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PubMed2006

From targeted "black spots" to area-wide pedestrian safety.

OBJECTIVE: To describe and illustrate the geographic distribution of pedestrian crash sites in an urban setting (Montreal, Canada) with an alternative data source. METHODS: Data on pedestrian victims were extracted for a 5-year period (1999-2003) from ambulance services information systems. The locations of crash sites and pedestrian victim density were mapped using a geographic information system. Pedestrian "black spots" were defined as sites where there had been at least eight pedestrian victims. RESULTS: The 22 identified black spots represent only 1% of all city intersections with at least one victim and 4% of all injured pedestrians, whereas 5082 victims were injured at >3500 different crash sites. The number and population rates of injured pedestrians are greater in central boroughs. Accordingly, the density of pedestrian victims is much higher in central boroughs. Over the 5-year period, in some central boroughs, pedestrian crashes occurred in up to 26% of intersections. CONCLUSIONS: Ambulance information systems were relevant to map pedestrian crash sites. Most pedestrians were injured at locations that would have been missed by the black spot approach. This high-risk preventive strategy cannot substantially reduce the total number of injured or the insecurity that many pedestrians experience when walking. Considering the large number and widespread occurrence of pedestrian crashes in Montreal, prevention strategies should include comprehensive environmental measures such as global reduction of traffic volume and speed.

Accidents, Traffic↗

PubMed2003

Safety in numbers: more walkers and bicyclists, safer walking and bicycling.

OBJECTIVE: To examine the relationship between the numbers of people walking or bicycling and the frequency of collisions between motorists and walkers or bicyclists. The common wisdom holds that the number of collisions varies directly with the amount of walking and bicycling. However, three published analyses of collision rates at specific intersections found a non-linear relationship, such that collisions rates declined with increases in the numbers of people walking or bicycling. DATA: This paper uses five additional data sets (three population level and two time series) to compare the amount of walking or bicycling and the injuries incurring in collisions with motor vehicles. RESULTS: The likelihood that a given person walking or bicycling will be struck by a motorist varies inversely with the amount of walking or bicycling. This pattern is consistent across communities of varying size, from specific intersections to cities and countries, and across time periods. DISCUSSION: This result is unexpected. Since it is unlikely that the people walking and bicycling become more cautious if their numbers are larger, it indicates that the behavior of motorists controls the likelihood of collisions with people walking and bicycling. It appears that motorists adjust their behavior in the presence of people walking and bicycling. There is an urgent need for further exploration of the human factors controlling motorist behavior in the presence of people walking and bicycling. CONCLUSION: A motorist is less likely to collide with a person walking and bicycling if more people walk or bicycle. Policies that increase the numbers of people walking and bicycling appear to be an effective route to improving the safety of people walking and bicycling.

Accidents, Traffic↗

PubMed1981

Length measurement by means of a grid.

Experiments were performed which demonstrate some of the length-measuring characteristics of a grid consisting of two sets of parallel lines crossing each other at right angles. If lines of various shapes are repeatedly superimposed at random on such a grid, the mean number of intersections each of these lines makes with the lines of the grid is directly proportional to the length of the line. There is a property of two-dimensional curves, which may be called directional bias, the presence of which in a test curve will increase the variability of the number of intersections made with the grid lines and skew the frequency distribution curve to the right. This effect is markedly reduced by taking the mean of three measurements if the second and third are taken with the grid rotated 30 degrees and 60 degrees respectively from its original position in relation to the test curve. Information gained from these experiments can be applied to stereological problems such as the estimation of particle densities in histological sections.

Cell Count↗

PubMed2005

Multilevel modelling of built environment characteristics related to neighbourhood walking activity in older adults.

