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PubMed2005

Rapid plasticity of binocular connections in developing monkey visual cortex (V1).

The basic sets of cortical connections are present at birth in the primate visual system. The maintenance and refinement of these innate connections are highly dependent on normal visual experience, and prolonged exposure to binocularly uncorrelated signals early in life severely disrupts the normal development of binocular functions. However, very little is known about how rapidly these changes in the functional organization of primate visual cortex emerge or what are the sequence and the nature of the abnormal neural events that occur immediately after experiencing binocular decorrelation. In this study, we investigated how brief periods of ocular misalignment (strabismus) at the height of the critical period alter the cortical circuits that support binocular vision. After only 3 days of optically imposed strabismus, there was a striking increase in the prevalence of V1 neurons that exhibited binocular suppression, i.e., binocular responses were weaker than monocular responses. However, the sensitivity of these neurons to interocular spatial phase disparity was not significantly altered. These contrasting results suggest that the first significant change in V1 caused by early binocular decorrelation is binocular suppression, and that this suppression originates at a site(s) beyond where binocular signals are initially combined.

Animals↗
PubMed1987

The clinical assessment of binocular vision before and after a working day.

This paper looks at the clinical investigative and measurement procedures used in assessing binocular vision at the beginning of a normal working day and compared with the same procedures carried out in the afternoon which included several hours of close work. In 84 young adults, it was found that dissociated heterophoria, associated heterophoria and fixation disparity increased. This was accompanied by visual symptoms. It is likely that any decompensation of heterophoria is worse at the end of a working day. The results of this investigation raise the question of the value of some clinical tests for binocular assessment carried out at the beginning of the day.

Adolescent↗
PubMed2003

Stereoscopic processing in the human brain as a function of binocular luminance rivalry.

We investigated the neural substrates of a recent model of human stereodepth perception by obtaining measurements of regional cerebral blood flow (rCBF) using PET. Subjects experienced the perceptual properties of stereopsis by viewing rival-luminance stereograms displaying an identical random-dot pattern in their central portion while the backgrounds exhibited correspondent dots contrasting in black/white luminance. The stereoscopic vision induced by retinal luminance rivalry coincided with a significant elevation of rCBF in the dorsal visual pathway. Area V5 (MT) was activated bilaterally by the experimental condition while the remaining active loci were restricted to the right hemisphere. The neural sites that responded to this novel stereoscopic stimulus are similar to those activated by traditional stereograms containing horizontal disparities.

Adult↗
PubMed1987

Binocular interaction in the dorsal lateral geniculate nucleus of the cat.

We have investigated binocular interaction in the dorsal lateral geniculate nucleus (LGN) of the cat. Neurons were recorded extracellularly during visual stimulation with sinusoidal gratings which were presented at different interocular phases (disparities). The large majority of cells (91%) exhibited some type of binocular interaction. For 75% and 16% of the total number of cells, the binocular interaction was inhibitory or facilitatory, respectively. For the remaining 9% of cells, no interaction was evident. In marked distinction from visual cortex, the facilitatory and inhibitory interactions in the LGN are independent of the relative interocular phase of the patterns. Neurons in the LGN are therefore insensitive to the stereoscopic depth cue, retinal disparity.

Animals↗
PubMed1993

Spatial aspects of vertical phoria adaptation.

Vergence adaptation to vertical disparity spreads to unadapted directions of gaze. The spatial spread function for prism adaptation was estimated from aftereffects of a vertical disparity presented at a single position. Constraints limiting the spatial spread of adaptation were investigated with two stimuli of opposite disparity (hyper and hypo), presented at two different eye positions with a separation that varied from 6 to 18 deg in either the horizontal or vertical meridian. On average, phoria adaptation to the single point paradigm spread uniformly across the entire 18 deg test field. A resolution limit for adaptation to the two point paradigm was demonstrated by a reduction of phoria aftereffects with decreasing target separation (crowding). Vertical phoria aftereffects were reduced more by horizontal than by vertical crowding. A disparity gradient limit was demonstrated for a fixed target separation by a reduction of gain (phoria change/stimulus disparity) with increasing stimulus disparity.

Adaptation, Ocular↗
PubMed2002

Scale invariance in near space: pointing under influence of context.

