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PubMed2005

Seeing motion in depth using inter-ocular velocity differences.

An object moving in depth produces retinal images that change in position over time by different amounts in the two eyes. This allows stereoscopic perception of motion in depth to be based on either one or both of two different visual signals: inter-ocular velocity differences, and binocular disparity change over time. Disparity change over time can produce the perception of motion in depth. However, demonstrating the same for inter-ocular velocity differences has proved elusive because of the difficulty of isolating this cue from disparity change (the inverse can easily be done). No physiological data are available, and existing psychophysical data are inconclusive as to whether inter-ocular velocity differences are used in primate vision. Here, we use motion adaptation to assess the contribution of inter-ocular velocity differences to the perception of motion in depth. If inter-ocular velocity differences contribute to motion in depth, we would expect that discriminability of direction of motion in depth should be improved after adaptation to frontoparallel motion. This is because an inter-ocular velocity difference is a comparison between two monocular frontoparallel motion signals, and because frontoparallel speed discrimination improves after motion adaptation. We show that adapting to frontoparallel motion does improve both frontoparallel speed discrimination and motion-in-depth direction discrimination. No improvement would be expected if only disparity change over time contributes to motion in depth. Furthermore, we found that frontoparallel motion adaptation diminishes discrimination of both speed and direction of motion in depth in dynamic random dot stereograms, in which changing disparity is the only cue available. The results provide strong evidence that inter-ocular velocity differences contribute to the perception of motion in depth and thus that the human visual system contains mechanisms for detecting differences in velocity between the two eyes' retinal images.

Adaptation, Physiological↗
PubMed2001

Cortical representation of acoustic motion in the rufous horseshoe bat, Rhinolophus rouxi.

Responses of neurons to apparent auditory motion in the azimuth were recorded in three different fields of auditory cortex of the rufous horseshoe bat. Motion was simulated using successive stimuli with dynamically changing interaural intensity differences presented via earphones. Seventy-one percent of sampled neurons were motion-direction-sensitive. Two types of responses could be distinguished. Thirty-four percent of neurons showed a directional preference exhibiting stronger responses to one direction of motion. Fifty-seven percent of neurons responded with a shift of spatial receptive field position depending on direction of motion. Both effects could occur in the same neuron depending on the parameters of apparent motion. Most neurons with contralateral receptive fields exhibited directional preference only with motion entering the receptive field from the opposite direction. Receptive field shifts were opposite to the direction of motion. Specific combinations of spatiotemporal parameters determined the motion-direction-sensitive responses. Velocity was not encoded as a specific parameter. Temporal parameters of motion and azimuth position of the moving sound source were differentially encoded by neurons in different fields of auditory cortex. Neurons with a directional preference in the dorsal fields can encode motion with short interpulse intervals, whereas direction-preferring neurons in the primary field can best encode motion with medium interpulse intervals. Furthermore, neurons with a directional preference in the dorsal fields are specialized for encoding motion in the midfield of azimuth, whereas direction-preferring neurons in the primary field can encode motion in lateral positions. The results suggest that motion information is differentially processed in different fields of the auditory cortex of the rufous horseshoe bat.

Acoustic Stimulation↗
PubMed2003

Direction-selective motion blindness after unilateral posterior brain damage.

Motion blindness (MB) is defined as the selective disturbance of visual motion perception despite intact perception of other features of the visual scene. MB is characterized by a pandirectional deficit of motion direction discrimination and is assumed to result from damage to the visual motion pathway, especially area MT/V5. However, the most characteristic feature of primate MT/V5 neurons is not their motion selectivity but their preference for one direction of motion (direction selectivity), which changes incrementally at neighbouring columns. In addition to this microscopic directional organization, studies in nonhuman and human primates suggest that single directions of motion are also coded at a more macroscopic level. We thus hypothesized that if MB in humans results from damage to direction-selective neurons in the visual motion pathway, posterior brain damage might cause MB which is direction selective, not pandirectional. The present study investigated motion direction discrimination in patients with posterior unilateral brain damage and determined separate psychophysical thresholds for the four cardinal directions. In addition, we analysed whether the direction of erroneous motion perception (i.e. the perception of right motion for upward motion) was random or showed a directional bias. We report three principal findings. First, motion direction discrimination was severely impaired in one or two directions while it was normal in the other directions. This constituted direction-selective MB. Second, MB was characterized not only by a quantitative direction-selective increase in psychophysical thresholds but also by a qualitative impairment of perceiving motion direction systematically in wrong directions. Both findings suggest that the cortical modules specialized for the perception of a single direction of motion might be larger than previously thought. Third, lesion analysis showed that unilateral damage, not only the human homologue of MT/V5 but also to parieto-occipital cortex, leads to MB.

