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At least 271 records · Page 15Linked to original sources
PubMed2003

Rapid-motion-perception based cardiac navigators: using the high flow blood volume as a marker for the position of the heart.

Navigators have been developed as one of the many approaches to reducing motion artifacts due to respiration. A typical navigator approach applies a pencil-beam style profile crossing the diaphragm to track the superior-inferior (SI) motion of the diaphragm, and subsequently applying correlations to determine the heart location. This approach necessitates a priori knowledge of the correlation coefficients between heart and diaphragm motion, a variable parameter among patients. This paper presents an alternative navigator method based on Rapid Motion Perception (RaMP). This method acquires profiles of the ventricular blood volume based on its high flow velocity. The position of the blood volume is a direct representation of the position of the heart. This method allows cardiac navigation in two orthogonal directions simultaneously, eliminates the need to obtain correlations to the diaphragm motion, and increases tracking reliability for individual patients. A prospective version of RaMP navigators has been implemented on a clinical 1.5 T scanner, and preliminary tests on human volunteers show that this method can successfully track the heart position over the entire respiratory period. This navigation scheme is tested for predicting superior-inferior and anterior-posterior (AP) motion of the heart for breath-hold and free breathing conditions. Bland-Altman plots comparing the motion predicted by the navigators and that computed from single-shot images immediately following the navigators, show that the accuracy of this method is +/- 1.43 mm in the SI direction and +/- 0.84 mm in the AP direction. The RaMP navigator is suited for real-time tracking of the bulk translational motion of the heart.

Adult↗
PubMedDate not supplied

Effects of directional expectations on motion perception and pursuit eye movements.

Expectations about future motions can influence both perceptual judgements and pursuit eye movements. However, it is not known whether these two effects are due to shared processing, or to separate mechanisms with similar properties. We have addressed this question by providing subjects with prior information about the likely direction of motion in an upcoming random-dot motion display and measuring both the perceptual judgements and pursuit eye movements elicited by the stimulus. We quantified the subjects' responses by computing oculometric curves from their pursuit eye movements and psychometric curves from their perceptual decisions. Our results show that directional cues caused similar shifts in both the oculometric and psychometric curves toward the expected motion direction, with little change in the shapes of the curves. Prior information therefore biased the outcome of both eye movement and perceptual decisions without systematically changing their thresholds. We also found that eye movement and perceptual decisions tended to be the same on a trial-by-trial basis, at a higher frequency than would be expected by chance. Furthermore, the effects of prior information were evident during pursuit initiation, as well as during pursuit maintenance, indicating that prior information likely influenced the early processing of visual motion. We conclude that, in our experiments, expectations caused similar effects on both pursuit and perception by altering the activity of visual motion detectors that are read out by both the oculomotor and perceptual systems. Applying cognitive factors such as expectations at relatively early stages of visual processing could act to coordinate the metrics of eye movements with perceptual judgements.

Adult↗
PubMed2006

Vernier acuity is normal in migraine, whereas global form and global motion perception are not.

PURPOSE: A recent study has demonstrated that some people with migraine display impairments of intermediate stages of motion and form processing. Deficits were identified by using tasks that required that local stimulus attributes be integrated into global percepts. Neurons capable of global processing of form and motion are known to be present in extrastriate cortical areas V4 and V5, respectively. It is not clear from the literature whether deficits of global processing in migraineurs are likely to arise from reduced input to extrastriate cortex from primary visual cortex (V1). The purpose of the study was to compare presumed measures of V1 performance (vernier acuity) to measures of global form and motion perception in migraineurs. METHODS: Thirty migraineurs (17 with aura, and 13 without) and 20 age-matched nonheadache control subjects participated. Intermediate level motion and form perceptions were measured using global dot motion stimuli and Glass patterns, respectively. Vernier stimuli were broad vertical bars composed of small dot elements. Both a static luminance stimulus and a motion defined form vernier stimulus were used. RESULTS: Mean migraine and control group performance were not significantly different for either vernier task (static: t(48)=0.39, P=0.70; motion: t(48)=0.29, P=0.77). Mean migraine group performance was significantly worse than in control subjects for both the global form (t(48)=2.06, P=0.04) and global motion (t(48)=2.87, P<0.01) tasks. CONCLUSIONS: On average, migraineurs demonstrate abnormalities of intermediate stages of both motion and form processing. These abnormalities do not appear to arise from dropout of performance at V1, as vernier acuity was normal in the same individuals.

Adolescent↗
PubMed1991

Two carriers for motion perception: color and luminance.

