An additional worry stems from the observation that sometimes the lack of a property being instantiated is an integral component of what is learned. To deal with the problem of missing properties, contemporary associationists have introduced an important element to the theory: inhibition. For example, if a US and a CS only appear when the other is absent, the organism will learn a negative relationship holds between them; that is, the organism will learn that the absence of the CS predicts the  US.[40] Here the CS becomes a “conditioned inhibitor” of the US. Inhibition, using associations as modulators and not just activators, is a central part of current associationist thinking. For example, in connectionist networks, inhibition is implemented by the activation of certain nodes inhibiting the activation of other nodes. Connection weights can be positive or negative, with the negative weight standing in for the inhibitory strength of the association.