Adult↗
PubMed1979

Induced motion as a function of the speed of the inducing object, measured by means of two methods.

Induced motion is illustrated by the apparent motion imparted to a stationary disc by the horizontal motion of an enclosing frame. The present study examined the effect of frame speed, with a constant extent of frame motion, on the magnitude of induced motion for average frame speeds varying from 0.17 to 2.85 deg s-1. The induced motion was measured by two methods. (i) The observer adjusted the horizontal distance between comparison posts to indicate the extent of the horizontal induced motion in a physically stationary disc. (ii) The observer adjusted the slant of a comparison rod to indicate the apparent path of motion of the disc physically moving straight up and down in phase with the horizontal motion of the surrounding frame. The latter method requires the observer to integrate two apparent components of motion of the disc, one from its physical vertical motion and the other from its induced horizontal motion. The results from both methods show that substantial amounts of induction occurred at all of the frame speeds. The vector addition of apparent motions from real and from induced motion obtained from the second method, and the substantial amounts of induction found with the high frame speeds are interpreted to indicate that the processes underlying the perception of real and induced motion are essentially the same.

Adult↗
PubMed1982

Spatial induction of illusory motion.

Induced motion is not limited to continuous motions presented on a frontoparallel plane. Experiments were conducted to investigate several varieties of induced motion to which theories of induced motion must apply. The observer indicated the perceived path of motion of a vertically moving test point to which induced motion at right angles to the physical motion was added by the motion of two inducing points. In experiment 1 all motions (both apparently and physically) were in a frontoparallel plane. It was found that discrete displacement as well as continuous motion of the test and inducing points produced substantial amounts of induction. In experiment 2 the inducing points were continuously moved in stereoscopic distance rather than remaining in an apparent frontoparallel plane. A large amount of apparent motion in depth was found in the vertically moving test point and was interpreted as an induced motion in depth. In experiment 3 an alternative interpretation of the phenomenon of experiment 2, in terms of an apparent vergence for the two images of the test point, was investigated and found to be unlikely. In experiment 4, with all the points moving continuously in a frontoparallel plane, eye motions as well as induced motions were measured, with the observer fixating either the test point or an inducing point. Substantial amounts of induction were obtained under both conditions of fixation. The consequences of these findings for theories of induced motion are discussed.

Adult↗
PubMed1999

Perceptual organization of apparent motion in the Ternus display.

A typical Ternus display has three sequentially presented frames, in which frame 1 consists of three motion tokens, frame 2 (blank) defines the interstimulus interval, and frame 3 has similar motion tokens with their relative positions shifted to the right. Interestingly, what appears to be a seemingly simple arrangement of stimuli can induce one of two distinct apparent-motion percepts in the observer. The first is an element-motion perception where the left-end token is seen to jump over its two neighboring tokens (inner tokens) to the right end of the display. The second is a group-motion perception where the entire display of the three tokens is seen to move to the right. How does the visual system choose between these two apparent-motion perceptions? It is hypothesized that the choice of motion perception is determined in part by the perceptual organization of the motion tokens. Specifically, a group-motion perception is experienced when a strong grouping tendency exists among the motion tokens belonging to the same frame. Conversely, an element-motion perception is experienced when a strong grouping tendency exists between the inner motion tokens in frames 1 and 3 (i.e. the two tokens that overlap in space between frames). We tested this hypothesis by varying the perceptual organization of the motion tokens. Both spatial (form similarity, 3-D proximity, common surface/common region, and occlusion) and temporal (motion priming) factors of perceptual organization were tested. We found that the apparent-motion perception of the Ternus display can be predictably affected, in a manner consistent with the perceptual organization hypothesis.

