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A069 PNAS最新 | 知觉收缩偏差不只在PFC，S2早期已显现

来源：公众号 PSY-Brain_Frontier ｜发布：2025-12-26 ｜原文链接

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基本信息：

Title: Early emergence of perceptual biases in the secondary somatosensory cortex

发表时间：2025.12.23

Journal:Proceedings of the National Academy of Sciences（PNAS）

影响因子：9.1

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引言

工作记忆（working memory, WM）让我们在刺激消失后仍能短暂保留信息，并据此做出比较与决策。经典观点认为，前额叶皮层（prefrontal cortex, PFC）等高阶区域是WM的“主舞台”，而感觉皮层更多负责输入。但在延迟比较任务中，行为常出现系统性偏差：对第一个刺激的估计会向整体分布的均值靠拢，即“收缩偏差”（contraction bias/central tendency）。作者此前发现，PFC群体活动在状态空间（state space）里呈现“弯曲/压缩”的几何结构，对应贝叶斯观察者模型（Bayesian observer model）里“先验+噪声观测”的整合结果。

这就引出关键问题：这种“带偏的表征”究竟是PFC等高阶区加工后才出现，还是在更早的感觉通路就已萌芽？本研究把目光投向次级躯体感觉皮层（secondary somatosensory cortex, S2），它参与触觉处理，也被认为与触觉WM有关。若S2在刺激呈现甚至早期延迟就已出现类似的“几何弯曲”，就意味着偏差可能是分布式网络的早期产物，而非纯粹由高阶决策区“后期塑形”。

核心结果

1）行为学：收缩偏差直接体现在表现上。

两只猴子都出现典型收缩偏差（contraction bias）：低f1被估高、高f1被估低，导致不同刺激类别正确率呈V形起伏（Fig. 1）。

2）模型：贝叶斯解释成立，延迟拉长偏差更强。

贝叶斯模型能拟合各类别曲线，并显示延迟Δ越长，f1不确定性相对更大（Σ1−Σ2变化），先验对f1后验的拉拽增强（Fig. 2，Table 1）。

3）神经元：S2不靠“上一试次记忆”来制造偏差。

S2单元对当前f1有信息，但对上一试次f1的信息接近基线，提示偏差不需要S2显式保留短期历史（Fig. 3）。

4）群体表征：S2早期就呈现“贝叶斯式几何”。

群体轨迹距离随f1呈S形扭曲（sigmoid），并在f1呈现期与延迟早期更贴近贝叶斯估计f1（f1,Bayes）而非真实f1；但S2随延迟更快变噪，PFC更稳定（Fig. 5–7，重点Fig. 6）。

Fig. 1. Task design, stimulus sets, and behavior.

Fig. 2. Bayesian fits of the accuracy curves.

Fig. 3. S2 neurons exhibit significant mutual information (MI) about currenttrial f1, but not about the previous-trial f1.

Fig. 4. State-space trajectories and their relative distances.

Fig. 5. Averaged relative distances for monkey one.

图10

Abstract

Working memory (WM) is distributed across multiple cortical areas, suggesting that behaviors relying on WM arise from interactions between these regions. In a recent study, we found that during delayed comparison tasks, the first stimulus is not represented veridically in the prefrontal cortex (PFC), but instead is encoded in a systematically warped manner–biased toward the mean of the stimulus distribution. This neural distortion, which emerges already during the stimulus presentation and persists throughout the delay period, closely mirrors a contraction bias observed in behavior. Furthermore, the behavioral responses could be explained by a Bayesian observer model, in which the observer integrates prior expectations with noisy sensory inputs. These results suggest that the geometry of PFC neural trajectories embodies Bayesian estimates that underlie biased decisions. Here, we investigate whether the secondary somatosensory cortex (S2)–a lower-level sensory area also implicated in tactile WM–exhibits a similar encoding structure. Our analyses reveal that although WM-related signals in S2 are less robust than in PFC, the neural state space in S2 shares key geometric features with that of PFC, including a similarly warped representation of stimulus values. These findings suggest that perceptual biases may originate early in the cortical processing stream and are not exclusively shaped by higherorder associative areas. More broadly, our results support a distributed organization of representational warping, in which even sensory areas contribute to the formation of bias-prone representations that guide behavior.

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