Starting with the experiments of Ramachandran and Gregory (Nature, 275, 55-56, 1978), several psychophysical studies in apparent motion (AM) have established that the perception of motion is significantly impaired at equiluminance. Still debated, however, is whether color alone can resolve ambiguities in AM. We report here on several psychophysical experiments, the quantitative results of which indicate that color does play a substantial role in AM. These findings seem to support recently proposed neurophysiological frameworks according to which there exist significant interactions among the neuronal pathways mediating the perception of basic visual attributes such as color, motion, form and depth.

Color Perception↗
PubMed2000

Differences between motion-direction perception and unspecific motion perception in the human knee joint.

Perception has commonly been seen as a conscious performance. Thus, regarding proprioception in some publications, it has been proposed that the term is properly used only when subjects are able to report on the direction as well as presence of imposed movements. Consequently, detections of movements without movement-direction perception have not been accepted as perceived, since these detections were regarded as unspecific. Unspecific sensation has been suggested to precede perception. From this "two-state model", it follows that threshold values should be lower for unconscious unspecific perception than for conscious specific perception. The aim of the present study was to test this suggested dichotomy. Proprioception was compared in unspecific detection trials (only the occurrence of a movement had to be detected, not its direction) and direction-specific detection trials (the occurrence of a movement of a specific direction had to be detected). Two types of specific detection trials and two types of unspecific detection trials were studied. Pairs of threshold values were determined, regarding amplitude detection using different angular velocities and regarding velocity detection using different angular displacements, for flexion and extension. Our results showed that, independent of each other, both threshold paradigms (amplitude detection and velocity detection) revealed the same perception characteristics. In specific detection paradigms, the proprioceptive thresholds were two times lower than in unspecific detection paradigms. Thus, movements of a particular type could be detected more easily than movement per se. The suggested "two-state model" might, therefore, not be appropriate in describing proprioceptive perception.

Adult↗
PubMed2005

Motion perception getting better with age?

Older people can discriminate visual motion of large, high-contrast stimuli better than young adults. This surprising result, reported by Betts et al. in this issue of Neuron, suggests weaker center-surround antagonism in senescence, perhaps attributable to age-related reduction in GABA-mediated inhibition.

Aged↗
PubMed1998

Interaction of vestibular, somatosensory and visual signals for postural control and motion perception under terrestrial and microgravity conditions--a conceptual model.

This article considers the intersensory interaction mechanisms and biomechanical aspects of human spatially oriented behavior and asks to what extent these are interrelated on earth by gravity and how they might be affected under microgravity. The interactions between vestibular, somatosensory and visual inputs for postural control are obscured by several complications (biomechanics, multi-body dynamics, multimodal feedback control, cognition etc.). However, they can be revealed in psychophysical studies on human self-motion perception. Based on such studies, we present a conceptual model, which we think is valid also for postural control. It accounts for the multi-segmental structure of the body, allowing local control of inter-segmental joints, but uses one global reference system for all segments, which is derived from the intersensory interactions. We hold that, at a sensory level, the system is tied together by linkages between vestibular, visual and somatosensory information which develop through experience of inertial and gravitational reaction forces. On earth these linkages are established even in the absence of active behavior by gravity, allowing the incorporation of one's body and its support into a notion (Gestalt) of ourselves in the environment. Under microgravity, in contrast, the linkages have to be actively established for postural and perceptual stability in the environment (e.g., by grasping a handle on the wall). From this work we recommend that future research under altered gravity conditions should be guided by models that include biomechanics, considerations of intersensory interaction and dynamic control mechanisms. Such an integrative conceptual framework will be helpful for reaching a general understanding of spatially oriented behavior.

Gravitation↗
PubMed1994

An analysis of the temporal integration mechanism in human motion perception.

We present a model for the temporal integration of apparent motion information. The model is constructed by considering psychophysical and neurophysiological data, and consists of the leaky integration of pulsatile motion detector responses to apparent motion stimuli. Each pulse represents a motion detector populational response to a discrete spatial displacement of the spatial pattern. Temporal contrast sensitivity determines the shape of constant-stimulus-duration threshold curves for image frame exposure durations less than about 133 msec. The shape of the threshold curve for image frame exposure durations greater than about 133 msec is determined by the leaky integrator time constant and the shape of the pulses emitted by the motion detectors. The leaky integrator model exhibits threshold saturation behaviour (the reaching of a maximum sensitivity or minimum threshold) seen in psychophysical data as well as dependence of saturation time on the frame rate of the apparent motion stimulus. A low frame rate results in a longer time-to-saturation because the leaky integrator discharges more between detector output pulses. When the motion detector output pulses are far enough apart there is effectively no temporal integration and therefore no threshold improvement over time. Finally, the behaviour of the psychophysical threshold curves across spatial displacement sizes is consistent with a populational-response threshold mechanism combined with spatial summation over a non-uniform distribution of detector types across the visual field.

