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Biomedical subjects

M Cynader

Publications and source records attributed to M Cynader.

At least 73 records · Page 4Linked to original sources
PubMed1981

Inter-hemispheric competition during postnatal development.

Functional asymmetries between the two sides of the brain, a well documented phenomenon in species as different as frog and man, are thought to arise from genetically determined anatomical differences which, at least in humans, may be observed in utero. Functional asymmetries can, however, be reversed after damage to one side of the brain. Here we report that rearing of kittens with the optic chiasm sectioned and one eyelid sutured during postnatal development results in a functional asymmetry in the corpus callosum, a bidirectional pathway which inter-connects the visual cortices on the two sides of the brain. Visual input originating on the side of the brain ipsilateral to the sutured eye loses the ability to influence cells on the other side of the brain. Conversely, visual input originating on the side of the brain ipsilateral to the exposed eye markedly increases its influence in the other hemisphere.

Animals↗
PubMed1981

The eye movements of the dark-reared cat.

Cats reared in total darkness to adulthood have abnormal eye movements. A spontaneous nystagmus is found in the dark before any visual experience. The eye movements evoked by vestibular or optokinetic stimulation are less effective at compensation than for a normal cat. The vestibulo-ocular reflex (VOR) has a low gain (around 0.3) and a frequency dependent phase relation. The efficiency of optokinetic nystagmus (OKN) is poorer than for a normal cat, except for downwards stimulus movement which is followed better than normal. OKN is poorest in response to a stimulus viewed monocularly moving in the nasal to temporal direction. Neither VOR nor OKN of a dark-reared cat recover in efficiency within 5 months of the animal being brought into the light. A normal cat put into the dark for 135 days shows none of these abnormalities except an occasional spontaneous nystagmus.

Acclimatization↗
PubMed1981

Modification of the balance and gain of the vestibulo-ocular reflex in the cat.

The characteristics of the vestibulo-ocular reflex (VOR) of a normal cat can be modified in response to visual demands. Two aspects of the VOR are modifiable independently by a normal cat: the gain and the balance. An imbalance results in a spontaneous nystagmus and an asymmetric VOR. Neither the gain nor the balance of a dark-reared cat's VOR is susceptible to visual modification. A cat whose crossed visual pathways are severed at the level of the optic chiasm is able to modify the gain of the VOR but not its balance. Both dark-reared and split-chiasm cats have only very short-lasting optokinetic after-nystagmus.

Acclimatization↗
PubMed1981

Depth perception, eye alignment and cortical ocular dominance of dark-related cats.

On first exposure to light, animals that have been reared from birth until about 4 months of age in total darkness exhibit substantial visual and visuomotor deficits, which decline in severity during the first few months following exposure to light. In order to determine whether dark-reared animals eventually acquire stereoscopic vision following exposure to light we examined the binocular status of 5 dark-reared animals two of which developed convergent eye alignment. The binocular status was assessed behaviorally by measurements of the ability of each animal to perceive depth using either one or both eyes, and physiologically by documentation of the distribution of cortical ocular dominance of a sample of visual acuity, their binocular depth perception remained very poor, comparable to the monocular performance of normal cats. In marked contrast to normal animals none of the dark-reared animals, even those with normal eye alignment, performed substantially better binocularly than monocularly, a result indicating the absence of a uniquely binocular mechanism for depth perception in these animals. Although the dark-reared animals were found to retain a substantial (but reduced) complement of binocularly influenced cortical neurons, the tuning of these cells for retinal disparity must be insufficiently precise to mediate depth perception under binocular viewing conditions that is superior to that which can be achieved monocularly.

Animals↗
PubMed1981

Strabismus disrupts binocular convergence in cat nucleus of the optic tract.

Rendering kittens strabismic by surgical section of the medial rectus muscles results in disruption of binocular convergence in the nucleus of the optic tract. Visual input through the ipsilateral eye of strabismic cats does not influence cells in this nucleus although nearly one half of the cells encountered in normal cats receive inputs from both eyes. The results provide a basis for the deficits in optokinetic nystagmus found in strabismic cats and suggest a new mechanism underlying competition between inputs from the two eyes during postnatal development.

Animals↗
PubMed1981

[Interhemispheric competition during postnatal development].

The corpus callosum is a bidirectional pathway interconnecting the visual cortices on the two sides of the brain. Rearing kittens with the optic chiasm sectioned and one eyelid sutured during early development results in a functional asymmetry in this pathway. Visual input originating on the side of the brain ipsilateral to the sutured eye looses the ability to influence cells on the other side of the brain. Conversely, visual input originating on the side of the brain ipsilateral to the exposed eye markedly increases its influence in the other hemisphere. These physiologic findings are paralleled by anatomic results indicating that the terminal field of the corpus callosum is markedly expanded in the cortical hemisphere ipsilateral to the deprived eye and reduced in the other hemisphere.

Aging↗
PubMed1980

Eye movement in strabismic cats.

Strabismus is a common clinical condition in which the visual axes of the eyes do not intersect on the object being viewed. As such, the ability to achieve single binocular vision by fusing the images of a single object in the two eyes is lost. In most cases of strabismus, one of the two eyes is clearly deviated and the other eye is used for fixation, although in some cases each eye is used alternately for fixation. While much attention has been devoted to the motor capabilities of the deviating eye in strabismus, little attention has been given to the visuomotor competence of the other eye. We report here that, if one eye of a kitten is made to deviate by surgery, the visuomotor capacities of the other, "normal", eye are affected. A reduction in the ability to follow the movement of a large striped drum is observed with binocular viewing, even when stimuli are viewed monocularly with the normal eye. This means that anomalous visual input from the deviated eye during stimulation is not the cause of the reduced oculomotor capacities.

