neural coding of behavior: evidence for precise timing of spikes ?

David Sparks sparks at cattell20.psych.upenn.edu
Wed Dec 13 20:00:31 EST 1995


In article <4ai75a$5h8 at eis.wfunet.wfu.edu>,
Mark Laubach <laubach at biogfx.neuro.wfu.edu> wrote:
>rcb1 at LEX.LCCC.EDU (Ron Blue) wrote:
>
>>Assume for that your brain is like a radio.  If you remain on the

	stuff deleted
>
>This is a good metaphor.
>
>I guess I've been a bit too harsh on the _precise timing_ thing.  I
>should have been more clear.  Indeed, precise timing may be a real
>basis for coding info in the brain.  Its just that I have found that
>precisely timed patterns do not show up consistently over trials in a
>behavioral task.  Nevertheless, this is really not evidence against
>precise timing in neurotransmission.  Rather, the complexity of inputs
>to a neuron may give rise to many precisely timed patterns and these
>patterns may be the basis of the "richness" that allows for
>flexibility in the CNS.
>
>Moreover, I found that, for neurons in the rat basal ganglia and
>cerebral cortex, there are trials, in a reaction-time task, on which
>the neurons fail to fire at all.  This occurs for cells that fire
>phasically even at high rates (from less than 5 to nearly 100 Hz)!
>These same sorts of failures occur if I 'build' spike trains from
>Poisson processes.  Maybe these failures are a basic limitation for
>information transmission in the CNS.  Could it be that info trans is
>limited by the poissonian nature of the generators underlying spike
>activity?
>
>(I doubt that I am the first to say of such a thing. Does anyone know
>of refs for previous statements like mine above from the
>neurophysiological or computational literature?)
>
>Mark Laubach
>Dept. of Physiology & Pharmacology
>Bowman Gray School of Medicine
>Wake Forest University
>Winston-Salem, NC 27157
>laubach at biogfx.neuro.wfu.edu
>-----------------------------------
>
	But consider motoneurons in the abducens nucleus. They have
firing rates that are proportional to the position of the eye in the
orbit and when the eye is in a stable position, the interspike
intervals are remarkably constant. Many years ago, while recording
from a motoneuron, I made a mistake on ketamine dose and gave an
animal a much larger dose than intended (the lack of negative
consequences with large dosages is one reason we use ketamine). This
had the effect of "locking" the eye in one position for approximately
30 minutes. I triggered the sweep of an oscilloscope trace with the
spike and noted the interspike intervals. The interspike intervals
were almost as constant as the multivibrators we were using in those
days to measure the interspike intervals. The point of this, is that
it does seem possible for neurons to maintain firing rates with
regular intervals. Consider too the discharge of pacemaker cells in
respiratory circuit. Does anyone have an estimate of variability in
interspike interval in these or other pacemaker cells?

	David Sparks


-- 
			David Sparks



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