Percept of the duration of a vibrotactile stimulus is altered by changing its amplitude

Abstract

There have been numerous studies conducted on time perception. However, very few of these have involved tactile stimuli to assess a subject's capacity for duration discrimination. Previous optical imaging studies in non-human primates demonstrated that increasing the duration of a vibrotactile stimulus resulted in a consistently longer and more well defined evoked SI cortical response. Additionally, and perhaps more interestingly, increasing the amplitude of a vibrotactile stimulus not only evoked a larger magnitude optical intrinsic signal (OIS), but the return to baseline of the evoked response was much longer in duration for larger amplitude stimuli. This led the authors to hypothesize that the magnitude of a vibrotactile stimulus could influence the perception of its duration. In order to test this hypothesis, subjects were asked to compare two sets of vibrotactile stimuli. When vibrotactile stimuli differed only in duration, subjects typically had a difference limen (DL) of approximately 13%, and this followed Weber's Law for standards between 500 and 1500 ms, as increasing the value of the standard yielded a proportional increase in DL. However, the percept of duration was impacted by variations in amplitude of the vibrotactile stimuli. Specifically, increasing the amplitude of the standard stimulus had the effect of increasing the DL, while increasing the amplitude of the test stimulus had the effect of decreasing the DL. A pilot study, conducted on individuals who were concussed, found that increasing the amplitude of the standard did not have an impact on the DL of this group of individuals. Since this effect did not parallel what was predicted from the optical imaging findings in somatosensory cortex of non-human primates, the authors suggest that this particular measure or observation could be sensitive to neuroinflammation and that neuron-glial interactions, impacted by concussion, could have the effect of ignoring, or not integrating, the increased amplitude.

Keywords: amplitude discrimination; duration discrimination; somatosensory; tactile; vibrotactile.

Figure 1

Summary of optical intrinsic signal (OIS) imaging results. Prior studies of non-human primate demonstrate that increasing the amplitude of a vibrotactile stimulus makes the OIS longer.

Figure 2

Photo of the multi-site vibrotactile stimulator. During an experimental session, subject was seated comfortably in a chair with right arm resting on the arm rest attached to the head unit of the stimulator. Vibrotactile stimulation was conducted via 5 mm probes that come in touch with subject’s index and middle finger.

Figure 3

Schematics of the duration discrimination protocol used in this study. During each trial of the duration discrimination task, two 40 Hz vibrotactile stimuli—the standard and test—were delivered sequentially to either D2 or D3. Subject was instructed to choose the stimulus that was perceptually longer with a response box in the response interval following the two stimuli intervals.

Figure 4

Adherence to Weber’s Law. Averaged difference limen (DL) values of the twenty subjects at various standard durations (with s.e. bars). The plotted linear regression has a correlation coefficient of 0.99191, a slope of 0.122 and a y-intercept of 7.4206.

Figure 5

Effect of amplitude confound on duration discrimination. The measured difference limens (DLs) for the duration discrimination protocol are shown normalized on a subject-by-subject basis to the 300 μm condition (both test and standard are at an amplitude of 300 μm). The 350 μm confound had a normalized value of 1.537 ± 0.173 and the 400 μm confound had a normalized value of 2.428 ± 0.263 (mean ± SE).

Figure 6

The measured DLs for the duration discrimination protocol are compared to two different amplitude confounds. In the Test 400 μm condition, a 400 μm stimulus was used for the test (longer) stimulus (*p < 0.05), and for the Std 400 μm condition, the 400 μm stimulus was used for the standard (shorter) stimulus (**p < 0.0005).

Figure 7

Comparison of amplitude confound impact on concussed and non-concussed individuals. The measured DLs for the duration discrimination protocol of control subjects from Figure 5 are compared to pilot data obtained from nineteen concussed subjects. Only the 400 μm Amplitude confound was used for this pilot study.

Figure 8

Effect of duration confound on amplitude discrimination. The measured DLs for the amplitude discrimination protocol are compared to two different duration confounds. In the test longer condition, the test was either +100 ms or +250 ms longer than the standard. The standard was 500 ms in these conditions. In the standard longer condition, the standard was either +100 ms or +250 ms longer than the test. The duration of the test stimulus was 500 ms for these conditions. (*p < 0.005).