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PubMed1975

Behavioral study of the visual cortex of Galago senegalensis.

An ablation study of the visual cortex of Galago senegalensis was undertaken in the hope of finding clues about the evolution of primate visual cortex. Removal of area 17 resulted in a profound sensory loss manifested by, first, the failure to discriminate between simple patterns; second, a deficit in localizing objects; third, a deficiency in tracking moving objects; and fourth, symptoms attributable to a deficiency in depth perception, such as misreaching and inaccurate jumping. Thus, the effects of ablating area 17 are similar in bushbabies and monkeys. In contrast, minimal sensory loss is produced by ablating area 17 in squirrels or tree shrews. This difference between primates and other mammals may depend on differences in the extent of the cortical target of the tecto-pulvinar path; in Galago and perhaps in all primates, more of the extrastriate visual cortex is entirely dependent on area 17. Removal of the ventral temporal cortex resulted in a loss of learned visual discriminations and in retardation in learning new visual discriminations. These symptoms seem related to the inferotemporal syndrome in monkeys.

Animals↗

PubMedDate not supplied

An artificial compound eye for stereoendoscopy.

A scanning endoscope has been designed with the ability to both project three-dimensional images and provide precise measurements of internal structures. This is beneficial for minimally invasive surgery, where surgeons suffer from a lack of depth perception, limited field of view, and the absence of a reference frame for dimensional measurements. Borrowing from the insect compound eye, the design uses an array of prisms, each facing a different direction but with overlapping fields of view. The prisms redirect their respective images normal to a fibre optic imaging plane and are individually controlled by electro-chromic shutters. The device thereby retains the ability to scan in multiple directions without mechanical parts and uses only a single camera. Comparison of the overlapping images with known prism positions allows for the calculation of absolute coordinates. Results from large-scale models show that the technology is plausible, and fabrication methods for a smaller device are discussed.

Depth Perception↗

PubMed1989

Visual orientation estimation.

A systematic error is reported in orientation estimation, in that on average, estimates are closer to the vertical axis than are the stimuli by up to 6 degrees. This systematic error results from a specific mechanism that may be related to depth perception, and that is avoided in certain circumstances or when other mechanisms take over. For example, the estimates of one observer who was a well-trained professional draughtsman did not show this systematic error. Furthermore, for all observers tested, estimation of clock time is not subject to the regular orientation estimation error. Rather, observers tend to estimate times as slightly further from the quarter hour than they really are. Orientation judgement channel capacity was also studied under various conditions. The number of discriminable orientations is far above the magic number "7" limit, reaching over 20 in optimal circumstances. The distribution of discriminable orientations is nonlinear, in that these are more closely packed about the horizontal and vertical axis than at the oblique.

Attention↗

PubMed1990

Effect of luminance and contrast on stereoscopic acuity.

The threshold of binocular depth perception was measured in 11 healthy volunteers. A three-rod arrangement was employed in which both the luminance of the rods and that of the adapting field could be adjusted independently. This allowed fixing the contrast when the effect of luminance was studied or fixing the luminance when the effect of contrast was investigated. The observation distance was 400 mm. Thresholds were expressed as angular disparities and were based on 75% correct responses. Points of subjective equality were also determined. Lowest thresholds (2.85 +/- 0.67 s of arc) were found for a moderate contrast of 0.5 whereas low (0.05) and high (0.95) contrast both produced significantly higher thresholds (luminance 250 cd/m2). Altering the field luminance (50, 250, 1600 cd/m2) under constant contrast conditions (0.95) did not measurably influence stereoscopic acuity.

Adult↗

PubMedDate not supplied

Visual contributions to postural stability in older adults.

BACKGROUND: With advancing age, there is a generalized reduction in visual functioning which has been associated with impaired postural stability and increased risk of falls. However, little is known about which visual abilities are the most important in the control of postural sway when standing. OBJECTIVE: To determine whether specific visual abilities predict stability when standing on firm and compliant surfaces. METHODS: Tests of visual function, peripheral sensation, strength, reaction time and sway were administered to 156 community-dwelling men and women aged 63-90 years. The visual tests included high- and low-contrast visual acuity, contrast sensitivity, depth perception, stereopsis and lower visual field size. Postural sway was measured with eyes open on a firm and a compliant foam rubber surface. RESULTS: On the firm surface, sway was significantly associated with only one sensorimotor measure: proprioception in the lower limbs. In contrast, on the compliant surface, sway was associated with all of the visual measures, quadriceps strength and reaction time. Multiple regression analysis revealed that contrast sensitivity, stereopsis and quadriceps strength were significant independent predictors of total sway when subjects stood on the compliant surface. CONCLUSION: The study findings confirm the importance of vision, in particular contrast sensitivity and stereopsis, in the control of posture under challenging conditions, and suggest some mechanisms for the association between impaired vision and falls in older people.

