Trace Mechanisms in the Nervous System
A stimulus trace persists in the nervous system after a stimulus ends.
When the Stimulus Is Gone
In some conditioning arrangements, the conditioned stimulus, or CS, ends before the unconditioned stimulus, or US, begins. At first, this timing seems problematic: the original stimulus is no longer physically present when the US arrives. Trace mechanisms explain how learning can still be possible. The nervous system can retain a stimulus trace after the stimulus ends, allowing the trace to overlap with the later US.
What do you think happens?
If the CS ends before the US begins, what could still be present when the US arrives?
Reveal answer
Answer: A stimulus trace left by the CS
A stimulus trace persists in the nervous system after a stimulus ends. Learning is possible when that trace remains present as the US arrives.
The Trace After CS Offset
A stimulus trace is a representation of a stimulus that persists in the nervous system after the stimulus itself has ended.
The physical CS and its trace are related, but they are not the same event. During the CS, the original stimulus is present. When the CS ends, the original stimulus is no longer physically present, but its trace can remain in the nervous system for some time. The trace therefore provides continuity across the interval between CS offset and US onset.
Crossing the Temporal Gap
Trace conditioning includes a temporal gap between CS offset and US onset. During this gap, the original CS is no longer physically present. The important possibility is that the CS trace remains present during at least part of that interval and is still present when the US begins. In this arrangement, the relevant simultaneous event is the presence of the CS trace and the US, not necessarily the physical CS and the US together.
Following One Trace-Conditioning Sequence
Track what is physically present and what may persist as the CS ends before the US begins.
CS begins: The conditioned stimulus is physically present and produces a corresponding stimulus trace in the nervous system.
CS ends: The original CS is no longer physically present. This event is CS offset.
Temporal gap: There is an interval between CS offset and US onset. The stimulus trace can persist during this interval.
US begins: Learning is possible if the CS trace remains present when the US arrives.
The CS and US do not need to be physically present together. The relevant overlap is between the persisting CS trace and the US.
Trace and US Overlap
The central timing relationship is not simply CS followed by US. It is CS, then CS offset, then persistence of the CS trace, followed by the arrival of the US while that trace is still present. This simultaneous presence of the CS trace and the US explains why a temporal gap does not automatically prevent learning.
| Timing question | Answer in trace conditioning |
|---|---|
| Is the original CS physically present when the US begins? | Not necessarily; the CS can end before the US begins. |
| What can remain after CS offset? | A stimulus trace can persist in the nervous system. |
| What can be present when the US arrives? | The CS trace and the US. |
| What makes learning possible across the gap? | The CS trace remains present when the US arrives. |
Common Timing Mistakes
Assuming the original CS must still be physically present when the US begins.
Trace conditioning specifically includes arrangements in which the CS ends before the US begins.
Fix:
Ask whether the CS trace, rather than the physical CS, is still present when the US arrives.Treating the temporal gap as proof that no learning can occur.
Learning is possible when the CS trace persists across the gap and remains present when the US arrives.
Fix:
Track the trace during the gap instead of treating CS offset as the end of all CS-related activity.Using CS and CS trace as if they were interchangeable terms.
The physical CS has ended, while the stimulus trace persists in the nervous system.
Fix:
Use CS for the original stimulus and CS trace for what persists after it ends.
When analyzing a conditioning sequence, mark four points separately: CS onset, CS offset, the temporal gap, and US onset. Then ask whether the CS trace is still present at US onset. This keeps the physical stimulus, its offset, and its persisting trace from being confused.
Check the Sequence
A CS ends before a US begins. Explain whether this arrangement can support learning using the terms CS offset, temporal gap, CS trace, and US. Your explanation should identify which two events can be simultaneously present when the US begins.
Hints
- First identify what happens at CS offset.
- Then describe what can persist during the temporal gap.
- Finally identify what is present when the US begins.
A correct explanation should state that the original CS has ended, the CS trace can persist across the temporal gap, and learning is possible when the CS trace remains present as the US arrives.
Trace Mechanism Summary
- A stimulus trace persists in the nervous system after a stimulus ends.
- Trace conditioning includes a temporal gap between CS offset and US onset.
- The original CS does not have to remain physically present when the US begins.
- Learning is possible when the CS trace remains present as the US arrives.
- The relevant simultaneous event is the CS trace together with the US.
Key Takeaways
- A stimulus trace is what persists in the nervous system after a stimulus ends.
- In trace conditioning, the CS can end before the US begins, creating a temporal gap.
- The CS trace can persist during that gap and remain present when the US arrives.
- The important overlap is between the CS trace and the US, not necessarily between the original CS and the US.