Visual Pathway

Most of current concepts for a visual prosthesis are based on neuronal electrical stimulation at different locations along the visual pathways within the central nervous system. The different designs of visual prostheses are named according to their locations (i.e., cortical, optic nerve, subretinal, and epiretinal). Visual loss caused by outer retinal degeneration in diseases such as retinitis pigmentosa or age-related macular degeneration can be reversed by electrical stimulation of the retina or the optic nerve (retinal or optic nerve prostheses, respectively). On the other hand, visual loss caused by inner or whole thickness retinal diseases, eye loss, optic nerve diseases (tumors, ischemia, inflammatory processes etc.), or diseases of the central nervous system (not including diseases of the primary and secondary visual cortices) can be reversed by a cortical visual prosthesis. The intent of this article is to provide an overview of current and future concepts of retinal and optic nerve prostheses. This article will begin with general considerations that are related to all or most of visual prostheses and then concentrate on the retinal and optic nerve designs. The authors believe that the field has grown beyond the scope of a single article so cortical prostheses will be described only because of their direct effect on the concept and technical development of the other prostheses, and this will be done in a more general and historic perspective.. ( Surv Ophthalmol 47 :335-356, 2002.

We present the case of a patient having dissociative identity disorder (DID) who – after 15 years of misdiagnosed cortical blindness –  step by step regained sight during psychotherapeutic treatment. At first only a few personality states regained vision whereas others remained blind. This could be confirmed by electrophysiological measurement, in which visual evoked potentials (VEPs) were absent in the blind personality states but were normal and stable in the seeing states. A switch between these states could happen within seconds. We assume a top-down modulation of activity in the primary visual pathway as a neural basis of such psychogenic blindness, possibly at the level of the thalamus. VEPs therefore do not allow separating psychogenic blindness from organic disruption of the visual pathway. In summary, psychogenic blindness seems to suppress visual information at an early neural stage.

Visual evoked potentials (VEPs) offer reproducible and quantitative data on the function of the visual pathways and the visual cortex. Pattern reversal VEPs to full-field stimulation are best suited to evaluate anterior visual pathways while hemi-field stimulation is most effective in the assessment of post-chiasmal function. However, visual information is processed simultaneously via multiple parallel channels and each channel constitutes a set of sequential processes. We outline the major parallel pathways of the visual system from the retina to the primary visual cortex and higher visual areas via lateral geniculate nucleus that receive visual input. There is no best method of stimulus selection, rather visual stimuli and VEPs' recording should be tailored to answer specific clinical and/or research questions. Newly developed techniques that can assess the functions of extrastriate as well as striate cortices are discussed. Finally, an algorithm of sequential steps to evaluate the various levels of visual processing is proposed and its clinical use revisited.

Over the past decade much controversy has surrounded the hypothesis that the megachiroptera, or megabats, share unique neural characteristics with the primates. These observations, which include similarities in visual pathways, have suggested that the megabats are more closely related to the primates than to the other group of the Chiropteran order, the microbats, and suggests a diphyletic origin of the Chiroptera. To contribute data relevant to this debate, we used immunohistochemical techniques to reveal the architecture of the neuromodulatory systems of the Egyptian rousette (Rousettus aegypticus), an echolocating megabat. Our findings revealed many similarities in the nuclear parcellation of the cholinergic, putative catecholaminergic and serotonergic systems with that seen in other mammals including the microbat. However, there were 11 discrete nuclei forming part of these systems in the brain of the megabat studied that were not evident in an earlier study of a microbat. The o...

Ascending sensory information reaches primary sensory cortical areas via thalamic relay neurons that are organized into modality-specific compartments or nuclei. Although the sensory relay nuclei of the thalamus show consistent modalityspecific segregation of afferents, we now show in a wild-type mouse strain that the visual pathway can be surgically "rewired" so as to induce permanent retinal innervation of auditory thalamic cell groups. Applying the same rewiring paradigm to a transgenic mouse lacking the EphA receptor family ligands ephrin-A2 and ephrin-A5 results in more extensive rewiring than in the wild-type strain. We also show for the first time that ephrin-A2 and ephrin-A5 define a distinct border between visual and auditory thalamus. In the absence of this ephrin-A2/A5 border and after rewiring surgery, retinal afferents are better able to invade and innervate the deafferented auditory thalamus. These data suggest that signals that induce retinal axons to innervate the denervated auditory thalamus may compete with barriers, such as the ephrins, that serve to contain them within the normal target. The present findings thus show that the targeting of retinothalamic projections can be surgically manipulated in the mouse and that such plasticity can be controlled by proteins known to regulate topographic mapping.

Physiological research with monkeys and psychophysical research with humans has demonstrated the ability of diffuse red light to suppress activity in the magnocellular (M) visual pathway. In contrast, a previous psychophysical study found that a subset of nonpsychotic relatives of persons with schizophrenia showed the opposite effect, suggesting a novel biobehavioral marker for the disorder. Other research suggests that persons with schizophrenia have a dysfunctional M pathway under neutral (non-red) light conditions. However, it does not appear that these findings have been explored with physiological methods in nonpsychotic relatives, which would provide support for genetic contributions. The current study used physiological methodology to explore whether, as a group healthy adults show suppression of the M pathway in response to diffuse red light; 2) nonpsychotic relatives of persons with schizophrenia have a dysfunctional M pathway under neutral light conditions; and 3) nonpsychotic relatives of persons with schizophrenia have a differential M pathway response to red light. INDEX WORDS: schizophrenia, relatives, magnocellular visual pathway, red light, V5 iv ACKNOWLEDGEMENTS I would like to express sincere gratitude toward: my Major Professor, Dr. L. Stephen Miller -for continual helpful insight, support, and guidance on this project; Dr. James M. Brown -for crucial contributions regarding knowledge pertaining to the effect of red light on the magnocellular visual pathway and general functioning of the visual system; Dr. Nathan E. Yanasak -for assistance with operation of fMRI system (including physics parameter guidance) and fMRI data analysis; Drs. Nader Amir and Amos Zeichner -for helpful suggestions in the planning of the project and interpretation of findings; Dr. Jennifer McDowell -for helpful suggestions in the interpretation of background literature and current findings; Dr. Sharon Esposito and Advantage Behavioral Health Systems (Athens, GA) for help with recruitment of nonpsychotic relatives of persons with schizophrenia; and Gregory Strauss and Yfat Kessel for help with data collection.