OBJECTIVE: To examine the relation between built environment factors (representing several dimensions of urban form of neighbourhoods) and walking activity at both the neighbourhood level and the resident level, in an older adult sample. DESIGN, SETTING, PARTICIPANTS: A cross sectional, multilevel design with neighbourhoods as the primary sampling unit and senior residents as the secondary unit. Five hundred and seventy seven residents (mean age = 74 years, SD = 6.3 years) participated in the survey, which was conducted among 56 city defined neighbourhoods in Portland, Oregon, USA. Neighbourhood level variables were constructed using geographical information systems. Resident level variables consisted of a mix of self reports and geocoded data on the built environment. MAIN OUTCOME MEASURE: Self reported neighbourhood walking. MAIN RESULTS: A positive relation was found between built environment factors (density of places of employment, household density, green and open spaces for recreation, number of street intersections) and walking activity at the neighbourhood level. At the resident level, perceptions of safety for walking and number of nearby recreational facilities were positively related to high levels of walking activity. A significant interaction was observed between number of street intersections and perceptions of safety from traffic. CONCLUSIONS: Certain neighbourhood built environment characteristics related to urban form were positively associated with walking activity in the neighbourhoods of senior residents. Public health promotion of walking activity/urban mobility and the design of interventions need to consider the contribution of neighbourhood level built environment influences.

Aged↗

PubMed2001

The co-occurrence of AIDS and homelessness: results from the integration of administrative databases for AIDS surveillance and public shelter utilisation in Philadelphia.

STUDY OBJECTIVE: Administrative databases from the City of Philadelphia that track public shelter utilisation (n=44 337) and AIDS case reporting (n=7749) were merged to identify rates and risk factors for co-occurring homelessness and AIDS. DESIGN: Multiple decrement life tables analyses were conducted, and logistic regression analyses used to identify risk factors associated with AIDS among the homeless, and homelessness among people with AIDS. SETTING: City of Philadelphia, Pennsylvania, USA. MAIN RESULTS: People admitted to public shelters had a three year rate of subsequent AIDS diagnosis of 1.8 per 100 person years; nine times the rate for the general population of Philadelphia. Logistic regression results show that substance abuse history (OR = 3.14), male gender (OR = 2.05), and a history of serious mental disorder (OR = 1.62) were significantly related to the risk for AIDS diagnosis among shelter users. Among people with AIDS, results show a three year rate of subsequent shelter admission of 6.9 per 100 person years, and a three year rate of prior shelter admission of 9%, three times the three year rate of shelter admission for the general population. Logistic regression results show that intravenous drug user history (OR = 3.14); no private insurance (OR = 2.93); black race (OR = 2.82); pulmonary or extra-pulmonary TB (OR = 1.43); and pneumocystis pneumonia (OR = 0.56) were all related to the risk for shelter admission. CONCLUSIONS: Homelessness prevention programmes should target people with HIV risk factors, and HIV prevention programmes should be targeted to homeless persons, as these populations have significant intersection. Reasons and implications for this intersection are discussed.

Acquired Immunodeficiency Syndrome↗

PubMed1985

Multisection-multiecho MR imaging: effect on image quality.

An analysis of the effects of multisection-multiecho imaging on signal intensity is presented using a gaussian excitation pulse. The number of sections acquired, size of the intersection gap, and the number of echoes were all varied independently to evaluate their effect on image intensity. The results indicate that one should take a conservative approach in multisection-multiecho imaging, leaving at least a 2-mm-intersection gap and approximately a 20-msec delay between section excitations to avoid image signal loss owing to partial saturation effects.

Magnetic Resonance Spectroscopy↗

PubMed1993

Segmental calculation of left ventricular wall stresses.

A procedure for calculating left ventricular wall stresses segmentally was devised. Rectangular coordinates of the wall surfaces as seen in longitudinal section were plotted with the long axis as the x-axis. For each cavity point, a third-order polynomial (cubic spline) was fitted to the point together with several adjacent points on either side of it; the cavity radius (normal to cavity surface) at the point was found algebraically from the spline's coefficients. Each cavity radius was matched with the most symmetrical one from the opposite cavity surface. The point of intersection of the cavity radius with the outer surface was found, and a midwall point was identified from logarithmic means of cavity and outer radial lengths. For each midwall point, a cubic spline was fitted to that point together with several adjacent points on either side of it, and the midwall radius at that point was determined algebraically from the spline's coefficients. Each midwall radius was matched to the most symmetrical one from the opposite midwall. The locus of points at equal radial distances from opposite midwalls forms the axis. The midwall radius of curvature (r theta) orthogonal to the meridian at each point was taken as the radial distance from the midwall to the axis. Midwall meridional radius of curvature (r phi) was calculated from the spline's coefficients. Thickness (h) was calculated from intersections between the midwall radius and the inner and outer surfaces. For each point, meridional tension (T phi) was calculated as T phi = Pr theta/2, and hoop tension (T theta) was calculated as T theta = (Pr theta/2)(2 - r theta/r phi) where P is transmural pressure. Stresses were calculated as tensions divided by thicknesses (sigma phi = T phi/h, sigma theta = T theta/h), or more directly as sigma phi = Pr theta/2h and sigma theta = (Pr theta/2h)(2 - r theta/r phi). This procedure was validated with simple chamber shapes, and it has been applied to left ventricles.

