Stimulus Traces in Instrumental Conditioning

A delayed primary reward does not have to be the only influence operating during the delay.

Programming

Why the Delay Matters

In instrumental conditioning, an action and its reward do not always occur together. As the interval between them becomes longer, it can become more difficult for the reward to influence learning across the entire interval. The important question is therefore not simply whether a reward is delayed. We must also ask what occurs during the delay.

A delayed primary reward does not have to be the only influence operating between an action and its reward.

Signals Inside the Delay

Hull's account addresses delayed reinforcement by proposing that learning can be supported not only by the primary reward at the end of the delay, but also by secondary reinforcement during the interval. If stimuli regularly occur during the delay, they can favor the development of a more immediate reinforcing influence. This gives the learner something relevant to learning before the primary reward arrives.

What happens between an instrumental response and a delayed primary reward, and how can regularly occurring stimuli bridge that delay?

Secondary reinforcement is a more immediate reinforcing influence that can develop from regularly occurring stimuli during the delay between an action and a delayed primary reward.

Following the Action-to-Reward Sequence

Track how learning may be supported when a primary reward is delayed and regularly occurring stimuli appear during the delay.

Action: An instrumental action occurs first.

Delay: The primary reward does not occur immediately, so there is an interval for other events to influence learning.

Regular stimuli: Stimuli occur during the interval. Because they regularly occur during the delay, they can favor the development of secondary reinforcement.

Immediate influence: Secondary reinforcement provides a more immediate reinforcing influence instead of requiring the learner to rely only on the primary reward at the end.

Primary reward: The delayed primary reward still occurs at the end of the sequence.

The delay contains an additional reinforcing influence, so learning is not described as depending only on the primary reward at the end.

Extending the Goal Gradient

Molar stimulus traces refer to the lingering presence of a stimulus in Hull's theory. The source presents these traces as part of the explanation for how a goal gradient can span time. Hull's proposal is that secondary reinforcement works together with these traces, allowing the resulting gradient to extend beyond the period that stimulus traces alone would cover.

In practical terms, the regularly occurring stimuli provide intermediate support during the delay. The primary reward remains at the end, but the learner is not left with only that distant event as the relevant influence. This is why secondary reinforcement can extend the goal gradient across a longer interval.

How does responding change across the delay to a primary reward when secondary reinforcement provides intermediate signals?

When Secondary Reinforcement Is Blocked

The effect of a delay depends partly on whether conditions support or obstruct secondary reinforcement. Animal experiments described in the source support this distinction: when conditions favor secondary reinforcement during a delay, learning decreases with increased delay less than it does when those conditions obstruct secondary reinforcement.

Condition	What occurs during the delay	Effect of increasing the delay
Secondary reinforcement supported	Regularly occurring stimuli can favor a more immediate reinforcing influence.	Learning decreases with increased delay less than when secondary reinforcement is obstructed.
Secondary reinforcement obstructed	The additional reinforcing influence during the delay is not supported.	Learning decreases with increased delay more than under conditions that favor secondary reinforcement.

What differs in the learning process when a secondary reinforcer occurs during the delay versus when access to that stimulus is obstructed?

When analyzing delayed reinforcement, compare delays that differ in their opportunities for secondary reinforcement. Do not treat every delay as equivalent, because the conditions inside the delay can change how strongly learning is affected.

Stimulus Traces After CS Offset

A stimulus trace is a lingering presence of a stimulus in the nervous system after the physical stimulus ends.

In some classical-conditioning arrangements, the conditioned stimulus ends before the unconditioned stimulus begins. This creates a temporal gap: the original CS is no longer physically present when the US arrives. Pavlov proposed that learning can still occur because the CS leaves a trace in the nervous system that persists for some time after the CS ends.

What changes over time when the physical CS is turned off but an internal stimulus trace remains available before the US begins?

The crucial distinction is between the original CS and its trace. The original CS and the US need not be physically present at the same time. Learning is possible when the CS trace remains present when the US arrives.

How can a CS trace persist after the CS ends, and how does its overlap with the later US support learning across a temporal gap?

Check the Timing

What do you think happens?

A CS ends, a temporal gap follows, and then a US begins. Which event must still be present when the US arrives for learning to be possible according to the stimulus-trace account?

The original physical CS
The CS trace
The delay itself
The action that occurred before the CS

Reveal answer

Answer: The CS trace

The original CS can have ended before the US begins. Learning is possible when the CS trace remains present when the US arrives.

MEDIUM

Compare two delayed-reinforcement situations. In the first, regularly occurring stimuli during the delay favor secondary reinforcement. In the second, conditions obstruct secondary reinforcement. Explain why the second situation should show a greater decrease in learning as the delay increases.

Hints

Identify what is available during the interval before the primary reward.
Ask whether the delay contains a more immediate reinforcing influence.
Compare the two delays rather than comparing only delay with no delay.

Your attempt (not saved)

EASY

Describe the timing of trace conditioning using these terms: CS, CS offset, temporal gap, CS trace, and US. Your explanation should identify which two events are simultaneously present when learning can occur.

Hints

The physical CS ends before the US begins.
The CS leaves a lingering presence in the nervous system.
Identify the trace and the US as the simultaneous events.

Your attempt (not saved)

Mistakes About Delays and Traces

Treating the delayed primary reward as the only influence during the delay.
Hull's account proposes that regularly occurring stimuli during the delay can favor secondary reinforcement and provide a more immediate reinforcing influence.
Fix: Examine what occurs during the interval, not only what occurs at its end.
Assuming that all delays have the same effect.
The source states that learning decreases with increased delay less when conditions favor secondary reinforcement than when those conditions obstruct it.
Fix: Compare the opportunities for secondary reinforcement within each delay.
Saying that the original CS must still be physically present when the US begins.
Trace conditioning includes a temporal gap between CS offset and US onset.
Fix: Ask whether the CS trace remains present when the US arrives.
Confusing the CS with the CS trace.
A stimulus trace is the lingering presence of a stimulus in the nervous system after the stimulus ends.
Fix: Separate the physical CS, its offset, and the trace that persists afterward.

Key Takeaways

Secondary reinforcement can provide a more immediate reinforcing influence during a delay before a primary reward.
Regularly occurring stimuli during the delay can favor secondary reinforcement, and secondary reinforcement can help extend a goal gradient across time.
The effect of a delayed reward depends partly on whether conditions support or obstruct secondary reinforcement.
A stimulus trace is a lingering presence of a stimulus in the nervous system after the physical stimulus ends.
In trace conditioning, learning can occur across a temporal gap when the CS trace is still present as the US begins.

Key Takeaways

Delayed primary reinforcement is not necessarily the only influence operating during the delay.
Regularly occurring stimuli can favor secondary reinforcement and provide more immediate support for learning.
Secondary reinforcement works with stimulus traces to help a goal gradient span more time.
A stimulus trace can persist after a CS ends, allowing learning when the trace and the later US overlap.
The critical comparison is between delays with and without opportunities for secondary reinforcement.