Source retrieval is not properly differentiated from object retrieval in early schizophrenia: an fMRI study using virtual reality

Abstract

Source memory, the ability to identify the context in which a memory occurred, is impaired in schizophrenia and has been related to clinical symptoms such as hallucinations. The neurobiological underpinnings of this deficit are not well understood. Twenty-five patients with recent onset schizophrenia (within the first 4.5 years of treatment) and twenty-four healthy controls completed a source memory task. Participants navigated through a 3D virtual city, and had 20 encounters of an object with a person at a place. Functional magnetic resonance imaging was performed during a subsequent forced-choice recognition test. Two objects were presented and participants were asked to either identify which object was seen (new vs. old object recognition), or identify which of the two old objects was associated with either the person or the place being presented (source memory recognition). Source memory was examined by contrasting person or place with object. Both patients and controls demonstrated significant neural activity to source memory relative to object memory, though activity in controls was much more widespread. Group differences were observed in several regions, including the medial parietal and cingulate cortex, lateral frontal lobes and right superior temporal gyrus. Patients with schizophrenia did not differentiate between source and object memory in these regions. Positive correlations with hallucination proneness were observed in the left frontal and right middle temporal cortices and cerebellum. Patients with schizophrenia have a deficit in the neural circuits which facilitate source memory, which may underlie both the deficits in this domain and be related to auditory hallucinations.

Keywords: Associative memory; First episode; Hallucinations; Schizophrenia; Source memory; Virtual reality.

Fig. 1

(A) Aerial view showing the layout of the virtual city, with green dashes indicating the path participants followed for the study. (B) An example of an encounter with a character paired with an object at a location during encoding (prior to fMRI scanning). (C) An example of a recognition trial from the “Person” condition during fMRI scanning.

Fig. 2

Activations in memory contrasts (Person vs. Object or Place vs. Object) for both controls and SCZ patients. Areas showing greater activity for source memory are shown on the left, while regions showing greater activity to object memory are shown on the right.

Fig. 3

Results of the group analysis comparing SCZ patients and controls. β values were plotted for selected regions, labeled by letter (error bars represent standard error of the β values). Regions A, B, and C are areas in which controls showed larger β values for both Person vs. Object and Place vs. Object in the between group contrasts, while D, E, and F show areas in which SCZ patients showed larger β values than controls. Patients with schizophrenia show a pattern of not differentiating between object memory and source memory, even in regions in which we observed SCZ > controls. This apparent increase in activity in SCZ appears to be driven by a decrease in activity which is present in controls but not in SCZ patients.

Fig. 4

Significant clusters in the regression analysis, global hallucinations vs. Place > Object. Graphs show calculated β values for Place > Object plotted against global hallucination score (item 7 on the SAPS, rated from 0 (none) to 5 (severe)). All coordinates are in MNI space, k = number of active voxels in the cluster.