Understanding Degeneration and the Capacity for Reorganisation in the Adult Human Visual Cortex

Loss of vision in infancy results in functional reorganisation of the visual cortex1-3;
this shows how the brain can adapt to an altered sensory input early in life. Much less
is known about how the adult brain reorganises in response to abnormal visual input
or how it might adapt to treatments that try to restore visual function. Since inherited
retinal diseases are the leading cause of blindness in the developed world, the demand
for therapy will increase in the next decade. Even where therapy is available, the brain
needs to be able to interpret the restored sensory input, and it is not clear whether this
will be possible following prolonged loss of input. In order to optimally restore
function it is vital to understand the principles of how the adult brain responds to a
loss of sensory input.
Stargardt disease (STGD) is a type of retinal disease in which central vision is most
affected. This project will measure the longitudinal changes in the visual system of
patients with STGD and will be completed under the supervision of Professor Holly
Bridge.
Aims
1) Cortical effects of retinal degeneration in STGD
Previous research has investigated structural and functional cortical changes in retinal
diseases4, 5. 25 STGD patients and 25 age-matched controls will be recruited to
undertake a cross-sectional investigation of the cortical changes associated with
STGD. This will be measured using Magnetic resonance spectroscopy imaging
(MRSI), structural MRI, diffusion-weighted imaging and population receptive field
(pRF) mapping. Clinical measures of retinal function will then be compared to these
measures.
2) Longitudinal investigation of reorganisation due to degeneration in STGD
Since retinal degeneration can be rapid in STGD, many patients show a decrease in
visual function after two years. To quantify the effects of this loss of input and
function on the visual cortex, patients will be rescanned after two years. By measuring
differences in the same MRI quantities, I will be able to directly demonstrate the
effects of retinal degeneration on the brain.
3) Assessing the potential for boosting plasticity in the healthy visual system
Increasing excitability in the brain is thought to boost the amount of reorganisation
that can take place; studies in adult animals have shown that this change can occur
when manipulating the neurochemical environment6. In order to maximise visual
function following visual restoration it may be necessary to boost the ability of the
cortex to respond to restored input. In the motor system tDCS has been used as an
adjunct procedure to increase behavioural effects of training or rehabilitation7, much
less is known of its effects in the visual system. 20 volunteers will be scanned using
MRSI and fMRI (7T) to investigate changes that occur following tDCS application
firstly to the occipital pole, and secondly to hMT+.
To summarise, this project will determine the direct consequences of visual loss due
to retinal degeneration, specifically the changes that occur in individual patients over
time. Additionally, this project will assess the potential to boost plasticity in the visual
system using tDCS, which may be important for optimising visual function.