Freedman David J, Assad John A
Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, Massachusetts 02115, USA.
Nature. 2006 Sep 7;443(7107):85-8. doi: 10.1038/nature05078. Epub 2006 Aug 27.
Categorization is a process by which the brain assigns meaning to sensory stimuli. Through experience, we learn to group stimuli into categories, such as 'chair', 'table' and 'vehicle', which are critical for rapidly and appropriately selecting behavioural responses. Although much is known about the neural representation of simple visual stimulus features (for example, orientation, direction and colour), relatively little is known about how the brain learns and encodes the meaning of stimuli. We trained monkeys to classify 360 degrees of visual motion directions into two discrete categories, and compared neuronal activity in the lateral intraparietal (LIP) and middle temporal (MT) areas, two interconnected brain regions known to be involved in visual motion processing. Here we show that neurons in LIP--an area known to be centrally involved in visuo-spatial attention, motor planning and decision-making-robustly reflect the category of motion direction as a result of learning. The activity of LIP neurons encoded directions of motion according to their category membership, and that encoding shifted after the monkeys were retrained to group the same stimuli into two new categories. In contrast, neurons in area MT were strongly direction selective but carried little, if any, explicit category information. This indicates that LIP might be an important nexus for the transformation of visual direction selectivity to more abstract representations that encode the behavioural relevance, or meaning, of stimuli.
分类是大脑为感觉刺激赋予意义的过程。通过经验,我们学会将刺激归为不同类别,如“椅子”“桌子”和“车辆”,这对于快速且恰当地选择行为反应至关重要。尽管我们对简单视觉刺激特征(如方向、朝向和颜色)的神经表征了解很多,但对于大脑如何学习和编码刺激的意义却知之甚少。我们训练猴子将360度的视觉运动方向分为两个离散类别,并比较了顶内沟外侧(LIP)和颞中区(MT)的神经元活动,这两个相互连接的脑区已知参与视觉运动处理。我们在此表明,LIP区的神经元——一个已知在视觉空间注意力、运动规划和决策中起核心作用的区域——由于学习而强烈地反映运动方向类别。LIP神经元的活动根据运动方向的类别成员关系对其进行编码,并且在猴子重新训练以将相同刺激归为两个新类别后,这种编码发生了变化。相比之下,MT区的神经元具有很强的方向选择性,但几乎不携带(如果有的话)明确的类别信息。这表明LIP可能是将视觉方向选择性转化为更抽象表征的重要枢纽,这些表征编码了刺激的行为相关性或意义。