In collaboration with Sparks, we sought to assess the effects of a cholesterol diet on trace conditioning of the rabbit nictitating membrane response .
In each of our trace conditioning experiments, acquisition of a conditioned response was a function of the trace interval and usually took many days of training to reach asymptote, and this asymptote tended to be lower than that seen using delay conditioning .
We next conducted a seminal experiment  in which we added 0.12 parts per million (ppm) copper as copper sulfate to distilled drinking water and found that the levels of beta amyloid in cholesterol-fed rabbits had increased over previous levels to the point of generating extracellular plaques and, importantly, these rabbits showed a deficit in trace conditioning relative to controls.
The data in Figure 3 show both the level of responding during trace conditioning and the number of beta amyloid immune-positive neurons in the cortex and hippocampus of the two groups.
Using a trace conditioning paradigm, we found that cholesterol facilitated rabbit HR conditioning and that the unconditioned HR response to shock was also modified by conditioning .
In an experiment by Darwish and colleagues, we trained rabbits to asymptotic levels of NMR tracing conditioning and then instituted an 8-week diet of 2% cholesterol or normal chow before assessing the memory of trace conditioning by presenting the tone alone over a period of days during extinction .
Because the conditioned stimulus (CS) is terminated prior to the unconditioned stimulus (US), taste-aversion conditioning is defined as a type of trace conditioning. If the interval between CS offset and US onset (trace interval) is prolonged (long-trace conditioning), conditioning is relatively weak in comparison with conditioning using brief (less than 1 min) trace intervals (short-trace conditioning).
Trace conditioning with X-rays as an aversive stimulus.
* Trace conditioning. This procedure has been suggested as a model for explicit (conscious) working memory [14,15].
During trace conditioning, significant activity to CS+ versus CS- was found in the anterior portion of the vermis cerebelli, putamen, and the inferior parietal cortex.
The CS+ in trace conditioning yielded a significant activity in the right IPL and STG.