Animals↗

PubMed1986

Medullospinal sympathoexcitatory neurons in normotensive and spontaneously hypertensive rats.

Aortic depressor nerve discharge (AND), lumbar sympathetic nerve discharge (SND), and single-unit activity of medullospinal pressure-sensitive neurons of nucleus paragigantocellularis lateralis (PGCL neurons) were recorded in halothane-anesthetized 16-wk-old spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats. The relationship between these variables and mean arterial pressure (MAP) was investigated. The gain of baroreceptor afferents was not significantly different between the strains, but corner MAP (intersection between linear ascending portion of curve and noise) was 38 mmHg higher in SHRs. The relationship between SND and MAP and that between PGCL neuronal activity and MAP were both characterized by a plateau (maximal activity) at low MAP followed by a linear reduction reaching zero at a level called cutoff MAP. The theoretical intersection between the plateau and the linear decremental portion of these curves was defined as a corner MAP. Values of corner MAP determined at the three levels (AND, SND, PGCL) were identical in a given strain and reset by a common value in SHRs (38-40 mmHg). The gain of baroreceptor reflex measured at all three sites as the slope of the linear incremental (AND) or decremental (PGCL and sympathetic chain) portion of the activity-MAP relationship was the same in WKY rats and in SHRs. Cutoff MAP measured in PGCL was identical to that measured in peripheral sympathetic system for a given strain. There was no significant difference in maximal discharge rate of PGCL neurons nor in lumbar SND and no interstrain difference in the proportion of SND that was suppressible by arterial baroreceptor feedback.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2004

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in the rat pre-Botzinger complex.

Previously, we reported that the pre-Bötzinger complex (PBC) exhibited a dramatic reduction in cytochrome oxidase activity at postnatal day (P) 12. This coincided in time with decreases in glutamate and NMDA receptor subunit 1 and increases in GABA, GABAB, glycine receptor, and glutamate receptor GluR2. To test our hypothesis that various alpha-subunits of GABAA receptors also undergo changes in their expression during postnatal development, as they do in other brain regions, we undertook an in-depth immunohistochemical study of GABAA receptor subunits alpha1, alpha2, and alpha3 in the PBC of P0 to P21 rats. We found that 1) GABAA alpha3-subunit was expressed at relatively high levels at P0, which then declined with age; 2) GABAA alpha1-subunit was expressed at relatively low levels at P0 but increased with age; 3) the developmental trends of subunits alpha1 and alpha3 intersected at P12; and 4) GABAA alpha2-subunit expression was moderate to light at P0 and remained quite constant during development, being lowest at P21. These findings suggest that the apparent switch in relative expressions of subunits alpha3 and alpha1 during development and the intersection of slopes around P12 may be associated with possible changes in GABAA receptor subtypes that would mediate different functional properties of GABA transmission, such as primarily a less efficient inhibitory transmission before P12 and a more mature inhibitory effect at P12 and thereafter, as suggested by the kinetics of distinct postsynaptic potentials. This mechanism may contribute partially to the dramatic reduction in cytochrome oxidase activity within the PBC at P12, as shown previously.

Aging↗

PubMed1998

Frameless laser-guided stereotaxis: A system for CT-monitored neurosurgical interventions.

