Hitting head, seeing stars

Bill Kemler bkemler at cabin.llcc.cc.il.us
Tue Oct 22 13:40:12 EST 1996

Schaap at rullf2.medfac.leidenuniv.nl (Jeroen Schaap) wrote:

>I would like to turn it a little bit around. Why would banging your head
>etc actually trigger (excite) neuronal events?
Any type of stimulation that produces sufficient depolarization at the axon hillock of
a neuron will trigger an action potential (nerve impulse). This stimulation could be
electrical, chemical, or mechanical energy. The neurons in the brain are normally
protected from mechanical stimulation by the skull and cerebrospinal fluid. An
exception occurs when you bang your head hard enough for the brain to bump against the
inside of the skull.

> It is not neccesary and
>in contradiction to absence of simular and/or parallel events for other
>sensory systems.

I'm not sure I understand what you are saying here. All of the somatosensory systems
normally respond to mechanical energy: touch, pain, deep pressure, proprioception, etc.
(and they can respond to electrical and chemical energy as well).
>Why would those bangs or those chemicals only trigger
>receptors in the eye?

They don't. Perhaps I didn't make myself clear. It is the cells of the visual cortex at
the back of brain that are responsible for "seeing", not the receptors in the eye.
Here's a crude analogy. Think of the eye as a TV camera and the visual cortex as the TV
monitor. What you "see" is the picture on the monitor screen. The camera converts light
energy into electrical energy that excites the phosphors on the TV screen and creates
the picture, but you can't see the picture without the monitor. Moreover, the signals
coming from the camera are not always what you see. Strong electrical interference (or
perhaps even banging the TV really hard) can alter the ways those phosphors are excited
and cause you to see "stars" on your TV screen. 

>In what extent is rubbing your eye equivalent to banging your
>head? Eye-rubbing is more site-specific, I would say, but also a
>complete different event. Higher pressure at the site of the
>photoreceptors maybe with other physical parameters of importance. But
>what happens after a head bang? A lot of things, but as far as I can
>imagine, no increase of local pressure on the retina. In conclusion, two
>completely unrelated (at first glance) events cause the same subjective
>experience. (Is it the same at all? Maybe, maybe not.)

Head banging and eye rubbing are the same in that they are both able to activate the
cells of the visual cortex (and thus produce visual sensations), but they do so in
different ways. Rubbing your eyes puts pressure on the cells in the retina, which in
turn send signals through the visual pathways to the cortex. Head banging puts direct
pressure on the cortical cells. If you will forgive another analogy, think of a cell in
the visual cortex as switch that turns on a visual experience much like a switch on a
wall that turns on a light. Under normal circumstances that wall switch is under the
control of the person in the room. But other things can move that switch as well. Your
dog might jump into it. You could be throwing a ball to your dog and accidentally hit
It doesn't matter what moves the switch. As long as it moves, the lights will go on or
off. Likewise, it doesn't matter how the cells of the visual cortex get activated.
Anything that activates them will produce visual sensations. For this reason, work is
under way to produce artificial vision for people who have lost the use of their eyes
by electrically stimulating the visual cortex using input from modified TV cameras. But
that's another topic.  

>Another conclusion, it is not the actual information coming from
>the photoreceptors that eventually gives rise to the stars, becauseÜv
>'surfing' the optic tract are not likely to be specifically excited by
>headbanging. So somewhere in the head a bang has the same effect as
>doing something like rubbing your eye. I would like to think this 'site'
>as being those area's that interpret visual information at some higher
>level. (with very diffuse borders).

The comment above should explain this.

>At least I can imagine depriving those areas from cohorent
>information can be accomplished by both eye rubbing and head banging and
>chemicals etc. Psychedelics are known to exert mostly inhibitory
>effects, could be consistent with a headbang deminishing all neuronal

Psychedelics such as LSD do, indeed, have inhibitory effects, but the question is
where? The inhibitory effects of interest occur mainly in the thalamus. One of the main
functions of the thalamus is to filter (inhibit) sensory information BEFORE it gets to
the cortex. When you inhibit inhibition you get the release of more activity downstream
(i.e. at the visual cortex). So, again, we are talking about increased activity, not
diminished activity at the cortex.

Increased activity of the visual cortex is also responsible for the visions we see when
we dream with eyes closed in the dark. In this case, the increased activity results
from nerve signals coming from the brainstem rather than the eye. So we see that visual
hallucinations can originate from many different sources, but involve a common terminal
mechanism, activation of cortical cells.

Hope your hallucinations are pleasant ones.

           * The greatest obstacle to learning is not ignorance *
           * but the illusion of knowledge.                     *

Bill Kemler   2.31u6Ä

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