Abstract
The ability to time intervals confers organisms, including humans, with many remarkable capabilities. A common method for studying interval timing is classification, in which a subject must indicate whether a given probe duration is nearer a previously learned short or long reference interval. This task is designed to reveal the probe duration that is equally likely to be labeled as short or long, known as the temporal bisection point. Studies have found that this bisection point is influenced by a variety of factors including the ratio of the target intervals, the spacing of the probe durations, the modalities of the stimuli, the attentional load, and the inter-trial duration. While several of these factors are thought to be mediated by memory effects, the prototypical classification task affords no opportunity to measure these memory effects directly. Here, we present a novel bisection task, termed the “Bisection by Classification and Production” (BiCaP) task, in which classification trials are interleaved with trials in which subjects must produce either the short or long referents or their midpoint. Using this method, we found a significant correlation between the means of the remembered referents and the bisection points for both classification and production trials. We then cross-validated the bisection points for production and classification trials by showing that they were not statistically differentiable. In addition to these population-level effects, we found within-subject evidence for co-variation across a session between the production bisection points and the means of the remembered referents. Finally, by using two sets of referent durations, we showed that only memory bias-corrected measures were consistent with a previously reported effect in which the ratio of the referents affects the location of the bisection point. These results suggest that memory effects should be considered in temporal tasks.
Highlights
Organisms rely on a wide range of temporal information to guide their behavior (Buhusi and Meck, 2005)
To assess the effects of memory on the bisection point, we developed the “Bisection by Classification and Production” (BiCaP) task (Figure 1A), which consists of classification and production trials
We examined the correlation between the bias-corrected mean and the bisection points across subjects for classification trials (p = 0.022, R2 = 0.26) and production trials (p = 1.9 × 10−7, R2 = 0.79) and found both to be significant (Figure 2A, top). (Separating by groups, we found: classification 1/5 s: p = 0.175, R2 = 0.22; production 1/5 s: p = 0.002, R2 = 0.70; classification 2/4 s: p = 0.098, R2 = 0.30, production 2/4 s: p = 3.14 × 10−5, R2 = 0.90.) We examined the co-variation between classification and production bisection points (Figure 2C) and found no significant difference between these groups (p = 0.15, two-tailed Mann–Whitney U-test, U18 = 356, z = −1.45)
Summary
Organisms rely on a wide range of temporal information to guide their behavior (Buhusi and Meck, 2005). Their circadian rhythms are entrained by temporal cycles that span days (Czeisler et al, 1999) In between these extremes, organisms must be able to evaluate the length of temporal intervals on the order of seconds to hours to guide their decision-making (Richelle and Lejeune, 1980; Gallistel, 1990; Kacelnik and Brunner, 2002; Sohn and Carlson, 2003). While the precise form of this task has been modified many times, its essential component is that subjects are required to classify sample temporal intervals as short or long This classification relies on the subject remembering previously learned short and long reference intervals (i.e., similarity method). The bisection point, or probe duration at which a subject is likely to choose “short” or “long,” can be inferred
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