Konecky R O, Smith M A, Olson C R
Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania.
Center for the Neural Basis of Cognition, Carnegie Mellon University, Pittsburgh, Pennsylvania; and.
J Neurophysiol. 2017 Jun 1;117(6):2269-2281. doi: 10.1152/jn.00541.2016. Epub 2017 Mar 22.
To explore the brain mechanisms underlying multi-item working memory, we monitored the activity of neurons in the dorsolateral prefrontal cortex while macaque monkeys performed spatial and chromatic versions of a Sternberg working-memory task. Each trial required holding three sequentially presented samples in working memory so as to identify a subsequent probe matching one of them. The monkeys were able to recall all three samples at levels well above chance, exhibiting modest load and recency effects. Prefrontal neurons signaled the identity of each sample during the delay period immediately following its presentation. However, as each new sample was presented, the representation of antecedent samples became weak and shifted to an anomalous code. A linear classifier operating on the basis of population activity during the final delay period was able to perform at approximately the level of the monkeys on trials requiring recall of the third sample but showed a falloff in performance on trials requiring recall of the first or second sample much steeper than observed in the monkeys. We conclude that delay-period activity in the prefrontal cortex robustly represented only the most recent item. The monkeys apparently based performance of this classic working-memory task on some storage mechanism in addition to the prefrontal delay-period firing rate. Possibilities include delay-period activity in areas outside the prefrontal cortex and changes within the prefrontal cortex not manifest at the level of the firing rate. It has long been thought that items held in working memory are encoded by delay-period activity in the dorsolateral prefrontal cortex. Here we describe evidence contrary to that view. In monkeys performing a serial multi-item working memory task, dorsolateral prefrontal neurons encode almost exclusively the identity of the sample presented most recently. Information about earlier samples must be encoded outside the prefrontal cortex or represented within the prefrontal cortex in a cryptic code.
为了探究多项目工作记忆背后的脑机制，我们在猕猴执行斯特恩伯格工作记忆任务的空间和颜色版本时，监测了背外侧前额叶皮层中神经元的活动。每个试验要求在工作记忆中保留三个依次呈现的样本，以便识别随后与其中一个匹配的探针。猕猴能够以远高于随机水平的准确率回忆起所有三个样本，表现出适度的负荷和近因效应。前额叶神经元在每个样本呈现后的延迟期内发出其身份信号。然而，随着每个新样本的呈现，先前样本的表征变得微弱并转变为异常编码。在最后延迟期基于群体活动运行的线性分类器，在需要回忆第三个样本的试验中，其表现大致与猕猴相当，但在需要回忆第一个或第二个样本的试验中，其表现下降的幅度比猕猴中观察到的要陡峭得多。我们得出结论，前额叶皮层的延迟期活动仅强烈表征了最近的项目。猕猴显然是基于除前额叶延迟期放电率之外的某种存储机制来完成这项经典工作记忆任务的。可能性包括前额叶皮层以外区域的延迟期活动以及前额叶皮层内未在放电率水平体现的变化。长期以来，人们一直认为工作记忆中保存的项目是由背外侧前额叶皮层的延迟期活动编码的。在此我们描述了与该观点相反的证据。在执行串行多项目工作记忆任务的猕猴中，背外侧前额叶神经元几乎只编码最近呈现样本的身份。关于较早样本的信息必须在前额叶皮层之外编码，或者以前额叶皮层内的隐秘编码形式呈现。