OBJECTIVE: Since a simple, rapid and accurate stereotactic system allowing CT monitoring would offer the neurosurgeon several advantages, the feasibility and application accuracy of frameless laser-guided freehand point stereotaxis for neurosurgical interventions was studied. METHODS: A Cartesian coordinate grid mounted upon the far wall of a CT scan room defined a plane. The scanner isocenter defined the origin of a three-dimensional coordinate system. Phantom entry point and target point coordinates were determined by the positional CT cursor. These coordinates were entered into a computer which determined the coordinates of the grid intersection point with a line passing through the entry and target points. A tripod-mounted laser assembly comprising two encased lasers oriented retrograde and antegrade along opposite vectors, was positioned near the grid. The retrograde laser was positioned to illuminate the marked grid intersection point while the antegrade laser simultaneously illuminated the entry point; the beams were thereby aligned along the line of trajectory. The tip of a probe was placed on the entry point; the hub was then moved into the path of the antegrade laser, thereby aligning the probe with the line of trajectory. The probe was then inserted to the target at a trigonometrically calculated distance. RESULTS: Ten consecutive phantom tests averaged 17.5 min. Phantom test application accuracy averaged +/-2 mm at an average insertion distance of 7.1 cm. CONCLUSION: Phantom tests indicate that simple, rapid and accurate CT-monitored frameless laser-guided freehand point stereotaxis is feasible. Clinical investigation is warranted.

Humans↗

PubMed2002

A new approach for transseptal catheterization in patients undergoing percutaneous balloon mitral valvuloplasty.

AIMS: To evaluate the safety and efficacy of a new approach for transseptal catheterization in patients undergoing percutaneous balloon mitral valvuloplasty (PBMV). METHODS: One hundred and two patients with rheumatic mitral stenosis were randomized into two groups. In the study group (RA approach), an imaginary horizontal line was drawn from the top end of the tricuspid valve under anteroposterior fluoroscopic view. The intersection of the horizontal line and the right edge of the corresponding thoracic vertebra was defined as the upper border of the Fossa ovalis. The atrial septum was punctured from a point 0.5 cm below the upper border of the Fossa ovalis. In the control group (LA approach), an imaginary horizontal line was drawn between the upper and middle third of the left atrium, and the intersection of this horizontal line and the right edge of the corresponding thoracic vertebra was used as an atrial septum puncture point. RESULTS: Atrial septum puncture succeeded in all patients in the study group and in 72.6% of the patients in the control group (p < 0.01). The average fluoroscopy times for transseptal catheterization in the study and the control groups were 2.0 +/- 0.5 and 3.0 +/- 1.0 min, respectively (p < 0.01). Transseptal catheterization was subsequently achieved using the RA approach in the 14 patients from the control group in whom the LA approach failed. CONCLUSIONS: The RA approach is a safe and effective means for transseptal catheterization in patients undergoing PBMV.

Cardiac Catheterization↗

PubMed1987

The relationship of mitral annular shape to the diagnosis of mitral valve prolapse.

The geometric or anatomic diagnosis of mitral valve prolapse, as opposed to the pathologic diagnosis of myxomatous valve disease, is based on the relationship of the mitral leaflets to the surrounding anulus. Current echocardiographic criteria for this diagnosis include leaflet displacement above the annular hinge points in any two-dimensional view; implicit in this equivalent use of intersecting views is the assumption that the mitral anulus is a euclidean plane. Prolapse by these criteria is found in a surprisingly large proportion of the general population. In most of these individuals, however, prolapse is present in the apical four-chamber view and absent in roughly orthogonal long-axis views of the left ventricle. This frequently observed discrepancy between leaflet-annular relationships in intersecting views suggests an underlying geometric property of the mitral apparatus that would produce the appearance of prolapse in one view without actual leaflet distortion. To address this possibility, a model of the mitral valve and anulus was constructed. When the model anulus was given a nonplanar, saddle-shaped configuration, the clinical observations were reproduced: the leaflets appeared to lie above the low points of the anulus in one plane, and below its high points in a perpendicular plane. Therefore, the appearance of mitral valve prolapse can occur without actual leaflet displacement above the most superior points of the mitral anulus if the anulus is nonplanar. To determine whether this pattern is reflected in the human mitral anulus, two-dimensional echocardiographic views of the mitral apparatus were obtained by rotation about the cardiac apex in 20 patients without evident annular or rheumatic valvular disease. In all cases the mitral anulus, as reconstructed from these views, had a nonplanar systolic configuration, with high points located anteriorly and posteriorly. This is consistent with the findings of other groups in animals, and would favor the appearance of prolapse in the four-chamber view and its absence in long-axis views that are oriented anteroposteriorly. This model can therefore explain the frequently observed discrepancy between leaflet-annular relationships in roughly orthogonal views. It challenges the assumption that the mitral anulus is planar as well as the diagnosis of prolapse in many otherwise normal individuals based on that assumption.