Accidental Falls↗

PubMed1997

A physiological model for motion-stereo integration and a unified explanation of Pulfrich-like phenomena.

Many psychophysical and physiological experiments indicate that visual motion analysis and stereoscopic depth perception are processed together in the brain. However, little computational effort has been devoted to combining these two visual modalities into a common framework based on physiological mechanisms. We present such an integrated model in this paper. We have previously developed a physiologically realistic model for binocular disparity computation (Qian, 1994). Here we demonstrate that under some general and physiological assumptions, our stereo vision model can be combined naturally with motion energy models to achieve motion-stereo integration. The integrated model may be used to explain a wide range of experimental observations regarding motion-stereo interaction. As an example, we show that the model can provide a unified account of the classical Pulfrich effect (Morgan & Thompson, 1975) and the generalized Pulfrich phenomena to dynamic noise patterns (Tyler, 1974; Falk, 1980) and stroboscopic stimuli (Burr & Ross, 1979).

Depth Perception↗

PubMed1988

Limits of binocular fusion in the short wave sensitive ("blue") cones.

Stereoscopic depth perception is possible when the short wave sensitive (SWS or "Blue") cones are isolated using a yellow adapting field. We have measured the maximum disparity that can be fused (the diplopia threshold) as a function of the separation between pairs of dots or lines. Under all conditions, these diplopia thresholds are the same for the isolated SWS cones as for the entire visual system. In addition, SWS diplopia thresholds vary as a linear function of dot or line separation, so that they exhibit disparity scaling. Further experiments show that disparity scaling is dependent upon the presence of low spatial frequencies in the stimulus and not upon the retinal eccentricity of stimulation. These data indicate that the SWS cones provide information to the disparity processing system through more than one low spatial frequency channel but not through high frequency ones.

Adaptation, Ocular↗

PubMedDate not supplied

Neural responses to relative speed in the primary visual cortex of rhesus monkey.

Relative motion information, especially relative speed between different input patterns, is required for solving many complex tasks of the visual system, such as depth perception by motion parallax and motion-induced figure/ground segmentation. However, little is known about the neural substrate for processing relative speed information. To explore the neural mechanisms for relative speed, we recorded single-unit responses to relative motion in the primary visual cortex (area VI) of rhesus monkeys while presenting sets of random-dot arrays moving at different speeds. We found that most VI neurons were sensitive to the existence of a discontinuity in speed, that is, they showed higher responses when relative motion was presented compared to homogenous field motion. Seventy percent of the neurons in our sample responded predominantly to relative rather than to absolute speed. Relative speed tuning curves were similar at different center-surround velocity combinations. These relative motion-sensitive neurons in macaque area VI probably contribute to figure/ground segmentation and motion discontinuity detection.

Action Potentials↗

PubMed2004

Long-term prediction of incident hip fracture risk in elderly white women: study of osteoporotic fractures.

OBJECTIVES: To identify independent risk factors for first hip fracture over 10 years of follow-up. DESIGN: Prospective cohort study. SETTING: Four U.S. clinical centers. PARTICIPANTS: A total of 6,787 women aged 66 and older in the Study of Osteoporotic Fractures. MEASUREMENTS: Total hip bone mineral density (BMD) using dual-energy x-ray absorptiometry and a comprehensive set of potential risk factors were collected. Incident hip fractures were identified prospectively and confirmed using radiographic report. RESULTS: Six hundred two women (8.9%) had a hip fracture during a mean +/- standard deviation (SD) follow-up of 10.1 +/- 3.2 years. Older age, previous self-reported fracture after age 50, maternal history of hip fracture after age 50, greater height at age 25, impaired cognition, slower walking speed, nulliparity, type II diabetes mellitus, Parkinson's disease, and depth perception each independently predicted a 1.17- to 1.83-fold increase in hip fracture risk, whereas each SD (0.13 g/cm2) decrease in hip BMD was independently associated with a 1.84-fold increase in risk. Lower body mass index also was associated with an increased risk of hip fracture, although lower hip BMD largely explained this association. CONCLUSION: Although hip BMD is strongly related to hip fracture risk in elderly white women, other clinical risk factors also are independent predictors of long-term risk and provide additional insight into the prevention of fracture in high-risk women. Clinicians should be alert to factors other than BMD that place older women at a high risk of hip fracture.

