Department of Psychology, University of California, Berkeley, CA 94720.
Department of Psychology, University of California, Berkeley, CA 94720;
Proc Natl Acad Sci U S A. 2018 May 8;115(19):5010-5015. doi: 10.1073/pnas.1805044115. Epub 2018 Apr 23.
Working memory is capacity-limited. In everyday life we rarely notice this limitation, in part because we develop behavioral strategies that help mitigate the capacity limitation. How behavioral strategies are mediated at the neural level is unclear, but a likely locus is lateral prefrontal cortex (LPFC). Neurons in LPFC play a prominent role in working memory and have been shown to encode behavioral strategies. To examine the role of LPFC in overcoming working-memory limitations, we recorded the activity of LPFC neurons in animals trained to perform a serial self-ordered search task. This task measured the ability to prospectively plan the selection of unchosen spatial search targets while retrospectively tracking which targets were previously visited. We found that individual LPFC neurons encoded the spatial location of the current search target but also encoded the spatial location of targets up to several steps away in the search sequence. Neurons were more likely to encode prospective than retrospective targets. When subjects used a behavioral strategy of stereotyped target selection, mitigating the working-memory requirements of the task, not only did the number of selection errors decrease but there was a significant reduction in the strength of spatial encoding in LFPC. These results show that LPFC neurons have spatiotemporal mnemonic fields, in that their firing rates are modulated both by the spatial location of future selection behaviors and the temporal organization of that behavior. Furthermore, the strength of this tuning can be dynamically modulated by the demands of the task.
工作记忆是容量有限的。在日常生活中，我们很少注意到这种限制，部分原因是我们开发了行为策略来帮助缓解这种容量限制。行为策略在神经水平上是如何被调节的尚不清楚，但一个可能的位置是外侧前额叶皮层（LPFC）。LPFC 中的神经元在工作记忆中起着重要作用，并且已经证明它们可以编码行为策略。为了研究 LPFC 在克服工作记忆限制方面的作用，我们记录了在接受训练以执行序列自我排序搜索任务的动物中 LPFC 神经元的活动。该任务测量了在有前瞻性地计划选择未选择的空间搜索目标的同时，回顾性地跟踪哪些目标之前被访问的能力。我们发现，单个 LPFC 神经元不仅可以编码当前搜索目标的空间位置，还可以编码搜索序列中几个步骤之外的目标的空间位置。神经元更有可能编码前瞻性而不是回溯性的目标。当受试者使用刻板的目标选择行为策略来减轻任务的工作记忆要求时，不仅选择错误的数量减少，而且 LPFC 中的空间编码强度也显著降低。这些结果表明，LPFC 神经元具有时空记忆场，因为它们的放电率既受到未来选择行为的空间位置的调制，也受到该行为的时间组织的调制。此外，这种调整的强度可以根据任务的要求动态调节。
