Studies of vestibular perception have historically focused on measuring perception at or near the threshold (Walsh 1961
; Clark 1967
; Gundry 1978
; Melvill Jones and Young 1978
; Benson and Brown 1989
; Gianna et al. 1996
; Grabherr et al. 2008
; Mallery et al. 2010
; Soyka et al. 2011
) with studies of suprathreshold perception focusing on heading perception (MacNeilage et al. 2010
) and cycles of sinusoids (Mallery et al. 2010
). Previous studies have not investigated the possibility of directional asymmetry in suprathreshold vestibular perception. It has been demonstrated that vestibular perception near the threshold can be asymmetric in normal controls (Benson et al. 1986
) and this has been recently demonstrated in the current laboratory (Roditi and Crane 2011
). All but one of the subjects used in this study also participated in the prior study (). A recent review suggested that that these asymmetries may be caused by a bias (Merfeld 2011
) although there is also evidence that they may be due to direction specific differences in sensitivity near the threshold (Roditi and Crane 2011
). The current study attempts to extend knowledge of vestibular thresholds to stimuli that are well above the threshold of human perception. It was found that the magnitude of yaw and sway motion in opposite directions was perceived as equal, and when the differences were significant they were small.
There were no significant directional asymmetries in this study despite these same subjects often showing directional asymmetries near the threshold of perception as described in a prior study in this laboratory (Roditi and Crane 2012
). Subject numbers are provided in . In the prior study, subject #1 had a significant and consistent asymmetry in sway thresholds, which was not evident here. Significant threshold asymmetries were also previously reported in subjects #4, 12, and 13. Yaw threshold asymmetry was less commonly present in subject #1. Thus, it seems asymmetries at the perceptual threshold do not translate into asymmetry in suprathreshold perception.
An unexpected result was that the order in which the stimuli were presented has a significant effect on their perceived magnitude in a large subset of subjects. In yaw motion, it was most common for the second interval to be perceived as larger than the first. Thus, the first interval was actually larger at the PSE. In sway, fewer subjects demonstrated the bias with a nearly equal number of subjects describing the first and second interval as appearing larger.
Using both MC and VA in otherwise similar conditions tested the possibility of the existence of the order effect being due to chance or the task itself. In 11 of the 12 conditions that were repeated using both methods, the results were similar (), indicating that the task did not lead to a difference in how these stimuli were perceived. Also, if asymmetries occurred by chance, we would also expect to see a similar rate of directional specific asymmetries and this was not observed.
The level at which this order asymmetry occurred was investigated using a visual only stimulus that was designed to be analogous to the motion stimulus. The results were similar to a motion only stimulus in that there was usually no directional asymmetry (the exception being subject #13 for a visual sway stimulus). The order asymmetry continued, but was not well correlated with the order asymmetry seen with motion. For example, in subject #1 there was a similar trend, but in subject #9 the effect reversed direction for a visual stimulus (). Thus, the order effects with motion stimuli were not directly duplicated with visual stimuli.
Order effects were not limited to comparisons of OD stimuli. With SD stimuli, about half the subjects demonstrated a significant order effect with both visual and motion stimuli, however, the effect was not well correlated between the visual and motion tests or between sway and yaw stimuli ().
One difference between OD and SD stimulus pairs was that the OD stimuli compared a variable interval to a constant interval while the SD stimuli compared two intervals of variable magnitude. This was necessary because the range of the motion platform was limited so that OD stimuli could potentially be of larger magnitude than SD stimuli. The largest differences () were such that one interval was just less than twice as large as the other, which would correspond to PSEs at 10 and 20 cm. Most stimulus pairs were considerably smaller such that both intervals would be much closer to 15 cm, the constant interval used in the OD blocks. Due to the similar range of motion between the two, it is thought that this difference did not substantially influence the results.
One possible explanation for the order effects seen would be a perceptual after effect. The most extensively described of these is the visual motion after effect (MAE) or “waterfall illusion” initially described more than 175 years ago (Addams 1834
). More recently, this has been found to influence other areas of visual perception (Kohn 2007
; Thompson and Burr 2009
). Similar perceptual after effects have been described with sound intensity (Reinhardt-Rutland 1998
), voice (Bestelmeyer et al. 2010
), proprioception (Seizova-Cajic et al. 2007
) and even the vestibular system (Crane 2011
). The visual component of this study was not designed to maximize visual MAE because the stimuli had an intentionally degraded coherence and the second stimulus was the same duration and coherence as the initial stimulus, which would tend to overpower any MAE. If a MAE were to occur in two similar visual stimuli in opposite directions, the second would appear larger, and this would cause the TAI to be positive. Although a slight positive was seen in sway () and for the visual SD stimuli (), the opposite effect was seen in yaw (), implying that this was not a major effect. After effects in the vestibular system with translation and are more subtle than visual MAE (Crane 2011
), and these results demonstrate only small and inconsistent effects ( & ). The most consistent vestibular effect was seen with the TAI with OD yaw stimuli (). A false perception of continued rotation after yaw rotation has been described as is thought to be related to velocity storage (Bertolini et al. 2011
), although the initial stimulus here was likely too short for velocity storage to have been in issue. The effect may have been due to an after effect in yaw, but such a phenomenon has not yet been described with the relatively short stimuli used here. Such effects would be expected to show up as a non-zero TAI, but the mean TAI was near zero ( & ) suggesting after effects likely did not have a significant influence. After effects may explain the positive TAI seen with yaw rotation in some subjects and in the average data ().
It is possible that these temporal asymmetries may have been reduced by including a gap or inter-stimulus interval between the stimulus pairs. This could occur because the magnitude of the vestibular after effect (Crane 2011
), as well as velocity storage effects (Bertolini et al. 2011
) decay with time although some effect can still be present several seconds after the initial movement. Including a long inter-stimulus interval may have caused some recall bias as subjects may not be able to accurately remember the initial stimulus. Due to the large number of stimulus presentations in each block, adding a long inter-stimulus interval would add significant time to the experiment which would contribute to subject fatigue. In this study, potential vestibular aftereffects were instead controlled for by including stimulus presentations in which the initial stimulus could be in either direction so that any effect would cancel out when determining direction specific asymmetries. However the observed temporal asymmetries are likely to be a result of an aftereffect.
The standard deviation of the psychometric function was a measure of the precision of differentiating the two stimuli. It is known that function of the vestibular reflexes declines with age (Stefansson and Imoto 1986
; Tian et al. 2001
), and this has been also shown for fore-aft perception (Kingma 2005
), as well as perception of motion in other directions (Roditi and Crane 2012
). Based on these data it would be expected that the precision of responses might decline with age. However, this did not occur with visual stimuli or with motion stimuli in opposite directions (). There was, conversely, a strong correlation between advancing age and decreased precision in differentiating same direction stimuli (). Although some of this may represent degeneration of the peripheral otolith organs, it cannot explain why the decline in precision with age is limited to comparing two stimuli in the same direction. It may be because comparison of opposite direction stimuli involves evaluation of vestibular signals that are less consistent than those from same direction. Thus, precision in comparison of opposite direction stimuli may be less dependent on end organ performance.