Absorptiometry, Photon↗

PubMed1994

Stereopsis and positional acuity under dark adaptation.

Though experience tells us we can perceive depth in dim light, it is not so obvious that one of the chief mechanisms for depth perception, stereopsis, is possible under scotopic conditions. The only studies on human stereopsis in the dark adapted state seem to be those of Nagel [(1902) Zeitschrift für Psychologie, 27, 264-266] and Mueller and Lloyd [(1948) Proceedings of the National Academy of Science, U.S.A., 34, 223-227], both of which used real objects or line stereograms. We tested stereopsis using both random-dot and line stereograms and, in agreement with these studies, found that stereopsis is indeed possible in dark adaptation. We also measured stereo acuity and positional acuity (both of which are examples of hyperacuity) and compared these with grating acuity at several levels of light and dark adaptation. At all illumination levels tested, acuities for stereopsis and relative line position were both higher than for grating acuity. As light levels decreased, positional and grating acuity declined in parallel fashion, whereas stereoacuity declined more steeply.

Dark Adaptation↗

PubMed1998

Medical, morphological and functional aspects of Greek football referees.

OBJECTIVE: The aim of the study was to investigate state of health, morphological and functional profile of football referees (Rs). EXPERIMENTAL DESIGN: A prevalence study. PARTICIPANTS: 188 Greek active male Rs of A, B, C and D football divisions have been evaluated. MEASURES: Standard clinical-instrumental and anthropometric methods were used. Appropriate functional tests were used for evaluation of visual performance, physical condition and mental ability. RESULTS: Mean age, height, weight, and BMI were 36.3 +/- 4.5 years, 177.4 +/- 5.7 cm, 81.6 +/- 7.8 kg, and 25.9 +/- 2.1 kg.m-2 respectively. 64% and 6% were overweight and obese respectively. In 27.2% resting blood pressure exceeded upper normal limits. Active (41.5%) and former smokers (17.0%) were identified. Visual acuity and colour discrimination were defective in 2.4% and 0.7% respectively. Stereo depth perception deficiently in 7.1%. Compared to international standards, Rs' mean scores in the 6 physical condition field tests were found satisfactory. All Rs were found efficient in the agility test, two-thirds in the maximal and prolonged speed tests and half of them in the endurance test. Relatively more Rs of A and B divisions were found fit as compared to Rs of C and D divisions. Average IQ score was 112.5 +/- 11.1, while about 90% and 34% of the Rs' IQ score was > or = 100 and > or = 120 respectively. CONCLUSIONS: Average male Greek football R is an apparently healthy, young to middle aged, rather overweight, and functionally efficient. The great majority of Rs of A and B divisions appear better trained and somewhat better functional than Rs of C and D divisions.

Adult↗

PubMed2004

[An event-related potential study the on information processing of binocular disparity in random-dot stereogram].

OBJECTIVE: To study the effects of discontinuous-continuous disparity gradient, uncrossed-crossed disparity and small-large disparity in random-dot stereogram (RDS) on the event-related potentials (ERPs). METHOD: Behavioral data and ERP were recorded when 16 healthy participants (8 males and 8 females) with normal depth perception were performing the task, in which the stereopsis were generated by red-blue glasses. RESULT: 1) The accuracy in discontinuous disparity gradient was higher than that in continuous disparity gradient, and the accuracy in large disparity was higher than that in small one. 2) Discontinuous disparity gradient elicited shorter N160 latency and larger N350 amplitude than continuous disparity gradient did. Compared with uncrossed disparity, crossed disparity elicited shorter N350 latency. In addition, large disparity elicited shorter N160 latency and larger P280 amplitude than small disparity did. CONCLUSION: The effect of disparity gradient on disparity information processing is significant, and the discontinuous disparity gradient has its processing advantage. Latency of N160 and amplitude of P280 is correlated with the information processing of the size of disparity. N350 component is correlated with the information processing of the uncrossed-crossed information of disparity.