Adaptation, Ocular↗
PubMed1994

Full-wave and half-wave rectification in second-order motion perception.

UNLABELLED: Microbalanced stimuli are dynamic displays which do not stimulate motion mechanisms that apply standard (Fourier-energy or autocorrelational) motion analysis directly to the visual signal. In order to extract motion information from microbalanced stimuli, Chubb and Sperling [(1988) Journal of the Optical Society of America, 5, 1986-2006] proposed that the human visual system performs a rectifying transformation on the visual signal prior to standard motion analysis. The current research employs two novel types of microbalanced stimuli: half-wave stimuli preserve motion information following half-wave rectification (with a threshold) but lose motion information following full-wave rectification; full-wave stimuli preserve motion information following full-wave rectification but lose motion information following half-wave rectification. Additionally, Fourier stimuli, ordinary square-wave gratings, were used to stimulate standard motion mechanisms. Psychometric functions (direction discrimination vs stimulus contrast) were obtained for each type of stimulus when presented alone, and when masked by each of the other stimuli (presented as moving masks and also as nonmoving, counterphase-flickering masks). RESULTS: given sufficient contrast, all three types of stimulus convey motion. However, only one-third of the population can perceive the motion of the half-wave stimulus. Observers are able to process the motion information contained in the Fourier stimulus slightly more efficiently than the information in the full-wave stimulus but are much less efficient in processing half-wave motion information. Moving masks are more effective than counterphase masks at hampering direction discrimination, indicating that some of the masking effect is interference between motion mechanisms, and some occurs at earlier stages. When either full-wave and Fourier or half-wave and Fourier gratings are presented simultaneously, there is a wide range of relative contrasts within which the motion directions of both gratings are easily determinable. Conversely, when half-wave and full-wave gratings are combined, the direction of only one of these gratings can be determined with high accuracy. CONCLUSIONS: the results indicate that three motion computations are carried out, any two in parallel: one standard ("first order") and two non-Fourier ("second-order") computations that employ full-wave and half-wave rectification.

Contrast Sensitivity↗
PubMed1996

Global-motion perception: interaction of chromatic and luminance signals.

A global dot-motion stimulus was employed in order to investigate the interaction between luminance and chromatic signals in motion processing. Thresholds are determined by measuring the minimum number of dots which need to move in a coherent fashion in a field of randomly moving dots in order for the observers to be able to determine the direction of coherent motion. We found that: (1) observers could not track an achromatic signal-dot which changes its luminance polarity between frame transitions. The addition of a consistent chromatic signal allowed observers to track such a dot when the dot contained low- (8%) luminance contrast but this ability was impaired as the luminance contrast was increased; (2) the addition of chromatic contrast to a dot which contained consistent low-luminance contrast could result in threshold elevation. For fixed contrast chromatic and luminance signals, the presence and degree of threshold elevation depended upon the spatiotemporal properties of the dot motion; (3) the ability of observers to extract a global-motion signal carried by a group of dots of one colour was impaired by the addition of a number of additional-noise dots of a different colour. These results are interpreted as indicating that: (1) the motion-selective cells that are sensitive to chromatic signals are also sensitive to luminance signals; (2) the combined chromatic and luminance and purely luminance motion cells are pooled to form a single pathway prior to global-motion extraction; and (3) the negative interaction observed between the chromatic and luminance signals is likely to be due to the differences in the processing speeds of the combined luminance and chromatic and the purely luminance sensitive motion cells.

Color↗
PubMed2003

Centric-minded templates for self-motion perception.

We propose a two-layer neuromorphic architecture by which motion field pattern, generated during locomotion, are processed by template detectors specialized for gaze-directed self-motion (expansion and rotation). The templates provide a gaze-centered computation for analyzing motion field in terms of how it is related to the fixation point (i.e., the fovea). The analysis is performed by relating the vectorial components of the act of motion to variations (i.e., asymmetries) of the local structure of the motion field. Notwithstanding their limited extension in space, such centric-minded templates extract, as a whole, global information from the input flow field, being sensitive to different local instances of the same global property of the vector field with respect to the fixation point; a quantitative analysis, in terms of vectorial operators, evidences this property as tuning curves for heading direction. Model performances, evaluated in several situations characterized by conditions of absence and presence of pursuit eye movements, validate the approach. We observe that the gaze-centered model provides an explicit testable hypothesis that can guide further explorations of visual motion processing in extrastriate cortical areas.

Computational Biology↗
PubMed1994

Spatial displacement limits for cyclopean (stereoscopic) apparent-motion perception.

