Page 382 - Handbook of Biomechatronics

376 Lilach Bareket et al.

risks of: infection (1%–2%) which could necessitate re-exploration and
removal of the cranial bone flap or its replacement, hemorrhage requiring
reoperation (1%), and possible injury to the sagittal or transverse venous
sinuses which are at the edge of an occipital craniotomy exposing the visual
cortex (Bjornsson et al., 2006). A craniotomy carries a small risk of infection
(1%–2%) which could necessitate re-exploration and removal of the cranial
bone flap or its replacement, and hemorrhage requiring reoperation (1%).
There is a small risk of injury to the sagittal or transverse venous sinuses
which are at the edge of an occipital craniotomy exposing the visual cortex.
Local brain injury due to the insertion of the electrodes may cause micro-
hemorrhage, local scarring, and loss of neurons. This would further impair
the transmission of current into the brain (Bjornsson et al., 2006).
Chronic focal stimulation of cerebral cortex may also cause development
of epilepsy through a process called kindling, where this focal area of cortex
becomes an independent generator of seizure activity (Goddard, 1967;
Morimoto et al., 2004). The seizure activity may spread to other parts of
the brain depending on the magnitude of electric currents passing through
the cortex, the duration of the stimulation, and the individual’s threshold for
seizure activation (Goddard, 1967; Morimoto et al., 2004). The potential for
kindling of epileptic seizures may be increased if the patient has a history of
epilepsy. The risk of seizures can be mitigated by low-stimulation currents
(10s of μA range), limiting temperature rise, intermittent stimulation pat-
terns rather than the same electrode firing constantly, and prophylactic anti-
epileptic drugs (AEDs). These drugs, however, may also render the neurons
less responsive (refractory) to the stimulation by the electrodes so a careful
balance should be achieved (Bezard et al., 1999).

4 ON AND LGN PROSTHESES
Besides the great progress in restoration of visual sensation through
retinal and cortical prostheses, electrical stimulation of the ON (Veraart
et al., 2003; Sakaguchi et al., 2012) or the LGN (Panetsos et al., 2011;
Pezaris and Eskandar, 2009) is also being investigated.
The first to attempt a visual prosthesis based on stimulation of the ON
were Veraart et al. (1998). The patient (blind due to RP) received a
four-contact silicon cuff array implanted around the intracranial section of
the ON. The four electrodes were located at 90-degree intervals, to enable
stimulation to the entire visual field. Colored phosphenes were reproducibly