bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pharmacological characterization of mouse GPRC6A, an L‐α‐amino‐acid receptor modulated by divalent cations
Authors: Christiansen B , Hansen K B , Wellendorph P , Bräuner‐Osborne H 
Published: January 2007

Background and purpose:GPRC6A is a novel member of family C of G protein‐coupled receptors with so far unknown function. We have recently described both human and mouse GPRC6A as receptors for L‐α‐amino acids. To date, functional characterization of wild‐type GPRC6A has been impaired by the lack of activity in quantitative functional assays. The aim of this study was thus to develop such an assay and extend the pharmacological characterization of GPRC6A.Experimental approach:We have engineered a novel cell‐based inositol phosphate turnover assay for wild‐type mouse GPRC6A based on transient co‐expression with the promiscuous GαqG66D protein, known to increase receptor signalling sensitivity. This assay allowed for measurements of L‐α‐amino acid potencies. Furthermore, in combination with an assay measuring inward currents at Ca2+‐activated chloride channels in Xenopus oocytes, the divalent cation‐sensing ability of the receptor was examined.Key results:Using our novel assay, we demonstrate that the basic L‐α‐amino acids ornithine, lysine, and arginine are the most potent agonists at wild‐type mouse GPRC6A. Using two different assay systems, we show that divalent cations do not activate the Gq signalling pathway of mouse GPRC6A per se but positively modulate the amino‐acid response.Conclusions and Implications:This is the first reported assay for a wild‐type GPRC6A successfully applied for quantitative pharmacological characterization of amino acid and divalent cation responses at mouse GPRC6A. The assay enables further search for GPRC6A ligands such as allosteric modulators, which may provide essential information about the physiological function of GPRC6A.British Journal of Pharmacology (2007) 150, 798–807. doi:10.1038/sj.bjp.0707121
Contrast Sensitivity and Increment Threshold
Authors: Bodis-Wollner I 
Published: January 2007

The threshold of rectangular and annular increments was established on square-wave gratings of 1 foot lambert mean luminance. In the range of 5–18 cycles/degree the threshold fell as the spatial frequency of the background increased. The threshold change was similar for rectangles and annuli, but the exponent of the curve differed according to the relative orientation of the test stimulus. The threshold curve of rectangles was related to the human contrast sensitivity curve, and it was found that threshold is directly proportional to the contrast of the grating and effective contrast is a function of the spatial frequency of the pattern. In the range below 5 cycles/degree of the grating pattern the threshold dropped. This is attributed to the low-frequency decline in contrast sensitivity, but this decrease occurred at a higher spatial frequency than expected. Apparently orientation and size specificity in field adaptation is compatible with functions of cells established in the visual cortex of primates and humans.
Use of Sensory Recombination and Somatosensory Deafferentation Techniques in the Investigation of Sensory-Motor Integration
Authors: Taub E , Goldberg I A 
Published: January 2007

Sensory rearrangement and somatosensory deafferentation experiments are being employed in this laboratory, both singly and in combination, as complementary research strategies for elucidating the role of direct sensory feedback in the learning and performance of various categories of movement. In deafferentation experiments on adolescent monkeys, we found that movements of almost all types can be learned and performed in the absence of guidance from the periphery. Moreover, recent work with primate infants deafferented on the first day of life has demonstrated that somatosensory feedback and spinal reflexes are also not necessary, after birth, for the ontogenetic development of most types of movement performed by the forelimbs. Sensory rearrangement studies with human subjects were directed at examining prism adaptation as a learning phenomenon. One experiment showed that, in controlling the rate of performance of an operant response, decreases in the amount of lateral displacement of vision could be used as a reward, and increases could be used as a punishment. These results support the central condition required by our previously formulated avoidance theory of prism adaptation, namely that the sensory discordance produced by prisms is aversive. In three further experiments, distribution of practice—a standard learning variable—was found to be a powerful means of manipulating such effects as (i) magnitude of adaptation, (ii) intermanual transfer, and (iii) adaptation when the subject observes his passively moved limb during the exposure period. In an experiment combining the two techniques, the amount of prism aftereffect in monkeys with deafferented forelimbs was compared with the amount in normal monkeys as a means of evaluating the ‘proprioceptive change’ hypothesis of prism adaptation. The results provided supporting evidence for the theory, indicating that the adaptation involved a recalibration of motor—kinesthetic systems rather than a change in visual perception.
The Contribution of Relational Factors to Line-Length Matches
Authors: Wenderoth Peter 
Published: January 2007

