For this reason it is not possible to see small or rapidly moving objects at very low light levels that allow only rod vision.

In designing image capture systems for prosthetic vision, researchers may want to borrow some of the principles employed by the retina. Increased integration times are commonly employed in CCD arrays and other electronic camera detectors, but integration across space is rarely employed for real-time image acquisition, as it runs counter to the designers’ wish to maximize spatial detail. Given the limited spatial resolution of early visual prostheses, however, such loss of spatial detail at the input should be of no consequence to the image perceived by the prosthesis wearer, and could be employed to achieve maximum sensitivity.

1.3.4  A Few Remarks Regarding Visual Development

Throughout this chapter we have seen that the principal pathway on which functional vision depends is that from the retina through the LGN to cortical area V1. Even the mechanisms of involuntary eye movement control (maintaining off-center gaze, microsaccades), which are served by pretectal pathways, can be compensated for – as appears from primate experiments –, presumably by virtue of corticosubcortical connections like those from V1 to the NOT and accessory optic nuclei. Hence, if visual impairment or blindness is caused by a disorder at the level of the eyes, optic nerve, or primary visual cortex, in a person whose visual function had followed its normal development earlier in life, one can assume that all cortical processing mechanisms and functions are intact, and may be successfully restored if adequate input signals are provided. If, on the other hand, normal visual development did not occur, as in the case of a congenital deficit of the retina or optic nerve, then a visual prosthesis implanted at a later age is unlikely to enable functional vision, similarly to the lack of functional vision documented in adult corneal transplant recipients who had congenital corneal opacities or corneal trauma in infancy [59]. Just like the cochlear prosthesis, however [54, 62], the visual prosthesis may provide opportunities for partial development of functional vision in children, provided the implantation takes place at a very young age, presumably in the first or second year of life. Obviously, this will require the technology to have been proven safe and effective in adults.

1.4  Prospects for Prosthetic Vision Restoration

On the basis of the architectural and functional layout of the human visual system described above, it should be clear that vision restoration through stimulation of intact structures along the retino-cortical pathway is feasible, in principle. Given the transformation of visual information that occurs at every stage along the visual pathway, the prudent approach in visual prosthetics would seem to be to implant as

Fig. 1.6  In this cross-section of the human retina-cortical pathway, seen from below, the numbers 1 through 4 indicate locations currently being considered for visual prosthesis implantation. Adapted from [46]

distally – that is, as early along the pathway – as feasible: If the photoreceptors are non-functional, then the bipolar or ganglion cells in the retina would be the best target for stimulation; if the retina is detached so a retinal prosthesis cannot be placed reliably, then an optic nerve implant would be indicated; if the retinal ganglion cells, and thus the optic nerve fibers, are damaged by glaucoma, then an implant in the LGN or primary visual cortex may be in order; etc. Figure 1.6 illustrates the four locations that seem best suited for the placement of visual prostheses.

In all these examples the assumption is that the visual pathway proximal to the lesion – that is, towards the brain – is intact, but their success will depend on the extent of secondary degeneration that may have occurred further along the visual pathway. Certainly it is known that many ganglion cells are lost after an extended period of outer retinal degeneration, but a substantial percentage survives, more

than enough to carry the small number of signals from today’s implants [52]. Moreover, the more central and the more invasive the surgery, the greater the risk of systemic and irreversible complications. Thus the idea to implant on the proximal side of the lesion, but as close to it as can be done safely, appears to be the implicit practice among most visual prosthesis groups.

The extent to which prosthesis recipients will be able to regain useful vision, and the duration required for functional rehabilitation, cannot be predicted until a larger number of patients has received a greater variety of implants than is currently the case; recent reports from two groups regarding letter recognition [18, 70], wayfinding [3], and maze tracing [5] by a small number of retinal implant recipients are encouraging indicators that a modest level of prosthetic vision is possible. From simulations in sighted volunteers (see Chap. 16) we have learned that seeing with pixelized vision is possible; yet the small electrode numbers in retinal arrays, the irregularity of phosphenes in cortical arrays, and the apparent differences between simulations with distinct dots and prosthetic percepts of broadly overlapping phosphenes will make the rehabilitation process an arduous one.

Acknowledgment  Supported in part by PHS grant # EY019991. This chapter is an adaptation of parts of an earlier chapter. [19] The author wishes to acknowledge the contributions of Eyal Margalit, M.D., who co-authored that chapter.

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Chapter 2

Vision’s First Steps: Anatomy, Physiology,

and Perception in the Retina, Lateral Geniculate

Nucleus, and Early Visual Cortical Areas

Xoana G. Troncoso, Stephen L. Macknik, and Susana Martinez-Conde

Abstract  This chapter reviews the functional anatomical bases of visual perception in the retina, the lateral geniculate nucleus (LGN) in the visual thalamus, the primary visual cortex (area V1, also called the striate cortex, and Brodmann area 17), and the extrastriate visual cortical areas of the dorsal and ventral pathways.

The sections dedicated to the retina and LGN review the basic anatomical and laminar organization of these two areas, as well as their retinotopic organization and receptive field structure. We also describe the anatomical and functional differences among the magnocellular, parvocelullar and koniocellular pathways.

The section dedicated to area V1 reviews the functional maps in this area (retinotopic map, ocular dominance map, orientation selectivity map), as well as their anatomical relationship to each other. Special attention is given to the modular columnar organization of area V1, and to the various receptive field classes in V1 neurons.

The section dedicated to extrastriate cortical visual areas describes the “where” and “what” pathways in the dorsal and ventral visual streams, and their respective physiological functions.

The temporal dynamics of neurons throughout the visual pathway are critical to understanding visibility and neural information processing. We discuss the role of lateral inhibition circuits in processing spatiotemporal edges, corners, and in the temporal dynamics of vision.

We also discuss the effects of eye movements on visual physiology and perception in early visual areas. Our visual and oculomotor systems must achieve a very delicate balance: insufficient eye movements lead to adaptation and visual fading, whereas excessive motion of the eyes produces blurring and unstable vision during fixation. These issues are very important for neural prosthetics, in which electrodes are stabilized on the substrate.

S. Martinez-Conde (*)

Barrow Neurological Institute, 350 W. Thomas Road, Phoenix, AZ 85013, USA e-mail: smart@neuralcorrelate.com