7 Computational EEG Analysis for the Diagnosis of Psychiatric …
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7.3 Psychiatric Disorders and EEG Biomarkers
7.3.1 Schizophrenia
7.3.1.1 Auditory Steady-State Response (ASSR)
As was mentioned in the previous section, abnormal gamma oscillation is an important biomarker of schizophrenia; it is thought to be related to NMDA receptor hypofunction and altered excitation and inhibition balance models. Most of human studies on 40-Hz ASSR reported reductions of evoked power and phase measure in
schizophrenia [130]. However, these results are not entirely consistent. Hong and
colleagues rather found enhanced evoked powers in patients with schizophrenia who
were taking new generation antipsychotics [43]. Hamm et al. also reported higher
phase measure in patients with schizophrenia compared with healthy controls, where
they employed novel stimulus parameters including broadband auditory noise and
wide inter-stimulus interval (ISI) [35]. Notably, a new line of research investigating
the level of spontaneous (non-phase locked) gamma-band activity, found enhanced
gamma activity during both ASSR stimulus presentation and task baseline, but not
in a resting-state [42]. This finding could be significant because it resonates with
animal findings where NMDA receptor antagonists and genetic reduction of NMDA
receptor function induced increases in spontaneous gamma power [22, 66].
A reason underlying the inconsistency might be the obscure definition of ASSR
components as researchers use different ASSR components from classifications that
have not reached consensus. Recently, Mathalon and Sohal systematically classified
the ASSR components into stimulus-evoked oscillations (evoked power), stimulusinduced oscillations (total power), and stimulus independent oscillations (baseline
or resting-state power) [86]. This classification holds importance because the signals reflect different aspects of information processing. For example, the evoked
power reflects bottom-up sensory encoding, while spontaneous gamma (the stimulusinduced and stimulus independent oscillations) are related to emerging dynamic processes in cortical networks [46]. Although the argument to distinguish ASSR signals
has been already established, only a few studies have explicitly used it to date. In
order to reduce the inconsistency in the results, future research ought to take this into
consideration.
Considering that cortical pyramidal cell and interneurons are major sources of
gamma band oscillations [133], another important question is how the 40-Hz ASSR
would be associated with well-known neuropathological features of schizophrenia
such as gray matter loss and symptoms [34]. A major brain area thought to be
involved in generating the 40-Hz ASSR is the primary auditory cortex located in
the superior temporal gyrus (STG) [40]. Recently, Kim and colleagues reported that
the ASSR gamma power was significantly increased in patients with schizophrenia
compared with healthy controls [63] (see Fig. 7.2). The discrepancy about ASSR
gamma power might have occurred because the inter-train interval of ASSR stimuli
train was different among studies. The study of Kim and colleagues used longer
155
7.3 Psychiatric Disorders and EEG Biomarkers
7.3.1 Schizophrenia
7.3.1.1 Auditory Steady-State Response (ASSR)
As was mentioned in the previous section, abnormal gamma oscillation is an important biomarker of schizophrenia; it is thought to be related to NMDA receptor hypofunction and altered excitation and inhibition balance models. Most of human studies on 40-Hz ASSR reported reductions of evoked power and phase measure in
schizophrenia [130]. However, these results are not entirely consistent. Hong and
colleagues rather found enhanced evoked powers in patients with schizophrenia who
were taking new generation antipsychotics [43]. Hamm et al. also reported higher
phase measure in patients with schizophrenia compared with healthy controls, where
they employed novel stimulus parameters including broadband auditory noise and
wide inter-stimulus interval (ISI) [35]. Notably, a new line of research investigating
the level of spontaneous (non-phase locked) gamma-band activity, found enhanced
gamma activity during both ASSR stimulus presentation and task baseline, but not
in a resting-state [42]. This finding could be significant because it resonates with
animal findings where NMDA receptor antagonists and genetic reduction of NMDA
receptor function induced increases in spontaneous gamma power [22, 66].
A reason underlying the inconsistency might be the obscure definition of ASSR
components as researchers use different ASSR components from classifications that
have not reached consensus. Recently, Mathalon and Sohal systematically classified
the ASSR components into stimulus-evoked oscillations (evoked power), stimulusinduced oscillations (total power), and stimulus independent oscillations (baseline
or resting-state power) [86]. This classification holds importance because the signals reflect different aspects of information processing. For example, the evoked
power reflects bottom-up sensory encoding, while spontaneous gamma (the stimulusinduced and stimulus independent oscillations) are related to emerging dynamic processes in cortical networks [46]. Although the argument to distinguish ASSR signals
has been already established, only a few studies have explicitly used it to date. In
order to reduce the inconsistency in the results, future research ought to take this into
consideration.
Considering that cortical pyramidal cell and interneurons are major sources of
gamma band oscillations [133], another important question is how the 40-Hz ASSR
would be associated with well-known neuropathological features of schizophrenia
such as gray matter loss and symptoms [34]. A major brain area thought to be
involved in generating the 40-Hz ASSR is the primary auditory cortex located in
the superior temporal gyrus (STG) [40]. Recently, Kim and colleagues reported that
the ASSR gamma power was significantly increased in patients with schizophrenia
compared with healthy controls [63] (see Fig. 7.2). The discrepancy about ASSR
gamma power might have occurred because the inter-train interval of ASSR stimuli
train was different among studies. The study of Kim and colleagues used longer