Analysis of Variance↗
PubMed2003

Visual motion illusions, eye movements, and the search for objectivity.

The initial interpretations of motion phenomena are often in terms of eye movements. It reflects a desire to link an objective expression of motion (of the eyes) with a subjective experience of motion. This approach is examined historically with respect to induced motion, motion aftereffects, visual vertigo, autokinetic effects and stroboscopic motion. All but one of these phenomena were initially observed in the natural environment and they have subsequently been studied under laboratory conditions. Eye movements are not correlated with induced motion, the motion aftereffect, or stroboscopic motion, but they do correspond to the direction and extent of visual vertigo. The extent of apparent visual motion in the autokinetic effect far exceeds that of involuntary eye movements but there might be a weak relation between them. If eye movements are to provide a reasonable account of motion illusions then they should apply to all the contours that are visible. This does apply to visual vertigo, but not to induced motion, the motion aftereffect and stroboscopic motion, all of which involve relative motion in a visible display. The autokinetic phenomenon involves the perception of isolated stimuli, and so interpretations in terms of some internal eye movement signals remain sustainable.

Eye Movements↗
PubMed2002

Constraints on the source of short-term motion adaptation in macaque area MT. II. tuning of neural circuit mechanisms.

Neurons in area MT, a motion-sensitive area of extrastriate cortex, respond to a step of target velocity with a transient-sustained firing pattern. The transition from a high initial firing rate to a lower sustained rate occurs over a time course of 20-80 ms and is considered a form of short-term adaptation. In the present paper, we compared the tuning of the adaptation to the neuron's tuning to direction and speed. The tuning of adaptation was measured with a condition/test paradigm in which a testing motion of the preferred direction and speed of the neuron under study was preceded by a conditioning motion: the direction and speed of the conditioning motion were varied systematically. The response to the test motion depended strongly on the direction of the conditioning motion. It was suppressed in almost all neurons by conditioning motion in the same direction and could be either suppressed or enhanced by conditioning motion in the opposite direction. Even in neurons that showed suppression for target motion in the nonpreferred direction, the adaptation and response direction tuning were the same. The speed tuning of adaptation was linked much less tightly to the speed tuning of the response of the neuron under study. For just more than 50% of neurons, the preferred speed of adaptation was more than 1 log unit different from the preferred response speed. Many neurons responded best when slow motions were followed by faster motions (acceleration) or vice versa (deceleration), suggesting that MT neurons may encode information about the change of target velocity over time. Finally, adaptation by conditioning motions of different directions, but not different speeds, altered the latency of the response to the test motion. The adaptation of latency recovered with shorter intervals between the conditioning and test motions than did the adaptation of response size, suggesting that latency and amplitude adaptation are mediated by separate mechanisms. Taken together with the companion paper, our data suggest that short-term motion adaptation in MT is a consequence of the neural circuit in MT and is not mediated by either input-specific mechanisms or intrinsic mechanisms related to the spiking of individual neurons. The circuit responsible for adaptation is tuned for both speed and direction and has the same direction tuning as the circuit responsible for the initial response of MT neurons.

Acceleration↗
PubMedDate not supplied

Intersegmental sagittal motion in the lower cervical spine and discogenic spondylosis: a preliminary study.

