Study On Inhibitory Synaptic Plasticity In The Visual Cortex

Humans urgently want to know how the brain works,but so far,the brain is still the black hole of human cognition.On April 2,2013,U.S.President Barack Obama announced BRAIN Initiative(Brain Research through Advancing Innovative Neurotechnologies),along with the Human Brain Project in the European Union and Brain/Minds project in Japan.In October 2015 China announced China Brain Project is about to start,and all of these show that brain science has become a hot research field.There are hundreds of billions of neurons in the human brain,and synaptic connection from a neuron to another one plays an important role in learning and memory.Synaptic plasticity is the ability of synapses to strengthen or weaken over time,in response to increases or decreases in neural activity.Vision is a very important organ to receive signals from the outside world.So the investigation of visual cortex and synaptic plasticity is more attractive.Both excitatory and inhibitory synapses have great influence on the visual system.Because of the limited research on the mechanism of inhibitory synaptic plasticity,the effects of the synaptic plasticity of inhibitory neurons on the visual system are not determined,so researching the inhibitory synaptic plasticity and its effect on the critical period for ocular dominance are meaningful.In this thesis,we use a simplified visual signal transmission system that is a three-layer feedforward model.The first layer is two groups of the excitatory neurons,the second layer one group of the inhibitory neurons,and both of them are simulated by Poisson neurons.The last layer is used to simulate them,and the last layer neuron in visual cortex is simulated by Leaky integrate-and-fire model.In this model,two groups of the excitatory input stand for left and right eyes,and one group of the inhibitory input is excited by the excitatory input.In this simplified model,both the excitatory and inhibitory synaptic plasticity are considered.Hebbian and anti-Hebbian learning windows for inhibitory synaptic plasticity are investigated.The results show that the average weight of inhibitory synapses delay to zero,close to the disconnected state of biological neurons,thus Hebbian learning of inhibitory synaptic is not suitable for this system.However,for anti-Hebbian learning window,the average weight of inhibitory synapses increases gradually and reaches a stable value,and the critical period for ocular dominance is short.Further,the effect and regulation of inhibitory synaptic plasticity on the critical period for ocular dominance are investigated.With the growth and development,the inhibitory synaptic weight increases,the range of the critical period is gradually reduced.That means inhibitory synaptic plasticity is a key cause that the critical period for ocular dominance is closed.In biological experiments,the critical period can be extended and reopened after closure by the drug or gene regulation.A regulation experiment is designed and the numerical simulation shows the regulation of decreasing inhibitory synaptic weighs can reopen the plasticity of ocular dominance.The regulation may provide a theoretical basis for the treatment of strabismus and amblyopia.