Assignment 1: The basics of the brain: the numbers, the basics of neural firings, etc.
Assigned Tuesday, 1/24.
Due Monday, 1/30 by 11:59pm - submit electronically.

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 facts and numbers. Listening carefully during class and section will also help you find the answers. Note: many of the answers may be inexact.

1.
a. What is the minimum time interval between firings of cortical neurons? Identify two factors that govern this number.
b. What is the minimum time for a person to give a forced choice response to a simple stimulus?
c. What do these facts suggest about the number of steps in a minimal neural computation?
d. What is the speed of neural signal transmission? How does this compare to the speed of a computer?

2.
a. About how many neurons are in the human brain?
b. What is the fan-out of cortical neurons (on the average, to how many other neurons is each neuron directly connected)?
c. From these two numbers, estimate the number of synapses crossed that suffice to connect any two neurons in the brain. (Note: [fan-out] ^ [# of synapses] should be greater than or equal to the [# of neurons].)
d. About how many synapses are there in the brain?
e. At about what age is the number of synapses at its maximum.
f. About how many synapses per second are generated to this point.

3.
a. About how many genes are in the human genome? Why is this controversial?
b. Since there are vastly more neural connections than genes, it has been claimed that very little of our brain is pre-specified. Show that this argument fails if one takes into account the fact that much of cortex is laid out in systematic maps with regular connection patterns.
c. Describe how neural connectivity is not fully specified in the genetic code, although it could be in principle. Recall that a newborn colt or calf functions almost immediately.

d. What would you predict if Sperry’s experiment on rotating the frog retina by 180 degrees were performed on monkeys?

4.
a. Why does an action potential not go backward on a neuron?
b. What is the role of myelin in propagating the action potential?
c. What is the role of neurotransmitters in establishing the post-synaptic potential?
d. Describe one possible mechanism for generating an excitatory and one mechanism for an inhibitory post-synaptic potential.