Microstimulation with Chronically Implanted Intracortical Electrodes - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 308

as small spots of light, called phosphenes, were produced by 34 of the 38
implanted microelectrodes. Threshold currents for phosphene generation with
trains of biphasic pulses were as low as 1.9 A, and most of the microelec-
trodes had thresholds below 25 A. The phosphenes' brightness could be modified
with stimulus amplitude, frequency, and pulse duration. The phosphenes did
not flicker during the stimulation and ended quickly when the stimulation was
terminated. The apparent size of phosphenes ranged from a “pin-point” to a
“nickel” (20-mm-diameter coin) held at arm's length. Distinct phosphenes could
be elicited by microelectrode spaced as closely as 500m, suggesting that a
prosthesis based on ICMS might restore vision with good spatial detail.
The Anatomy and Physiology of the Visual System,
as they Relate to a Cortical Visual Prosthesis
The axons of the optic nerve and tract project onto the lateral geniculate nucleus of
the thalamus,which in turnprojects inanorderly fashiononto the striate cortexat the
posterior (occipital) pole of the brain. This “visuotopic” projection from the retina
onto the striate cortex creates a map of the corresponding half of the visual field in
the cortex. The macula lutea, the portion of the retina that mediates high-resolution
vision and thus the perception of spatial detail, is represented in the posterior part of
the striate cortex, while more peripheral regions of the retina (the visual field) are
representedmore anteriorly (Figure16.1). Themaculaoccupies onlya small portion
of the retina but is represented by a disproportionately large region of the cerebral
cortex, which is commensurate with its role in the perception of fine spatial detail;
this “cortical magnification factor” reflects the high density of photoreceptors in
and around the macula. The small size of the macula (a few mm in diameter) may
make it difficult to develop a retinal prosthesis that can deliver electrical stimulation
into this region with sufficient spatial detail so as to convey to a blind person a
facsimile of the high-resolution vision that this region subserves in a sighted person.
However, due to the cortical magnification factor, the macula is represented by
many square centimeters of cerebral cortex. Further, since this region is located at
the extreme posterior pole of the brain, surgical access is relatively easy, so a cortical
prosthesis that incorporates a large number of intracortical microelectrodes may
offer the best prospects for restoring useful central vision to blind person. However,
the topology of the cortical projection of more peripheral regions of the visual field
ismuch less favorable for a cortical-level prosthesis, since they project onto cortical
regions deeper within the central sulcus between the cerebral hemispheres and into
the depths of the calcarine sulci of both hemispheres.
There also is the intriguing possibility that such a prosthesis could convey
higher order visual percepts, many of which are mediated by neural circuits in
the “secondary” or “extra-striate” visual areas that surround the primary visual
cortex. For example, the perception of the speed and direction of a moving object
in the visual field appears to be mediated in the middle temporal cortex, usually
designated as visual area MT or V5 [2]. The high spatial selectivity afforded
by ICMS is well suited to access this neuronal circuitry. Thus microstimulation