Neural bases of visual processing of moving and stationary stimuli presented to the blind hemifield of hemianopic patients

Strabismus outcomes in pediatric patients undergoing disconnective hemispheric surgery for intractable epilepsy: a systematic review

BACKGROUND

Children undergoing hemispheric surgery for intractable seizures are susceptible to visual complications including strabismus. This systematic review aims to investigate the rates and characteristics of strabismus development after hemispheric surgery and evaluate clinical implications for ophthalmologic care.

METHODS

A systematic search of MEDLINE, EMBASE, Cochrane, PsychINFO, and Web of Science databases was performed from database inception to May 2022. Included articles referred to strabismus outcomes in pediatric populations after hemispherectomy or hemispherotomy. Reviews and non-English-language publications were excluded. Risk of bias was assessed using Joanna Briggs Institute critical appraisal tools. Demographic data and characteristics of strabismus were extracted and tabulated.

RESULTS

Of 41 articles identified, 10 studies consisting of 384 pediatric participants (48% females) and age at surgery between 6 months and 16 years were included. Preoperative strabismus rates ranged between 3% and 56%, whereas postoperative rates ranged between 38% and 100%. With respect to the site of hemispheric surgery, contralateral exodeviation was the most common (16%-67%; n = 7) and then ipsilateral exodeviation (16%-56%; n = 2), whereas ipsilateral esodeviation was infrequent (4%-9%; n = 3).

CONCLUSIONS

Contralateral exotropia and ipsilateral esotropia may occur after hemispheric surgery and may have the potential to be field expanding. Concerns regarding negative social reactions should be balanced with the risk of visual field reduction and (or) diplopia by strabismus surgery. Higher-quality articles with large, homogeneous, and well-described populations (i.e., complete pre- and postoperative ophthalmologic assessments) are required to establish the risks and rates of strabismus development after hemispheric surgery.

Tractometry of Human Visual White Matter Pathways in Health and Disease

Diffusion-weighted MRI (dMRI) provides a unique non-invasive view of human brain tissue properties. The present review article focuses on tractometry analysis methods that use dMRI to assess the properties of brain tissue within the long-range connections comprising brain networks. We focus specifically on the major white matter tracts that convey visual information. These connections are particularly important because vision provides rich information from the environment that supports a large range of daily life activities. Many of the diseases of the visual system are associated with advanced aging, and tractometry of the visual system is particularly important in the modern aging society. We provide an overview of the tractometry analysis pipeline, which includes a primer on dMRI data acquisition, voxelwise model fitting, tractography, recognition of white matter tracts, and calculation of tract tissue property profiles. We then review dMRI-based methods for analyzing visual white matter tracts: the optic nerve, optic tract, optic radiation, forceps major, and vertical occipital fasciculus. For each tract, we review background anatomical knowledge together with recent findings in tractometry studies on these tracts and their properties in relation to visual function and disease. Overall, we find that measurements of the brain's visual white matter are sensitive to a range of disorders and correlate with perceptual abilities. We highlight new and promising analysis methods, as well as some of the current barriers to progress toward integration of these methods into clinical practice. These barriers, such as variability in measurements between protocols and instruments, are targets for future development.

A Scientific Approach to Conscious Experience, Introspection, and Unconscious Processing: Vision and Blindsight

Although subjective conscious experience and introspection have long been considered unscientific and banned from psychology, they are indispensable in scientific practice. These terms are used in scientific contexts today; however, their meaning remains vague, and earlier objections to the distinction between conscious experience and unconscious processing, remain valid. This also applies to the distinction between conscious visual perception and unconscious visual processing. Damage to the geniculo-striate pathway or the visual cortex results in a perimetrically blind visual hemifield contralateral to the damaged hemisphere. In some cases, cerebral blindness is not absolute. Patients may still be able to guess the presence, location, shape or direction of movement of a stimulus even though they report no conscious visual experience. This "unconscious" ability was termed "blindsight". The present paper demonstrates how the term conscious visual experience can be introduced in a logically precise and methodologically correct way and becomes amenable to scientific examination. The distinction between conscious experience and unconscious processing is demonstrated in the cases of conscious vision and blindsight. The literature on "blindsight" and its neurobiological basis is reviewed. It is shown that blindsight can be caused by residual functions of neural networks of the visual cortex that have survived cerebral damage, and may also be due to an extrastriate pathway via the midbrain to cortical areas such as areas V4 and MT/V5.

Resolving visual motion through perceptual gaps

Perceptual gaps can be caused by objects in the foreground temporarily occluding objects in the background or by eyeblinks, which briefly but frequently interrupt visual information. Resolving visual motion across perceptual gaps is particularly challenging, as object position changes during the gap. We examine how visual motion is maintained and updated through externally driven (occlusion) and internally driven (eyeblinks) perceptual gaps. Focusing on both phenomenology and potential mechanisms such as suppression, extrapolation, and integration, we present a framework for how perceptual gaps are resolved over space and time. We finish by highlighting critical questions and directions for future work.

Visual fixation in disorders of consciousness: Development of predictive models to support differential diagnosis

The visual fixation represents a doubtful behavioral sign to discriminate Vegetative from Minimally Conscious State (MCS). To disentangle its meaning, we fitted univariate and multivariable logistic regression models matching different neurophysiological and neuroimaging data of 54 patients with Disorders of Consciousness to select the best model predicting which visual performance (visual blink or pursuit) was shown by patients and the best predictors set. The best models found highlighted the importance of the structural MRI and the visual evoked potentials data in predicting visual pursuit. Then, a qualitative pilot test was made on four patients showing visual fixation revealing that the obtained models correctly predict whether the patients' visual performance could support/correlate to a cognitively mediated behavior. The present pilot models could help clinicians to evaluate if the visual fixation response can support the MCS diagnosis.

Blindsight

For over a century, research has demonstrated that damage to primary visual cortex does not eliminate all capacity for visual processing in the brain. From Riddoch's (1917) early demonstration of intact motion processing for blind field stimuli, to the iconic work of Weiskrantz et al. (1974) showing reliable spatial localization, it is clear that secondary visual pathways that bypass V1 carry information to the visual brain that in turn influences behavior. In this chapter, we briefly outline the history and phenomena associated with blindsight, before discussing the nature of the secondary visual pathways that support residual visual processing in the absence of V1. We finish with some speculation as to the functional characteristics of these secondary pathways.