Animals↗
PubMed1980

Period of susceptibility of kitten visual cortex to the effects of monocular deprivation extends beyond six months of age.

The duration of the sensitive period of the kitten visual cortex to the effects of monocular deprivation was explored by studies of the behavioral and physiological recovery from extended periods of monocular occlusion imposed from birth, and by examination of the physiological effects of a 3 month period of monocular occlusion imposed on animals at either 4, 5, 6, 7 or 8 months of age. Animals monocularly deprived until 4 months of age eventually show considerable behavioral and physiological recovery from the severe deficits observed immediately following termination of the period of deprivation. The conclusion that binocular connectivity may still be altered by the nature of the animal's visual input beyond 4 months of age was supported by the results obtained from animals that were monocularly deprived at 4 months of age or older. Animals deprived at either 4, 5 or 6 months showed a clear shift of cortical ocular dominance in favour of the non-deprived eye, but those deprived at 7 or 8 months showed approximately normal ocular dominance distributions. It is concluded that the sensitive period lasts at least twice as long as previously thought, to between 6 and 8 months of age.

Aging↗
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↗
PubMed1979

Interocular alignment following visual deprivation in the cat.

Kittens were placed in the dark just after birth and then removed at various ages for the study of interocular alignment. It was found that kittens dark-reared for 4 months or longer were characteristically incyclotorted with respect to normal animals. Deprivation periods of less than 2 months were ineffective in producing these changes. Divergence of the visual axes was also observed in some dark-reared cats. Pupillary constriction in response to light was much more pronounced in dark-reared cats than in normal cats. This enhanced pupillary reaction persisted for at least 3 weeks after the deprived animals were brought into an illuninated environment. When dark-reared cats were allowed a recovery period in a normally lit visual environment, their ocular alignment changed markedly. The incyclotorsion and divergence of the visual axes disappeared, and instead cats allowed recovery from deprivation could display excyclotorsion and/or convergence of the visual axes. These anomalies of ocular alignment associated with the recovery from visual deprivation could occur following periods of initial deprivation as short as 30 days or as long as 2 years. The mechanisms and possible significance of such anomalies are considered.

Animals↗
PubMed1979

Role of visual cortex in interocular alignment.

The role which the visual cortex plays in the development of interocular alignment in the cat was examined by removing this structure bilaterally in 4 groups of subjects. These included (1) kittens 10 to 14 days of age, (2) 10- to 14-day-old kittens in which one eyelid was sutured shut at the same time, (3) normally reared adult cats, and (4) cats dark-reared until 4 months of age. If the cortex is removed in young kittens, interocular alignment appears to develop normally until the kittens are 60 to 80 days of age. At this time, an abrupt change in alignment resulting in incyclotorsion of the optic axes is observed. If binocular vision is prevented in kittens with neonatal visual cortex lesions by suturing one eyelid shut, convergent strabismus and/or incyclotorsion are frequently observed. This characteristic incyclotorsion does not develop if similar lesions are made in adult cats; no significant alterations of eye alignment occur in these animals even after postoperative survival times of more than 6 months. Incyclotorsion characterizes dark-reared cats when they are first brought into the light, but this diminishes with time and may even be replaced by excyclotorsion after the animals spend a few weeks in the light. If dark-reared cats are decorticated on being brought into the light, these changes are largely prevented. Such animals remain permanently incyclotorted relative to normal cats. The results indicate that the visual cortex plays an important role in the development of torsional alignment of the eyes.

Animals↗
PubMed1978

Neurones in cat parastriate cortex sensitive to the direction of motion in three-dimensional space.

1. On psychophysical grounds, Beverley & Regan suggested that in man different neural mechanisms mediate the binocular perception of movement in depth and the binocular perception of positional (static) depth. They proposed that the human visual pathway contains several neural mechanisms, each sensitive to a different direction of motion in space. These mechanisms compute the direction of motion from the relative speeds and directions of movement of the left and right retinal images.2. We have recorded from 101 units in area 18 of cat visual cortex, searching for neurones tuned to the direction of motion in three dimensions, with properties that could account for the proposed directionally tuned binocular motion detectors in man. The cat's left eye viewed one bar, while its right eye simultaneously viewed a second bar. Single units were stimulated by independently oscillating the bars from side to side. The apparent direction of movement in three dimensions was altered by varying the relative speeds of the bars and their relative directions of motion. The mean (positional) disparity of the bars could also be varied.3. For one class of neurone (twenty cells), binocular stimulation inhibited firing for trajectories parallel to the frontoparallel plane over a large volume of space. Strong firing was produced by oppositely directed bar movements. Some of these neurones were especially narrowly tuned to the direction of movement in depth, responding only to a range of 2-3 degrees , i.e. to moving bodies that would hit or only narrowly miss the cat. These cells emphasized the direction of movement at the expense of positional information.3. These units occurred in clusters. On the perpendicular penetrations in which they were found, they comprised a substantial majority of all cells encountered.5. For a second class of neurone (nine cells), binocular facilitation produced selective responses to objects moving along trajectories that missed the head.6. The two classes of neurone provide a basis for four proposed directionally tuned binocular motion detectors.7. A third class of neurone (seventeen cells) was selectively sensitive to movements parallel to the frontoparallel plane. There was strong binocular facilitation when the bars moved at the same speeds in the same directions: oppositely directed movements might be more than 100 times less effective. These neurones may signal positional disparity.8. These three classes of neurone cut across established categories. Only when both eyes were stimulated simultaneously with targets moving in different speeds and directions was it possible to demonstrate the binocular interactions described here.

Action Potentials↗