The magic number 70 (plus or minus 20): Variables determining performance in the Rodent Odor Span Task

Abstract

The olfactory span task (OST) uses an incrementing non-matching to sample procedure such that the number of stimuli to remember increases during the session. The number of consecutive correct responses (span length) and percent correct as a function of the memory load have been viewed as defining rodent working memory capacity limitations in several studies using the OST. However, the procedural parameters of the OST vary across experiments and their effects are not well understood. For example, in several studies, the number of stimuli to remember is confounded with the number of comparison stimuli displayed in the test arena. Experiment 1 addressed whether performance is influenced by the number of comparison choices available on any given trial (2, 5, 10) as well as the number of odor stimuli to remember during a session (12, 24, 36). Performance was most accurate when the number of stimuli to remember was low, as would be expected from a working memory interpretation of OST. However, accuracy was also affected by the number of comparison stimulus choices. High levels of accuracy were seen even with 36 odors, suggesting that the capacity for odor memory in rats was greater than suggested by previous research. Experiment 2 attempted to define this capacity by programming sessions with 36, 48 or 72 stimuli to remember in a group of rats that had previously received extensive OST training. Highly accurate performance (80% correct or better) was sustained throughout the session at even the greatest memory loads, arguing strongly against the notion that the OST models the limited capacity of human working memory. Experiment 3 explored the possibility that stimulus control in the OST is based on relative stimulus familiarity, rather than recognition of stimuli not yet presented during the current session. Number of odor cups visited increased with the number of comparisons in the arena, but rats rarely sampled all of the comparison odors before responding. However, on probe trials which included only stimuli that had been presented during the session, latency to respond and number of comparisons sampled was sharply increased. These data suggest that responding in the OST is determined not just by relative familiarity, but rather by a more specific “what-when” or perhaps “how long ago” form of stimulus control.

Influence of pharmacological manipulations of NMDA and cholinergic receptors on working versus reference memory in a dual component odor span task

“…This adaptation of the OST procedure allowed for a comparison of performance across trials unbiased by concurrent variation in chance performance. Average baseline performance was similar to earlier studies using the dualcomponent procedure both in terms of span and percent correct accuracy (MacQueen et al 2011;April et al 2013;Galizio et al 2013). Accuracy declined as the number of stimuli to remember increased, demonstrating that two-comparison odor span performance was capacity dependent.…”

What-Where-When memory in the rodent Odor Span Task

“…However, using the OST procedure, April, Bruce and Galizio (2013) found that if the number of distracting stimuli is controlled and held constant throughout the testing session, the number of stimuli that rats can remember does not appear to be limited; indeed, rats showed accurate performance on the task with over 70 odor stimuli to remember. This finding is inconsistent with a limited capacity, working memory account.…”

“…April et al (2013) used a probe procedure in which no new stimuli were presented on certain trials to assess the relative importance of recent familiarity with odor stimuli. They found that rats tended to select the odor that had been encountered least recently on such trials, but that number of stimuli visited and latencies to respond were much longer.…”

“…Further, by using probe procedures, the April et al (2013) approach also addresses a concern voiced by Zentall and colleagues. Zentall, Clement, Bhatt, and Allen (2001) argued that what, where, when memory tasks may be interpreted as inflexible trained relations and therefore, may not be sufficient to meet the criteria for episodic-like memory.…”

“…Pigeons were probed with an “unexpected question” which required the subject to respond to a stimulus complex not previously reinforced in training. The probes used in April et al (2013), and in the current study, may similarly be interpreted as “unexpected questions,” because at unpredictable points during a session, the rat is probed with an unfamiliar stimulus arrangement.…”