Associative Strength in Conditioning Models

Why Order Matters

Classical conditioning is not simply produced by presenting two events together. The order and timing of the events matter. In the usual arrangement, a previously neutral stimulus, called the conditioned stimulus or CS, begins before the unconditioned stimulus or US. After learning, the conditioned response usually begins before the US arrives. This arrangement makes the CS useful as a prediction of what is coming next.

Prediction Across Time

The temporal-difference model, or TD model, treats temporal differences in prediction as learning signals. At one moment, a stimulus produces a prediction about a later outcome. At a later moment, the system has another prediction, influenced by what has occurred in between. Learning depends on the difference between the prediction at the current state and the value backed up from a later state.

This gives the model a way to represent changing associative value. A stimulus does not need to be judged only by whether the US eventually appears. Its value can change as the trial progresses. A later stimulus may already have learned value, and that value can create a learning difference for an earlier stimulus.

Remote Associations

Facilitation of remote associations occurs when a CS is followed by an empty trace interval and then by the US. Conditioning to the first stimulus can improve when a second CS occupies part of that otherwise empty interval. The second stimulus does more than add another event to the trial: it fills part of the temporal path between the first CS and the US.

Overlapping Predictors

The Egger-Miller effect shows that a closer temporal relationship with the US does not guarantee stronger conditioning to a stimulus. In the relevant arrangement, CSA and CSB overlap. CSB is temporally better placed relative to the US, yet adding CSA reduces conditioning to CSB compared with an arrangement in which CSA is absent.

Moving a Blocked Stimulus

Blocking normally occurs when one CS has already been trained to predict the US. When a second stimulus is added during later pairings, much less learning occurs about that new stimulus. The pretrained stimulus already accounts for the outcome, so the added stimulus receives little new associative strength in the usual arrangement.

The TD model predicts reversal of blocking when the newly added stimulus is moved earlier so that it begins before the pretrained stimulus. In that arrangement, the new stimulus can acquire associative strength. As its strength grows, the strength associated with the previously trained stimulus decreases.

Second-Order Learning

Second-order conditioning extends prediction beyond direct CS–US pairings. First, CSB is trained with the US. Later, CSA is paired with CSB without the US. CSA can nevertheless acquire associative strength, even though CSA is never directly paired with the US.

The TD explanation is that CSB already has learned value. When the trial moves from CSA to CSB, the later CSB state can provide a nonzero learning difference for CSA. In the second phase, CSA first gains substantial strength while the strength associated with CSB decreases. This pattern has also been reported in animal experiments.

Arrangement Checks

• Treating conditioning as a result of two events merely occurring on the same trial.

  The order and timing of the events matter. The usual arrangement has the CS before the US.

  Fix: Describe the temporal arrangement before predicting learning.

• Assuming that the stimulus closest to the US must always receive the most conditioning.

  The Egger-Miller effect shows that adding CSA can reduce conditioning to CSB despite CSB having the closer temporal relationship.

  Fix: Consider the full arrangement, including overlap and whether another stimulus is present.

• Treating blocking as unchanged when the newly added CS is moved earlier.

  The TD model predicts reversal of blocking under this timing change.

  Fix: Track which stimulus comes first and how associative strength changes during the altered arrangement.

• Assuming direct pairing with the US is required for every conditioned stimulus to gain strength.

  In second-order conditioning, the already conditioned CSB can provide a learning signal for CSA.

  Fix: Ask whether a later stimulus already carries learned predictive value.

What to Remember

1. Classical conditioning usually depends on the CS occurring before the US because the CS functions as a prediction of the later US.
2. The TD model treats the difference between current prediction and later backed-up value as a learning signal.
3. An intervening CS can facilitate conditioning to an earlier, remote CS by filling part of the temporal path to the US.
4. The Egger-Miller effect shows that adding an overlapping stimulus can reduce conditioning to another stimulus even when that other stimulus is closer to the US.
5. Moving a newly added CS before a pretrained CS can reverse blocking, while pairing an earlier CSA with an already conditioned CSB can produce second-order conditioning.