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.

Keywords: episodic-like memory; non-match-to-sample; odor span task; olfaction; rat; recognition memory; working memory.

Figure 1

Arena apparatus used for all experiments.

Figure 2

Experiment 1. Mean accuracy (percent correct; depicted in top panel) and mean span (bottom panel) across the three stimulus conditions (12, 24, 36) and as a function of number of comparison choices available (2, 5, 10). Black circles represent 12 stimulus conditions, white squares are 24 stimuli, and black triangles are 36 stimuli. Error bars represent SEM (in some cases the error bar is obscured by the data point symbol).

Figure 3

Percent correct in Experiment 1 as a function of the number of stimuli to remember. Panels depict within session accuracy for conditions involving 12 (top panel), 24 (middle panel), and 36 stimuli (bottom panel). Black circles represent the 2 comparison (comp) choice arrangement, white squares for 5 choices, and black triangles for 10 choices.

Figure 4

Percent correct for individual subjects in Experiment 2, listed in order according to Table 2. Performance on sessions with different numbers of stimuli to remember are shown in each panel: 36 (top panel), 48 (middle panel), and 72 stimuli (bottom panel). Black bars represent individual subject means; white bars depict the grand mean for each condition. For subjects that received two sessions, error bars represent SEM; all other subjects had single sessions at each condition.

Figure 5

Mean percent correct in Experiment 2 shown as a function of the number of stimuli to remember (bins of 12 trials). Black circles show performance on 36-stimulus sessions, 48-stimulus conditions are shown as white squares, and 72-stimulus conditions are shown as black triangles. Error bars represent SEM.

Figure 6

Mean latencies for 12 (top panel), 24 (middle panel), and 36 (bottom panel) stimulus conditions plotted as a function of the number of stimuli to remember from Experiment 1 (bins of 3 trials). Black circles show performances in the 2-comparison (comp) conditions, white squares for 5 comparisons, and black triangles for 10 comparisons. Error bars represent SEM.

Figure 7

Mean visits for 12 (top panel), 24 (middle panel), and 36 (bottom panel) stimulus conditions plotted as a function of the number of stimuli to remember from Experiment 1. Black circles show performances in the 2-comparison (comp) conditions, white squares for 5 comparisons, and black triangles for 10 comparisons. Error bars represent SEM.

Figure 8

Experiment 3. Mean latencies (top panel), mean visits per trial (middle panel), and percent target chosen (bottom panel) for both Random and Delayed Relative Familiarity Probe tasks. Performances on Relative Familiarity probe trials (RF) are shown with black bars while Baseline trials (BL) are white bars. Error bars represent SEM.