OBJECTIVE: The object of this investigation was to identify any correlation between discogenic spondylosis and the type of motion (normal, hypomobility, hypermobility, paradoxical motion) found in the sagittal plane of the intervertebral motion units of the lower cervical spine. DESIGN AND SETTING: A case control study was performed from the files of 100 patients (ages 15-73) with cervical spine-related symptomatology at the Anglo-European College of Chiropractic Clinic. PATIENTS: The cases were randomly selected from a cohort of patients with normal radiographic anatomy who attended the clinic from 1987-1990 and were known to have cervical spine neutral, flexion and extension lateral radiographs taken. MAIN OUTCOME MEASURES: Extended chi 2 was used to test the observed data. RESULTS: The findings from both the flexion and extension films suggested that intervertebral motion units with and without varying severities of discogenic spondylosis did differ with respect to the type of motion exhibited there (flexion: chi 2 = 39.399, p < .001; extension: chi 2 = 45.7424, p < .001). Intervertebral motion units which had discogenic spondylosis had a greater likelihood of exhibiting motion abnormalities (flexion: chi 2 = 5.665, p < .01; extension: chi 2 = 6.178, p < .01), and all types of motion seemed to be dependent on its severity (flexion: chi 2 = 16.464, p < .01; extension: chi 2 = 15.954, p < .02). In general, normal motion occurred approximately 60% of the time when there was absent or mild discogenic spondylosis and decreased precipitously as moderate and severe amounts of discogenic spondylosis appeared. In global cervical flexion, when there was either little or no discogenic spondylosis and abnormal motion was present, intersegmental hypermobility was predominant. Hypomobility became predominant overall as moderate and severe discogenic spondylosis was found. In global cervical extension, for all severities of discogenic spondylosis when there was abnormal motion, intersegmental hypomobility was predominant. Also of note was the presence of paradoxical motion, which occurred in 11% of the intervertebral motion units without discogenic spondylosis [usually at the C7-T1 intervertebral motion unit (86%)]. CONCLUSIONS: From the data it can be concluded that there are trends which occur with differing amounts of discogenic spondylosis when considering intersegmental cervical sagittal motion. However, additional detailed study is required to corroborate the findings and determine what their clinical significance is.

Adolescent↗
PubMed1992

Visual processing of motion-defined form: selective failure in patients with parietotemporal lesions.

The following psychophysical data were obtained from 13 patients with unilateral cerebral hemispheric lesions and 20 control subjects: speed thresholds for detecting and for recognizing motion-defined letters, speed thresholds for detecting coherent motion and for discriminating its direction, and visual acuity for recognizing letters of 96% and 11% contrast. Acuity was between 6/6 and 6/3 for all patients. Four patients showed a selective loss of ability to recognize motion-defined letters, while the ability to detect those same letters was spared, as was the ability to detect coherent motion and discriminate its direction (type I loss). Three patients showed a loss of ability both to recognize and to detect motion-defined letters, while the ability to detect coherent motion and discriminate its direction was spared (type II loss). All seven patients who failed to recognize motion-defined letters had extensive lesions in parietotemporal white matter underlying Brodmann cortical areas 18, 19, 37, 39, 21, and 22. The lesion was in the left hemisphere for three patients and in the right hemisphere for the remaining four. The region of overlap in these seven patients was not invaded by the lesion in any of the other six patients, and none of these six patients showed a loss of ability to recognize motion-defined letters. Three patients showed selective loss of acuity for low-contrast letters with normal Snellen acuity. The lesions in these three patients extended more posteriorly than in any other patient, and their region of overlap was in white matter underlying areas 18 and 19. We conclude that (1) the loss of ability to recognize letters in seven patients was specific to motion-defined letters rather than being a general loss of letter-recognition ability, (2) this visual loss was specific to motion-defined form rather than being a general failure of motion processing, and (3) the visual loss was not produced by lesions that did not involve the localized cerebral region specified above. To explain the existence of type I and of type II loss with sparing of the detection and discrimination of coherent motion, we propose that motion information is processed hierarchically. We further suggest that homologs of the socalled motion and color/form pathways (i.e., areas V1/MT/MST/7a and areas V1/V4/IT) are interconnected to form a distributed system that is important for the recognition of motion-defined form.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1998

The processing of first- and second-order motion in human visual cortex assessed by functional magnetic resonance imaging (fMRI).

