Deficits of hierarchical predictive coding in left spatial neglect

Right brain-damaged patients with unilateral spatial neglect fail to explore the left side of space. Recent EEG and clinical evidence suggests that neglect patients might suffer deficits in predictive coding, i.e. in identifying and exploiting probabilistic associations among sensory stimuli in the environment. To gain direct insights on this issue, we focussed on the hierarchical components of predictive coding. We recorded EEG responses evoked by central, left-side or right-side tones that were presented at the end of sequences of four central tones. Left-side and right-side deviant tones produce a pre-attentive Mismatch Negativity that reflects a lower-order prediction error for the ‘Local’ deviation of the tone at the end of the sequence. Higher-order prediction errors for the frequency of these deviations in the acoustic environment, i.e. ‘Global’ deviation, are marked by the P3 response. We show that when neglect patients are immersed in an acoustic environment characterized by frequent left-side deviant tones, they display no pre-attentive Mismatch Negativity both for left-side deviant tones and infrequent omissions of the last tone, while they have Mismatch Negativity for infrequent right-side deviant tones. In the same condition, neglect patients show no P300 response to ‘Global’ prediction errors for deviant tones, including those in the non-neglected right-side, and omissions. In contrast to this, when right-side deviant tones are predominant in the acoustic environment, neglect patients have pre-attentive Mismatch Negativity both for right-side deviant tones and infrequent omissions, while they display no Mismatch Negativity for infrequent left-side deviant tones. Most importantly, in the same condition neglect patients show enhanced P300 response to infrequent left-side deviant tones, notwithstanding that these tones evoked no pre-attentive Mismatch Negativity. This latter finding indicates that ‘Global’ predictions are independent of ‘Local’ error signals provided by the Mismatch Negativity. These results qualify deficits of predictive coding in the spatial neglect syndrome and show that neglect patients base their predictive behaviour only on statistical regularities that are related to the frequent occurrence of sensory events on the right side of space.伴有单侧空间忽略（unilateral spatial neglect）的右脑损伤患者，无法对空间左侧进行探索。近期脑电图（EEG）与临床证据表明，忽略症患者可能存在预测编码（predictive coding）缺陷，即无法识别并利用环境中感觉刺激间的概率关联。为直接探究这一问题，本研究聚焦于预测编码的层级结构组分。我们记录了在四段中央音调序列末尾呈现的中央、左侧或右侧音调所诱发的脑电图反应。左侧与右侧的偏差音调会诱发出前注意失匹配负波（Mismatch Negativity，MMN），该成分反映了序列末尾音调的“局部”偏差所对应的低层级预测误差。针对声学环境中此类偏差发生频率的高层级预测误差（即“全局”偏差），则以P3反应（P3 response）作为标识。我们发现，当忽略症患者处于以左侧偏差音调高频出现为特征的声学环境中时，其既无法对左侧偏差音调以及末尾音调的罕见缺失诱发出前注意失匹配负波，却可对右侧偏差音调的罕见出现诱发出该负波。在该实验条件下，忽略症患者也无法对偏差音调（包括非忽略侧的右侧偏差音调）以及音调缺失所对应的“全局”预测误差产生P300反应。与之相反，当声学环境中右侧偏差音调占主导时，忽略症患者可对右侧偏差音调以及罕见的音调缺失诱发出前注意失匹配负波，但无法对罕见出现的左侧偏差音调产生该负波。最为关键的是，在该实验条件下，尽管左侧偏差音调未诱发出前注意失匹配负波，忽略症患者却对这类罕见出现的左侧偏差音调表现出增强的P300反应。这一结果表明，“全局”预测独立于由失匹配负波所传递的“局部”误差信号。上述结果细化了空间忽略综合征中预测编码的缺陷机制，并表明忽略症患者的预测行为仅基于与空间右侧感觉事件高频出现相关的统计规律。