Complex sentence: They suggested that not only conditioned stimuli physically present on a given trial can undergo changes in their associative strength, the associative value of a CS can also be altered by a within-compound-association with a CS present on that trial.
Split into simpler sentences: They suggested that not only conditioned stimuli physically present on a given trial can undergo changes in their associative strength. The associative value of a CS can also be altered by a within-compound-association with a CS present on that trial.