We have examined the activity levels produced in various areas of the human occipital cortex in response to various motion stimuli using functional magnetic resonance imaging (fMRI) methods. In addition to standard luminance-defined (first-order) motion, three types of second-order motion were used. The areas examined were the motion area V5 (MT) and the following areas that were delineated using retinotopic mapping procedures: V1, V2, V3, VP, V3A, and a new area that we refer to as V3B. Area V5 is strongly activated by second-order as well as by first-order motion. This activation is highly motion-specific. Areas V1 and V2 give good responses to all motion stimuli, but the activity seems to be related primarily to the local spatial and temporal structure in the image rather than to motion processing. Area V3 and its ventral counterpart VP also respond well to all our stimuli and show a slightly greater degree of motion specificity than do V1 and V2. Unlike V1 and V2, the response in V3 and VP is significantly greater for second-order motion than for first-order motion. This trend is evident, but less marked, in V3A and V3B and absent in V5. The results are consistent with the hypothesis that first-order motion sensitivity arises in V1, that second-order motion is first represented explicitly in V3 and VP, and that V5 (and perhaps also V3A and V3B) is involved in further processing of motion information, including the integration of motion signals of the two types.

Brain Mapping↗
PubMed1977

Ventricular septal motion and left ventriclular dimensions during abnormal ventricular activation.

To determine the effect of abnormal ventricular activation on ventricular septal motion, left ventricular endocardial motion and left ventricular dimensions, 12 patients with normal motion were studied with echocardiography during incremental pacing of the right ventricular apex, outflow and inflow regions. Three types of abnormal ventricular septal motion were seen: The type I pattern was characterized by an early rapid preejection posterior ventricular septal motion followed by another posterior systolic motion that lasted throughout ejection, both of which were associated with septal thickening. In the type II pattern an early rapid preejection posterior ventricular septal motion was followed by an anterior ejection motion; the latter was not accompanied by septal thickening. The type III pattern consisted of an early preejection posterior ventricular septal motion followed by a mid and late systolic posterior motion: the latter motion extended through diastole. During right ventricular apical pacing, 8 of 11 patients showed a type 1 pattern, 1 a type II pattern and 2 a normal septal motion. During right ventricular outflow pacing,seven of nine patients showed a type II pattern, one a type III pattern and one a type I pattern. During right ventricular inflow pacing, eight of nine patients showed a type II pattern and one a type III pattern. At faster pacing rates patterns of types I and III changed to a type II pattern (five patients). End-diastolic dimensions decreased significantly during incremental right ventricular pacing when compared with those during sinus rhythm. End-systolic dimensions decreased significantly only during right ventricular apical and outflow pacing at maximal rates. In the seven patients who had pacing from all three sites, the decrease in left ventricular dimensions did not significantly differ when the three pacing sites were compared. These findings suggest that (1) abnormal ventricular septal motion during right ventricular pacing (induced left bundle branch block patterns) is dependent on the sequence of ventricular activation; (2) ventricular septal motion during right ventricular outflow and inflow pacing is similar to that seen in spontaneous left bundle branch block, whereas the pattern of septal motion during right ventricular apical pacing is different from that of spontaneous left bundle branch block; and (3) changes in left ventricular dimension are dependent on ventricular pacing rate but independent of pacing site.

Adult↗
PubMedDate not supplied

Rat epididymal sperm motion changes induced by ethylene glycol monoethyl ether, sulfasalazine, and 2,5-hexandione.