Adolescent↗

PubMed1992

Dissociation between left ventricular untwisting and filling. Accentuation by catecholamines.

BACKGROUND: Efficient early diastolic filling is essential for normal cardiac function. Diastolic suction, as evidenced by a decreasing left ventricular pressure during early filling, could result from restoring forces (the release of potential energy stored during systolic deformation) dependent on myofilament relaxation. Although these restoring forces have been envisioned within individual myofibers, recent studies suggest that gross fiber rearrangement involving the connective tissue network occurs easy in diastole. This may lead to the release of potential energy stored during systole and suction-aided filling. METHODS AND RESULTS: To establish precisely the timing and extent of restoration of the systolic torsional deformation of the left ventricle with respect to early filling at baseline and with enhanced relaxation, we studied untwisting during control conditions and with catecholamine stimulation. Using noninvasive and nondestructive magnetic resonance tagging, torsional deformation of the left ventricle was measured at 20-msec intervals in 10 open-chest, atrially paced dogs, starting at aortic valve closure. Eight equiangular tags intersected the epicardium and endocardium in three short-axis imaging planes (base, mid, and apex). From the intersection points, epicardial and endocardial circumferential chord and arc lengths were measured and angular twist of mid and apical levels with respect to the base (maximal torsion and its reversal, untwisting) was calculated. Echo-Doppler provided timing of aortic valve closure and of mitral valve opening. Zero torsion was defined at end diastole. Torsion at the apical level reversed rapidly between its maximum and the time immediately after mitral valve opening: from 7.0 +/- 5.8 degrees to 3.2 +/- 5.4 degrees and 12.0 +/- 8.5 degrees to 6.9 +/- 7.8 degrees (mean +/- SD, both p less than 0.01) at the epicardium and endocardium, respectively. During the same period, no significant circumferential segment length changes occurred. As expected, after mitral valve opening, filling resulted in significant circumferential segment lengthening, whereas further reversal of torsion was small and nonsignificant. During dobutamine infusion, torsion at end systole was greater and reversal during isovolumic relaxation was much more rapid and greater in extent (p less than 0.01). Torsion reversed from 11.5 +/- 4.3 degrees to 5.7 +/- 4.8 degrees and 17.4 +/- 6.4 degrees to 6.9 +/- 7.7 degrees at epicardium and endocardium. CONCLUSIONS: Untwisting occurs principally during isovolumic relaxation before filling and is markedly enhanced in speed and magnitude by catecholamines. This partial return of the left ventricle to its preejection configuration before mitral valve opening could represent an important mechanism for the release of potential energy stored in elastic elements during the systolic deformation. These myocardial restoring forces would be markedly enhanced by physiological changes consequent to catecholamines such as during exercise, offsetting the concomitant shortening of the filling period.

Animals↗

PubMed1995

A study in ventricular-ventricular interaction. Single right ventricles compared with systemic right ventricles in a dual-chamber circulation.

