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 focused 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 (MMN) 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 characterised by frequent left-side deviant tones, they display no pre-attentive MMN both for left-side deviant tones and infrequent omissions of the last tone, while they have MMN for infrequent right-side deviant tones. In the same condition, neglect patients show no P3 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 MMN both for right-side deviant tones and infrequent omissions, while they display no MMN for infrequent left-side deviant tones. Most importantly, in the same condition neglect patients show enhanced P3 response to infrequent left-side deviant tones, notwithstanding that these tones evoked no pre-attentive MMN. This latter finding indicates that “Global” predictions are independent of “Local” error-signals provided by the MMN. 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.