PNAS最新 | 知覺收縮偏差不只在PFC，S2早期已顯現(xiàn)

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

發(fā)表時(shí)間：2025.12.23

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

影響因子：9.1

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引言

工作記憶（working memory, WM）讓我們?cè)诖碳はШ笕阅芏虝罕Ａ粜畔ⅲ?jù)此做出比較與決策。經(jīng)典觀點(diǎn)認(rèn)為，前額葉皮層（prefrontal cortex, PFC）等高階區(qū)域是WM的“主舞臺(tái)”，而感覺皮層更多負(fù)責(zé)輸入。但在延遲比較任務(wù)中，行為常出現(xiàn)系統(tǒng)性偏差：對(duì)第一個(gè)刺激的估計(jì)會(huì)向整體分布的均值靠攏，即“收縮偏差”（contraction bias/central tendency）。作者此前發(fā)現(xiàn)，PFC群體活動(dòng)在狀態(tài)空間（state space）里呈現(xiàn)“彎曲/壓縮”的幾何結(jié)構(gòu)，對(duì)應(yīng)貝葉斯觀察者模型（Bayesian observer model）里“先驗(yàn)+噪聲觀測(cè)”的整合結(jié)果。

這就引出關(guān)鍵問題：這種“帶偏的表征”究竟是PFC等高階區(qū)加工后才出現(xiàn)，還是在更早的感覺通路就已萌芽？本研究把目光投向次級(jí)軀體感覺皮層（secondary somatosensory cortex, S2），它參與觸覺處理，也被認(rèn)為與觸覺WM有關(guān)。若S2在刺激呈現(xiàn)甚至早期延遲就已出現(xiàn)類似的“幾何彎曲”，就意味著偏差可能是分布式網(wǎng)絡(luò)的早期產(chǎn)物，而非純粹由高階決策區(qū)“后期塑形”。

核心結(jié)果

1）行為學(xué)：收縮偏差直接體現(xiàn)在表現(xiàn)上。

兩只猴子都出現(xiàn)典型收縮偏差（contraction bias）：低f1被估高、高f1被估低，導(dǎo)致不同刺激類別正確率呈V形起伏（Fig. 1）。

2）模型：貝葉斯解釋成立，延遲拉長(zhǎng)偏差更強(qiáng)。

貝葉斯模型能擬合各類別曲線，并顯示延遲Δ越長(zhǎng)，f1不確定性相對(duì)更大（Σ1?Σ2變化），先驗(yàn)對(duì)f1后驗(yàn)的拉拽增強(qiáng)（Fig. 2，Table 1）。

3）神經(jīng)元：S2不靠“上一試次記憶”來制造偏差。

S2單元對(duì)當(dāng)前f1有信息，但對(duì)上一試次f1的信息接近基線，提示偏差不需要S2顯式保留短期歷史（Fig. 3）。

4）群體表征：S2早期就呈現(xiàn)“貝葉斯式幾何”。

群體軌跡距離隨f1呈S形扭曲（sigmoid），并在f1呈現(xiàn)期與延遲早期更貼近貝葉斯估計(jì)f1（f1,Bayes）而非真實(shí)f1；但S2隨延遲更快變?cè)耄琍FC更穩(wěn)定（Fig. 5–7，重點(diǎn)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.

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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分享人：BQ

審核：PsyBrain 腦心前沿編輯部