In this study, researchers evaluated the sensory and cognitive functions of the brain of Auditory steady state response (ASSR) and inhibitory gating (IG). In some clinic examinations and animal experiments, general anesthesia is necessary to conduct electrophysiological recordings. However, the effects of anesthesia on ASSR and IG remain unclear.
Auditory steady state response (ASSR) and inhibitory gating (IG) are commonly used to evaluate the sensory and cognitive functions of the central nervous system. Electroencephalographic (EEG) signals entrained to periodic auditory stimuli (a train of clicks) are often referred to as the ASSR.
The power (magnitude) and phase locking ability (phase consistency across trials) of the ASSR can reflect the functional integrity of the neural circuits that support synchronization across frequencies. EEG measurement of ASSR, particularly in the gamma frequency range (30–80 Hz), has been commonly used in the clinical examination of mental illness and in neuropharmacological experiments in animal models.
IG is experimentally characterized by a reduced responsiveness to redundant stimuli. IG is estimated using evoked response potential (ERP) techniques in which identical pairs of auditory stimuli (tones, white noises or clicks) are presented and ERP responses to the stimuli are elicited and then compared.
The first stimulus in a pair is commonly identified as the conditioning stimulus (C) and the second one is called the test stimulus (T). Under conscious conditions, the amplitude of LFP evoked by T stimulus was smaller than that evoked by C stimulus, indicating an IG effect.
They quantified the LFP by measuring the mean negative deflection at about 25 ms (N25) after the stimulus onset, and the mean positive deflection at about 40 ms (P40). The T/C ratio of N25 was 0.75 and that of P40 was 0.90. A T/C ratio < 1.0 reflects an IG effect.
Under anesthetized conditions, the absolute amplitudes of LFP evoked by both C and T stimulus were reduced a little, but the T/C ratio of N25 and P40 remained similar. The primary goal of this study was to investigate the effects of general anesthesia on the widely used electrophysiological examinations.
ASSR and IG. We used microwires implanted into different brain areas to record sound-evoked LFPs, which can provide more accurate information from distinct functional areas.
Previous studies on surgical patients reported that the amplitude of the 40-Hz ASSR was attenuated or possibly abolished with a loss of consciousness after the application of anesthetics. Thus, it was proposed that the amplitude of the ASSR may be a reliable indicator of the level of consciousness.
The most surprising result of this study is that IG in all the tested regions was little affected by chloral hydrate anesthesia. The exact neural mechanism responsible for IG is not fully understood, but was proposed to be regulated by recurrent inhibitory and excitatory circuits involving multiple neurotransmitter system.