研究表明,海马细胞活动的内在触发与记忆的唤起有关。
人体海马神经元中的一些细胞能够增加它们响应特定目标、场景、范畴或人的活性,而在小鼠中,一套类似的神经细胞在特定的环境中也会被激活。如今,发表在《科学》杂志上的两项研究表明,这些细胞的活性同时也能够在内部产生,这可能与记忆的唤起有关。
美国新泽西州立大学的Gyorgy Buzsáki和同事训练小鼠穿过具有两个通道的迷宫,在这里小鼠被要求不是向左走就是向右走,以便获得奖励。在每次训练中,小鼠都能够记得上次训练所走的路,并选择相反的方向。在试验当中,这些小鼠有10到20秒的时间在一个转动的轮子中奔跑。与预期的一样,来自海马神经元的记录揭示了椎体细胞——即所谓的“位置细胞”——在小鼠在迷宫中行动时被短暂地激活了。然而,大约一半被记录的椎体细胞在小鼠于轮子中跑动时也表现出了类似的短暂活性:不同的神经细胞组合在特定的时间被激活,所以“情节神经细胞”的每种组合都有一个特定的“情节领域”。因此,在迷宫导航期间观察到的海马神经元的特定活性也发生在动物无需导航的时候。重要的是,这种情节活性与记忆和工作的编程特征相关,这是因为它并没有出现在转轮情节并未成为记忆的一部分的对照组中。
此外,作者指出,在轮子转动期间的细胞组合活性预示了动物在接下来的迷宫导航情节中将要作出的(向左或向右的)选择,即便对于那些在试验中错误的选择也是如此。这些发现表明,除了被环境线索触发外,连续的神经细胞组合行为能够在内部被触发并持续。据此推测,在轮子转动期间,小鼠回忆起它们之前的迷宫路径,为的是计划下一步所选择的方向。
在一项相关的研究中,美国加利福尼亚大学洛杉矶分校的Itzhak Fried和同事提供了直接证据,表明海马细胞活动的内在触发与记忆的唤起有关。他们对癫痫症患者进行了研究,这些人的大脑中被植入了电极。这些病人观看了有关名人、电视人物、事件或动物的一组声像短片。这些短片被放映了几遍,然后这些病人被要求口头回忆他们刚刚看过的片段。作为对一个或多个片段的响应,大脑内侧颞叶中超过一半被记录的神经细胞都表现出了短暂的活性提升。在这些细胞的激活速度以一种持续不断的形式上升期间,这些神经细胞中的一部分看起来具有一个“首选的”片段,至关重要的是,在海马和内嗅皮质中,选择性神经细胞在受试者口头回忆观看的短片前平均3秒钟也增加了它们的激活速度,但是其他神经细胞则在回忆“非首选的”短片时保持沉默。
这两项研究表明,海马神经元的选择活性不但发生在对特定环境线索的响应中,同时也能够在内部产生,这可能是回忆一段记忆引发了神经细胞更早前的活动。单个海马神经元内部产生的再活化可能表现为与神经系统有关的记忆唤起,而未来的研究将必须建立神经活动所表现的环境线索(或声像片段)的特征。(生物谷Bioon.com)
生物谷推荐原始出处:
Science 5 September 2008: DOI: 10.1126/science.1159775
Internally Generated Cell Assembly Sequences in the Rat Hippocampus
Eva Pastalkova, Vladimir Itskov,* Asohan Amarasingham, Gy?rgy Buzsáki
A long-standing conjecture in neuroscience is that aspects of cognition depend on the brain's ability to self-generate sequential neuronal activity. We found that reliably and continually changing cell assemblies in the rat hippocampus appeared not only during spatial navigation but also in the absence of changing environmental or body-derived inputs. During the delay period of a memory task, each moment in time was characterized by the activity of a particular assembly of neurons. Identical initial conditions triggered a similar assembly sequence, whereas different conditions gave rise to different sequences, thereby predicting behavioral choices, including errors. Such sequences were not formed in control (nonmemory) tasks. We hypothesize that neuronal representations, evolved for encoding distance in spatial navigation, also support episodic recall and the planning of action sequences.
Center for Molecular and Behavioral Neuroscience, Rutgers, The State University of New Jersey, 197 University Avenue, Newark, NJ 07102, USA.
Science 3 October 2008: DOI: 10.1126/science.1164685
Internally Generated Reactivation of Single Neurons in Human Hippocampus During Free Recall
Hagar Gelbard-Sagiv,1 Roy Mukamel,2 Michal Harel,1 Rafael Malach,1 Itzhak Fried2,3*
The emergence of memory, a trace of things past, into human consciousness is one of the greatest mysteries of the human mind. Whereas the neuronal basis of recognition memory can be probed experimentally in human and nonhuman primates, the study of free recall requires that the mind declare the occurrence of a recalled memory (an event intrinsic to the organism and invisible to an observer). Here, we report the activity of single neurons in the human hippocampus and surrounding areas when subjects first view cinematic episodes consisting of audiovisual sequences and again later when they freely recall these episodes. A subset of these neurons exhibited selective firing, which often persisted throughout and following specific episodes for as long as 12 seconds. Verbal reports of memories of these specific episodes at the time of free recall were preceded by selective reactivation of the same hippocampal and entorhinal cortex neurons. We suggest that this reactivation is an internally generated neuronal correlate for the subjective experience of spontaneous emergence of human recollection.
1 Department of Neurobiology, Weizmann Institute of Science, Rehovot, 76100, Israel.
2 Department of Neurosurgery, David Geffen School of Medicine, and Semel Institute for Neuroscience and Human Behavior, University of California Los Angeles, Los Angeles, CA 90095, USA.
3 Functional Neurosurgery Unit, Tel Aviv Medical Center and Sackler School of Medicine, Tel Aviv University, Tel Aviv, 64239, Israel.