Stimulus Traces and Secondary Reinforcement
A stimulus trace persists in the nervous system after a stimulus ends.
When the Stimulus Has Ended
A difficult timing question in classical conditioning occurs when the conditioned stimulus, or CS, ends before the unconditioned stimulus, or US, begins. The original stimulus is no longer physically present when the US arrives, yet learning can still occur. The proposed explanation is that the earlier stimulus leaves a stimulus trace in the nervous system. That trace can persist after the stimulus ends and can remain present when the US begins.
The central issue is not whether the original CS and the US are physically present at the same moment. The key issue is whether a trace of the CS remains when the US arrives.
The Trace Across Time
A stimulus trace is a continuing nervous-system trace of a stimulus that persists after the stimulus itself has ended.
Trace conditioning is defined by a temporal gap between CS offset and US onset. During that gap, the original CS is no longer physically present. Pavlov proposed that the stimulus nevertheless leaves a trace in the nervous system. If that trace remains when the US arrives, the trace and the US are simultaneously present in the relevant sense. This provides a basis for learning across the delay.
Physical Stimulus and Nervous-System Trace
The physical CS and the stimulus trace should not be treated as the same thing. The CS is the stimulus occurring in the environment. The trace is what the stimulus leaves in the nervous system after the CS has ended. The physical CS can therefore be absent while its trace remains present.
Following one conditioning arrangement
Identify what is physically present and what may persist across a delay when the CS ends before the US begins.
CS begins: The conditioned stimulus is present.
CS ends: The original stimulus is no longer physically present.
Temporal gap: There is an interval between CS offset and US onset.
Trace persists: A nervous-system trace of the earlier CS can remain during the interval.
US begins: Learning can still be possible if the CS trace remains present when the US arrives.
The relevant simultaneous event is the presence of the CS trace and the US, not necessarily the original CS and the US.
Trace and US at the Same Moment
This distinction explains why the temporal gap does not automatically prevent learning. The gap separates CS offset from US onset, but it does not necessarily separate the CS trace from the US. The proposed learning relation is therefore between the continuing trace and the arriving US.
Delay-Period Stimuli
Secondary reinforcement occurs when stimuli present during a delay acquire reinforcing value through association with a primary reinforcer.
A delay can contain stimuli other than the original CS. Those delay-period stimuli may acquire reinforcing value because they are associated with a primary reinforcer that occurs later. This is a different explanatory role from a stimulus trace. A trace preserves the earlier CS in the nervous system across the gap; secondary reinforcement concerns a stimulus in the delay gaining reinforcing value through association with the later primary reinforcer.
Two Accounts of Delayed Learning
| Feature | Stimulus trace | Secondary reinforcement |
|---|---|---|
| What it refers to | A nervous-system trace of an earlier stimulus | A delay-period stimulus with acquired reinforcing value |
| What happens after the CS ends | The trace persists | A stimulus occurring during the delay can become associated with a later primary reinforcer |
| Role in delayed learning | Helps connect the earlier CS with the later US | Gives a delay-period stimulus reinforcing value through association with a primary reinforcer |
When analyzing a delayed-conditioning arrangement, first mark CS onset, CS offset, the temporal gap, and US onset. Then ask two separate questions: Does a CS trace remain when the US arrives? Do any stimuli during the delay acquire reinforcing value through association with a primary reinforcer? Keeping these questions separate prevents the two concepts from being conflated.
Check the Timing
What do you think happens?
The CS ends before the US begins. What must remain for trace conditioning to remain possible?
Reveal answer
Answer: A CS trace must remain present when the US arrives.
Trace conditioning includes a temporal gap between CS offset and US onset. Learning can still occur when the CS trace remains present as the US arrives, even though the original CS is no longer physically present.
A conditioning arrangement has a CS, then a period in which the CS is no longer physically present, and then a US. Explain whether this arrangement fits the definition of trace conditioning and identify what could support learning across the gap.
Hints
- Look for the relationship between CS offset and US onset.
- Distinguish the original CS from its nervous-system trace.
- Ask whether the trace remains when the US begins.
Key Takeaways
- A stimulus trace persists in the nervous system after a stimulus ends.
- Trace conditioning includes a temporal gap between CS offset and US onset.
- Learning can remain possible when the CS trace is present as the US arrives, even though the original CS is no longer physically present.
- Stimuli occurring during a delay can acquire reinforcing value through association with a primary reinforcer; this is secondary reinforcement.
- Stimulus traces bridge the timing gap, whereas secondary reinforcement describes acquired reinforcing value in a delay-period stimulus.
Key Takeaways
- A stimulus trace is a nervous-system trace that persists after the original stimulus ends.
- Trace conditioning is defined by a temporal gap between CS offset and US onset.
- The relevant overlap for learning can be between the CS trace and the US, not necessarily between the original CS and the US.
- Secondary reinforcement occurs when delay-period stimuli acquire reinforcing value through association with a primary reinforcer.
- Stimulus traces and secondary reinforcement explain different aspects of learning across delays.