|其他题名||The Cortical Plasticity of Visual Motion Processing|
|关键词||视觉运动 自身运动知觉 顶内沟腹侧 客体运动知觉 皮层可塑性|
训练是影响皮层变化的一种因素，自然老化也会造成皮层信息加工能力的改变。在客体运动知觉领域，研究者发现老化造成了老年人的在视运动知觉实验任务中存在空间抑制效应减弱的现象(Betts, et al, 2009; Betts, et al, 2005)。那么老化是否必然导致神经系统抑制效应的减弱呢?也许并非如此。我们分别采用运动互斥效应范式与空间抑制效应范式比较了年轻人与老年人在同样实验条件下的效应值，由于运动互斥效应被认为主要源于MT脑区对不同方向敏感神经元群体间的相互抑制，空间抑制效应源于中高级皮层脑区对初级皮层(V1)的反馈抑制，因此这项研究旨在探讨自然老化对视觉系统背侧通路中级皮层MT区和初级皮层V1区抑制功能的影响。我们发现尽管老年人比年轻人在空间抑制效应上表现出减弱趋势，在运动互斥效应上则表现为显著增强，意味着老化使对初级视皮层的反馈抑制减弱，但中级视皮层(MT)内侧抑制效应反倒是增强的。其神经机制可能是由于侧抑制源于对不同运动方向敏感的神经元群体间的相互抑制，而老化降低了神经元对运动方向编码的准确性，表现为调制曲线带宽增加且基线增高，进而使两个相互抑制的神经元群体调制曲线的重叠区域增加。因此，老化所引起的皮层功能变化并不一定必然导致神经系统抑制功能的减弱，要结合抑制类型来判断。
Although the development of neural system has a "critical period", there is still strong plasticity of the individual's cerebral cortex after the critical period. Recently, more and more studies showed that the plasticity of the cortex might occur at various stages of the visual pathway. The development of cortical plasticity not only requires to be driven by the bottom-up visual stimulation, but also requires top-down control, in order to promote and improve the ability of visual perception .
It is essential for most organisms to identify the motion of the object and to judge the heading direction, which is attributed to a series of areas in the dorsal pathway for the non-human primates and human beings. During the self-motion, the judgment of heading direction based on the optic flow pattern is accomplished by the MST area and VIP area, which belong to the higher level visual cortex. However, the direction of gabor and moving points could be recognized by the primary and secondary visual cortex.
In this study, we used the extracellular recording technique of the neural electrophysiology on the awake monkey, to examine the change of response characteristics for VIP neurons during the training of heading direction and the correlation between neuronal response and behavioral response. We would discuss
these following aspects: first, how does perceptual training affect the behavior of macaques and the response of VIP neurons? Second, could the plasticity of the neurons in VIP area sufficiently support the plasticity of the behavior? If not, what factors attribute to this enhancement?
As the possible function of the VIP area is involved in the neural computation of eye movement compensation during the heading perception. Our experiment used the optic flow to simulate people moving to different directions. We designed the fixation condition, the real pursuit condition, and the simulated pursuit condition in order to investigate the training effects of eye movement compensation on the behavioral level and the neuronal level. In the simulated pursuit condition, macaques focused a fixation point, the optic flow on the screen were the optic flow pattern on retina when the monkeys did the real pursuit. This manipulation ensured that the retina image during the simulated pursuit condition was the same as the real pursuit condition.
The results showed that, on the behavioral level,，there is a significant perceptual learning effect of the discrimination ability. The thresholds of rhesus monkeys for discriminating optic flow angle decreased. The sensitivity of behavior enhanced and the discrimination ability gradually increased. Moreover, the enhancement of discrimination ability was dependent on the condition of eye movement. For the eye movement compensation ability, the improvement of the eye movement compensation ability in the real pursuit condition cannot be transferred to the simulated pursuit condition. In other words, the macaques might adopt different direction discrimination strategies to solve the distortion of the retinal image problem, in order to identify the actual heading direction.
On the neural level, for the discrimination ability, perceptual training changed the information representation of heading perception in the VIP region. Perception training could make difference between maximum and minimum responses larger. The discrimination ability of VIP neurons could be significantly improved by the perceptual training. However, there is no significant interaction effects between the eye movement condition and the training stage which means that these improvements are common. For the eye movement compensation ability, perceptual training did not has significant effect on those neurons.
Comprehensive analysis of perception training effect on macaque behavior and response of VIP neurons, we found that although the correlation between the VIP neurons' response and the choice of macaque continuously improved, this enhancement might be derived from the improvement of discrimination ability of VIP neurons, rather than the ability of the eye movement compensation. Combination the result which the macaques might adopt different direction discrimination strategies to solve the distortion of the retinal image problem, we inferred that the functional role of VIP is mainly to encode perceptual information, rather than how to use this information to form accurate judgment and even to adjust reading-out rules.
Training is one of the factors that affect the change of cortex, and aging could also change the information processing ability of cortex. In the study of visual motion perception, researchers have found that aging has caused the weakening of the spatial inhibition effect of the elderly (Betts, et al, 2009; Betts, et al, 2005). However, is aging necessarily lead to a reduction in the inhibitory effect of the neural system? It is still unclear. In the second study, we used motion repulsion and surround suppression paradigm to compare the effect values of young and old people under the same experimental conditions. As the motion repulsion effects are thought to stem from mutual inhibition of MT areas neurons with different preferred directions, and the spatial inhibition effect is the result of the interaction between excitatory feedback connections and inhibitory horizontal connections, so our study aimed to explore the aging effect of inhibition mechanisms of MT area and V 1 area.
We found that motion repulsion effect was increased with aging and the surround suppression effect was reduced. These results suggested that aging caused the reduction of inhibitory effects of inter cortical feedback between the MT and the V 1 region. However, the cortical inhibition function of MT area was increased by aging. Because motion repulsion has been interpreted as arising from local mutual inhibition, which is that two groups of neurons with different preferred directions inhibit each other, our result suggest that aging may leads to enhanced local mutual inhibition. This might be due to the decrease of direction coding accuracy caused by aging, which is represented as the increase of the bandwidth and baseline of the neuron tuning curve, and then the overlapping regions of tuning curves increased. Therefore, the change of cortex function caused by aging does not necessarily lead to a reduction of the neural system inhibitory function, which should be judged by combining the suppression type.
|邓虎. 视觉运动信息加工的皮层可塑性[D]. 中国科学院大学. 中国科学院心理研究所,2018.|
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