With any hallucinations, if you can do functional brain imagery while they're going on, you will find that the parts of the brain usually involved in seeing or hearing - in perception - have become super active by themselves. And this is an autonomous activity; this does not happen with imagination.
Interpretation
The quote explains that during hallucinations, specific brain areas responsible for perception become highly active, distinguishing these experiences from mere imagination.
Oliver Sacks highlights the neurological basis of hallucinations, identifying that they trigger heightened activity in the brain regions typically associated with sensory perception. This points to the idea that hallucinations are not simply figments of imagination, but rather autonomous activities that can be observed and measured in the brain, providing insight into how our mind processes reality and illusions differently.
In practice
In a lecture on mental health, you can quote Sacks to explain the neurobiological differences between hallucinations and imagination.
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