Both Künnapas and Rock and Ebenholtz investigated the effects of surrounding-frame size upon line-length judgments. Whereas Künnapas obtained errors of the order of 10–15%, typical of those which occur in other geometrical illusion figures, Rock and Ebenholtz reported errors closer to 100%. Rock and Ebenholtz claimed that perceived size is largely relationally determined and that this fact was obscured in Künnapas' experiment by allowing observers to compare the test lines directly with each other and within a common framework provided by the wall of the room. On the contrary, the experiments reported here suggested that errors in the Rock and Ebenholtz study may have been inflated by their use of nondirective instructions and by the confounded effects of other variables. When the Rock and Ebenholtz experiments were repeated with adequate controls over these variables, the effect of surrounding frames on line-length matches were of the order of 15–19%, similar to those of Künnapas but much smaller than those reported by Rock and Ebenholtz. The complexity of the stimuli in the Rock and Ebenholtz type of experiment is such, however, that the effects of a number of variables and possible cue conflicts remain to be investigated.
The Geometry of the Correspondence between Two Retinal Projections
Authors: Ninio Jacques 
Published: January 2007

Since each retinal projection of an object has some definite geometric relation to it, the two projections of a same object are related to each other. The correspondences between points, linear segments, and orientations on the two retinas are discussed thoroughly in three-dimensional geometry, and the constraints that tie the projections (the ‘projective invariants’) are presented. Some widely held conceptions on disparity or the ambiguity problem in binocular stereopsis appear to be based upon representations that are correct in particular situations, but are misleading in the general case. I suggest that in order to achieve binocular stereopsis, the brain does not proceed by complete trial and error, but may guide its search for correspondences by taking advantage of the geometric constraints. There are at least four major possible strategies: (i) a metric strategy, as initially proposed by Julesz; (ii) a projective strategy based on the law of invariance of the anharmonic ratio and Desargue's theorem; (iii) a perspective strategy discussed in relation to the homology relationships between vanishing points and in relation to physiological studies on cells of visual cortex; and (iv) a more dynamic strategy based upon the geometric properties of the Zöllner illusion.
Depicted Angle of Forms and Perception of Line Drawings
Authors: Kaess Dale W , Deregowski Jan B 
Published: January 2007

Subjects were presented with projections of either rectangular or elliptic forms which appeared either on their own or upon the following figures: (a) a face of a portrayed cube, (b) a parallelogram identical with (a), and (c) a rectangle of equal width and area to (a). Four different angles of projection were used for both the forms and for the background figures. In the composite figures these angles were varied independently. The subjects were required to judge the width of the forms. The forms were judged to be wider when presented on backgrounds (a) and (b) than when presented on background (c) or alone, especially at greater angles of projection. This suggests that the implicit-shape constancy effect is independent of relational effects resulting from the frame surrounding the form.
Visual Sensitivity to the Shape and Size of a Moving Object: Implications for Models of Object Perception
Authors: Beverley Kenneth I , Regan David 
Published: January 2007

Subjects adapted to rectangular targets whose opposite edges moved in opposite directions at any given instant (as when an object moves directly towards or away from the head). The distance that adaptation spread from the vertical edges was up to three times less when the horizontal edges were also moving than when the horizontal edges were stationary. Furthermore, for a square target (1 deg × 1 deg), adaptation spread least when the horizontal edges moved at the same speed as the vertical edges, whereas for a target whose height was half its width (0·5 deg × 1 deg) adaptation spread least when the horizontal edges moved at half the speed of the vertical edges. We propose that the human visual system acts as though it contains detectors sensitive to the size and shape of an object and that these detectors enhance this sensitivity to shape and size by comparing the velocities of the horizontal and vertical edges. When a nonrotating solid object moves in three dimensions its shape severely restricts the possible relationships between the velocities of the vertical and horizontal edges of its retinal image. Our hypothetical detectors utilise these geometrically determined velocity relationships as a basis for their selective sensitivity to shape. More speculatively, object perception and the perception of shape and size in everyday vision might involve visual sensitivity to the relationship between the velocities of an object's edges as well as sensitivity to the locations of these edges: the visual system may recognise which of the many edges in the visual field belong to a single object by comparing the velocities of orthogonal pairs of edges.
Relation of Brightness to Threshold for Light-Adapted and Dark-Adapted Rods and Cones: Effects of Retinal Eccentricity and Target Size
Authors: Drum Bruce 
Published: January 2007

