The influence of prior experience and expected timing on vibrotactile discrimination

Muhsin Karim,Michael Breakspear,Angela Langdon,Justin A. Harris

doi:10.3389/fnins.2013.00255

Abstract

Vibrotactile discrimination tasks involve perceptual judgements on stimulus pairs separated by a brief interstimulus interval (ISI). Despite their apparent simplicity, decision making during these tasks is biased by prior experience in a manner that is not well understood. A striking example is when participants perform well on trials where the first stimulus is closer to the mean of the stimulus-set than the second stimulus, and perform comparatively poorly when the first stimulus is further from the stimulus mean. This “time-order effect” suggests that participants implicitly encode the mean of the stimulus-set and use this internal standard to bias decisions on any given trial. For relatively short ISIs, the magnitude of the time-order effect typically increases with the distance of the first stimulus from the global mean. Working from the premise that the time-order effect reflects the loss of precision in working memory representations, we predicted that the influence of the time-order effect, and this superimposed “distance” effect, would monotonically increase for trials with longer ISIs. However, by varying the ISI across four intervals (300, 600, 1200, and 2400 ms) we instead found a complex, non-linear dependence of the time-order effect on both the ISI and the distance, with the time-order effect being paradoxically stronger at short ISIs. We also found that this relationship depended strongly on participants' prior experience of the ISI (from previous task titration). The time-order effect not only depends on participants' expectations concerning the distribution of stimuli, but also on the expected timing of the trials.

Highlights

Vibrotactile discrimination tasks have been used to examine the behavioral and neural responses involved in perceptual decision making
We examined the temporal dynamics of perceptual representation by observing performance in a vibrotactile discrimination task
By varying the interstimulus interval (ISI) across four relatively short intervals, we found a complex, nonlinear dependence of the time-order effect on both the ISI and the distance in magnitude between the first stimulus and the global mean

Summary

Introduction

Vibrotactile discrimination tasks have been used to examine the behavioral and neural responses involved in perceptual decision making. Earlier presentations of vibrotactile stimuli during a task (Sinclair and Burton, 1996; Preuschhof et al, 2010), or training/titration prior to a vibrotactile discrimination study (Karim et al, 2012), have been shown to establish the conditions for biased decision making to occur Decisions in these tasks are influenced by an implicit mechanism for evaluating current sensory information in the context of past information garnered from the task. Depending on the stimulus-set used, all other trial sequences are “nonpreferred.” While Stim is held in memory during the inter-stimulus interval (ISI) prior to Stim onset, the representation of Stim effectively “drifts” toward the global mean (Preuschhof et al, 2010) Preferred trials are those in which the Stim representation drifts away from the Stim representation, causing the two stimuli to be perceived as more distinct. Nonpreferred trials are those in which the Stim representation drifts toward the Stim representation, causing the two stimuli to be perceived as less distinct, resulting in decreased accuracy and slower responses (Sinclair and Burton, 1996; Preuschhof et al, 2010)

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