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At least 307 records · Page 17Linked to original sources
PubMed2006

Motion illusion activates the visual motion area of the brain: a near-infrared spectroscopy (NIRS) study.

Near-infrared spectroscopy (NIRS) enables noninvasive measurement of concentration changes of oxy- and deoxy-hemoglobin. The present study investigated cerebral representations of motion illusion by NIRS and examined several experimental procedures to determine an efficient procedure that can shorten the experimental time. We compared hemodynamic responses to figures with and without motion illusion. The number of repetitions of the tasks in the experiments and other factors were also examined. Results showed significant responses around area MT/V5 to the motion illusion from the analyses of three cycles (blocks) of presentation of illusion induction stimulus. These findings indicate that motion illusion can be detected by NIRS, and we propose a concise and efficient procedure for NIRS.

Adult↗
PubMed2005

Perceptual manifestations of fast neural plasticity: motion priming, rapid motion aftereffect and perceptual sensitization.

Visual neurons show fast adaptive behavior in response to brief visual input. However, the perceptual consequences of this rapid neural adaptation are less known. Here, we show that brief exposure to a moving adaptation stimulus-ranging from tens to hundreds of milliseconds-influences the perception of a subsequently presented ambiguous motion test stimulus. Whether the ambiguous motion is perceived to move in the same direction (priming), or in the opposite direction (rapid motion aftereffect) varies systematically with the duration of the adaptation stimulus and the adaptation-test blank interval. These biases appear and decay rapidly. Moreover, when the adapting stimulus is itself ambiguous, these effects are not produced. Instead, the percept for the subsequent test stimulus is biased to the perceived direction of the adaptation stimulus. This effect (perceptual sensitization) builds gradually over the time between the adaptation and test stimuli. Our results indicate that rapid adaptation plays a role mainly within early motion processing, whereas a slow potentiation controls the sensitivity at a later stage.

Adaptation, Physiological↗
PubMed2005

Motion-onset VEPs reflect long maturation and early aging of visual motion-processing system.

Pattern-reversal and motion-onset visual evoked potentials (VEPs) were simultaneously tested in a group of 70 healthy subjects between the ages of 6-60 years to verify suspected differences in maturation and aging dynamics of the pattern and motion processing subsystems of the visual pathway. The motion-onset VEPs displayed dramatic configuration development and shortening of latencies up to 18 years of age (correl. coeff. -0.85; p < 0.001) and systematic prolongation from about 20 years of age (correl. coeff. 0.70; p < 0.001). This confirms long-lasting maturation of the magnocellular system and/or motion processing cortex and their early age related changes. Less significant changes of pattern-reversal VEPs in the tested age range can be interpreted as a sign of early maturation of the parvocellular system and its enhanced functional endurance in the elderly.

Adolescent↗
PubMed1997

Visual motion perception after brain damage: II. Deficits in form-from-motion perception.

We investigated form-from-motion perception (FFM perception) in a sample of 39 patients with acquired brain damage. Pronounced FFM deficits were found in two patients (FM1 and FM2) with biparietal lesions. Both patients were able to identify the relevant figure, when it was not embedded in obstructive texture. Moreover, they could localize the figures in the FFM condition, although they could not reliably identify them. The two patients had normal motion coherence thresholds. Their performance in a static figure-ground task did not differ from that of other patients. These findings imply that the FFM deficits are not caused by impairment of basic visual motion or form perception but are the consequence of damage to a parietal brain structure involved in the combined analysis of visual motion and form information. The nature and functional role of this brain structure as well as the implications of our results for models of FFM perception are discussed.

Adult↗
PubMed2001

Directional tuning of human motion adaptation as reflected by the motion VEP.

Motion onset evoked visual potentials are dominated by a negativity (N2) at occipital electrodes and a positivity (P2) at the vertex. The degree of true motion processing reflected by N2 and P2 was estimated from the direction specificity of motion adaptation. Adapting stimuli moved to the right and test stimuli (random dot patterns of 26 degrees diameter; 10% contrast; 10.5 degrees /s velocity) moved in one of eight directions, which differed by 45 degrees. VEPs were recorded from occipito/temporal and central sites in eight subjects. Two adaptation effects were observed for N2 (P<0.01): a global amplitude reduction by 47% and a direction-specific reduction by a further 28%. For P2, only the global effect (54%; P<0.01) was observed. The global adaptation effect could also be induced by pattern reversal and pattern-onset adaptation, i.e. stimuli containing ambiguous or very little motion energy, respectively. We conclude that at least 28% of the N2 amplitude reflects the activity of direction-specific elements, whereas P2 does not at all.

