And as I lay there I kept thinking there was someone in the bed with me. I was not emotionally disturbed particularly by the thought – I knew there was no-one else in the room and that I was imagining this in my fever. But I wasn’t fully aware of this other person beside me as I could only hear the sounds they were making, I had no sense of their “presence”. Quite distinctly there was the sound of their breathing loud in sleep, the occasional toss and a turn or rolling lurch for more comfort, the shuffling of feet at the end of the bedclothes. Just a half metre to my right. Then it would be just me again, and I could lie there in peace for a while, suffering the usual crummy symptoms of illness. Maybe for a second the feeling would come back – but as I said – I knew this was an hallucination caused by my fever so there was no dread involved – it was a bit like having a lucid dream only a lucid auditory hallucination instead.
Except it wasn’t an hallucination caused by the fevered imaginings of my brain- or at least I don’t think it was. Instead I think my illness had disturbed my perceptual system so that my brain was interpreting the inputs from the ears correctly, just the information coming from the ears was wrong.
Spatial location is derived in hearing from phase differences in the inputs to the two ears. Sound arrives at each ear at minutely different times according to how far away each ear is from the sound source. So if the sound source is equidistant from each of the ears then it sounds directly in front, or along the midline. But if the sound source is to the right a few metres and we are facing it then the sound will reach our right ear slightly before the left ear. And those differences in time are quite tiny but our brains handle them quite easily for lower sorts of frequencies (This actually gets quite complex but for the sake of this note, we only need assume that time differences between the ears are important in perception of the location of a sound source).
Well I was sick and one of the symptoms was very “blocked” ears – to the point where I was having great difficulty hearing. And this was not symmetrical – one side was more affected than the other. So it struck me that whatever was causing my deafness may also have been disturbing the natural time differences between my ears – that sounds to one side were being delayed inside my head and that my brain – quite reasonably – was interpreting the location of the sound source by interpreting the signal as if all was right with the apparatus – oblivious to the problems my illness was causing to the input stream. My brain was right, it was my ears that were wrong, and at night in bed, with no vision to cross check the source location my brain had no way of knowing what troubles my ears were having.
So I tested this. I wiggled my feet under the sheets and heard them wiggle – a couple of feet to my right. I waggled my hands and there they were – on the other side of the bed. Problem solved, I was not hallucinating in my fevered state (although I was a bit) – but instead I was interpreting the world correctly according to my past analyses of sound inputs – it was not a problem with the receiver, it was the the mics that were faulty. 🙂
And I wondered how many times in our lives we mis-perceive the world because our sensory systems are in error at the level of the receptors and our brains are actually correct – given the faulty inputs they are receiving