Similarities between cortical “up” states during slow wave sleep and wakefulness: the implications for schizophrenia

NMDA receptor hypofunction and the thalamus in schizophrenia

The thalamus can be subdivided into two kinds of nuclei, the higher order (HO) and the first order (FO) relays, which are distinguished based on the origin of their main or driver inputs. The driver inputs to the HO nuclei arrive from the cortex, and the messages they deliver are then relayed to other cortical areas. As the origin of these inputs is the cortical layer V, whose axons branch and innervate lower motor centers in the CNS, the messages are copies of motor instructions issued to those lower motor centers. These copies are thus an integral part of perceptual processes. In schizophrenia, the HO nuclei are shrunken suggesting that a reduced ability to integrate copies of ongoing motor commands in perceptual processes may be one part of the underlying pathophysiology. The driver inputs in the thalamus utilize ionotropic glutamate receptors such as the NMDAR. NMDAR antagonists may exert their pro-psychotic effects by impairing the function of the HO nuclei. Here, we argue that such agents (or the proposed NMDAR hypofunction in schizophrenia) weaken the driver inputs in the HO nuclei, thereby producing a cortico-thalamo-cortical disconnection and impairing sensorimotor integration.

Schizophrenia as a disturbance of cortical sensory maps

Schizophrenia is a disorder characterized by a variety of symptoms, which among others include hallucinations, delusions and passivity experiences. It has been found that individuals with schizophrenia misattribute their own thoughts and actions to an outside agency (source monitoring deficits), which could account for psychotic experiences such as that of hearing voices. In order to explain the source-monitoring deficits as well as psychosis, it has been proposed that mechanisms that enable anticipation and recognition of sensory consequences of one’s own actions are impaired in schizophrenia. Importantly, such mechanisms may require accurate cortical sensory representations such as in the primary somatosensory cortex (S1). The establishment and maintenance of cortical sensory representations has been found to utilize a sleep-related brain rhythm known as spindling. Namely, in the perinatal period in humans and animals, and possibly also thereafter, spontaneous activity in the sensory periphery drives spindle activity in the developing cortical sensory areas, which then contributes to the formation of sensory representations that match bodily features. For example, muscle twitch-spindle sequences during sleep facilitate the formation and maintenance of S1 in accordance with the layout of musculature. This process has been proposed to continue throughout the lifespan and may be particularly important during periods of bodily changes (adolescence, menopause). In schizophrenia, the amount of sleep spindle activity is markedly reduced, which would be expected to result in insufficient cortical sensory representations and have relevance for the relative inability of individuals with schizophrenia to accurately recognize self-initiated actions.