A NEW study has shown how patients with long-term blindness could respond to visual stimuli with the use of a retinal prosthetic implant after years of sight deprivation.
The Argus II Retinal Prosthesis System, a commercially available prosthetic, was surgically implanted into seven blind patients affected by retinitis pigmentosa, a hereditary condition leading to an incurable loss of sight. The prosthetic can detect visual signals and transmit them to the brain by stimulating axons of retinal ganglion cells. This allowed Ms Elisa Castaldi, a PhD candidate at the University of Pisa, Pisa, Italy, and her colleagues to explore whether the brains of adults with long-term blindness could process restored visual input after years of being unable to do so.
To be included in the study, individuals also had to have existing visual memory. “All of our participants had experienced vision for many years before becoming blind and their experience of vision was well beyond a ‘critical period’, which is a stage in the life-span where you need to be exposed to certain environmental stimuli,” Ms Castaldi told the EMJ. “Otherwise, your nervous system will never learn how to interpret these stimuli which might be very difficult, maybe even impossible to learn later in life,” she said.
After surgery, the research team found no improvement in the ability of the participants to discriminate motion direction but an improvement was measured in simple tasks related to visual detection. These included spatially localising the presence of high-contrast visual stimuli. An increase in blood oxygenation level-dependent (BOLD) activity in the primary visual cortex and the lateral geniculate nucleus areas of the brain was also found among participants, whereas prior to the surgery, activity in these regions were weak or absent. Higher increases of BOLD activity were seen in patients more sensitive to the visual detection tasks. “However, this change in brain activity, observed at both the thalamic and cortical level, took extensive training over a long period of time to become established,” Ms Castaldi said. “The more the patient practiced, the more their brain responded to visual stimuli, and the better they perceived the visual stimuli using the implant. In other words, the brain needs to learn to see again.”
Jack Redden, Reporter