Neurons in the PFC are typically activated by different cognitive tasks, and also by different stimuli and abstract variables within these tasks. A single neuron's selectivity for a given stimulus dimension often changes depending on its context, a phenomenon known as nonlinear mixed selectivity (NMS). It has previously been hypothesized that NMS emerges as a result of training to perform tasks in different contexts. We tested this hypothesis directly by examining the neuronal responses of different PFC areas before and after male monkeys were trained to perform different working memory tasks involving visual stimulus locations and/or shapes. We found that training induces a modest increase in the proportion of PFC neurons with NMS exclusively for spatial working memory, but not for shape working memory tasks, with area 9/46 undergoing the most significant increase in NMS cell proportion. We also found that increased working memory task complexity, in the form of simultaneously storing location and shape combinations, does not increase the degree of NMS for stimulus shape with other task variables. Lastly, in contrast to the previous studies, we did not find evidence that NMS is predictive of task performance. Our results thus provide critical insights on the representation of stimuli and task information in neuronal populations, in working memory.

SIGNIFICANCE STATEMENT How multiple types of information are represented in working memory remains a complex computational problem. It has been hypothesized that nonlinear mixed selectivity allows neurons to efficiently encode multiple stimuli in different contexts, after subjects have been trained in complex tasks. Our analysis of prefrontal recordings obtained before and after training monkeys to perform working memory tasks only partially agreed with this prediction, in that nonlinear mixed selectivity emerged for spatial but not shape information, and mostly in mid-dorsal PFC. Nonlinear mixed selectivity also displayed little modulation across either task complexity or correct performance. These results point to other mechanisms, in addition to nonlinear mixed selectivity, representing complex information about stimulus and task context in neuronal activity.

PFC 中的神经元通常由不同的认知任务以及这些任务中的不同刺激和抽象变量激活。单个神经元对给定刺激维度的选择性通常会根据其上下文而变化，这种现象称为非线性混合选择性 (NMS)。以前曾假设 NMS 是作为在不同环境中执行任务的训练的结果而出现的。我们通过在雄性猴子被训练执行涉及视觉刺激位置和/或形状的不同工作记忆任务之前和之后检查不同 PFC 区域的神经元反应来直接检验这一假设。我们发现训练诱导 PFC 神经元的比例适度增加，NMS 专门用于空间工作记忆，但不适用于形状工作记忆任务，其中 9/46 区域的 NMS 细胞比例增加最为显着。我们还发现，工作记忆任务复杂性的增加，以同时存储位置和形状组合的形式，不会增加 NMS 对刺激形状与其他任务变量的程度。最后，与之前的研究相比，我们没有发现 NMS 可以预测任务绩效的证据。因此，我们的结果为工作记忆中神经元群体中刺激和任务信息的表示提供了重要的见解。我们没有发现 NMS 可以预测任务绩效的证据。因此，我们的结果为工作记忆中神经元群体中刺激和任务信息的表示提供了重要的见解。我们没有发现 NMS 可以预测任务绩效的证据。因此，我们的结果为工作记忆中神经元群体中刺激和任务信息的表示提供了重要的见解。

重要性声明如何在工作记忆中表示多种类型的信息仍然是一个复杂的计算问题。据推测，在受试者接受复杂任务训练后，非线性混合选择性允许神经元在不同环境中有效地编码多种刺激。我们对训练猴子执行工作记忆任务之前和之后获得的前额叶记录的分析仅部分同意这一预测，因为非线性混合选择性出现在空间而非形状信息上，并且主要出现在背侧 PFC 中。非线性混合选择性在任务复杂性或正确性能方面也几乎没有变化。这些结果指向除了非线性混合选择性之外的其他机制，代表了神经元活动中关于刺激和任务背景的复杂信息。