Studying the PDP Books


Hello, I’ve taken to studying the PDP books on recommendation from a member of this community. However, I’m stuck on the pattern associator. This is on page 35.

As I understand it, the +1 and -1 within the A and B units represent their current state of activation, analogous to being excited or inhibited. The +.25 and -.25 represent the strength of the connection between A units and B units, with the + being excitatory and the - being inhibitory. I have a few questions about this.

First, it appears as though the B units are influenced by the A units.Do the B units influence the A units or is it a one way only thing?

Second, is a B unit activated by the sum of its A units, or is a single A unit sufficient to activate it?

Third, and this depends on the answer to Second, I understand how an excited +1 A unit with a +.25 excitatory connection strength can excite a B unit. I understand how an excited +1 A unit with a -.25 inhibitory connection strength can inhibit a B unit. I don’t understand how a -1 A unit with either -.25 or +.25 can inhibit or excite a B unit. According to the text,

“For present purposes, we’ll assume that the units can be positive or negative values, with 0 representing a kind of neutral intermediate value.”.

Wouldn’t a B unit influenced by an inhibited inhibitory A unit, or an inhibited excitatory A unit (inhibitory and excitatory determined by the sign of its .25 value), receive no input from either case and simply be 0, because in both cases the A unit is inhibited and presumably sending no signal?

Thank you for reading,


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