Epididymal sperm was examined using the Hamilton-Thorne Sperm analyzer (HTM-IVOS, version 10.6) in male rats treated with known male reproductive toxicants that act by different mechanisms to detect effects on sperm motion. Three agents known to produce changes in sperm motion at high exposure levels were administered at lower levels. Ethylene glycol monoethyl ether (EGEE), sulfasalazine (SASP), and 2,5-hexandione (2,5-HD) were administered by oral gavage to adult male Sprague-Dawley rats at 250 or 500 mg/kg/day, at 300 or 600 mg/kg/day, or at 100 or 250 mg/kg/day, respectively. The males were treated with EGEE, SASP, and 2,5-HD for 35, 28, and 28 days, respectively. The males treated with EGEE and SASP were mated with untreated females to assess male fertility. All males were examined for body weight, testicular and epididymal weight, epididymal sperm count, and sperm motion. The sperm motion parameters included percentage of motile sperm, percentage of progressively motile sperm (progressive motility), curvilinear velocity (VCL), average path velocity (VAP), straight line velocity (VSL), amplitude of lateral head displacement (ALH), beat cross frequency (BCF), linearity (LIN), and straightness (STR). For the male rats treated with SASP, no treatment-related effects on percentages of motile sperm or sperm count were observed despite impaired male fertility. However, abnormal motion of epididymal sperm from the SASP treated males was detected by a significant reduction in mean progressive motility, VAP, and ALH, and an increase in BCF and STR. For the males treated with 2,5-HD for 4 weeks, most parameters generated by the HTM-IVOS indicated decreased sperm motion despite no remarkable changes in testicular weight, epididymal weight, or sperm count. In the EGEE-treated males at 250 mg/kg/day for 5 weeks, abnormal motion of epididymal sperm was detected by decreased progressive motility and increased BCF, although there were no treatment-related effects on testicular weight or male fertility. Progressive motility was decreased in all treated groups and the difference from the control value was of the greatest magnitude among the sperm motion parameters generated by the HTM-IVOS. Velocity parameters (VAP, VSL, VCL) responded sensitively to abnormal sperm motion in the SASP and 2,5-HD studies. In spite of decreased sperm motion, BCF values were significantly increased in all treated groups except the 7-week EGEE high-dose group, where there were no motile sperm to evaluate. ALH was significantly decreased in the treated groups in which remarkable effects on sperm motion were noted. There were no significant changes in ALH at the low-dose of EGEE at which only mild effects on sperm motion were observed. STR was increased for epididymal sperm from the males treated with SASP when compared with the controls. For the males treated with EGEE and 2,5-HD, however, STR was decreased when compared with the controls. There were no significant differences in LIN in any of the groups treated with SASP, in which remarkably reduced sperm motion was detected by the other parameters. In conclusion, among the parameters generated by the HTM-IVOS, progressive motility was significantly decreased in all treated groups and the most valuable for detecting slight changes in sperm motion induced by these three different target toxicants. Further investigation with a larger set of compounds is needed to evaluate which IVOS parameters are the most sensitive in detecting motion changes.

Animals↗
PubMed2000

Pulsatile wall motion and blood pressure in aneurysms with open and thrombosed endoleaks--comparison of a wall track system and M-mode ultrasound scanning: an in vitro and animal study.

OBJECTIVE: Pulsatile wall motion has been suggested as a means by which to evaluate abdominal aortic aneurysms after exclusion from the circulation to determine whether the treatment has been effective. The objective of this study was to investigate the relations between pulsatile wall motion and both the mean and pulse pressures within the aneurysmal sac for both patent and thrombosed endoleaks. Furthermore, we compared the measurements of pulsatile wall motion by means of M-mode ultrasound scanning and a wall track system to determine the most reliable technique. METHODS: First, interobserver and intraobserver variability of M-mode ultrasound scan measurements was determined at different pressure levels in a cow iliac artery placed in an in vitro circulation. M-mode ultrasound scanning and a wall track system were compared in the same model. Second, in an animal experiment, an aneurysm and endoleak model with both patent and thrombosed endoleaks was created. Systemic and aneurysmal mean and pulse pressures were recorded synchronically with pulsatile wall motion by means of M-mode ultrasound scanning and a wall track system. RESULTS: The intraobserver and interobserver variability values for M-mode ultrasound scan measurement in vitro were 0.11 mm (SD = 0.10 mm) and 0.15 mm (SD = 0.13 mm), respectively. In the animal study, a significant difference existed with respect to the level of pulse pressure within the aneurysmal sac between the group with pulsatile wall motion and the group without such motion (P <.0001). The presence of pulsatile wall motion was not correlated with the level of aneurysmal mean pressure. The level of pulsatile wall motion determined by means of M-mode ultrasound scanning correlated well with the level determined by means of the wall track system (r = 0. 74; P =.01). For the level of pulsatile wall motion determined by means of M-mode ultrasound scanning, a significant difference between patent and thrombosed endoleaks existed (P =.04). For detecting endoleaks, the sensitivity and specificity of pulsatile wall motion as determined by means of the wall track system were 52% and 100%, respectively, and the sensitivity and specificity of pulsatile wall motion as determined by means of M-mode ultrasound scanning were 64% and 67%, respectively. For the detection of pulse pressure in the aneurysmal sac, the sensitivity and specificity of pulsatile wall motion as determined by means of the wall track system were 76% and 100%, respectively, and the sensitivity and specificity of pulsatile wall motion as determined by means of M-mode ultrasound scanning were 90% and 71%, respectively. CONCLUSIONS: We found that pulsatile wall motion is correlated with aneurysmal pulse pressure but not with the mean level of pressure inside the aneurysm. Although measurements of pulsatile wall motion are of great theoretic value when groups of patients who have undergone endovascular aneurysm repair are being compared, this method appears to be unreliable in a clinical setting with respect to determining whether the aneurysmal sac is still pressurized in individual patients.

