation of the orientation of short lines seen in visual images. We construct a model of the response of populations of neurons in the human primary visual cortex by combining realistic response properties of individual neurons and cortical maps of orientation and location preferences. The encoding error, which characterizes the difference between the parameters of a visual stimulus and their cortical representation, is calculated using Fisher information as the square root of the variance of a statistically efficient estimator. The error of encoding orientation varies considerably with the location and orientation of the short line stimulus as modulated by the underlying orientation preference map. The average encoding error depends only weakly on the structure of the orientation preference map and is much smaller than the human error of estimating orientation measured psychophysically. From this comparison we conclude that the actual mechanism of orientation perception does not make efficient use of all the information available in the neuronal responses and that it is the decoding of visual information from neuronal responses that limits psychophysical performance.

Action Potentials↗
PubMed1983

Development of the orientation of the visuo-tectal map in Xenopus.

Eye rudiments from Xenopus embryos of stage 28 or younger were explanted on to the flank of similar embryos with normal or nasotemporally reversed orientation. After some hours the eyes were retransplanted to an orbit of a stage 32/34 embryo, with either normal or reversed nasotemporal orientation. Later the visuotectal projections through the operated eyes were mapped electrophysiologically. The maps obtained were oriented as if the mapping orientation of the eye had already been determined in the donor orbit before the first transplantation; i.e. if normally oriented in the final host orbit the eye gave a normal map, and if the eye was reversed in the final host orbit it gave a reversed map. In only one out of 25 cases did it seem that the orientation of the map could have been influenced by the orientation of the eye on the flank of the intermediate host, and here the evidence was weak. Two eyes gave reduplicated maps and 4 maps were uninterpretable. It was concluded that the orientation of the map is determined before stage 26 and is not altered by information derived from the flank during stages 26-34.

Animals↗
PubMed1983

Orientation bias in the response of kitten LGNd neurons to moving light bars.

The orientation sensitivity to moving light bars was determined for 113 neurons in laminae A, A1 and C of the dorsal part of the lateral geniculate nucleus (LGNd) of kittens 7-42 days old. Forty neurons (35.4%) were biased to contrast orientation (OB neurons), i.e. their response to an optimally oriented bar was 2-10 times stronger than their response to a bar oriented orthogonally to the optimal. The remaining 73 neurons were not sensitive to contrast orientation. Evidence is presented that orientation bias in the LGNd develops prior to visual experience. Orientation biased responses in the LGNd strongly depended on stimulus parameters; preferred stimuli were light bars having a length of 5 degrees or more and moving at velocities slower than 5 degrees/s. Our findings suggest that the OB neurons of the LGNd could be effective in generating the early orientation sensitivity in the visual cortex.

Animals↗
PubMed1987

Neuronal organization underlying visually elicited prey orienting in the frog--II. Anatomical studies on the laterality of central projections.

A complete transverse hemisection of the neuraxis just caudal to the optic tectum in the frog, Rana pipiens, results in a failure to orient toward stimuli in one visual hemifield [Kostyk and Grobstein (1986) Neuroscience 21, 41-55]. The extent of the deficit area implies disturbances in the outputs triggered by both tectal lobes. In this paper we report studies aimed at determining more precisely what damage is involved in producing the hemisection deficit, with the broader objective of identifying particular neural structures which may be important in visually elicited orienting. Small lesions at the level of the hemisection which are restricted to the ventromedial white tracts result in an orienting deficit identical to that produced by a complete hemisection. Large lesions which spare the ventromedial white tracts are without significant effect on orienting turns. The finding is consistent with the hypothesis that the hemisection deficit results from interruption of tectal outflow paths. Interestingly, partial damage of the ventromedial white tracts does not result in disconnection of any local tectal region from premotor circuitry but instead systematically alters the turns triggered from all tectal regions. Ventrolateral lesions at the same level do not produce deficits in orienting but do disturb optokinetic behavior. Introduction of horseradish peroxidase into ventromedial lesions produces retrograde labeling in a large number of structures both rostral and caudal of the lesion. Labeling patterns following introduction of horseradish peroxidase into ventrolateral lesions, which do not affect orienting turns, were qualitatively similar but differed quantitatively. The observed patterns of tectal cell labeling make it unlikely that the hemisection deficit can be accounted for in terms of interruption of direct projections deriving from complementary regions of the two tectal lobes. They also indicate that if there exists an uncrossed tectal outflow adequate to trigger orienting turns, it must be by way of an indirect projection. A more general analysis of the labeling patterns suggests that a crossed tectal projection and uncrossed projections from three midbrain tegmental nuclei (the anterodorsal tegmental nucleus, the nucleus profunds lateralis and the nucleus of the medial longitudinal fasciculus) are likely to be involved in triggering orienting turns. The three midbrain tegmental nuclei are of particular interest in that they provide possible anatomical substrates for an indirect uncrossed descending tectal outflow path.