Adaptation, Physiological↗
PubMed2003

Motion coherence thresholds in infants--different tasks identify at least two distinct motion systems.

Optokinetic nystagmus (OKN) can be demonstrated from birth, but behavioural discrimination tasks such as habituation and preferential looking do not reveal any sensitivity to motion direction until a few weeks of age. This study compared coherence threshold for motion direction for OKN and preferential looking responses using closely comparable stimuli, in infants between 6 and 27 weeks of age. Infants were tested with two random dot motion displays, a uniform area of moving dots for OKN responses and a display in which a region was segmented on one side by differential motion direction for preferential looking responses. Coherence thresholds for each response were determined by a staircase method. For OKN responses, mean coherence thresholds were between 20% and 25%, with no significant improvement in OKN performance throughout the age range. Preferential looking thresholds were significantly higher than OKN thresholds. Preferential looking thresholds improved significantly with age, but remained higher than OKN thresholds throughout the age range tested. Experiments varying direction reversal frequency and stimulus area indicated that these differences were not simply a consequence of the spatial and temporal non-uniformity of the preferential looking stimulus. The differences in sensitivity levels and age trends for OKN and preferential looking responses we have found suggest that different directional mechanisms are involved in the two responses. We discuss the possibility that, in early infancy, OKN and preferential looking reflect the performance of subcortical and cortical directional mechanisms respectively.

Adult↗
PubMed1997

Motion-reversal reveals two motion mechanisms functioning in scotopic vision.

We studied scotopic motion mechanisms, using a two-frame sinusoidal grating separated by various ISIs equated for mean luminance level. Perceived direction of displacement varied with both ISI and luminance. As luminance decreased, apparent motion reversal disappeared. This is predicted by a first-order motion model if the underlying temporal impulse response function varies from biphasic under photopic conditions to monophasic under scotopic conditions. Performance at long (but not short) ISIs depends upon stimulus contrast, suggesting there is also a scotopic feature-tracking mechanism. With isoluminant and high spatial frequency gratings, where the temporal impulse response function is monophasic, no motion reversal was observed.

Adaptation, Ocular↗
PubMed1997

Motion blur and motion sharpening in the human visual system.

The effect of motion sharpening upon blur discrimination thresholds was examined for a range of speeds and blur widths. Blur discrimination thresholds were measured for drifting edges whose blur was either physically or perceptually constant. Under conditions where edges were kept at a constant physical blur width, discrimination thresholds rose as a function of speed as previously reported. However, when the perceived blur of edges was held constant, discrimination performance was more-or-less constant for speeds up to at least 6.3 deg sec-1. The results indicate that the deterioration of blur discrimination performance with speed may be due to motion sharpening and not motion blur as has previously been suggested. The results are discussed in terms of a scheme whereby a non-linearity in motion processing serves to sharpen moving edges, whilst the finite integration time of the system tends to smear them.

Discrimination, Psychological↗
PubMed1999

Oscillatory motion but not pattern reversal elicits monocular motion VEP biases in infantile esotropia.

Patients with early disruptions of binocularity show cortical directional asymmetries in their steady state monocular VEP response to oscillatory motion. The VEP directional asymmetry is characterized by significant first harmonic components that show a 180 degrees difference in the response phase between the two eyes. By contrast, the normal response is dominated by even-order response harmonics, although some normal observers also have measurable responses at the first harmonic. Experiments and simulations were conducted to determine if the first harmonic in patients could reasonably be attributed to direction selective mechanisms. A secondary goal was to determine whether the first harmonic response of normals was also due to imbalances in direction selective mechanisms. Monocular steady state VEPs were elicited by oscillating 3 c/deg gratings presented at 6 and 10 Hz in normal observers and observers with infantile esotropia. Responses were also obtained to phase-reversing gratings of the same spatial and temporal frequencies. Phase reversal eliminated the majority of first harmonic responses which were recorded for normal observers to oscillatory motion. However, phase reversal did not elicit the cortical motion asymmetry in infantile esotropia. Modeling results suggest that the first harmonic response to oscillatory motion arises due to non-linearities in both direction selective and non-direction-selective mechanisms, with the latter being dominant in patients with early onset strabismus.

