纹外皮层PMLS区反馈投射对初级视皮层神经元反应特性的影响

神经系统中存在大量下行投射,与上行输入一起形成复杂的前馈与反馈回路,调控神经信号的传导和处理,但目前对皮层内反馈投射的功能作用认识还比较薄弱.通过微量注射抑制性神经递质γ-氨基丁酸(γ-aminobutyric acid,GABA),使猫纹外皮层后内侧外上雪氏区(area posteromedial lateral suprasylvian,PMLS)局部可逆性失活,使用胞外记录方法,研究初级视皮层17区神经元反应特性的变化.实验结果显示,PMLS区失活后,17区细胞对运动刺激的反应总体减弱,反应的相对稳定性基本不变,最高发放率/自发之比有所下降.与此同时,细胞的方向选择性指数减小,朝向选择性无显著变化.除少数"双向"反应细胞外,绝大部分细胞的最优方向基本不变.进一步分析发现,细胞对各个方向刺激的反应普遍下降,最优方向上的下降程度最大,是导致方向选择性减弱的主要原因.这些结果表明,PMLS区反馈投射可增强初级视皮层的方向选择性,而对朝向选择性影响有限.这一作用特点体现了PMLS区在皮层中偏重处理运动视觉信息的功能.

Effects of feedback projection from cortical area PMLS on response properties of striate neurons

In the nervous system, numerous feedforward and feedback circuits are formed by descending projections together with ascending inputs. It has been shown that the transmission and processing of neural signals are modulated by these extremely complicated pathways, but little is known about the functional roles of feedback projections in the cortex. Area posteromedial lateral suprasylvian (PMLS) in the extrastriate cortex of the cat was inactivated locally and reversibly by microinjection of inhibitory neurotransmitter GABA, and extracellular recording was performed to investigate the changes in visual response properties of neurons in the striate cortex. The PMLS inactivation resulted in an overall reduction in the responses of striate neurons to moving stimuli, accompanied by little change in the relative stability of responsiveness and a decline in the ratio of maximal firing rate to spontaneous activity level. In the meanwhile, the direction selectivity index decreased, but the change of orientation tuning strength was insignificant, the preferred direction kept unvaried except for a few bi-directional cells. Further analysis revealed that the neuronal responses were reduced in all the stimulus directions, but most largely in the preferred direction, which might serve as the key factor to weaken the direction selectivity. These findings indicate that the feedback projection from PMLS can enhance the direction selectivity of striate neurons but exert little effect on the orientation tuning. It appears that the feedback modulation is characterized by the important role of PMLS in visual motion processing in the cerebral cortex.