A study on polysomnographic observations and subjective experiences under sensory deprivation
Abstract
We investigated the relationship between subjective experiences and polysomnographic observations under sensory deprivation (SD). The results can be divided into two types: (i) visual images were perceived frequently, accompanied by alpha waves or theta waves in electroencephalogram and frequent rapid eye movements (REM); (ii) the subjects fell in deep sleep without perception of visual images. The perceptions of visual images with alpha waves or theta waves and REM are supposed to be different from those in usual wakefulness and sleep. These results suggest that the SD could disclose different aspects of consciousness from the conventionally categorized states such as wakefulness and sleep.
INTRODUCTION
The past studies on sensory deprivation (SD) reported that subjects perceived hallucinations or images.1 Coincidentally, in our recent studies on SD the subjects reported ‘feeling as though staying on the border between sleep and wake’, ‘feeling like neither sleep nor wake’, or ‘dreaming while recognizing it as a dream’.2 These studies suggest that an unusual sleep or dreaming, or some sort of altered state of consciousness occurs during SD. However, physiological characterizations of these phenomena have not yet been performed in detail. Therefore, it is meaningful to investigate the corresponding polysomnogram as an objective measure of the state of consciousness from the viewpoint of sleep physiology. In this paper, the relationship between subjective experiences and polysomnographic observations is investigated under SD.
MATERIALS AND METHOD
We used the flotation tank method to realize the SD. The tank (2.5 × 1.8 × 1.2 m) is made of fiber reinforced plastic. The tank is filled with water (35.5°C) up to 30 cm from the bottom. The subject floats on the water, which could significantly reduce possible physical stimuli. Subjects were left for 1 h in the SD environment. Simultaneously, a polysomnograph (EEG: C3, C4, O1 and O2, EOG: L and R, EMG: chin muscle, ECG) was recorded. An interview with the subject was done before and after the SD. In particular, the later interview was devoted to collecting the subject’s experiences during their stay in the tank. The subjects were five (three males and two females) healthy adult volunteers aged 25–32 years. The experiment was performed twice for each subject (there were two exceptional subjects: once for one subject, and three times for another). In total, 10 experiments were done.
RESULTS
Here, we are confined to visual images and sleep as subjective experiences during SD, although other aspects of the experiences were also found. The result can be classified into two groups. Among a total of 10 cases, one group consists of 6 cases in which the subjects perceived visual images with no sleep (2 cases) and only light sleep (Stage-1 and/or Stage-2; 4 cases), denoted by VI (VI: visual images). Some subjects of this group reported that the images had been somewhat different from their usual dreams. The other consists of 3 cases in which the subjects fell into light or deep sleep (Stage-2 in 1 case and Stage-4 in 2 cases) without perceptions of visual images, denoted by NVI (NVI: non-visual images). In exceptional case 1, neither VI nor sleep was reported but Stage-4 appeared on polysomnogram. Regarding group VI, we found characteristics of the polysomnogram summarized as follows: (i) frequent rapid eye movements (REM) were observed; (ii) alpha waves predominantly appeared in EEG; (iii) theta wave trains sustained over 5 min were observed. We made an ad hoc scoring to show explicitly the changes in aspects of the polysomnogram. Scoring criteria is as follows: [Stage α]: the percentage of alpha wave is more than 50%. [Stage θ]: alpha wave is less than 50% and humps or spindles do not appear. [Stage Hump]: humps appear. [Stage Spindle]: sleep spindles appear. [Stage δ]: delta waves appear. Additionally, REM and slow eye movements (SEM) are shown in the lower part of the chart. Examples of both groups are explained below.
VISUAL IMAGE EXAMPLE
In this case, the subject reported that he had perceived VI in the first half of the SD session, and had slept in the second half. According to his report, he gave a signal by intentional ‘zigzag’ eye movement when he came to himself after he had perceived the clear and vivid VI. On the polygraph, the peculiar ‘zigzag’ eye movements were recorded in first 10 min (Fig. 1c), and theta waves in EEG and REM were observed before the signal (Fig. 1d). Alpha waves were observed concurrently with the signal (Fig. 1b). Theta waves and REM, or alpha waves and REM were shown to be present, when he perceived the VI. The REM might be generated for scanning the subjective VI.
a.
NON-VISUAL IMAGE EXAMPLE
In this case, the subject reported that she had deeply slept, and had not perceived VI. On the polysomnograph, delta waves were predominantly observed (Fig. 2b).
(a).
DISCUSSION
The subjective experiences and polysomnographic characteristics during SD can be divided into the following two types: (i) VI were perceived frequently, which was accompanied by the alpha wave trains in EEG and frequent REM, or the theta wave trains in EEG and REM; (ii) the subjects fell in deep sleep without perception of VI. The perceptions of VI with REM and alpha waves or theta waves are supposed to be different from those in usual wakefulness and sleep. Sometimes, the perceived VI were more vivid or clearer than usual dreams. These results suggest that sensory deprivation could disclose different aspects of consciousness from the conventionally categorized states such as wakefulness and sleep. We will perform more detailed comparison between SD and usual sleep from the viewpoint of both subjective experiences and polysomnograph.