Adolescent↗
PubMed1999

Enhanced motion aftereffect for complex motions.

We measured the magnitude of the motion after effect (MAE) elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. MAE magnitude was estimated by three methods: the duration of the MAE; the contrast required to null the MAE and the threshold elevation for detecting an abrupt jump. All three techniques showed that MAEs for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions.

Adaptation, Ocular↗
PubMed2003

Separate neural pathways for contour and biological-motion cues in motion-defined animal shapes.

To determine whether contour and biological motion (BM) cues for motion-defined shapes are subserved by two separate mechanisms, we used PET to measure regional cerebral blood flow in nine human subjects. Subjects were scanned in the following four conditions: (1) contour-defined animals with natural movements (running), (2) motion-defined animals in which the contours were removed and dots were placed on the limbs and moving parts (BM; running); (3) drifting static animal shapes (contours); and (4) drifting dots. The results demonstrate that the perception of BM involves the superior frontal gyrus, the precuneus, the fusiform, the lingual and the medial temporal gyri, the inferior parietal lobe, the hippocampal and parahippocampal areas, and the cerebellum. In addition, the anterior cingulate cortex and the amygdala were significantly activated. The perception of contour-defined shapes produced significant elevation of rCBF in some areas similar to the BM condition, namely the fusiform, medial occipital, medial temporal, and lingual gyri. Only the occipital pole and the inferior temporal cortex were specifically activated by contour-defined shapes. These results are congruent with previous findings that the dorsal visual pathway is important for the perception of BM. They further support psychophysical results showing that contour and BM cues for motion-defined animal shapes are processed by independent channels.

Adult↗
PubMed1997

Just-noticeable difference in the speed of cyclopean motion in depth and the speed of cyclopean motion within a frontoparallel plane.

Weber fractions for discriminating the speed and displacement of a cyclopean target moving in depth ranged, respectively, from .07-.17 and .06-.13 over 6 observers. Corresponding data for a noncyclopean target were .07-.20 and .06-.12. For motion parallel to the frontal plane, corresponding data were .09-.20 .06-.16, and .05-.13. All Weber fractions were independent of the direction of motion and of near versus far disparity. All observers based their judgments entirely on the task-relevant variable and ignored task-irrelevant variables in all cases. We conclude that speed and displacement are encoded independently and in parallel for motion in depth and for motion within a frontoparallel plane.

Adult↗
PubMed1993

Apparent motion perception: the contribution of the binocular and monocular systems. An improved test based on motion aftereffects.

Research concerning the perception of apparent motion is not easy to conduct: it is hard to obtain quantitative results that can be easily interpreted. A solution to this problem is the use of motion aftereffects (MAEs). Adapting subjects to a specific type of motion leads to apparent motion in the opposite direction when the stimulus is removed. However, subjects are aware of the change in stimulus conditions. A new dynamic test stimulus is proposed in order to avoid artefacts introduced by the awareness of the conditions by the subject. A model, derived from earlier observations, is described which includes contributions from monocular and binocular systems. Results from an experiment in which the dynamic test stimulus was used show that they do not necessarily reproduce the results obtained with a static test stimulus. Central monocular systems are added to the model to account for this discrepancy. The 'pooling hypothesis', which states that the MAE is a weighted mean of the processes involved, permits the estimation of the weights of the individual subsystems. The results of the experiments are explained in terms of this hypothesis by the new model.

Adaptation, Ocular↗
PubMed2006

Does elastic tissue intrafraction motion with density changes forbid motion-compensated radiotherapy?

Intrafraction organ motion disturbs the otherwise highly conformal dose distributions planned for conformal radiotherapy and intensity-modulated radiotherapy. If the organ motions are rigid-body translations and preserve voxel density, techniques to make the beam follow the target by synchronous breathing can be arranged. Several potential difficulties disturb this oversimple statement and one of these is addressed in this paper. The effects of general elastic motion without preserving density are investigated. The effects are complex. Dose-space voxels disconnect from the planned irradiation bixel; voxels in a set aligned with a particular bixel cease to be so aligned even with a different bixel on elastic motion. Furthermore the density changes to conserve mass and this further perturbs the dose distribution. These effects are demonstrated here via simple examples but it may be impossible to make 'beam-breathing' strategies to compensate for them. Consequently understanding (and possibly ignoring) these second-order effects may be all that is possible.

