This is a hand calculation exercise to help familiarize you with some of the most basic computational properties of the brain. You may need to do some hunting in books or the WWW for numbers. Listening carefully during class and section will also help you find the answers.

a) What is the minimum time interval between firings of cortical neurons? What is the minimum time for a person to give a forced choice response to a simple stimulus? How do these facts yield the intuitive hundred-step-rule of neural computation? Does the speed of neural signal transmission change the intuition?

b) About how many neurons are in the human brain? What is the fan-out of cortical neurons (on the average, to how many other neurons is each neuron directly connected)? From these two numbers, estimate the minimum number of links to reach all cortical neurons from some starting cell ( [fan-out] ^ [# of links] should be >= to the [# of neurons]).

c) About how many base pairs are there in the human genetic code? About how many synapses are there in the brain?
The significant difference in these two values has been taken to prove that the genetic code could not specify all of our neuralconnections. Show that this argument fails if one takes into account the fact that much of cortex is laid out in systematic mapswith regular connection patterns.

Can you think of a biologically plausible way for the neural wiring to be done during development? Provide other arguments that neural connectivity is not fully specified in the genetic code, although it could be in principle.
 
 

 This assignment is due in class on Monday, February 1.

Readings:

Regier: Ch. 1, Reader: 1, 2.