BACKGROUND: Ventricular-ventricular interaction is known to occur in normal human heart. To determine whether it plays a role in the function of single right ventricles, systemic right ventricles were compared with and without a left ventricle mechanically coupled to it. METHODS AND RESULTS: A noninvasive magnetic resonance tagging technique (spatial modulation of magnetization [SPAMM]) that lays intersecting stripes down on the myocardium was used to examine 18 patients with systemic right ventricles: 7 with a single right ventricle who have undergone the Fontan procedure (age, 38.8 +/- 8.9 months) and 11 with transposition of the great arteries who have undergone an atrial inversion operation (age, 16.3 +/- 3.9 years). The motion of the intersection points was tracked through systole to determine regional twist and radial shortening. Shortening rates also were evaluated. Finite strain analysis was applied to the grid lines using Delaunay triangulation, and the two-dimensional strain tensor and principal E1 strains were derived for the various anatomic regions. Basal and apical short-axis planes through the ventricular wall were categorized into four distinct regions spaced equally around the circumference of the slice. We observed the following results. (1) Strain was greatest and heterogeneity of strain was least in patients with transposition of the great arteries who were status post atrial inversion operation (six of eight regions). Marked differences were noted in the distribution of strain within a given region, from endocardium to epicardium, and from atrioventricular valve to apical plane between patient subtypes and those with a normal left ventricle. (2) Contrary to the normal subject studied by the use of the same method, for both patient subtypes, there was counterclockwise twist in one region, clockwise twist in the posterior or inferior wall, and a transition zone of no twist at which the two regions of twist met. Normal human adult left ventricles studied in short-axis twist uniformly counterclockwise as viewed from apex to base. (3) Radial inward motion was greatest in the superior wall of both types of systemic right ventricle. The inferior walls of Fontan patients and the posterior (ie, septal) walls of patients with transposition of the great arteries, status post atrial inversion, moved paradoxically in systole. The shortening rate at the atrioventricular valve of patients with transposition of the great arteries, status post atrial inversion, was significantly lower than at the apex or in Fontan patients. CONCLUSIONS: Marked differences in regional wall motion and strain were demonstrated in systemic right ventricles, depending on whether a left ventricle was present to augment its function. Ventricular-ventricular interaction appears to play an important role in affecting the biomechanics of systemic right ventricles. These observations were markedly different from those in the normal systemic left ventricle. These techniques demonstrate tools with which we can begin to evaluate surgical outcomes using regional myocardial mechanics and may provide a clue to single right ventricle failure.

Adolescent↗

PubMed2000

Relating MRI changes to motor deficit after ischemic stroke by segmentation of functional motor pathways.

BACKGROUND AND PURPOSE: Infarct size on T2-weighted MRI correlates only modestly with outcome, particularly for small strokes. This may be largely because of differences in the locations of infarcts and consequently in the functional pathways that are damaged. To test this hypothesis quantitatively, we developed a "mask" of the corticospinal pathway to determine whether the extent of stroke intersection with the pathway would be more closely related to clinical motor deficit and axonal injury in the descending motor pathways than total stroke lesion volume. METHODS: Eighteen patients were studied > or =1 month after first ischemic stroke that caused a motor deficit by use of brain T2-weighted imaging, MR spectroscopic (MRS) measurements of the neuronal marker compound N-acetyl aspartate in the posterior limb of the internal capsule, and motor impairment and disability measures. A corticospinal mask based on neuroanatomic landmarks was generated from a subset of the MRI data. The maximum proportion of the cross-sectional area of this mask occupied by stroke was determined for each patient after all brain images were transformed into a common stereotaxic brain space. RESULTS: There was a significant linear relationship between the maximum proportional cross-sectional area of the corticospinal mask occupied by stroke and motor deficit (r(2)=0.82, P<0.001), whereas the relationship between the total stroke volume and motor deficit was better described by a cubic curve (r(2)=0.76, P<0.001). Inspection of the data plots showed that the total stroke volume discriminated poorly between smaller strokes with regard to the extent of associated motor deficit, whereas the maximum proportion of the mask cross-sectional area occupied by stroke appeared to be a more discriminatory marker of motor deficit and also N-acetyl aspartate reduction. CONCLUSIONS: Segmentation of functional motor pathways on MRI allows estimation of the extent of damage specifically to that pathway by the stroke lesion. The extent of stroke intersection with the motor pathways was more linearly related to the magnitude of motor deficit than total lesion volume and appeared to be a better discriminator between small strokes with regard to motor deficit. This emphasizes the importance of the anatomic relationship of the infarct to local structures in determining functional impairment. Prospective studies are necessary to assess whether this approach would allow improved early estimation of prognosis after stroke.

Adult↗

PubMed1993

Perception of transparency in stationary and moving images.

