Several GSK458 manufacturer considerations in the design of penetrating cortical electrode arrays for a visual prosthesis have been discussed throughout previous sections. Several additional major concerns are worthy of discussion, and these are briefly covered here. Multiple studies report a clear depth–threshold relationship for phosphenes elicited by electrical stimulation with penetrating microelectrodes (Bak et al., 1990, Bartlett and Doty, 1980, Bartlett et al., 2005, DeYoe et al., 2005, Koivuniemi et al., 2011 and Tehovnik et al., 2003). These
studies consistently show a dramatic reduction in threshold with increasing depth from the surface, to the extent that the ratio of maximum to minimum thresholds may be as high as 100:1 (Bak et al., 1990). Thus, penetration of electrodes to a depth at which the stimulus threshold for phosphene perception is minimized will be an important consideration in not only preventing current spread overlap and therefore maintaining the discriminability of phosphenes, but also for reducing total power consumption by the device. This latter point may be of critical importance http://www.selleckchem.com/products/ly2157299.html in future implant designs employing many hundreds of electrodes. The precise cortical depth at which phosphene detection thresholds reach a minimum remains a point of some conjecture. The early macaque studies of
Bartlett and Doty (1980) concluded that the lowest thresholds were found in layers V/VI of macaque visual cortex, corresponding to a depth of 1.5 mm. More recently, DeYoe et al. (2005) reported that layers III–IVb of macaque visual cortex consistently demonstrated the lowest thresholds. Conversely, Tehovnik et al. (2003) reported the lowest thresholds from the border of
layers V/VI (at a depth of 1.75 mm), later contending that the significant variation in threshold beyond layer III reported by DeYoe et al. (2005) may have been due to electrode damage (Tehovnik and Slocum, 2013). Bradley et al. (2005) implanted electrodes varying in length between 0.7 and 1.5 mm into the visual cortex of a macaque, however they made no specific comment on differences in stimulus current threshold at these varying depths. Torab et al. (2011) implanted 2 arrays of 100 electrodes each into the visual cortex of a macaque, noting that behavioral Phosphatidylethanolamine N-methyltransferase responses could only be elicited from 5/37 stimulated electrodes in one array, and 3/45 electrodes in the other. Notably, the electrodes were 1 mm in length, and the authors commented that the plane within which the electrode tips were situated was likely not parallel with that of the cortical laminae, resulting in variable penetration depth across the array. This also correlated with differences in the level of background neuronal activity, with those electrodes recording the highest levels of activity tending to be those that produced behavioral responses (Torab et al., 2011).