Scientific

Stereopsis – deriving information about distance by comparing views from two eyes – is widespread in vertebrates but so far known in only class of invertebrates, the praying mantids. Understanding stereopsis which has evolved independently in such a different nervous system promises to shed light on the constraints governing any stereo system. Behavioral experiments indicate that insect stereopsis is functionally very different from that studied in vertebrates. Vertebrate stereopsis depends on matching up the pattern of contrast in the two eyes; it works in static scenes, and may have evolved in order to break camouflage rather than to detect distances. Insect stereopsis matches up regions of the image where the luminance is changing; it is insensitive to the detailed pattern of contrast and operates to detect the distance to a moving target. Work from my lab has revealed a network of neurons within the mantis brain which are tuned to binocular disparity, including some that project to early visual areas. This is in contrast to previous theories which postulated that disparity was computed only at a single, late stage, where visual information is passed down to motor neurons. Thus, despite their very different properties, the underlying neural mechanisms supporting vertebrate and insect stereopsis may be computationally more similar than has been assumed.

Dual sensory impairment (DSI) refers to combined vision and hearing impairment. DSI concerns a large population and its prevalence increases drastically with age. In the US, it is estimated that 40% of patients seeking vision rehabilitation also have hearing loss. The majority of people with DSI still have functional vision and hearing and would therefore benefit from rehabilitation to maximize the use of their residual senses. However, vision and hearing rehabilitation traditionally have been two separate systems that are not readily addressing the unique challenges faced by people with DSI. The long-term goal of our research on DSI is to understand the interaction between impaired vision and impaired hearing in important everyday activities, develop novel assessment instruments to assess vision and hearing functions in a unified framework, and establish new rehabilitation strategies to maximize the use of residual vision and hearing in this population.   The specific topic I will present focuses on spatial localization, which refers to the ability to determine the direction and distance of people and objects around us. Spatial localization is critical for safe navigation and effective social interaction. For people with vision impairment, the challenges in spatial localization are often addressed by Orientation and Mobility training and assistive devices, both of which emphasize the use of hearing. For people with DSI, it is critical that we understand the interaction between residual vision and hearing before making rehab recommendations. Using a combination of screening tools, laboratory spatial localization tasks, and questionnaires, we asked how impaired vision and impaired hearing affect an individuals’ spatial localization ability, both independently and in combination. I will comment on the implications of our findings for DSI rehabilitation, and also share the challenges in conducting DSI research and some future directions. https://www.hopkinsmedicine.org/profiles/details/yingzi-xiong

Macular Degeneration (MD) represents the leading cause of visual impairment in the Western World. Late-stage MD eventually leads to the development of a central region of blindness (scotoma), which compromises their ability to perform everyday life activities like reading and recognizing faces. Patients spontaneously cope with this challenge by adopting compensatory oculomotor strategies, including the development of a new fixation region in their healthy peripheral retina, called Preferred Retinal Locus (PRL), that they use as a functional replacement of the fovea. Given the role of the fovea as a perceptual and oculomotor reference in the healthy visual system, consequences of central vision loss are far-reaching. Moreover, visual deficits in MD can be extensive yet subjectively subtle, often manifesting as distortions of the visual field or being masked by perceptual filling-in mechanisms. Patients are often unaware of characteristics of their visual deficits. Studies with simulated central vision loss in healthy participants suggest that increased awareness of these deficits could be beneficial for visual rehabilitation. Hence, it becomes apparent that perception, eye movements and visual field awareness are all altered by loss of central vision. Therefore, characterization of MD and designing of rehabilitative interventions should address the multifaceted nature of this condition. Here I will present a series of studies investigating perceptual, oculomotor and attentional characteristics of central vision loss, at baseline and after different training interventions. These interventions aimed at improving visual abilities, eye movements and scotoma awareness, in both patients with MD and healthy participants tested with gaze-contingent, simulated scotoma. Taken together, results suggest that a multidimensional approach to low vision might hold the keys for a better understanding of the effects of central vision loss on perceptual, attentional and oculomotor systems. Furthermore, such approaches might help develop successful interventions for patients with MD. 

We have developed robust and objective measures that do not depend upon the young patient’s cooperation or provider’s assessments of visual acuity and strabismus angle and quantifies the entire spectrum of visual function deficits in a fast, reliable, and pediatric-friendly way. The systematic analysis of fixation eye movement traces obtained in the lab in patients with binocular vision disorders has revealed several features that can be utilized to detect the presence and severity of amblyopia and angle and control of strabismus. We have found that fixation eye movement abnormalities correlate with reduced light sensitivities and depth perception and extent of suppression of vision experienced by these patients. We have also found that assessment of fixation eye movement characteristics can be a useful tool to predict functional improvement post amblyopia and strabismus repair. https://fescenter.org/team/investigators/ghasia-fatema-md/

The psychophysical and oculomotor literatures have long proposed that stereoscopic vision is served by multiple underlying subsystems. The subsystems are usually construed in terms of dichotomies, e.g  patent vs qualitative, local vs global,  first-order vs second-order, transient vs sustained, absolute vs relative, etc.  Whether these multiple sub-systems are all independent is a matter of debate and moreover, there is very little known about their neural basis.  Here I will describe recent work in which we attempt to unify two of the more prominent dichotomies — one spatial — absolute vs relative disparity and the other temporal — transient vs sustained.  In a series of high-density EEG experiments using dynamic random dot stereograms that alternate between a flat plane and a stereo grating at 2 Hz, we find that odd harmonic components of the response reflect relative disparity extraction and that even harmonic components reflect absolute disparity extraction.   Moreover, the odd harmonics reflect a sustained temporal mechanism, while the even harmonics reflect a transient mechanism. Taken together our results indicate that relative disparities are processed by sustained mechanisms while absolute disparity processing is transient in nature. https://profiles.stanford.edu/anthony-norcia

I will discuss human fMRI and MEG studies of binocular rivalry, including some recent work on individual differences. Alternations between bistable percepts provide a powerful platform to study visual awareness and suppression. The frequency tagging technique is used to record whole brain SSVEPs with analyses of power, coherence, and connectivity.  In untagged data, we explore the role of alpha band signals in perceptual stabilization. One theme is comparison of well matched binocular states, such as different types of rivalry. Another theme is the non-invasive measurement of interocular inhibition, which may be a partially heritable trait. There is also the possibility to discuss potential relationships to binocular vision disorders such as amblyopia.  http://mvr.mcgill.ca/Janine/research.html