Brain Responses at Encoding Predict Limited Verbal Memory Retrieval by Persons with Schizophrenia

Abstract

Objective: Special attention has been given to verbal memory deficits in schizophrenia because they are apparent in healthy biological relatives of affected individuals, indicating a link to genetic risk for the disorder. Despite a growing consensus that encoding abnormalities contribute to poor verbal memory in the disorder, few studies have directly examined how neural responses during encoding contribute to later memory performance.

Method: We evaluated event-related potentials (ERPs) during encoding of verbal material by patients with schizophrenia, healthy first-degree biological relatives of patients, and healthy controls. The extent to which N1, N400, and anterior and parietal Late Positive Components (LPCs) explained encoding accuracy and later memory of material was investigated.

Results: Encoding accuracy was associated with asymmetry in anterior LPCs toward right frontal brain regions and was most evident in relatives. N1 was abnormal at encoding in schizophrenia and differentially accounted for later memory performance. In controls better recall of verbal material was predicted by a larger early occipital (N1) encoding response; however, in patients with schizophrenia smaller N1 encoding responses were related to better recall. Interestingly, better recognition of verbal material across groups was also predicted by smaller N1 amplitudes during encoding of word stimuli.

Conclusion: Separable patterns of electrophysiological response during encoding appear to differentially support recall and recognition of material from memory. Similar patterns of electrophysiological response across patient and relative groups suggest that those who carry genetic liability for schizophrenia share deviations in the neural activity related to encoding of material into episodic memory.

Fig. 1.

Left: Patients with schizophrenia had significantly smaller N1 than controls in response to words at encoding, as seen in the waveform plot at left. This was in part explained by the interaction between participant group and N1 in predicting later free recall performance. Right: As shown in the scatterplot at right, “smaller” (i.e., less negative) encoding N1 amplitudes predicted better free recall performance in the patient group while a “larger” encoding N1 predicted better recall in controls.

Fig. 2.

Late brain responses over anterior scalp sites during the encoding of words were lateralized toward the right hemisphere. Group contrasts showed that patients and relatives had larger right anterior LPCs than controls (site F8). The groups did not differ at midline parietal LPC. The scalp topography illustrates the grand average across all participants across the full LPC time course (450–700 ms).

Fig. 3.

Left: The LPC anterior lateralization, calculated as the difference between amplitude at sites F8 and F7 (450–700 ms), was significantly greater for relatives than controls. This represents substantially greater voltage response at right versus left hemisphere anterior scalp sites during the encoding of words. Right: Greater right lateralization was generally associated with better performance on the size judgment task during encoding. There were no group differences in the association between LPC anterior lateralization and encoding task performance.