Adult↗

PubMed1994

Analysis of stereothresholds for stimuli below 2.5 c/deg.

We analyze published data on disparity detection thresholds for a wide range of conditions. This type of detection changes behavior dramatically at the spatial frequency of 2.5 c/deg; above this frequency threshold remains constant while below it threshold grows at a uniform rate. Many other types of threshold, such as upper disparity limits for depth perception and threshold amplitudes for stereo and monocular motion, show similar behavior. These data lead to the postulate that there are no foveal stereo channels peaking below 2.5 c/deg, so that foveal stimuli in the whole range below 2.5 c/deg are processed by a single channel tuned to this frequency. Consequently, disparity detection thresholds at frequencies below this frequency are controlled by the single parameter of effective contrast in the 2.5 c/deg channel, whose output depends jointly on the contrast and spatial frequency of the stimuli. We develop this idea to explain the relations between spatial and contrast tuning functions for disparity thresholds. To validate our conclusions, we describe an experiment with difference-of-Gaussian stimuli over a range of interocular widths and contrast differences. For a dichoptic width ratio of 2:1, the dichoptic contrast ratio required to minimize disparity detection thresholds was 1:4, just as predicted by the model.

Contrast Sensitivity↗

PubMedDate not supplied

Encoding of both vertical and horizontal disparity in random-dot stereograms by Wulst neurons of awake barn owls.

In binocular vision, the lateral displacement of the eyes gives rise to both horizontal and vertical disparities between the images projected onto the left and right retinae. While it is well known that horizontal disparity is exploited by the binocular visual system of birds and mammals to enable depth perception, the role of vertical disparity is still largely unclear. In this study, neuronal activity in the visual forebrain (visual Wulst) of behaving barn owls to vertical disparity was investigated. Single-unit responses to global random-dot stereograms (RDS) were recorded with chronically implanted electrodes and transmitted via radiotelemetry. Nearly half of the cells investigated (44%, 16/36) varied the discharge as a function of vertical disparity. Like horizontal-disparity tuning profiles, vertical-disparity tuning curves typically exhibited periodic modulation with side peaks flanking a prominent main peak, and thus, could be fitted well with a Gabor function. This indicates that tuning to vertical disparity was not caused by disrupting horizontal-disparity tuning via vertical stimulus offset, but by classical disparity detectors whose orientation tuning was tilted. When tested with horizontal in addition to vertical disparity, almost all cells investigated (92%, 12/13) were tuned to both kinds of disparity. The emergence of disparity detectors sensitive in two dimensions (horizontal and vertical) is discussed within the framework of the disparity energy model.

Animals↗

PubMed2003

Robust judgement of inter-object distance by an arthropod.

Animals use several strategies for depth vision, reflecting the constraints imposed by body size, the structure of the visual system and the visual geometry of the environment. Arthropods in particular have restricted depth perception, because they are small and possess closely set, low-resolution compound eyes. Yet, here we show that fiddler crabs defending their burrows from conspecifics can judge how close other crabs are to their burrow. When confronted with small dummy crabs, the burrow owners assess the dummy's position and motion relative to their burrow and not relative to themselves--in other words, by using an allocentric rather than an egocentric frame of reference. Irrespective of their own distance from the dummy, the likelihood that the crabs rush back to defend their burrow increases strongly as the dummy approaches the burrow. In addition, the mean dummy-burrow distance at which the crabs respond is constant and independent of the dummy's direction of approach. We propose that to solve this sophisticated task of relative distance judgement, the crabs combine visual information on dummy position and direction with information on burrow location acquired during path integration. In doing so, the crabs, like humans, make clever use of the visual geometry of their environment.

Animals↗

PubMedDate not supplied

Mechanisms of stereopsis in monkey visual cortex.

