Michal Piekarczyk
0284129
Neural Nets CS667
last hw
Creating a Neural Net to interpret horizontal and vertical lines
We want to classify
and
as vertical lines and
and
as horizontal lines.
One way to do this is to associate the four quadrants with four input neurons into a neural
net and develop a neural net to make the needed decisions. Using bipolar vectors, the
above cases can be represented as [1,1,1,1] , [1,1,1,1] , [1,1,1,1] , and
[1,1,1,1] . If we choose that the threshold is 1, then we would need to satisfy the
following equations to create this neural net with a single layer architecture:
w1 + w2 - w3 - w4 > 1,
-w1 - w2 + w3 + w4 > 1,
w1 - w2 + w3 - w4 < 1, and
-w1 + w2 - w3 + w4 < 1,
A difficulty in applying the equations is that they represent a four dimensional problem
and that means it’s not easy to graph them to figure out the weights. However, if the first
two or last two equations are added, there’s a contradiction, that is, the result becomes
either 0 > 2 or 0 < 2, so the equations probably can’t work to solve the problem.
Setting up another architecture to handle the problem
One method for figuring out the system can be to have specific hidden neurons react to
specific inputs. In this way, the system behaves like a deterministic finite state automata,
except that most of the input combinations are not accounted for. The system will be built
to handle the vectors mentioned above, but wouldn’t know what to do with [1,1,1,1]
or [1,1,1,1] for example. There twelve of the sixteen combinations are unaccounted
for.
Michal Piekarczyk
0284129
Neural Nets CS667
last hw
If there’s one error in the input, there will always be two equally likely choices of
what it really was, so the problem can’t have an error-handling solution. For example,
can be either
or
.
The design chosen—with the weight diagram below—identifies each line as only
depending on two inputs. For example, the neuron for
depends on A and C, and the
weights on B and D are 0 and aren’t drawn. If A is +1, and C is +1, then
y in 1 / 2 1 / 2 1 / 2 1 / 2 . The bias attached only lets the sum to be positive if two of
the inputs are positive. The same kind of decision is made for each line. Next, another
layer with two neurons has each neuron performing an OR function. If the inputs to the
neural net are only limited to the four lines, then both
wouldn’t fire +1, so
and
really needs to use an XOR, but an OR is simpler and will do just fine. This time
the biases are positive. For example, if
then
indicates the line was a top horizontal line,
will receive y in 1 / 2 1 / 2 1 / 2 1 / 2 . This yields that the line is
horizontal. But, if neither
nor
fire +1, then
will receive
y in 1 / 2 1 / 2 1 / 2 1 / 2 , yielding a negative one, or a false for a horizontal, but
also true for a vertical—if we limit the input to the four line types. Finally, the output
neuron uses the policy that horizontal lines will be denoted with +1, but verticals will get
-1. This time no bias is necessary, but the weights are opposite signs, so a horizontal line
will
cause
y in 1 1 / 2 (1) (1 / 2) 1 ,
y in 1 1 / 2 (1) (1 / 2) 1 .
while
any
vertical
gives
Michal Piekarczyk
0284129
Neural Nets CS667
last hw
**note that all
connections are feedforward**
A comment can be made about the need for hidden layers to reiterate why a single layer
approach wouldn’t work. The above is the minimum number of layers needed to solve
the problem if we want to limit one decision per layer. Above, the decisions were in
identifying which of the four lines was represented, and then in classifying the lines into
different types. Without a hidden layer, there’s no decision-making phase.
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