Animals↗
PubMed1979

Stereoscopic subsystems for position in depth and for motion in depth.

We describe psychophysical evidence that the human visual system contains information-processing channels for motion in depth in addition to those for position in depth. These motion-in-depth channels include some that are selectively sensitive to the relative velocities of the left and right retinal images. We propose that the visual pathway contains stereoscopic (cyclopean) motion filters that respond to only a narrow range of the directions of motion in depth. Turning to the single-neuron level we report that, in addition to neurons turned to position to depth, cat visual cortex contains neurons that emphasize information about the direction of motion at the expense of positional information. We describe psychophysical evidence for the existence of channels that are sensitive to change size, and are separate from the channels both for motion and for flicker. These changing-size channels respond independently of whether the stimulus is a bright square on a dark ground or a dark square on a bright ground. At the physiological level we report single neurons in cat visual cortex that respond selectively to increasing or to decreasing size independently of the sign of stimulus contrast. Adaptation to a changing-size stimulus produces two separable after-effects: an illusion of changing size, and an illusion of motion in depth. These after-effects have different decay time constants. We propose a psychophysical model in which changing-size filters feed a motion-in-depth stage, and suppose that the motion-in-depth after-effect is due to activity at the motion-in-depth stage, while the changing-size after-effect is due to to activity at the changing-size and more peripheral stages. The motion-in-depth after-effect can be cancelled either by a changing-size test stimulus or by relative motion of the left and right retinal images. Opposition of these two cues can also cancel the impression of motion in depth produced by the adapting stimulus. These findings link the stereoscopic (cyclopean) motion filters and the changing-size filters: both feed the same motion-in-depth stage.

Animals↗
PubMed2003

The effects of intra-fraction organ motion on the delivery of intensity-modulated field with a multileaf collimator.

