Figure 1.

Temporal relationships between stimulus events mediate the ability to acquire associative memories. The conditioned stimulus (CS) is depicted in blue and followed by an unconditioned stimulus (US) in yellow. (a) Delay conditioning is represented as two stimuli that immediately follow one another. The stimuli can also overlap in time as long as the US follows the CS. In either case, the association is usually acquired rapidly, is not associated with awareness and, as such, is considered a procedural memory. (b) Trace conditioning is represented as two stimuli that are not contiguous in time. The stimuli are typically separated from one another by a trace interval, often referred to as the ‘gap’. This association is more difficult to learn and because of its dependence on the hippocampal formation [3,13,14] and association with awareness is often considered a declarative memory [51]. (c) Explicitly unpaired stimuli are presented one after another but the time between stimuli is random and unpredictable. After repeated exposure to these stimuli, an animal learns that the CS is followed by a US but does not know when the US will occur other than that it will not occur during the CS, and so the animal does not display excitatory responses to the CS. As illustrated, the stimulus events and their relationships to one another are remarkably similar between trace and unpaired conditioning, emphasizing the amazing ability of animals to detect predictive relationships among stimuli as they encounter them in their environment.