This paper describes the results of a pointing task close to the observer. Unlike most research reported in literature, the stimuli were composed of real solid objects in an unambiguous context, free of any cue conflict. Moreover, the stimuli were either within arm-reach or just beyond arm-reach, thus having strong binocular depth cues. Surprisingly, systematic errors up to four times the standard deviation were found. These errors depended mainly on the variations in context and hardly on egocentric distance. In good first approximation, the results were scale invariant. These results are in direct conflict with classic theories about visual space. The stimuli were unambiguous real 3-D equivalents of the computer-generated stimuli we used in a previous experiment. A comparison of the results did not reveal a clear effect of a depth cue conflict between accommodation and disparity.

Adolescent↗
PubMed2001

Stereoprocessing of cyclopean depth images: horizontally elongated summation fields.

The study evaluated how the detection efficiency varies with the length and width of Gabor wavelets of depth ripple in random element patterns. For local (one-cycle) wavelets, there was no anisotropy with respect to wavelet orientation, implying equal efficiency for processing shear versus compression disparities at threshold. The processing for larger patterns of depth ripple did not correspond to a fixed summation field but varied in size with spatial frequency, and in shape with orientation of depth ripples, up to four cycles of a horizontal bar at 0.5 cy/deg, or 8 degrees of visual angle. The presence of such extended summation fields in only one orientation is incompatible with the idea of local attentional processing, with a single disparity channel or with an adaptive mechanism that could accommodate to any form of disparity image. Thus, these results suggest the presence of a multichannel hypercyclopean level of processing specialized for horizontal depth contours, whose only disparity information is in their surface texture.

Attention↗
PubMed1980

Parameters of fixation disparity.

This paper reviews the definition, causation, and significance of fixation disparity and suggests that the magnitude and character of fixation disparity depend on several major parameters: individual differences in the efficiency of motor and sensory processes of binocularity; natural or prism-induced demands on motor fusion; the presence or absence of foveal or parafoveal fusion clues; and the presence or absence of vergence adaptation. The clinical and research usefulness of fixation disparity as an indicator of binocular stress is emphasized.

Adaptation, Ocular↗
PubMed1985

Clinical evaluation of vertical fixation disparity. Part III. Adaptation to vertical prism.

The purpose of this study was to determine if any relation exists between the ability to adapt to vertical prism and the slope of the fixation disparity curve, and if ocular discomfort, while wearing the inappropriate vertical prism, is correlated with the adaptation ability. Patients who showed the greatest adaptation ability generally had flatter slopes than patients who showed the least adaptation ability. The presence or absence of symptoms did not indicate how the patient would adapt to vertical prism. We conclude that greater care and concern should be used in the consideration of an induced vertical prism if the slope of the fixation disparity curve is steep.

Adaptation, Ocular↗
PubMed2002

Health and peace.

Health and peace are closely linked. One cannot have one without the other. Although health and peace are desirable conditions, we human beings often thwart our best intentions to achieve and maintain them. War has profound impacts on human health. In addition to direct consequences, including the fact that 90% of all deaths related to recent wars were among civilians, war has several indirect consequences, including long-term physical and psychological adverse health effects, damage to the social fabric and infrastructure of society, displacement of people, damage to the environment, drainage of human, financial, and other resources away from public health and other socially productive activities, and fostering of a culture of violence. Many public health issues can be both a consequence and a cause of war, including infectious diseases, mental health disorders, vulnerability of population groups, disparities in health status within and among countries, and weakening of human rights. We, health professionals, can promote peace in many ways and facilitate this work by demonstrating our values, vision, and leadership.

Croatia↗
PubMed1998

Multiple-location access in vision: evidence from illusory line motion.

Four experiments with undergraduates used illusory line motion (ILM) to contrast Z. W. Pylyshyn's (1989) FINST theory of spatial indexing with predictions made by unitary attention models. Multiple-onset stimuli were able to cause ILM at disparate, noncontiguous spatial locations. Consistent with gradient explanations of ILM and with FINST theory predictions, varying line-drawing speed and the number of stimuli revealed a decrease in ILM and a capacity limitation, respectively. Modeling analyses suggested a limit in the number of locations (5-7) that could elicit the illusion. Requiring participants to report the locations of all stimuli exhibiting illusory motion in a specified direction suggested parallel access to between 2 and 5 display locations simultaneously. The results of all 4 experiments were predicted by FINST theory but not by a broad class of unitary attention hypotheses.

