# Afferent data rates

Larry Hunter hunter at work.nlm.nih.gov
Wed Aug 14 17:02:00 EST 1991

```Konrad Weigel responds to my more carefully stated query about
empirically justified estimate of the rate of information flow into
the nervous system by proposing an information theoretic analysis that
led to the formula

I = (N * Log S)/t
2

where N = total number of sensory neurons times
S = number of states the output of the neuron can have
t = time it takes to transmit a single message
I = the information rate in bits/second.

Assuming that the outputs of each sensory neuron are independent, and
that all of the states an neuron can have are equiprobable.

He goes on to offer possible interpretations in terms of firing
frequencies of neurons.

I think it might be simpler to consider an action potential (or lack
thereof) as a single message.  That way, a neuron has 2 output states,
and the time it takes to transmit a single message is the period
between action potentials when the cell is firing at its maximum rate.
Since S is 2, this reduces to

I = N/t = Nf

where f = maximum firing frequency of a neuron, as action potentials/second.

Granted, this ignores signal propagation via voltage changes that are
not sufficient to generate action potentials.  There is some evidence
that this mode of transmission may be important, but let's skip that
for the time being.

So, in yet another attempt to rephrase my question:

1. How many sensory neurons are there in an adult human being?

2. What is the maximum firing rate of a neuron?  If this varies among
populations of neurons, what is the distribution of maximum firing
rates?

Surely these are empirical questions that someone must have addressed.

Larry

--
Lawrence Hunter, PhD.
National Library of Medicine
Bldg. 38A, MS-54
Bethesda. MD 20894
(301) 496-9300
(301) 496-0673 (fax)
hunter at nlm.nih.gov (internet)

```