A substantial proportion of neurons in the striate and prestriate cortex of monkeys have stereoscopic properties; that is, they respond differentially to binocular stimuli that are known in humans to provide cues for stereoscopic depth perception. Stereoscopic neurons, as these cells may be called, are selective for horizontal positional disparity (i.e., display disparity selectivity) and for the textural correlation between images over their receptive fields (i.e., they show correlation selectivity). Many neurons have tuned disparity response profiles that collectively cover the entire range of physiological disparities. Neurons with peak responses at or about the zero disparity ("tuned zero neurons," excitatory or inhibitory) have narrow and symmetrical profiles. Neurons that are tuned to larger disparities, either crossed ("tuned near neurons") or uncrossed ("tuned far neurons"), have broader excitatory profiles that are asymmetrically wider toward the smaller disparities, and commonly include an inhibitory component about the zero disparity. Other stereoscopic neurons have reciprocal profiles ("near" or "far" neurons, respectively) in the sense that they respond with excitation to crossed or uncrossed disparities, and with suppression to disparities of opposite sign. Stereoscopic neurons can also signal the textural correlation between paired retinal images by giving different responses to random-dot patterns that have, and to those that do not have, the same dot distribution over the neuron's left and right receptive fields. Tuned-zero excitatory neurons characteristically respond to uncorrelation with suppression; tuned-zero inhibitory neurons, with excitation; and both types give the opposite responses to correlated stereopatterns. Neurons selective for nonzero disparities, both tuned and reciprocal, also give excitatory responses to uncorrelated stimuli, but these responses are smaller and more variable than those evoked by correlated patterns at the effective disparities. These findings suggest that stereoscopic neurons in the visual cortex of the macaque comprise three operational systems: (1) a zero-disparity system that is involved in fine depth discrimination with the obligatory singleness of vision, and the maintenance of vergence; and (2) a near-, and (3) a far-disparity system that together signal qualitative estimates of depth with double vision, and vergence responses to large disparities.

Animals↗

PubMed1997

A simple integrative method for presenting head-contingent motion parallax and disparity cues on intel x86 processor-based machines.

Rogers and Graham (1979) developed a system to show that head-movement-contingent motion parallax produces monocular depth perception in random dot patterns. Their display system comprised an oscilloscope driven by function generators or a special graphics board that triggered the X and Y deflection of the raster scan signal. Replication of this system required costly hardware that is no longer on the market. In this paper the Rogers-Graham method is reproduced with an Intel processor based IBM PC compatible machine with no additional hardware cost. An adapted joystick sampled through the standard game-port can serve as a provisional head-movement sensor. Monitor resolution for displaying motion is effectively enhanced 16 times by the use of anti-aliasing, enabling the display of thousands of random dots in real-time with a refresh rate of 60 Hz or above. A color monitor enables the use of the anaglyph method, thus combining stereoscopic and monocular parallax on a single display without the loss of speed. The power of this system is demonstrated by a psychophysical measurement in which subjects nulled head-movement-contingent illusory parallax, evoked by a static stereogram, with real parallax. The amount of real parallax required to null the illusory stereoscopic parallax monotonically increased with disparity.

Computer Terminals↗

PubMed1994

Clinical implications of the erroneous use of the Verhoeff Stereopter: a case report.

BACKGROUND: The Verhoeff Stereopter test is currently used by some branches of the military to evaluate depth perception for career placement. A 22-year-old male presented for consultation after failing the Verhoeff stereopter test during an examination for Marine flight training school. METHODS: A complete vision examination was conducted. Stereopsis was evaluated using the Verhoeff Stereopter, Randot Stereotest and the Multi-stereo test. RESULTS: A complete visual analysis revealed a mild refractive error, no ocular pathology, and normal binocularity. Stereopsis testing with the Verhoeff stereopter, Randot stereotest, and Multi-Stereo tests were normal. Subsequent investigation revealed that the original military tester did not follow proper testing protocol. When the erroneous testing condition was analyzed, we found that the stimulus horizontal disparity was eliminated. This lack of horizontal disparity makes it impossible to pass the test. We confirmed our findings by presenting the correct testing conditions and the erroneous testing conditions to five subjects with normal stereopsis. All subjects passed the Verhoeff test under standard protocols but failed under the erroneous condition. The critical factor that causes the elimination of horizontal disparity is identified. CONCLUSION: Although this instrument is not popular among vision care practitioners, it is being utilized by various organizations. An understanding of the theoretical basis for the instrument would therefore be beneficial.

Adult↗