Neurocognitive mechanisms of conceptual processing in healthy adults and patients with schizophrenia

Abstract

This overview outlines findings of cognitive and neurocognitive studies on comprehension of verbal, pictorial, and video stimuli in healthy participants and patients with schizophrenia. We present evidence for a distinction between two complementary neurocognitive streams of conceptual analysis during comprehension. In familiar situations, adequate understanding of events may be achieved by mapping the perceived information on the associative and similarity-based connections between concepts in semantic memory - a process reflected by an N400 waveform of event-related electrophysiological potentials (ERPs). However, in less conventional contexts, a more flexible mechanism may be needed. We suggest that this alternative processing stream, reflected by a P600 ERP waveform, may use discrete, rule-like goal-related requirements of real-world actions to comprehend relationships between perceived people, objects, and actions. This neurocognitive model of comprehension is used as a basis in discussing studies in schizophrenia. These studies suggest an imbalanced engagement of the two conceptual streams in schizophrenia, whereby patients may rely on the associative and similarity-based networks in semantic memory even when it would be more adaptive to recruit mechanisms that draw upon goal-related requirements. Finally, we consider the roles that these conceptual mechanisms may play in real-life behavior, and the consequences that their dysfunction may have for disorganized behavior and inability to plan actions to achieve behavioral goals in schizophrenia.

Figure 1

ERPs time-locked to unexpected target words in sentences, compared to ERPs time-locked to predictable target words (A), and ERPs time-locked to verb-argument violations, compared to ERPs time-locked to predictable target words (B). Note: negative voltages are plotted upward. Shown are waveforms at frontal, central, and parietal electrode sites whose relative locations on the scalp are indicated on the head diagram. Adapted from Kuperberg et al. (2003).

Figure 2

ERPs time-locked to different types of verb-argument violations in sentences, compared to ERPs time-locked to predictable target words. Violated target verbs were semantically related to the preceding sentence context, and were preceded by noun arguments that were either compatible (the ball would throw) or incompatible (the gym would throw) with the role of an undergoer of the action conveyed by the verb (A), or were not semantically related to the preceding sentence context and were incompatible with the role of an undergoer of the action conveyed by the verb (B). Shown are ERPs at a parietal electrode site. Adapted from Kuperberg et al. (2006a; 2007b)

Figure 3

ERPs time-locked to verb-argument violations in sentences, compared to ERPs time-locked to predictable target words, in healthy control participants (A), and in patients with schizophrenia (B). Shown are ERPs at a parietal electrode site. Adapted from Kuperberg et al. (2006c).

Figure 4

Frames taken from video clips (produced using Canon-GL1 digital video camcorder and Adobe digital-editing software) used in our video comprehension paradigm. For each video scenario, shown are two frames illustrating real-world events depicted as a context, followed by a single frame illustrating the predictable final scene [1], the unexpected final scene [2], and the less comprehensible [3] or more comprehensible [4] final scenes that were both unexpected and showed unconventional object-action combinations. The corresponding video clips may be viewed at http://www.nmr.mgh.harvard.edu/~tatiana/IJP. Adapted from Sitnikova et al. (2008b).

Figure 5

ERPs time-locked to unexpected final scenes in video scenarios, compared to ERPs time-locked to predictable final scenes. Shown are waveforms at frontal, central, and parietal electrode sites whose relative locations on the scalp are indicated on the head diagram. Adapted from Sitnikova et al. (2008b).

Figure 6

ERPs time-locked to the final video scenes that were both unexpected and showed unconventional object-action combinations, compared to ERPs time-locked to predictable final scenes. Shown are waveforms at frontal, central, and parietal electrode sites whose relative locations on the scalp are indicated on the head diagram. Sitnikova et al. (2008b).

Figure 7

ERPs time-locked to the less comprehensible or more comprehensible final video scenes that were unexpected and showed unconventional object-action combinations, compared to ERPs time-locked to predictable final scenes.

Figure 8

Scatter plots showing relationships in patients with schizophrenia between the N400 ERP priming effect to predictable relative to unexpected/unconventional final scenes in video scenarios and the disorganization symptoms (A), and between the P600 ERP effect to unexpected/unconventional relative to predictable final scenes in video scenarios and the impersistence at work or school (B). Mean absolute value of voltage differences in each time window of interest were averaged across three electrode sites in a frontal midline region to quantify the N400 effect and in a parietal midline region to quantify the P600 effect. Adapted from Sitnikova et al. (2009).

Figure 9

ERPs time-locked to the final video scenes that were unexpected and showed unconventional object-action combinations, compared to ERPs time-locked to predictable final scenes, in healthy control participants (A), and in patients with schizophrenia (B). Adapted from Sitnikova et al. (2009).

Figure 10

ERPs time-locked to the less comprehensible final video scenes that were unexpected and showed unconventional object-action combinations, compared to ERPs time-locked to predictable final scenes, in healthy control participants (A), and in patients with schizophrenia (B). Adapted from Sitnikova et al. (2009).