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At least 685 records · Page 38Linked to original sources
PubMed1976

A home pleoptic method.

A method of initiating an after-image in an amblyopic eye with eccentric fixation by using Haidinger's brush for fixation control is described. This technique makes home pleoptics possible.

Afterimage↗
PubMed1977

Stereoscopic localization with the eyes asymmetrically converged.

When the eyes fixate a point in the median plane of the head, the physical surface to which the stereoscopic disparities of "nearer" and "farther" points are referred is the so-called frontal plane horopter. When, however, a point is fixated with the eyes in asymmetrical convergence, the horopter is now rotated with respect to the objective normal plane--that is, the surface that is normal to the direction of gaze. This rotation is believed to pose a problem with veridical stereoscopic localization. It has, therefore, been postulated that in asymmetrical convergence some physiological compensation takes place in order to preserve accurate stereoscopic perception. Some experimental evidence apparently supports this view. The logic of the arguments and the experiments are critically assessed with the conclusion that there is little evidence of any visual modification, nor would any seem likely. A reinterpretation of the stereoscopic axes of reference does occur, but this can be accounted for purely by innervational factors, the visual aspects remaining unchanged.

Afterimage↗
PubMed1987

Clinical method for measuring adaptation of tonic accommodation and vergence accommodation.

Previous work indicates that tonic adapters of accommodation and vergence have indirect effects on accommodative vergence and vergence accommodation and that these crosslink responses interact with one another to produce clear and single vision. Clinical measurements of tonic accommodation, tonic vergence, vergence accommodation (CA/C ratio), and accommodative vergence (AC/A ratio) are therefore of value in determining possible binocular vision abnormalities. Currently, clinical methods are only available for measuring the tonic vergence (phoria) and the AC/A ratio. We have determined a fast and accurate method for evaluating both tonic accommodation (accommodative phoria) and vergence accommodation (CA/C ratio). In both procedures, the loop of accommodation was opened with a 0.2 cpd difference of Gaussian (DOG) target. This target was found to be incapable of stimulating any reflex accommodation, yet it is localized in space and can easily stimulate binocular fixation. Our results indicate that opening the loop of accommodation with a pinhole Maxwellian view or with a 0.2 cpd DOG produces similar response values for tonic accommodative aftereffects and vergence accommodation responses. The decay rate of tonic accommodative aftereffects as well as changes in the resting focus of accommodation caused by vergence accommodation (CA/C) can be measured clinically using dynamic retinoscopy in combination with the DOG target. Clinical values of the CA/C ratio obtained with these techniques were highly correlated (r = 0.92) with laboratory measures obtained with an objective infrared optometer and eye trac monitor.

Accommodation, Ocular↗
PubMed2000

A functional angle on some after-effects in cortical vision.

The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons.

Adaptation, Ocular↗
PubMed1980

After-effects and the integration of patterns of neural activity within a channel.

Prolonged inspection of an adapting simulus changes the appearance of a subsequent test stimulus. There are five distinct viewing conditions under which such 'after-effects' may be generated. There are MON-MON (inspect with one eye, same eye), BIN-BIN (inspect with both eyes, test both eyes), BIN-MON (inspect with both eyes, test only one eye), MON-BIN (inspect with one eye, test with both) and TRANSFER (inpect with one eye, test with the other eye). A model based upon the assumption of the linearly additive effects of adaptation generated in 'dominance classes' or cortical units that are driven either by one eye, or the other eye, or by either or both eyes together, is described. This model generates predictions concerning the expected ralative magnitudes of after-effects generated under the five viewing modes described above, and experiments are described that confirm these predictions. The model can be extended to gaenerate predictions about other experimental conditions. A more complex version of the model is consistent with electrophysiologically derived estimates of the proportion of cortical units in each dominance class.

Afterimage↗
PubMed1980

The persistences of vision.

Human observers continue to experience a visual stimulus for some time after the offset that stimulus. The neural activity evoked by a visual stimulus continues for some time after its offset. The information extracted from a visual stimulus continues to be registered in a visual form of memory ('iconic memory') for some time after its offset. We may thus distinguish three distinct senses in which a visual stimulus may be said to persist after its physical offset: there is phenomenological persistence, neural persistence and informational persistence. Various assumptions have been made about the relation between these forms of visual persistence. The most frequent assumption is that they correspond simply to three different methods for studying a single entity. Detailed consideration of what is known about the properties of these three forms of persistence suggests, however, that this assumption is not correct. It can reasonably be proposed that visible persistence is the phenomenological correlate of neural persistence occurring at various stages of the visual system: photoreceptors, ganglion cells and the stereopsis system. Iconic memory on the other hand, does not correspond to visible persistence, nor to neural persistence in any stage of the visual system. Recent work, in fact, suggests that iconic memory is a property of some relatively late stage in the visual information-processing system, rather than being a peripheral sensory buffer store. This suggestion raises some fundamental theoretical issues concerning the psychology of visual perception, issues with which cognitive psychology has yet to come to grips.

Afterimage↗
PubMed1980

The perception of apparent movement.

When two similar pictures, overlapping but slightly displaced, were projected on a screen in alternation, apparent movement could be seen. How similar must successive pictures be to give apparent movement? This is the 'correspondence problem'. Manipulations of the local and global correspondences between pictures included motion phenomena such as reversed apparent movement; a four-stroke oscillatory cycle which gave an illusion of continuous motion in one direction; edges defined by texture, stereoscopic depth, or flicker, kinetic edges; and wave motion. It was concluded that human motion perception may comprise two separate mechanisms. Local point-by-point correlations between pictures are detected by a relatively peripheral system, probably based on directonally selective neural units. More subtle global correspondences are analysed by a more cognitive system which extracts edges before it process motion.

Afterimage↗
PubMed2005

Binding of motion and colour is early and automatic.

At what stages of the human visual hierarchy different features are bound together, and whether this binding requires attention, is still highly debated. We used a colour-contingent motion after-effect (CCMAE) to study the binding of colour and motion signals. The logic of our approach was as follows: if CCMAEs can be evoked by targeted adaptation of early motion processing stages, without allowing for feedback from higher motion integration stages, then this would support our hypothesis that colour and motion are bound automatically on the basis of spatiotemporally local information. Our results show for the first time that CCMAE's can be evoked by adaptation to a locally paired opposite-motion dot display, a stimulus that, importantly, is known to trigger direction-specific responses in the primary visual cortex yet results in strong inhibition of the directional responses in area MT of macaques as well as in area MT+ in humans and, indeed, is perceived only as motionless flicker. The magnitude of the CCMAE in the locally paired condition was not significantly different from control conditions where the different directions were spatiotemporally separated (i.e. not locally paired) and therefore perceived as two moving fields. These findings provide evidence that adaptation at an early, local motion stage, and only adaptation at this stage, underlies this CCMAE, which in turn implies that spatiotemporally coincident colour and motion signals are bound automatically, most probably as early as cortical area V1, even when the association between colour and motion is perceptually inaccessible.

Afterimage↗