To be able to function, the brain have a way of changing differences in
stimulation or thoughts into different patterns of firing in the brain.
This is called neural coding. To make this concept concrete, the color red
and green have to somehow have a different impact on the brain, cause a
different pattern of effects on the brain, or these two colors cannot be
experienced as different.
This interactivity introduces a few basic types of neural coding. These
types are introduced just as a means of indicating ways that the brain can use
to track the differences in our experiences. You can select type of neural
coding you wish to illustrate with the drop-down menu on the upper left corner
of the window that opens. Each type of neural coding illustrated is listed
below with a brief description and instructions on how to use the illustration.
In all cases, you run the illustration you have set up by pressing the
Animate button on the left side of the screen. In all cases, the
neurons on the left are going to be called input neurons and all neurons on the
right are called output neurons.
Coding type
Description
Instruction
Law of Specific Nerve Energies
This method of coding, first
proposed by Johannes Mueller in the 19th Century, proposes that the
brain can tell what is going on by which neurons are firing. So,
if one given neuron is firing, you see, if another given neuron is
firing you hear.
Use the check boxes to the left side
of the screen to indicate which input neuron is to fire when the
Animate button is pressed.
Number Responding
This method of coding proposes that
the brain can tell differences by the number of neurons that are firing.
This can be used to indicate the intensity of a stimulus. The more
neurons firing the more intense.
Use the Number of PSPs
menu on the left to indicate how many PSPs are fired by the input
neuron.
Firing Rate (or Frequency Coding)
This method of coding proposes that
the brain tells differences by the firing rate. One place this
method of coding is used is in hearing as is one way we can hear
different sounds as having different pitches.
Select the input neuron to fire
using the menu on the left and seeing how each input neuron has a
different effect on the single output neuron.
Relative Firing Rate
This method of coding indicates that
the brain can tell what is being experience not absolutely which neuron
is firing like the Law of Specific Nerve Energies, but by which neurons
fire more relative to some other neuron. For example, a red
stimulus might cause all the neurons to fire but some more than others.
Green would have a different pattern of effects. Not surprisingly,
color is partly indicated in this manner.
Select the input neuron on the left
and see how depending of which of the six input neurons fire, you get a
different pattern of action potentials on the output neurons.
Opponent Processing
In this final method, some insight
into the role of inhibition is indicated. In opponent processing,
one input does not just cause a neuron to fire but it inhibits other
neurons in the same system from firing. The two inputs tend to
cancel each other outs. This coding method can be used to make
inputs more distinct and is also used in part of our processing of
color.
Select the input neuron on the left
(A, B, or Both) and see the
different results on the two output neurons. Notice the inhibitory
connections.