Animals↗
PubMed2004

How adolescents construct their future: the effect of loneliness on future orientation.

This study examined the effect of loneliness, gender, and two dimensions of prospective life domains on adolescent future orientation. Future orientation was studied in four prospective domains: social relations, marriage and family, higher education and work and career. These domains are described in terms of two dimensions: theme (relational vs. instrumental) and distance (near vs. distant future). Data collected from Israeli Jewish adolescents (11th graders) were analysed by repeated measures ANOVAs and ANCOVAs (covariate: depressive experiences) for seven future orientation variables: value, expectance, control (motivational variables), hopes, fears (cognitive representation variables), exploration, commitment (behavioural variables). As predicted, lonely adolescents scored lower than socially embedded adolescents on future orientation variables applied to the relational and near future domains and lonely boys scored lower than lonely girls. However, effects were found only on the three future orientation motivational variables and not on the cognitive representation and behavioural variables. Contrary to prediction controlling for the effect of depressive experiences did not reduce the effect of loneliness on the future orientation variables, but reduced the tendency of adolescents to score higher on all future orientation variables in the instrumental than in the relational prospective domains. The contribution of these findings to the understanding of adolescent loneliness and future orientation was discussed and directions for future research were suggested.

Adolescent↗
PubMed2006

The radial bias: a different slant on visual orientation sensitivity in human and nonhuman primates.

It is generally assumed that sensitivity to different stimulus orientations is mapped in a globally equivalent fashion across primate visual cortex, at a spatial scale larger than that of orientation columns. However, some evidence predicts instead that radial orientations should produce higher activity than other orientations, throughout visual cortex. Here, this radial orientation bias was robustly confirmed using (1) human psychophysics, plus fMRI in (2) humans and (3) behaving monkeys. In visual cortex, fMRI activity was at least 20% higher in the retinotopic representations of polar angle which corresponded to the radial stimulus orientations (relative to tangential). In a global demonstration of this, we activated complementary retinotopic quadrants of visual cortex by simply changing stimulus orientation, without changing stimulus location in the visual field. This evidence reveals a neural link between orientation sensitivity and the cortical retinotopy, which have previously been considered independent.

Animals↗
PubMed2005

Contributions of local orientation and position features to shape integration.

Contour integration plays an important role in linking local elements into global shape and the binding strength among local elements depends on both orientation and position features. The very high sensitivity reported for detecting the sinusoidal deformation of circular contours may result from the presence and concordance of both orientation and position cues to shape difference. In this study, position and orientation-defined micropatch-sampled radial frequency (MSRF) patterns were employed, which permit the independent assessment of the contributions of local orientation and position features to shape integration. It was demonstrated that, while both local orientation and position features can encode shape deformation, the human visual system is more sensitive to orientation-defined shape difference than to position-defined shape difference. Furthermore, integration of the local orientation feature into shape is more than two times stronger than that of local position, and may involve a global pooling mechanism. Nevertheless, optimal shape discrimination performance requires the analysis of both local orientation and position features.

Adult↗
PubMed2005

Visually perceived vertical (VPV): induced changes in orientation by 1-line and 2-line roll-tilted and pitched visual fields.

We report a series of nine experiments which show that a single roll-tilted line in darkness induces changes of the orientation perceived as vertical (VPV) that are similar in magnitude and direction to those measured by Witkin and Asch (1948a) [Studies in space orientation. I. Perception of the upright with displaced visual fields. Journal of Experimental Psychology, 38, 762-782] with the classical square 4-sided frame, and we describe the configuration-independent mass-action rules by which the influences of the individual lines influences are combined. Clockwise (cw) and counterclockwise (ccw) orientations of a line produce cw and ccw displacements of the VPV setting, respectively, with effect magnitude increasing approximately linearly with line orientation (e.g., a 66.25 degrees - long line at 25 degrees horizontal eccentricity that varies in roll-tilt through +/-13.2 degrees around vertical generates a systematic variation in VPV over +/-7 degrees). The slope of the VPV-vs-roll-tilt function increases with line length along a negatively accelerated exponential function (length constant = 17.1 degrees). The influences of two bilaterally symmetric lines combine linearly and algebraically and the combined influence is linearly related to the sum of the VPVs for the 1-line components with a slope equal to 0.91 for short lines and 0.66 for long lines; thus, VPV for short lines manifests nearly complete additive summation, but for long lines, the 2-line VPV is nearer to the average of the VPV values for the two components measured separately. The effectiveness of the conjunction of two line segments within a visual scene does not depend on their separate orientations, only on their sum. Individual lines from pitched-only planes or from combinations of such planes generate identical influences to those generated from lines in frontoparallel planes with the same image orientations at the eye of the observer (their "retinal orientations"). Retinal orientation is the key to the induction of VPV change independently of the line's plane of origin.

Adult↗