The range of spatial displacements over which cyclopean (stereoscopic) apparent motion is perceived was investigated. The cyclopean stimuli were created from retinal disparity embedded in dynamic random-dot stereograms. In one experiment, the displacement range for crossed-disparity cyclopean motion was compared with that for luminance-domain motion. The results showed that cyclopean motion was perceived over spatial displacements that were about two to three times larger than the displacements over which luminance motion was perceived. In a second experiment, the displacement range for crossed-disparity cyclopean motion was compared with that for uncrossed-disparity cyclopean motion. The results revealed that the displacement range was restricted (motion quality was poor) for uncrossed motion relative to crossed motion. It is inferred that cyclopean motion from crossed disparity is represented at a coarse spatial scale, relative to luminance motion, and that cyclopean motion from uncrossed disparity is suppressed due to occlusion cues present when uncrossed stimuli are seen behind a textured background.

Female↗
PubMedDate not supplied

Transient and permanent deficits in motion perception after lesions of cortical areas MT and MST in the macaque monkey.

We examined the nature and the selectivity of the motion deficits produced by lesions of extrastriate areas MT and MST. Lesions were made by injecting ibotenic acid into the representation of the left visual field in two macaque monkeys. The monkeys discriminated two stimuli that differed either in stimulus direction or orientation. Direction and orientation discrimination were assessed by measuring thresholds with gratings and random-dots placed in the intact or lesioned visual fields. At the start of behavioral testing, we found pronounced, motion-specific deficits in thresholds for all types of moving stimuli, including pronounced elevations in contrast thresholds and in signal-to-noise thresholds measured with moving gratings, as well as deficits in direction range thresholds and motion coherence measured with random-dot stimuli. In addition, the accuracy of direction discrimination was reduced at smaller spatial displacements (i.e. step sizes), suggesting an increase in spatial scale of the residual directional mechanism. Subsequent improvements in thresholds were seen with all motion stimuli, as behavioral training progressed, and these improvements occurred only with extensive behavioral testing in the lesioned visual field. These improvements were particularly pronounced for stimuli not masked by noise. On the other hand, deficits in the ability to extract motion from noisy stimuli and in the accuracy of direction discrimination persisted despite extensive behavioral training. These results demonstrate the importance of areas MT and MST for the perception of motion direction, particularly in the presence of noise. In addition, they provide evidence for the importance of behavioral training for functional recovery after cortical lesions. The data also strongly support the idea of functional specialization of areas MT and MST for motion processing.

Animals↗
PubMed1996

Cyclopean motion perception produced by oscillations of size, disparity and location.

UNLABELLED: For cyclopean and monocularly-visible targets we measured psychophysical thresholds for perceptions produced by the following three stimuli: oscillations of disparity (DO), oscillations of size (SO) and oscillatory motion within the frontoparallel plane (FPO). RESULTS: thresholds for motion in depth perception produced by DO were similar for cyclopean and non-cyclopean targets over the entire 0.25-8 Hz frequency range investigated. Thresholds for perceiving motion in depth produced by SO were considerably (up to 2.5 times) higher for cyclopean targets than for monocularly-visible targets, as were thresholds for perceiving size oscillations. For both cyclopean and monocularly-visible target the perception of motion in depth could be canceled by pitting DO vs SO. Thresholds for perceiving FPO were similar to DO thresholds for the monocularly-visible target, but for the cyclopean targets FPO thresholds rose more steeply than DO thresholds for oscillation frequencies above 1 Hz. CONCLUSIONS: (1) for our subjects, the effective binocular stimulus for motion in depth perception is a rate of change of disparity; an interocular velocity difference is significant only to the extent that it produces a rate of change of disparity. (2) The sensations of motion in depth produced by DO and SO are qualitatively identical. (3) Neural signals produced by DO and SO converge onto a single neural mechanism that signals motion in depth.

Adaptation, Ocular↗
PubMed2005

Deficient motion perception in the fellow eye of amblyopic children.

The extent of motion processing deficits and M/dorsal pathway involvement in amblyopia is unclear. Fellow eye performance was assessed in amblyopic children for motion-defined (MD) form, global motion, and maximum displacement (Dmax) tasks. Group performance on MD form was significantly worse in amblyopic children than in control children. Global motion deficits were significantly related to residual binocular function. Abnormally elevated Dmax thresholds were most prevalent in children with anisometropia. Our findings from these three uncorrelated tasks implicate involvement of binocular motion-sensitive mechanisms in the neural deficits of amblyopic children with strabismic, anisometropic, and aniso-strabismic etiologies.

Amblyopia↗