据physorg网站2006年8月28日报道,短袜要放在短袜的抽屉里,衬衣要放在衬衣的抽屉里,这个从青少年阶段就要开始学起的古老训练教会了我们如何整理衣物。但是,大脑中的哪些部位是用来给像短袜、衬衫或其它这样的分类事物进行编码,它们又是怎样发生的呢?
美国哈佛医学院(HMS)的最新研究已经在大脑中确定了与这些记忆相联系的区域。研究员在提前出版的在线《自然科学》杂志中报道说,他们已经确定是神经元细胞的参与,将来自视觉的刺激分门别类。他们将大脑中某一部位神经元细胞的活性称为表皮层分类编码,也就是说,在学习的过程中,熟悉的视觉影像极大地改变了大脑的活动模式。他们的研究结果显示事物的分类是通过单独神经元(大脑细胞)的活性被存入大脑,而表皮层作为大脑电路系统的一部分用来学习和整理我们所看到的事物。
“以前人们并不知道大脑表皮层的活性模式能够在学习新事物的过程中发生如此大的变化,” 报告的主编、HMS神经生物学博士后研究员大卫•富莱德曼博士说。“大脑中的一些区域,特别是前叶和颞叶都与视觉分类记忆相关。正因为大脑中的这些区域全都内部互连,那么下一个重要步骤就是确定它们在分类记忆过程中各自所扮演的角色。”
我们并不是天生就具备像桌子、椅子和照相机那样的分类认知能力,相反,大脑中大多数的分类记忆都是通过经验的学习所产生。分类记忆是一切复杂行为的基础,因为它给我们周围视觉和听觉信息的意思。举例来讲,当你被告知一种新型的电子设备就是电话时,大脑中会立即产生一系列与其相关的部件信息(诸如听筒、话筒、拨号按键等等)以及它的功能。
我们基本上了解大脑处理如颜色、角度和运动方向等简单视觉特征的过程,但是对大脑是如何学习并分类记忆外界刺激却知之甚少。对相关视觉信息的分类处理能够使大脑根据它们的含义对外界信号进行整理以便我们快速地弄清发生在周围的一切。
试验中,猴子被训练完成一种简单的计算机游戏,它们要将分为两类的一系列视觉运动模式整理归一。随后富莱德曼和《自然科学》杂志高级作者、HMS神经生物学副教授约翰•阿撒德博士对游戏中的猴子进行研究,监视它们大脑中相互连接的外表皮层和中部颞叶层中神经元细胞的活性。外表皮层中的神经元活性完全反映了猴子在归类每种运动模式时的决定情况,相比之下,中部颞叶层的神经元则对每种类别中一系列运动模式的视觉区别更加敏感,而并不对它们各自的类别进行编码。
大脑表皮层的分类形式还随着学习和经验的增长而发生显著的变化。数个星期之后,试验的猴子被再次训练将归为一类的模式系列再重新分为两类。其大脑表皮层的编码分类被这次再训练彻底打破,而对新分类的视觉模式进行重新编码。
“这项研究帮助人们更深入的理解大脑是怎样学习和认识视觉影像的含义或意思并演示了新事物的学习过程如何引起大脑活性巨大而持久的变化,” 富莱德曼说。“我们将继续从事这项研究来确定大脑表皮只是单纯的进行运动类视觉刺激的分类处理还是能够更普遍的对包括形状在内的其它类视觉刺激进行分类处理。”
富莱德曼对这项研究持乐观态度并相信它最终会为神经学疾病及对神经紊乱的理解做出贡献。“了解大脑的学习、存储和认识过程并通过回忆视觉信息来帮助我们恢复由撞击、老年痴呆症及精神分裂症等大脑损伤所引起的功能障碍。”
英文原文:
Brain's Filing System Uncovered
New research from Harvard Medical School (HMS) investigators has identified an area of the brain where such memories are found. They report in the advanced online Nature that they have identified neurons that assist in categorizing visual stimuli. They found that the activity of neurons in a part of the brain called the parietal cortex encode the category, or meaning, of familiar visual images and that brain activity patterns changed dramatically as a result of learning. Their results suggest that categories are encoded by the activity of individual neurons (brain cells) and that the parietal cortex is a part of the brain circuitry that learns and recognizes the meaning of the things that we see.
“It was previously unknown that parietal cortex activity would show such dramatic changes as a result of learning new categories,” says lead author David Freedman, PhD, HMS postdoctoral research fellow in neurobiology. “Some areas of the brain, particularly the frontal and temporal lobes, have been associated with visual categorization. Since these brain areas are all interconnected, an important next step will be to determine their relative roles in the categorization process.”
We are not born with a built-in ability to recognize categories like table, chair, and camera. Instead, most categories such as these are learned through experience. Categories are a cornerstone of complex behavior, because they give meaning to the sights and sounds around us. For example, if you are told that a new electronic gadget is a telephone, this instantly provides a great deal of information about its relevant parts (speaker, microphone, keypad for dialing, etc.) and functions.
While much is known about how the brain processes simple visual features such as colors, angles, and motion-directions, less is known about how the brain learns and recognizes the meaning of stimuli. The process of grouping related visual images into categories allows the brain to organize stimuli according to their meaning and makes it possible for us to quickly make sense of our surroundings.
In these experiments, monkeys were taught to play a simple computer game in which they grouped members of a set of visual motion patterns into one of two categories. Freedman and senior author John Assad, PhD, HMS associate professor of neurobiology, then monitored the activity of neurons in two interconnected brain areas, the parietal cortex and the middle temporal area, while the monkeys played the categorization game. The activity of parietal neurons mirrored the monkeys’ decisions about which of the two categories each visual pattern belonged. In contrast, neurons in the middle temporal area were more sensitive to differences in the visual appearance among the set of motion patterns and did not encode their category membership.
Category representations in the parietal cortex also changed dramatically with learning and experience. Over the course of several weeks, the monkeys were retrained to group the same visual patterns into two new categories. Parietal cortex activity was completely reorganized as a result of this retraining and encoded the visual patterns according to the newly learned categories.
“This research helps to further the understanding of how the brain learns and recognizes the significance, or meaning, of visual images and demonstrates that learning new categories can cause dramatic and long-lasting changes in brain activity,” says Freedman. “We are continuing this work to determine if the parietal cortex is specialized for processing motion-based categories or if it plays a more general role in categorizing other types of visual stimuli, such as shapes, as well.”
Freedman is optimistic that research of this type will eventually contribute to a better understanding of neurological diseases and disorders. “Understanding how the brain learns, stores, recognizes and recalls visual information will help us overcome impairments to these functions caused from brain damage and diseases, including strokes, Alzheimer’s disease, and schizophrenia,” Freedman says.