‘Equal-brightness' functions of retinal eccentricity and target diameter were measured by a matching procedure, and compared with the corresponding threshold functions for four different adaptation conditions: light-adapted cones (LAC), dark-adapted cones (DAC), light-adapted rods (LAR) and dark-adapted rods (DAR). The separation between log brightness matches and log thresholds decreased with eccentricity and increased with target size for all adaptation conditions, but overall separation was substantially greater for the DAR condition than for the other three. A two-channel model of achromatic brightness is proposed to explain the results. The model assumes ‘strong’ and ‘weak’ channels, which contribute unequally to brightness. These channels are tentatively identified with tonic and phasic classes of retinal ganglion cells.
A Dichoptic Edge Effect Resulting from Binocular Contour Dominance
Authors: Welpe Edwin , von Seelen Werner , Fahle Manfred 
Published: January 2007

If one eye sees a bright field on a dark surround while a homogeneous background is presented to the other eye, then a distinct dark zone appears at the outer borders of the field. If the surround is coloured, then the zone is of corresponding hue. The effect disappears if the edge of the field can be fused with a bar of appropriate contrast. The influence of parameters such as pattern contrast and size were studied quantitatively. A model is proposed relating the effect to receptive-field organisation and the fusion process.
Self-Referent and Movement Cues in Coding Spatial Location by Blind and Sighted Children
Authors: Millar Susanna 
Published: January 2007

The study tested the hypothesis that movement and self-referent information differ, and that children can use both to code unseen locations. In one experiment blindfolded sighted children made more errors with changed than with unchanged accessing movements, despite unchanged reference conditions. Rotation decreased accuracy further, although most responses were to the correct side. Age did not interact with experimental conditions. In another experiment blind children and matched blindfolded controls were less accurate with changed movements; and rotations were more difficult still. Visual experience, but not age, interacted with conditions. The blind showed significantly more self-reference (responses to the original side in rotations), and were somewhat worse also with changed movements alone but did not differ from controls when movements and references were unchanged. The results support the hypothesis that movement and self-referent information differ. Visuospatial experience and task conditions rather than age seem to determine the extent to which children use either form of information.
The Effects of Temporal Modulation on the Perceived Spatial Structure of Sine-Wave Gratings
Authors: Parker Andrew 
Published: January 2007

Evidence is presented that two distinct effects are generated when a sine-wave grating is temporally modulated. These effects may differentiated on the grounds of different responses with different forms of temporal modulation (drift or phase reversal), different behaviour with changes of visual eccentricity and with changes of contrast. One effect is roughly equivalent to spatial-frequency doubling, as described by Kelly. The other is seen as a fractional increase in apparent spatial frequency ie 1.2 × or 1.4 × the original. Earlier papers are reviewed to lend support to these conclusions. The results obtained for spatial-frequency doubling are consistent with Kelly's model of a nonlinearity followed by a temporal integration. Spatial-frequency doubling emerges under conditions where a fractional increase in apparent spatial frequency would generally be expected but the particular spatiotemporal conditions are such that a breakdown in the motion-correspondence process may be suspected. Reasons for the existence of fractional shifts are not entirely clear, but it is argued that the similarity of the shifts induced by various manipulations (decreases of luminance, decreases of contrast, temporal modulation) are such that a general explanation must be sought, rather than one specific to temporal modulation alone.
Information about Three-Dimensional Shape and Direction of Illumination in a Square-Wave Grating
Authors: Bergström Sten Sture , Gustafsson Karl-Arne , Putaansuu Jorma 
Published: January 2007

