, 2007). Screening techniques may include tests of residual vision and the measurement of thresholds for light perception in response to retinal electrical stimulation (Yanai et al., 2003); the majority of potential cortical implant recipients will likely be those with complete failure of both retinae or optic nerves,
in whom no responses to light will be observed. Potential recipients of a cortical visual prosthesis will need further assessment to determine the likelihood of successfully eliciting visuotopically ordered phosphenes via ICMS of visual cortex. In the normally-sighted, the functional development of visual cortex is guided by the presence of both spontaneous (prior to eye opening) and stimulated (after eye opening) retinal and cortical activity (Espinosa and Stryker, 2012). In the absence of visual input, the connectivity and architecture of visual cortex are altered. While magnetic buy PLX3397 resonance imaging (MRI) studies of the congenitally blind (CB) have shown preservation of geniculocalcarine tract fiber integrity (Schoth et al.,
2006 and Zhang et al., 2012), reductions in the volume of the LGN, geniculocalcarine tract and visual cortex (Ptito Carfilzomib datasheet et al., 2008b and Qin et al., 2013), increased thickness of primary visual cortex (Anurova et al., 2014 and Qin et al., 2013), and increased functional connectivity between visual and non-visual cortices (Collignon et al., 2013 and Qin et al., 2013) are seen in this subject group. From a functional perspective, Grape seed extract this reorganization of visual cortex is believed to reflect the process of sensory cross-modal adaptation, in which visual cortex is recruited for non-visual tasks, including Braille reading and auditory
processing (Burton et al., 2002 and Collignon et al., 2013). Such changes clearly have significant implications for the selection of potential visual prosthesis recipients, and the preoperative evaluation of responses to visual cortical stimulation will be an important component of the process. Direct electrical stimulation of visual cortex in the preoperative setting is not feasible, however transcranial magnetic stimulation (TMS) is a tool that may offer a method for noninvasively assessing potential cortical visual prosthesis implant recipients prior to surgery. Previous studies of occipital TMS in normally-sighted subjects have demonstrated that it can elicit simple phosphenes (Marg, 1991 and Merabet et al., 2003), while in blind subjects the responses to TMS differ between the early (EB) and late blind (LB). Gothe et al. (2002) used TMS to stimulate the occipital cortex of blind individuals subgrouped by the presence or absence of residual vision. Notably, no EB study participants without memory of vision reported phosphenes from occipital TMS.