Abnormal Neural Synchrony and Perception
In our laboratory, we have examined γ oscillations in schizophrenia in auditory and visual stimulus paradigms. Kwon et al. (1999) studied the responses to auditory steady-state stimulation. Steady-state stimuli consist of repetitive trains of simple stimuli presented at a constant rate, and analysis of the EEG responses to these stimuli can provide information regarding the operation of the stimulated cortical circuits. Click trains were delivered to subjects at stimulation rates of 20 Hz, 30 Hz and 40 Hz.
In healthy control participants, 40 Hz (γ band) stimulation elicited the largest response compared with the other stimulation rates, as is typically found. In contrast, the responses of the patients with schizophrenia were reduced but only for 40 Hz stimulation (Kwon et al., 1999). This finding was the first demonstration of abnormal neural synchrony particular to the γ band in schizophrenia and was of particular interest given the prevalence of auditory processing abnormalities in this disorder.
Since γ-band synchrony has been classically linked to perceptual feature binding, we investigated γ oscillations in schizophrenia using a task designed to invoke visual feature-binding mechanisms (Spencer et al., 2003). In this experiment, participants discriminated between squares formed by illusory contours (Illusory Square) and a control condition (No-Square).
As can be seen in Figure 2, the stimuli in each condition are physically identical, but the rotation of the "pac-men" determines whether or not observers perceive a coherent object. In healthy controls, the Illusory Squares but not the No-Squares elicited a γ oscillation phase locked to the stimuli at occipital electrodes (Figure 3). For patients with schizophrenia, however, neither stimulus elicited an oscillation, even though the stimuli were correctly identified.
In a follow-up study, we examined response-locked γ oscillations in the same paradigm (Spencer et al., 2004). We reasoned that the neural mechanisms underlying conscious perception might be more correlated with reaction time than stimulus onset, as found in single-unit recording studies.
We found that in healthy controls, the Illusory Squares elicited a response-locked γ oscillation approximately 250 ms before reaction time (Figure 3), also at occipital electrodes. No such oscillation was elicited by the No-Square stimuli. Thus, the response-locked oscillation may be a correlate of visual feature-binding processes involved in conscious perception.
For the group with schizophrenia, a response-locked oscillation was also elicited by the Illusory Squares at occipital electrodes. However, the response-locked oscillation occurred in a lower frequency range for the group with schizophrenia (22 Hz to 24 Hz) than healthy controls (34 Hz to 40 Hz). This difference in synchronization frequency suggests that synchrony was necessary for the coherent object to be perceived, but the cell assemblies coding the object were unable to synchronize in the normal γ range for the patients with schizophrenia.
One possible cause of this effect is reduced cortical connectivity and/or conductivity delays, as evinced by diffusion tensor and magnetization transfer imaging (Kubicki et al., 2005). Selemon and Goldman-Rakic (1999) have suggested that reduced connectivity is an important neural substrate of schizophrenia, and a study modeling γ oscillations found that increased conduction delays lowered the synchronization frequency of a cell assembly (Kopell et al., 2000).
Evidence for a close relationship between the response-locked oscillation and core cognitive and neural abnormalities in schizophrenia was found in strong correlations between positive symptoms (visual hallucinations, thought disorder and disorganization) and the degree of phase-locking in the response-locked oscillation. These data are consistent with studies that have found correlations between thought disorder and disorganization symptoms and psychophysical measures of visual perception (e.g., Silverstein et al., 2000; Uhlhaas et al., 2004).
Conclusions and Clinical Implications
There is increasing evidence that schizophrenia is a disorder that impairs the communication of information within the brain. Recent studies suggest that γ-band synchrony is sensitive to core neural circuit abnormalities and symptoms of schizophrenia. Gamma oscillations in the EEG may, therefore, be a promising tool for studying the neural substrates of schizophrenia and other neuropsychiatric disorders.
It will be important for future studies to further establish these links by examining whether γ oscillations are sensitive to symptom states and antipsychotic drugs targeting various receptor systems.
As this area of research is still in its infancy, it is difficult to predict how findings of abnormal γ synchrony might influence the treatment of schizophrenia in the near future. In the long term, as neurorehabilitation methods become more advanced, it might be possible to restore neural circuits to their healthy states of function using brain stimulation, such as through transcranial magnetic stimulation (a noninvasive method). Perhaps even direct stimulation of particular brain areas through implanted electrodes will help improve neural circuit function, as in the treatment of Parkinson's disease with deep-brain stimulation.
Another avenue of treatment suggested by this research is cognitive rehabilitation (Silverstein and Wilkniss, 2004). If the integration of information in the brain is impaired in schizophrenia, cognitive rehabilitation strategies might emphasize treatments that enable patients to improve integrative processes that presumably rely on γ oscillations. For instance, patients can be taught to better attend to entire aspects of percepts and social interactions, rather than seizing on one part as giving all the information needed.
In addition to formal cognitive training, many therapists will recognize the utility of working with patients to assist the integration of all the information available and alternative interpretations. Whatever the results of this research, we expect that the scope of treatments available to the clinician will only grow as our understanding of the neural bases of mental disorders continues to advance.
This work was supported by a Research Enhancement Award Program from the U.S. Department of Veterans Affairs, National Institutes of Health grant R01 MH40799 to Dr. McCarley, and a Young Investigator Award from the National Alliance for Research on Schizophrenia and Depression to Dr. Spencer.
Drs. Spencer and McCarley have indicated they have nothing to disclose.