Study On Visual Information Processing Mechanism On Fear Conditioning In Subcortical Visual Pathway Of Rat

Visual system is the most important system of animal to cognize the information in outside world. There are two visual pathways in the brain. One is cortical visual pathway, in which the cortical visual pathway is responsible for the fine processing and intelligent recognition of visual information. Another is subcortical visual pathway, including Superior Colliculus(SC) →Pulvinar(Pulv) →Amygdala(AMG).It is the edge structure of the early evolution of the brain, and mainly takes charge of the visual processing and transmission of affective information quickly, especially related to fear visual information. Fear visual stimuli can activate the subcortical visual pathway to transfer visual information quickly. The sources of the kind of stimuli include congenital visual fear information derived from animal nature,emotional faces obtained from long-term social activities, as well as conditioned fear visual information acquired according to animals’ postnatal training. In which, the visual information processing mechanism of fear conditioning acquired by animals’ postnatal training is similar to that of stress disorder.Current research about visual information processing mechanism on fear conditioning in subcortical visual pathway is still in the early stage. It is unclear about the structural connections and projecting relationship of neural loops in subcortical visual pathways. It has not been clearly revealed that how the subcortical visual pathway responded to the conditioned fear visual information, and under what kind of "trigger" condition the subcortical visual pathway could be activated by fear visual information. The interaction and influence between cortical and subcortical visual pathways are still unclear in the process of conditioned fear visual cognition.Therefore, the study on visual information processing mechanism of conditioned fear in subcortical visual pathway is an emerging issue in the field of neuroscience,psychology, intelligence science, and has high theoretical significance and practical value.In this study, Long Evans rats were used as experimental animals to track neuralloops in subcortical visual pathway with a neurotropic virus. A training experiment paradigm was established to build the relationship between unconditioned fear foot shock with image cognition. The effect of pulvinar in the subcortical visual pathway on triggering and transferring visual fear information is determined by comparing the responsive properties of neurons in the subcortical visual pathway before and after forming the fear visual formation. The information transfer mechanism of visual fear in the subcortical visual pathway is revealed through analyzing the effect of the correlation levels between the fear emotion and visual images on functional connectivity among neuronal population. The visual information transfer and interaction for fear were analyzed between three brain areas in cortical and subcortical pathways, under both unconscious and conscious cognitive levels. The main works and innovation of this paper are as follows:(1) A training paradigm of fear conditioned learning was established base on figure cognition. An automatical training system was designed and developed to relate the unconditioned fear punishment with specific visual image. The standard of the formed relation between fear and visual cognition was quantified using behavioral methods of animals. 9 experiment modes of "single figure" were designed for the learned fear by training, which were carried in different groups. The influences of the duration of conditioned stimulus and unconditioned stimulus on building learned visual fear were analyzed statistically, and then the experimental mode was optimized.The effect of training paradigm was verified with "double figures" training mode of learned visual fear.(2) The projections between neural loops in subcortical visual pathway of rats were tracked and the response signals of neuronal population in the key nucleus were collected. The structures of related neural circuit in subcortical visual pathway were tracked with neurotropic virus. The collection sites in both cortical and subcortical visual pathways were determined. The response signals of neuronal population in specific brain area were collected under both acute and chronic condition. The positions of implanted microelectrodes were verified using histological methods.(3) The new electrophysiological phenomenon was found that the delay time of response of SC and LP neurons in subcortical visual pathways decreased graduallyduring the period of visual formation of fear conditioning. It was implied that the pulvinar in subcortical visual pathway has played a “gating” effect on the information transfer of fear visual information after analyzing the peak value and peak latency of neuronal population response. The neural mechanism of rapid transmission of fear visual information in the subcortical visual pathways is revealed from the characteristics of response time.(4) It was found that the transmission efficiency of fear related visual information in subcortical pathways increased with the enhancement of fear emotion.According to initiative escape rate of rats in the process of fear conditioned training,the training process was divided into three phases, including before training, in the training and after training, which were used to represent the three different levels of fear emotion formed by cognizing specific visual image. Through comparing the functional connectivity of neuronal population of SC and LP in subcortical pathways under the same visual stimuli but different fear level, we found that as fear level enhanced, the characteristics of functional connectivity of neuronal population of SC and LP changed much, e.g. the connection strength, connection density and clustering coefficients increased significantly, while the average path length decreased.(5) The fear visual information transfer and interaction between brain areas of the two visual pathways were analyzed under two different cognitive levels of unconscious and conscious. It was found that subcortical visual pathway played a key role in forming fear information cognition under unconscious condition. The directed functional connection between brain areas of SC and LP in subcortical visual pathway and V1 area in cortical pathway was estimated using Granger Causality analysis method. The directed functional connection map showed that fear visual information was transmitted through synergy between the two visual pathways. The connection strength of SC→ LP in subcortical visual pathway was significantly enhanced, and the feedback connection strength from the primary visual cortex in the cortical visual pathway to SC and LP in the subcortical visual pathway decreased with the weaken of the cognitive level of consciousness. It was indicated that the subcortical visual pathway has played a major role in the process of unconscious fear visual cognition.