Algorithms↗
PubMed2006

Motion compensation associated with single-level cervical fusion: where does the lost motion go?

STUDY DESIGN: Seven adult human cadaveric cervical spines (C2-T1) were biomechanically tested in a programmable testing device. OBJECTIVE: Compare the effects of incremental single-level fusion at different levels of the cervical spine. SUMMARY OF BACKGROUND DATA: Clinical studies have reported degenerative symptomatic disc disease at disc levels adjacent to fusion. No known study has attempted to delineate the effects of single-level fusion at different levels of the cervical spine. METHODS: The spines were tested in flexion, extension, right and left lateral bending, and right and left axial rotation for 7 different conditions: harvested and 6 independent single-level fused conditions (i.e., C2-C3, C3-C4, C4-C5, C5-C6, C6-C7, and C7-T1). Segmental motion and global stiffness data were normalized to the harvested condition and compared using a 1-way analysis of variance followed by a SNK test (P < 0.01). RESULTS: Motion compensation was distributed among the unfused segments with significant compensation at the segments adjacent to fusion. Significant increases occurred at the level above C3-C4 and C4-C5 fusions, and below for C5-C6 and C6-C7 fusions in both flexion and extension. CONCLUSIONS: Increase motion compensation occurred at segments immediately adjacent to a single-level fusion. Significant differences occurred at the level above the fusion site for the C3-C4 and C4-C5 fusion in both flexion and extension. When the lower levels (C5-C6, C6-C7) were fused, a significant amount of increased motion was observed at the levels immediately above and below the fusion. However, greater compensation occurred at the inferior segments than the superior segments for the lower level fusions (C5-C6, C6-C7).

Aged↗
PubMed1996

Impairment of the perception of second order motion but not first order motion in a patient with unilateral focal brain damage.

Unlike first order motion, which is based on spatiotemporal variations in luminance, second-order motion relies on spatiotemporal variation of attributes derived from luminance, such as contrast. Here we show that a patient with a small unilateral cortical lesion adjacent to human cortical area MT (V5) has an apparently permanent disorder in perceiving several forms of second-order but not first-order motion in his contralateral visual field. This result indicates that separate pathways for motion perception exist, either as divergent pathways from area MT or even from primary visual cortex, or as separate pathways from subcortical areas to extrastriate visual areas.

Adult↗
PubMed2002

Seeing blur: 'motion sharpening' without motion.

It is widely supposed that things tend to look blurred when they are moving fast. Previous work has shown that this is true for sharp edges but, paradoxically, blurred edges look sharper when they are moving than when stationary. This is 'motion sharpening'. We show that blurred edges also look up to 50% sharper when they are presented briefly (8-24 ms) than at longer durations (100-500 ms) without motion. This argues strongly against high-level models of sharpening based specifically on compensation for motion blur. It also argues against a recent, low-level, linear filter model that requires motion to produce sharpening. No linear filter model can explain our finding that sharpening was similar for sinusoidal and non-sinusoidal gratings, since linear filters can never distort sine waves. We also conclude that the idea of a 'default' assumption of sharpness is not supported by experimental evidence. A possible source of sharpening is a nonlinearity in the contrast response of early visual mechanisms to fast or transient temporal changes, perhaps based on the magnocellular (M-cell) pathway. Our finding that sharpening is not diminished at low contrast sets strong constraints on the nature of the nonlinearity.

Humans↗
PubMed2003

Impulsive-motion model for computing the closing motion of mechanical heart-valve leaflets.

The speed of mechanical heart-valve leaflets is known to be an important quantity for predicting cavitation, yet no simple computational means exists for predicting the leaflet speed. In this study, a model for simulating the motion of heart-valve leaflets in rigid test systems is presented. The input for the simulations is the ventricular pressure trace, readily measured in heart-valve tests. The model is based upon an impulsive-motion approximation, wherein the motion within the system is produced by rapid acceleration at the boundary, e.g., by a moving piston. A set of quasisteady, linear equations for the pressure field that are decoupled from the leaflet equation of motion is derived. The pressure field and leaflet moment are computed without the need to treat moving boundaries. Model predictions of closing time compared favorably with those measured in a 1994 cavitation study. Computed values of leaflet tip speed were also compared with those of a previous study, at the same value of average pressure slope. The model values were in agreement with measured speeds, given the limitations of using the average pressure slope as a metric for comparison.

Bioprosthesis↗