From David Sapsford, Department of Anaesthesia, Addenbrooke's Hospital
Douglas Fox’s article explores the idea that we experience time through discrete snapshots of reality, and offers an explanation of why we recall time as having apparently slowed down during stressful events (24 October, p 32).
I have long thought that the idea of a snapshot nature of memory also comes into play during the apparent state of unconsciousness we call anaesthesia. It is my conjecture that anaesthesia slows the formation of perceptual memory, possibly by missing out some of the snapshots.
In modern anaesthesia there is always some electrical activity in the brain, as complete unconsciousness – where no electrical activity is detected by electroencephalography (EEG) – is impractical and carries higher risks. Patients have been observed to form implicit memories from sounds they experienced while anaesthetised, although they may not be explicitly aware of this being the case (BMJ, vol 309, p 967).
Steady-state measurements of the auditory evoked potential (AEP) made using an EEG have been found to peak roughly every 25 milliseconds. This value is close to the neural processing time required for perception that Fox describes in his article.
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The steady-state AEP frequency has been shown by Jackie Andrade and others to decrease during anaesthesia (Anesthesia & Analgesia, vol 83, p 1279); in other words, the time between peaks increases. If the peaks reflect the formation of perceptual memory snapshots, then replaying the sequence of snapshots at a normal speed would give the effect of accelerated time.
It is common for patients recovering from an anaesthetic to think they are about to have surgery or that it has only been seconds since the start of anaesthesia. The brain has still been active, as evidenced by the observation of implicit recall, but perceptual memory has been stored at a slower rate than normal, such that on waking time is perceived as having passed very rapidly.
From Alan Chapman
Fox’s article suggests that our brain collects more information during times of crisis. Taking this reasoning a step further, if we measure time not by the artificial ticking of a clock but by the passing of significant events, it might go some way to explaining why we often recall time as appearing to have passed more slowly during childhood than in later life.
For children, any given period of time will have many new or significant events. As we grow older, and life becomes more routine, we experience fewer new or significant events.
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