(1/1648) Brain activation during maintenance of standing postures in humans.
The regulatory mechanism of bipedal standing in humans remains to be elucidated. We investigated neural substrates for maintaining standing postures in humans using PET with our mobile gantry PET system. Normal volunteers were instructed to adopt several postures: supine with eyes open toward a target; standing with feet together and eyes open or eyes closed; and standing on one foot or with two feet in a tandem relationship with eyes open toward the target. Compared with the supine posture, standing with feet together activated the cerebellar anterior lobe and the right visual cortex (Brodmann area 18/19), and standing on one foot increased cerebral blood flow in the cerebellar anterior vermis and the posterior lobe lateral cortex ipsilateral to the weight-bearing side. Standing in tandem was accompanied by activation within the visual association cortex, the anterior and posterior vermis as well as within the midbrain. Standing with eyes closed activated the prefrontal cortex (Brodmann area 8/9). Our findings confirmed that the cerebellar vermis efferent system plays an important role in maintenance of standing posture and suggested that the visual association cortex may subserve regulating postural equilibrium while standing. (+info)
(2/1648) Optical imaging of functional domains in the cortex of the awake and behaving monkey.
As demonstrated by anatomical and physiological studies, the cerebral cortex consists of groups of cortical modules, each comprising populations of neurons with similar functional properties. This functional modularity exists in both sensory and association neocortices. However, the role of such cortical modules in perceptual and cognitive behavior is unknown. To aid in the examination of this issue we have applied the high spatial resolution optical imaging methodology to the study of awake, behaving animals. In this paper, we report the optical imaging of orientation domains and blob structures, approximately 100-200 micrometer in size, in visual cortex of the awake and behaving monkey. By overcoming the spatial limitations of other existing imaging methods, optical imaging will permit the study of a wide variety of cortical functions at the columnar level, including motor and cognitive functions traditionally studied with positron-emission tomography or functional MRI techniques. (+info)
(3/1648) Dynamics of horizontal vergence movements: interaction with horizontal and vertical saccades and relation with monocular preferences.
We studied the dynamics of pure vergence shifts and vergence shifts combined with vertical and horizontal saccades. It is known from earlier studies that horizontal saccades accelerate horizontal vergence. We wanted to obtain a more complete picture of the interactions between version and vergence. Therefore we studied pure version (horizontal and vertical), pure vergence (divergence and convergence) and combinations of both in five adult subjects with normal binocular vision and little phoria (< 5 degrees). The visual targets were LED's in isovergence arrays presented at two distances (35 and 130 cm) in a dimly lit room. Two targets were continuously lit during each trial and gaze-shifts were paced by a metronome. The two subjects with a strong monocular preference made vergence eye movements together with small horizontal saccades during pure vergence tasks. The other subjects, who did not have a strong monocular preference, made pure vergence movements (without saccades). These findings, suggest that monocular preferences influence the oculomotor strategy during vergence tasks. Vergence was facilitated by both horizontal and vertical saccades but vergence peak-velocity during horizontal saccades was higher than during vertical saccades. (+info)
(4/1648) Functional micro-organization of primary visual cortex: receptive field analysis of nearby neurons.
It is well established that multiple stimulus dimensions (e.g., orientation and spatial frequency) are mapped onto the surface of striate cortex. However, the detailed organization of neurons within a local region of striate cortex remains unclear. Within a vertical column, do all neurons have the same response selectivities? And if not, how do they most commonly differ and why? To address these questions, we recorded from nearby pairs of simple cells and made detailed spatiotemporal maps of their receptive fields. From these maps, we extracted and analyzed a variety of response metrics. Our results provide new insights into the local organization of striate cortex. First, we show that nearby neurons seldom have very similar receptive fields, when these fields are characterized in space and time. Thus, there may be less redundancy within a column than previously thought. Moreover, we show that correlated discharge increases with receptive field similarity; thus, the local dissimilarity between neurons may allow for noise reduction by response pooling. Second, we show that several response variables are clustered within striate cortex, including some that have not received much attention such as response latency and temporal frequency. We also demonstrate that other parameters are not clustered, including the spatial phase (or symmetry) of the receptive field. Third, we show that spatial phase is the single parameter that accounts for most of the difference between receptive fields of nearby neurons. We consider the implications of this local diversity of spatial phase for population coding and construction of higher-order receptive fields. (+info)
(5/1648) Configuration saliency revealed in short duration binocular rivalry.