If two neutral density (ND) filters overlap partially, the luminance of the overlapping region is given by a multiplicative relationship (e.g., if the filters have 50% transmittance then on a 100 cd m-2 background, the luminance of each filter would be 50 cd m-2 but the luminance of the overlapping region would be 50% of 50 cd m-2, i.e. 25 cd m-2). Does the visual system respect this relationship? Two grey rectangles were overlapped to form a 'cross' and the luminance of the intersection was randomly varied. Naive subjects' ratings of transparency showed a surprising consistency with physics. A similar relationship is seen if two moving square-wave gratings are superimposed. When the intersection luminances were close to the multiplicative (25 cd m-2) case, component motion was seen, and values less than 25 cd m-2 and higher than 50 cd m-2 caused a decline in perceived component motion. Two interpretations are offered. (a) The visual system has access to 'tacit knowledge' of transparency and shadows. (b) If you assume a log signal compression in the retina, then by multiplying the luminances you would null the Fourier energy from the 'blobs' (i.e. regions of overlap between the two gratings). This would prevent 'blob tracking' and lead to the perception of coherent motion. Since most researchers in the field had simply added the gratings linearly they would have inadvertently introduced extra Fourier energy in the blobs and this might account for all previous instances of coherent motion observed in such displays. Whatever the ultimate interpretation, the present results provide the first clear evidence that even the processing of certain primitive visual dimensions--such as motion--can be powerfully constrained by the perception of transparency (Ramachandran V. S. (1990) in: AI and the Eye, Wiley, Chichester; Stoner, G., Albright, T. and Ramachandran, V. S. (1990) Nature 344, 153-155). This is well in line with other recent results demonstrating that transparency can also constrain the processing of stereopsis (Nakayama, K., Shimojo, S. and Ramachandran, V. S. (1991) Perception 19, 497-513) and perceptual grouping (Ramachandran, V. S. (1990) in: AI and the Eye, Wiley, Chichester).

Humans↗

PubMed1993

The role of early mechanisms in motion transparency and coherence.

Perceptual dissociation of moving plaid patterns into independently moving bar gratings occurs most readily when the grating signals are combined as if the bars were semi-transparent objects (Ramachandran, V. S. (1990) in: AI and the Eye. Wiley, Chichester, pp. 21-77. Stoner, G. R., Albright, T. D. and Ramachandran, V. S. (1990) Nature 344, 153-155). These and other examples of motion transparency are exploited to constrain the set of viable models for human motion processing. For example, one may exclude any fixed recombination of local motion signals into a plaid motion signal. Broad classes of linear and non-linear mechanisms for tracking blobs, corners, and other unambiguous plaid motion cues can also be ruled out because they fail to reproduce the experimental results even qualitatively. The shifting balance between the 'coherent plaid' and 'sliding gratings' percepts are attributed to processing stages before any integration or combination of local motion signals. The first essential stage is a roughly logarithmic nonlinearity before orientation filtering. In general, the resulting cross-products (at the intersections) code unambiguously for the true plaid motion vector, but these signals will be nulled for multiplicatively combined plaid components. Supporting evidence for this idea is obtained in our measurements of detection thresholds for the 'plaid' and 'sliding' percepts. The essential element of the second stage consists of 'end-stop' cells which detect the nullable intersection signal, and so produce a plaid-motion signal with the required characteristics. Finally, it is argued that the ecological role of the proposed mechanism lies in the ability to handle movement of patterned objects in lighting conditions dominated by complicated cast shadows.

Humans↗

PubMed1993

The effect of disparity on motion coherence.

Many moving plaid stimuli are ambiguous, and perception switches between a coherent plaid pattern and two transparent gratings. Here, experiments are reported that examined the effect of stereodepth between the two gratings of the moving plaid stimulus on the perception of coherence or motion transparency. Increasing disparity increased the percentage of time that two independently drifting transparent gratings were perceived. This was studied for plaids with various levels of intersection luminance. Using intersection luminances beyond conditions of physical transparency increased the percentage of time that one coherent plaid was seen. These two opposing influences could be pitted against each other to achieve constant levels of coherence. An adaptation paradigm was also used in which observers adapted to a stationary stimulus with either zero, crossed or uncrossed disparity between the gratings, and then indicated the occurrence of coherence and motion transparency in test stimuli of drifting plaids with zero, crossed or uncrossed disparity. Adaptation to crossed and uncrossed stereo-depth increased relative perceived coherence equally, especially for zero test disparity. An analysis of the length of the episodes of coherence and motion transparency indicated that the effect of adaptation was to decrease the length of motion transparency episodes, while the length of coherence episodes did not change. It is concluded that mechanisms involved in the processing of stereo-depth must have an input to the integration stage of the motion channel and that pattern and component motion mechanisms can operate quite independently.

Adaptation, Ocular↗