A homogeneous grey picture and a ‘Mondrian’ type of picture were illuminated by a projector with square-wave gratings of thirty different contrast values used as slides. Ten observers reported whether the picture appeared three-dimensional (3-D) (pleated) or flat. 3-D responses in this situation indicate colour constancy ‘at the cost of’ nonveridical depth perception. The frequencies of 3-D responses were significantly higher for the structured picture than for the homogeneous grey one. In reports of the direction from which the apparent 3-D object appeared to be illuminated there was a significant preference for responses “from above” when the grating was horizontally oriented. With vertical orientation there was no preference for “from the left” or “from the right”. The results from the first experiment contradict traditional cue theories of depth perception since the projection of the borders between the fields of the structured picture was invariant and expected to inform about the flatness of the picture. They are, however, in line with a model for perceptual analysis of reflected light into common and relative components proposed earlier by Bergström. The difference in perceived direction of illumination between horizontally and vertically orientated gratings is discussed in connection with human ecology.
Binocular Cues in the Perception of Distance of a Point Source of Light
Authors: Morrison James D , Whiteside Thomas C D 
Published: January 2007

The ability to make egocentric distance estimates of a single point source of light, seen in darkness and without the cues of changing size and luminance, was investigated in sixteen observers. The attenuation required to maintain constant luminance, when the target was viewed from different distances, was shown to follow the inverse square law providing the angle subtended by the light was less than 20 s arc. Distance changes were also simulated by means of a split mirror which produced vergence cues, or by test lenses to provide accommodation cues. Over the range 0.5 to 9.2 m distance estimates were surprisingly accurate, although there was some overestimation of near and underestimation of far distances. Most observers made good judgements when only convergence cues were varied, whereas no observers made consistently good judgements when only accommodation cues were varied. The difficulties are discussed in terms of the accommodation–convergence link. When distance was simulated by changing convergence and accommodation cues, estimates were not as good as when real distance was changed. Since good estimates were made with brief target exposures, these judgements were not based on subsequent convergence or accommodation changes. It is suggested that the metric or reference against which the apparently absolute judgements were made was the efferent demand signal associated with a ‘resting’ position of convergence in darkness.
Perceptual Organization in Multistable Apparent Motion
Authors: Ramachandran Vilayanur S , Anstis Stuart M 
Published: January 2007

Is motion perception based on a local piecemeal analysis of the image or do ‘global’ effects also play an important role? Use was made of bistable apparent-motion displays in trying to answer this question. Two spots were flashed simultaneously on diagonally opposite corners of a 1 deg wide square and then switched off and replaced by two spots appearing on the other two corners. One can either see vertical or horizontal oscillation and the display is bistable just as a Necker cube is. If several such bistable figures are randomly scattered on the screen and presented simultaneously, then one usually sees the same motion axis in all of them, suggesting the presence of field-like effects for resolving ambiguity in apparent motion. While viewing a single figure observers experience hysteresis: they tend to adhere to one motion axis or the other and can switch the axis only by looking away and looking back after 10–30 s have elapsed. The figure can be switched off and made to reappear at some other random location on the screen and it is then always found to retain its motion axis. Several such demonstrations are presented to show that spatial induction effects in metastable motion displays may provide a particularly valuable probe for studying ‘laws’ of perceptual organization.
Suprathreshold Stereo-Depth Matches as a Function of Contrast and Spatial Frequency
Authors: Schor Clifton M , Howarth Peter A 
Published: January 2007

Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg−1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies. Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15–9.6 cycles deg−1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10–100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5–72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth. A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.
The Standard Deviation of Luminance as a Metric for Contrast in Random-Dot Images
Authors: Moulden Bernard , Kingdom Fred , Gatley Linda F 
Published: January 2007

Michelson's contrast, C, is an excellent metric for contrast in images with periodic luminance profiles, such as gratings, but is not suitable for images consisting of isolated stimulus elements, eg single bars; other metrics have been devised for such stimuli. But what metric should be used for random-dot images such as are commonly used in stereograms and kinematograms? Previously the standard deviation (SD) of the luminances (equivalent to the root mean square, RMS, of the amplitudes) has been taken as a measure of contrast, but on little more than intuitive grounds. The validity of this speculative usage is tested. Experiments are described in which a wide range of random-dot images of various compositions was used and the adapting power of these images measured. This was taken as an index of their visual effectiveness. The contrast and contrast-reducing effects of the stimuli were expressed in terms of six candidate metrics, including SD, to discover which would give the most lawful description of the experimental data. The usefulness and generality of the SD measure were confirmed. The effects of mean luminance were also measured and a general expression that would take them into account was derived. Finally, on the basis of computational modelling in which spatial filters with properties approximating those of retinal ganglion cells were used, a possible theoretical account for the success of the SD metric is offered.