Supra-threshold spatial integration was studied by testing the saliency of multi-Gabor element configurations in short duration binocular rivalry (dichoptic masking) conditions. Dichoptic presentations allow for a competition between spatially overlapping supra-threshold stimuli that involve non-overlapping monocular receptive fields in the first stage of visual filtering. Different spatial configurations of Gabor patches (sigma = lambda = 0.12 degree) were presented to one eye (target) together with a bandpass noise presented to the other eye (mask). After a short rivalry period (120 ms) in which a dominance of one eye was established, a probe (a randomly positioned small rectangle of reduced contrast in the target) was presented for additional detection period (80 ms). Probe detection performance was measured (two-alternative-forced choice paradigm (2AFC) by finding the mask contrast leading to 79% correct response. Results show that configuration saliency is consistently expressed as dominance in short-duration binocular rivalry, with similar results obtained for longer durations (200 ms and continuous presentations). We find that textures of high-contrast randomly oriented patches are more dominant than uniform textures where the effect decreases and eventually reverses with decreasing of contrast. For supra-threshold contours, however, we find that smooth collinear contours are more dominant than 'jagged' ones, regardless of phase and contrast. These findings suggest principles underlying early lateral integration mechanisms based on contrast dependent inhibitory and excitatory connections. This mechanism could be based on iso-orientation surround (2D) inhibition and collinear (1D) facilitation, with inhibition being more effective at high contrasts. (+info)
(6/1648) Instability of torsion during smooth asymmetric vergence.
Several categories of torsional eye movements obey Listing's law; however, systematic deviations from this law occur during vergence. Two kinematic models attempt to incorporate these deviations, both of which are supported by experimental evidence; however, they lead to different torsion predictions. These discrepancies have been explained in terms of experimental procedures, but it now seems likely from several recent studies that individual differences in torsion patterns may also be important. This study therefore examines the variation of torsion during a smooth asymmetric vergence task in which a fixation target was moved along the line-of-sight of the right eye at 15 degrees elevation; each of five subjects observed five trials of both inward and outward target motion, repeated in two sessions several weeks apart. There were no significant group differences in left or right eye torsion between trials or sessions, suggesting that monocular torsion patterns were relatively stable over time. When examined more closely, however, the torsion patterns shown by some individuals did vary for inward versus outward target motion. Hence, monocular torsion was idiosyncratic and depended on the direction in which fixation was changing (convergence or divergence). In a binocular analysis, cycloversion varied dramatically between subjects and depended on the direction of target motion; however, this was not the case for cyclovergence. In summary, cyclovergence is relatively stable and depends on where the eyes are looking, whereas cycloversion (and hence monocular torsion) is relatively unstable and depends on how they came to be in that particular horizontal and vertical orientation. These findings help to explain the controversy surrounding the torsional behaviour of the human eye during vergence. (+info)
(7/1648) The influence of large scanning eye movements on stereoscopic slant estimation of large surfaces.
The results of several experiments demonstrate that the estimated magnitude of perceived slant of large stereoscopic surfaces increases with the duration of the presentation. In these experiments, subjects were free to make eye movements. A possible explanation for the increase is that the visual system needs to scan the stimulus with eye movements (which take time) before it can make a reliable estimate of slant. We investigated the influence of large scanning eye movements on stereoscopic slant estimation of large surfaces. Six subjects estimated the magnitude of slant about the vertical or horizontal axis induced by large-field stereograms of which one half-image was transformed by horizontal scale, horizontal shear, vertical scale, vertical shear, divergence or rotation relative to the other half-image. The experiment was blocked in three sessions. Each session was devoted to one of the following fixation strategies: central fixation, peripheral (20 deg) fixation and active scanning of the stimulus. The presentation duration in each of the sessions was 0.5, 2 or 8 s. Estimations were done with and without a visual reference. The magnitudes of estimated slant and the perceptual biases were not significantly influenced by the three fixation strategies. Thus, our results provide no support for the hypothesis that the time used for the execution of large scanning eye movements explains the build-up of estimated slant with the duration of the stimulus presentation. (+info)
(8/1648) An orientation anisotropy in the effects of scaling vertical disparities.
Garding et al. (Vis Res 1995;35:703-722) proposed a two-stage theory of stereopsis. The first uses horizontal disparities for relief computations after they have been subjected to a process called disparity correction that utilises vertical disparities. The second stage, termed disparity normalisation, is concerned with computing metric representations from the output of stage one. It uses vertical disparities to a much lesser extent, if at all, for small field stimuli. We report two psychophysical experiments that tested whether human vision implements this two-stage theory. They tested the prediction that scaling vertical disparities to simulate different viewing distances to the fixation point should affect the perceived amplitudes of vertically but not horizontally oriented ridges. The first used elliptical half-cylinders and the 'apparently circular cylinder' judgement task of Johnston (Vis Res 1991;31:1351-1360). The second experiment used parabolic ridges and the amplitude judgement task of Buckley and Frisby (Vis Res 1993;33:919-934). Both studies broadly confirmed the anisotropy prediction by finding that large scalings of vertical disparities simulating near distances had a strong effect on the perceived amplitudes of the vertically oriented stimuli but little effect on the horizontal ones. When distances > 25 cm were simulated there were no significant differential effects and various methodological reasons are offered for this departure from expectations. (+info)