Current source density (CSD) old/new effects during recognition memory for words and faces in schizophrenia and in healthy adults

Abstract

We previously reported a preserved 'old-new effect' (enhanced parietal positivity 300-800 ms following correctly-recognized repeated words) in schizophrenia over mid-parietal sites using 31-channel nose-referenced event-related potentials (ERP) and reference-free current source densities (CSD). However, patients showed poorer word recognition memory and reduced left lateral-parietal P3 sources. The present study investigated whether these abnormalities are specific to words. High-density ERPs (67 channels) were recorded from 57 schizophrenic (24 females) and 44 healthy (26 females) right-handed adults during parallel visual continuous recognition memory tasks using common words or unknown faces. To identify and measure neuronal generator patterns underlying ERPs, unrestricted Varimax-PCA was performed using CSD estimates (spherical spline surface Laplacian). Two late source factors peaking at 442 ms (lateral parietal maximum) and 723 ms (centroparietal maximum) accounted for most of the variance between 250 and 850 ms. Poorer (76.6+/-20.0% vs. 85.7+/-12.4% correct) and slower (824+/-170 vs. 755+/-147 ms) performance in patients was accompanied by reduced stimulus-locked parietal sources. However, both controls and patients showed mid-frontal (442 ms) and left parietal (723 ms) old/new effects in both tasks. Whereas mid-frontal old/new effects were comparable across groups and tasks, later left parietal old/new effects were markedly reduced in patients over lateral temporoparietal but not mid-parietal sites, particularly for words, implicating impaired phonological processing. In agreement with prior results, ERP correlates of recognition memory deficits in schizophrenia suggest functional impairments of lateral posterior cortex (stimulus representation) associated with conscious recollection. This deficit was more pronounced for common words despite a greater difficulty to recall unknown faces, indicating that it is not due to a generalized cognitive deficit in schizophrenia.

Figure 1

Reference-free current source density (CSD) [μV/cm²] waveforms (−100 to 1400 ms, 100 ms pre-stimulus baseline) for word stimuli (averaged across old and new items) comparing 44 controls (solid gray lines) and 57 patients (dashed black lines) at all 67 recording sites. Horizontal and vertical electrooculograms (EOG) [μV] are shown before blink correction. Distinct CSD components included inferior lateral-parietal P1 sources (approximate peak latency 80 ms at PO7) and N1 sinks (145 ms at P7), occipital P2 sources (225 ms at O1), a central N2 sink (295 ms at FCz), and mid-parietal P3 sources (645 ms at Pz).

Figure 2

CSD waveforms as in Figure 1 for face stimuli. Distinct CSD components included inferior lateral-parietal P1 sources (approximate peak latency 90 ms at PO8) and N1 sinks (145 ms at P10), occipital P2 sources (220 ms at O2), a central N2 sink (295 ms at FCz), and mid-parietal P3 sources (645 ms at Pz).

Figure 3

CSD waveforms from 44 controls (left) and 57 patients (right) for words comparing old (dashed black lines) and new (solid gray lines) items at selected midline (Fz, Cz, POz) and left parietal (P3, P7) sites. Both groups showed more positive-going current sources for old than new words, overlapping the falling phase of the mid-frontal N2 sink (Fz, Cz) and the subsequent parietal P3 source (POz, P3, P7). The later old/new effects, however, were restricted to medial sites in patients (POz, P3).

Figure 4

CSD waveforms as in Figure 3 for face stimuli. As for words, both groups had more positive-going current sources for old than new faces, overlapping the falling phase of the mid-frontal N2 sink (Fz, Cz) and the subsequent parietal P3 source (POz, P3, P7), with the later old/new effects confined to medial sites in patients (POz, P3).

Figure 5

Unrestricted temporal PCA solution. Top: Time courses of Varimax-rotated covariance loadings for the first five CSD factors extracted (89.3% total variance explained). Labels indicate the peak latency of the factor loadings relative to stimulus onset. Bottom: Corresponding factor score topographies (nose at top) for both tasks and groups (pooled across old and new items) with percentage of explained variance. Factors 161, 220 and 442 clearly corresponded to lateral inferior-parietal N1 sinks (left-lateralized for words), task-specific occipital P2 sources, and parietal P3 sources, respectively. Factor 723 had a more complex topography, with a centroparietal (CP) source maximum accompanied by a mid-frontal and a right inferior occipitoparietal sink.

Figure 6

Statistical evaluation of topographic old/new effects using randomization tests for paired samples for CSD factors 442 and 723, performed across groups for each task (A, B), and separately for 44 controls and 57 patients for words (C, D) and faces (E, F). Task-dependent differences of the old/new effect were also evaluated separately for each group (G, H). Shown are the mean factor score topographies for new and old items and their respective old-minus-new difference (A–F; multivariate Hotelling’s T² statistics are reported for the total sample), or the old-minus-new difference for words minus the old-minus-new difference for faces (G, H), and squared univariate (channel-specific) paired samples T statistics thresholded at the 95^th quantile (p = 0.05) of the corresponding randomization distribution (maximum of all 67-channel squared univariate paired samples T statistics). To facilitate comparisons of the max(T²) topographies with the underlying sink-source difference topographies, the sign of the difference at each site was applied to the respective T² value, which is otherwise always positive. Please note that symmetric scales optimized for score ranges across new and old stimuli were used for the original topographies. To allow for better comparison of old/new effects across tasks and groups, the same symmetric scale range was used for all difference topographies for each factor, and within each set of max(T²) topographies. All topographies are two-dimensional representations of spherical spline interpolations (m = 2; λ = 0) derived from the mean factors scores or T² statistics available for each recording site. Inset topographies show the sites selected for repeated measures ANOVA models performed on CSD factors 442 (1–4) and 723 (5–6), as indicated by colored locations (red: old/new effects; blue: inverted old/new effects).