Acceleration↗
PubMed1980

Disparity-induced vergence responses in normal and strabismic subjects.

Latencies of vergence responses were obtained by presenting vertical line stimuli for 0.5 sec to the fovea of one eye and to various retinal locations of the other eye of four subjects with normal binocular vision, one exotrope with normal retinal correspondence, and four esotropes with anomalous correspondence. Vergence latencies were larger for squinters than nonsquinters and were especially long for the squinters with anomalous correspondence. For three of the four anomalous corresponders, the direction of the vergence movement (convergence or divergence) was based on the location of the stimulus with respect to the anomalous retinal point in the deviating eye--not with respect to the fovea as occurred for the subjects with normal correspondence.

Esotropia↗
PubMed1995

Association of acquired color vision defects in blue cone monochromatism.

The retinal functions of a 43-year-old man with diabetic retinopathy in a family affected by blue cone monochromatism were studied. While changes in visual acuity and color vision were similar to those seen in patients affected by rod monochromatism, and his spectral sensitivities on high intensity background and his threshold versus intensity curve indicated only suggestive blue cone function, mass electroretinograms (ERG) findings demonstrated well preserved blue cone function. The disparity in blue cone function between psychophysical and mass electroretinographical evaluation may be caused by the difference in how much the fovea influenced each testing. Diabetic retinopathy most likely caused significant reduction of the psychophysical blue cone function and resulted in clinical findings similar to rod monochromatism. The mass blue cone ERG was useful to differentiate blue cone monochromatism from rod monochromatism in this diabetic patient.

Adult↗
PubMed1985

Vision in the presence of known natural retinal image motion.

Previously we reported that failures of compensatory eye movements led to appreciable binocular retinal image motion during head rotation. Subjectively, the visual world appeared clear, fused, and stable under these conditions. The present experiments examined these impressions psychophysically. The spatial modulation transfer function of subjects with known retinal image motion was measured during head rotation. We found that contrast sensitivity was reduced for gratings over 6 cycles/degree and was increased for lower spatial frequencies. Our results, when compared with Kelly's [J. Opt. Soc. Am. 69, 1340-1349 (1979)] measurements made with artificially moving stabilized gratings, show that natural retinal image motion is less harmful to contrast sensitivity at high spatial frequencies and more beneficial at low spatial frequencies. Furthermore, we had previously found that natural retinal image motion was different in each eye during head movement but no diplopia was noticed. We confirmed this subjective impression by measuring forced-choice stereoacuity thresholds concurrent with binocular head and eye recordings. Stereoacuity was not disturbed by large fixation disparities or high vergence velocities. Recordings also were made while a fused Julesz stereogram was viewed during attempts to break fusion with violent head movements. Fusion could not be broken. Stereograms turned on during violent head movement fused rapidly. We conclude that vision is better with natural retinal image motion than expected from experiments done with stabilized heads.

Head↗
PubMed2003

Depth-related visually evoked potentials by dynamic random-dot stereograms in humans: negative correlation between the peaks elicited by convergent and divergent disparities.

Binocular visually evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to convergent and divergent stimuli, each having six disparity levels, using dynamic random-dot stereograms (DRDS). The VEP recorded consisted of a negative peak (N300) and a positive peak (P500) within intervals of 200-400 ms and 400-600 ms, respectively, with respect to the stimulus onset. For convergent disparities, the relationships between the amplitude of N300 and the degree of disparity showed convexity towards the disparity axis, whereas the same relationships displayed concavity for P500. For divergent disparities, on the other hand, the amplitude and the degree of disparity relationships showed concavity towards the disparity axis for N300, in contrast to those obtained by convergent disparities. Although the disparity profile of P500 displayed concavity in the left hemisphere, its right hemisphere counterpart turned out to be bi-modal in behaviour, indicating a relative loss of disparity sensitivity in the mid-disparity range. The significant negative correlation between the N300 and P500 behaviour in response to both stimulus modalities suggests that the activity of the N300 centre is effective in changing the synchronization level of the cell population comprising the P500 centre. The significant negative correlations between the profiles of the N300 wave in response to convergent DRDS and its divergent DRDS counterpart in both hemispheres, and between the profiles of the convergent and divergent P500 waves in the left hemisphere imply that the N300 and the P500 foci are sensitive to both the magnitude and the direction of the disparity. The latency differences between the two response modalities revealed that the N300 wave in response to convergent DRDS always led the N300 wave elicited by divergent DRDS, indicating that convergent disparities are processed faster than the divergent disparities.