Intensity-modulated radiation therapy can be conveniently delivered with a multileaf collimator. With this method, the entire field is not delivered at once, but rather it is composed of many subfields defined by the leaf positions as a function of beam on time. At any given instant, only these subfields are delivered. During treatment, if the organ moves, part of the volume may move in or out of these subfields. Due to this interplay between organ motion and leaf motion the delivered dose may be different from what was planned. In this work, we present a method that calculates the effects of organ motion on delivered dose. The direction of organ motion may be parallel or perpendicular to the leaf motion, and the effect can be calculated for a single fraction or for multiple fractions. Three breast patients and four lung patients were included in this study,with the amplitude of the organ motion varying from +/- 3.5 mm to +/- 10 mm, and the period varying from 4 to 8 seconds. Calculations were made for these patients with and without organ motion, and results were examined in terms of isodose distribution and dose volume histograms. Each calculation was repeated ten times in order to estimate the statistical uncertainties. For selected patients, calculations were also made with conventional treatment technique. The effects of organ motion on conventional techniques were compared relative to that on IMRT techniques. For breast treatment, the effect of organ motion primarily broadened the penumbra at the posterior field edge. The dose in the rest of the treatment volume was not significantly affected. For lung treatment, the effect also broadened the penumbra and degraded the coverage of the planning target volume (PTV). However, the coverage of the clinical target volume (CTV) was not much affected, provided the PTV margin was adequate. The same effects were observed for both IMRT and conventional treatment techniques. For the IMRT technique, the standard deviations of ten samples of a 30-fraction calculation were very small for all patients, implying that over a typical treatment course of 30 fractions, the delivered dose was very close to the expected value. Hence, under typical clinical conditions, the effect of organ motion on delivered dose can be calculated without considering the interplay between the organ motion and the leaf motion. It can be calculated as the weighted average of the dose distribution without organ motion with the distribution of organ motion. Since the effects of organ motion on dose were comparable for both IMRT and conventional techniques, the PTV margin should remain the same for both techniques.

Algorithms↗
PubMed2000

Recovery of global nonrigid motion: a model-based approach without point correspondences

We present a novel technique for the estimation of global nonrigid motion of an object's boundary without using point correspondences. A complete description of the motion of an object's boundary involves specifying a displacement vector at each point of the boundary. Such a description provides a large amount of information, which needs to be processed further for study of the global characteristics of the deformation. Nonrigid motion can be studied hierarchically in terms of global nonrigid motion and point-by-point local nonrigid motion. The technique presented gives a method for estimating a global affine or polynomial transformation between two object boundaries. The novelty of the technique lies in the fact that it does not use any point correspondences. Our method uses hyperquadric models to model the data and estimate the global deformation. We show that affine or polynomial transformation between two datasets can be recovered from the hyperquadric parameters. The usefulness of the technique is twofold. First, it paves the way for viewing nonrigid motion hierarchically in terms of global and local motion. Second, it can be used as a front end to other motion analysis techniques that assume small motion. For instance, most nonrigid motion analysis algorithms make some assumptions on the type of nonrigid motion (conformal motion, small motion, etc.) that are not always satisfied in practice. When the motion between two datasets is large, our algorithm can be used to estimate the affine transformation (which includes scale and shear) or a polynomial transformation between the two datasets, which can then be used to warp the first dataset closer to the second so as to satisfy the small-motion assumption. We present experimental results with real and synthetic two- and three-dimensional data.

Journal Article↗
PubMed2005

Comparison of tRNA motions in the free and ribosomal bound structures.

A general method is presented that allows the separation of the rigid body motions from the nonrigid body motions of structural subunits when bound in a complex. The application presented considers the motions of the tRNAs: free, bound to the ribosome and to a synthase. We observe that both the rigid body and nonrigid body motions of the structural subunits are highly controlled by the large ribosomal assembly and are important for the functional motions of the assembly. For the intact ribosome, its major parts, the 30S and the 50S subunits, are found to have counterrotational motions in the first few slowest modes, which are consistent with the experimentally observed ratchet motion. The tRNAs are found to have on average approximately 72-75% rigid body motions and principally translational motions within the first 100 slow modes of the complex. Although the three tRNAs exhibit different apparent total motions, after the rigid body motions are removed, the remaining internal motions of all three tRNAs are essentially the same. The direction of the translational motions of the tRNAs are in the same direction as the requisite translocation step, especially in the first slowest mode. Surprisingly the small intrinsically flexible mRNA has all of its internal motions completely inhibited and shows mainly a rigid-body translation in the slow modes of the ribosome complex. On the other hand, the required nonrigid body motions of the tRNA during translocation reveal that the anticodon-stem-loop, as well as the acceptor arm, of the tRNA enjoy a large mobility but act as rigid structural units. In summary, the ribosome exerts its control by enforcing rigidity in the functional parts of the tRNAs as well as in the mRNA.

Anticodon↗