Patients with PPPD showed a reduced vestibular perception threshold and expressed more vegetative symptoms during and after rotation in the rotary chair experiment compared to healthy controls. These objective perception threshold deficits and increased vegetative responses to a vestibular stimulus in a functional vestibular disorder add to the current understanding of PPPD. With regard to clinical characteristics, disease duration, mean scores of DHI, HADS and STAI and the percentage of patients with a peripheral vestibular disorder as the suspected triggering event for PPPD are consistent with the current consensus criteria of the Bárány Society and published reports of patients with PPPD [3, 4].
Perceptual thresholds consecutively improve, as the sinusoidal rotation frequency increases over 0.2 Hz, and plateau at about 0.5 Hz . This suggests that vestibular signals may undergo a high-pass filtering. This physiological threshold may be altered by psychological influences (i.e. anxiety, body vigilance) as suggested for visual cues during sensory integration previously . Based on increased comorbidity Balaban et al. suggested a concept for balance and anxiety in the context of migraine . He hypothesized that vestibular processing and anxiety share central pathways to the amygdala and cerebral cortex [25, 26]. Similar may apply for PPPD as anxiety here also is frequently comorbid , and increased connectivity within brain networks regulating interoception, emotional behavior and cognitive control in patients with functional vestibular disorder was previously demonstrated [27, 28]. In the current study patients showed a correlation between anxiety measure (STAI) and vegetative symptoms during the chair rotation. This sensitized vegetative response pattern may reflect one aspect of the affective mediation of vestibular processing in PPPD. However, PPPD and control groups had mean HADS-D-scores in the normal range and the mean HADS-A-score for the PPPD group was barely in the clinically meaningful range, similar accounts for the STAI.
Motion perception can be altered by psychological factors. In a different experiment subjects were asked to simply imagine themselves rotating in a chair before the actual rotation began. Vestibular perceptual thresholds were lower when the real rotations and the imagined rotations had the same direction, as were thresholds for the vestibular ocular reflex . This finding indicates that top-down modulation of low-level vestibular perception to physical rotation is possible by psychological influence alone. Thus, the vestibular system appears to be susceptible to modulation by higher order cognitive processes associated with attention, working memory or mental imagery, and potentially anxiety and expectation [30, 31].
A neuroscientific mechanism is currently lacking as to why some individuals rely more heavily upon visual cues , and others on vestibular (i.e. gravito-inertial cues) or somato-proprioceptive cues . The development of PPPD in certain patients may be related to a psychological trait in patients with predominant vestibular perception preference. Structural and functional imaging studies suggested an abnormal integration of visual and vestibular information as well as alterations in multisensory vestibular brain areas [27, 33–36]. This supports the assumption that alterations of vestibular processing in PPPD are based on widespread changes of the multisensory vestibular system. The affection of somatosensory perception was demonstrated by the detection of a habituation deficit in patients with PPPD following trigeminal pain stimuli at brainstem level applied with the nociceptive blink reflex . Galvanic vestibular stimulation showed a reduced perception threshold for body motion in PPPD patients, which was attributed to a lowered sensory feedback control . Our data imply reduced motion perception thresholds, which may be attributed to cortical and subcortical overexcitability and may explain the abnormal self-motion awareness. This may also lead to exacerbations of the subjective postural instability, which improve under cognitive distraction . Our findings may also support the concept of increased attentional effects on vestibular processing in PPPD. This is in line with the current consensus criteria, which explicitly allows subtyping of this multifaceted clinical entity (e.g. posturally predominant subtype, visually predominant subtype) . Therefore, it appears most likely that patients with PPPD have a multimodal dysfunction not limited to vestibular stimuli alone.
A change of the vestibular sensitivity is not specific to PPPD, but can also be found in other vertigo associated disorders, such as vestibular migraine  and motion sickness . This increased sensitivity of the vestibular system was hypothesized as correlate for a habituation deficit or cortical hyperexcitability. Kinetosis could be interpreted as a sensitivity threshold for the perception of vestibular stimuli that is adjusted too finely.
Our study has some limitations. The self-report instrument HADS measured a slightly higher rate of anxiety values in PPPD group. Nevertheless, we did not find evidence for fulfilling the diagnostic criteria for active anxiety disorder in these patients. This is in concordance with previous research, patients with functional dizziness score elevated anxiety values even though they do not suffer from anxiety disorder [40, 41]. However, the increased level of psychological factors in the PPPD group and the proportion of patients taking serotonergic medication as PPPD treatment could have influence to the perception threshold. However, the direction of this alteration could be contrary, as an anxious mode of vestibular control may decrease the perception threshold, while selective serotonin-reuptake inhibitors most likely increase it and are therefore used in this indication [3, 25, 42]. Our relatively small number of patients is a limitation, but we decided to exclude PPPD patients with clinically significant comorbid anxiety disorder or depression to prevent bias. In contrast to PPPD in general, we only included female participants, this should not influence the results, as there were no differences of vestibular perceptual thresholds between males and females reported in a previous investigation . Ultimately, an influence on these study results cannot be excluded here.
Concerning the debate as to whether PPPD should be termed psychogenic or functional, we think that both terms can be used depending on the clinical and individual situation of the patient. However, we find the term functional most suitable in cases when there is no identifiable psychological cause, the patient is accepting the diagnosis, and symptoms are interfering with everyday activities.