Adolescent↗
PubMed2001

Visual indexes, preconceptual objects, and situated vision.

This paper argues that a theory of situated vision, suited for the dual purposes of object recognition and the control of action, will have to provide something more than a system that constructs a conceptual representation from visual stimuli: it will also need to provide a special kind of direct (preconceptual, unmediated) connection between elements of a visual representation and certain elements in the world. Like natural language demonstratives (such as 'this' or 'that') this direct connection allows entities to be referred to without being categorized or conceptualized. Several reasons are given for why we need such a preconceptual mechanism which individuates and keeps track of several individual objects in the world. One is that early vision must pick out and compute the relation among several individual objects while ignoring their properties. Another is that incrementally computing and updating representations of a dynamic scene requires keeping track of token individuals despite changes in their properties or locations. It is then noted that a mechanism meeting these requirements has already been proposed in order to account for a number of disparate empirical phenomena, including subitizing, search-subset selection and multiple object tracking (Pylyshyn et al., Canadian Journal of Experimental Psychology 48(2) (1994) 260). This mechanism, called a visual index or FINST, is briefly discussed and it is argued that viewing it as performing a demonstrative or preconceptual reference function has far-reaching implications not only for a theory of situated vision, but also for suggesting a new way to look at why the primitive individuation of visual objects, or proto-objects, is so central in computing visual representations. Indexing visual objects is also, according to this view, the primary means for grounding visual concepts and is a potentially fruitful way to look at the problem of visual integration across time and across saccades, as well as to explain how infants' numerical capacity might arise.

Cognitive Science↗
PubMed1996

Binocularity in infancy.

A variety of behavioural and electrophysiological studies agree that the onset of functional binocular interaction in human visual cortex normally occurs between 10 and 16 weeks of age. Measures of sensitivity to binocular correlation and to disparity agree closely, and behavioural and visual evoked potential measures on the same infant show onset of binocularity within about a 2 week range. Beyond the initial onset, the maximum disparity to which infants are sensitive increases steadily and stereoacuity is found to increase very rapidly. The initial development of binocularity does not appear to be a consequence of improving alignment of the eyes and occurs even in the presence of strabismus. However, the connections subserving binocularity are plastic in early childhood; they can be disrupted by unilateral strabismus, although in some strabismic children who use both eyes for fixation, they can adapt to serve stereo function at the angle of deviation and re-adapt, albeit temporarily, to the surgical alignment of the eyes. These findings allow us to pose some as yet unanswered questions about the development of binocularity, including: How is the infant's visual system organised before the establishment of binocularity? How does the pre-binocular infant maintain vergence? And what neural changes underlie the increase in performance for small and large disparities following the initial onset of binocular function?

Aging↗
PubMed2001

Vergence dynamics predict fixation disparity.

The neural origin of the steady-state vergence eye movement error, called binocular fixation disparity, is not well understood. Further, there has been no study that quantitatively relates the dynamics of the vergence system to its steady-state behavior, a critical test for the understanding of any oculomotor system. We investigate whether fixation disparity can be related to the dynamics of opponent convergence and divergence neural pathways. Using binocular eye movement recordings, we first show that opponent vergence pathways exhibit asymmetric angle-dependent gains. We then present a neural model that combines physiological properties of disparity-tuned cells and vergence premotor cells with the asymmetric gain properties of the opponent pathways. Quantitative comparison of the model predictions with our experimental data suggests that fixation disparity can arise when asymmetric opponent vergence pathways are driven by a distributed disparity code.

Accommodation, Ocular↗