The Role of Predictability in Saccadic Eye Responses in the Suppression Head Impulse Test of Horizontal Semicircular Canal Function

Abstract

BackgroundIn the suppression head impulse paradigm (SHIMP) vHIT protocol, the participant is instructed to follow with his gaze a mobile target generated by a laser placed on the participant’s head. Recent studies have reported that the refixation saccade latencies are in relation with the time evolution of the vestibular dysfunction in both (standard and SHIMP) procedures. We hypothesized that some central mechanisms like head impulse prediction could be one of the causes for the differences in the saccadic eye responses.MethodsA prospective cohort non-randomized study was designed. For the SHIMP protocol, recorded with the ICS Impulse ver. 4.0® (Otometrics A/S, Taastrup, Denmark) vHIT device, three different algorithms were performed: “predictable,” “less predictable,” and “unpredictable” depending on the target’s predictability. A mathematical method was developed to analyze the SHIMP responses. The method was implemented as an additional tool to the MATLAB open source script for the extended analysis of the vHIT responses named HITCal.ResultsIn cohort 1, 52 participants were included in “predictable” SHIMP protocol. In cohort 2, 60 patients were included for the “less predictable” and 35 patients for the “unpredictable” SHIMP protocol. The participants made more early saccades when instructed to perform the “predictable” paradigm compared with the “less predictable” paradigm (p < 0.001). The less predictable protocol did not reveal any significant difference when compared with the unpredictable protocol (p = 0.189). For the latency of the first saccade, there was statistical difference between the “unpredictable” and “predictable” protocols (p < 0.001) and between the “less predictable” and “predictable” protocols (p < 0.001). Finally, we did not find any relationship between the horizontal vestibulo-ocular reflex (hVOR) gain and the latency of the saccades.ConclusionWe developed a specific method to analyze and detect early SHIMP saccades. Our findings offer evidence regarding the influence of predictability on the latency of the SHIMP saccadic responses, suggesting that early saccades are probably caused by a conditioned response of the participant. The lack of relationship between the hVOR gain and the latency of the saccades suggests that the predictive behavior that caused the early eye saccades are independent of the vestibular function.

Vestibulo‐ocular reflex gain values in the suppression head impulse test of healthy subjects

“…The presence of covert saccades on HIMP paradigm was controlled by including only healthy subjects in whom they are not expected to appear . The early SHIMP saccades were identified and removed from the test with a specific published method . It is worth noting that removal of covert saccades in patient data is reported to be one of the advantages of SHIMPs.…”

“…The laser source was aligned with the rotational axis. The subject was instructed to not anticipate the laser movement. The experimenter was trained to ensure unpredictable head impulses. For this, after a head impulse, the explorer was trained to perform a slow return to 0 position and to ensure a random duration pause of at least 5 seconds before the next impulse was performed …”

“…After the data had been collected, all the SHIMP head impulses were reviewed to ensure the absence of early saccades that could influence in VOR gain calculation. To assess and control the effect of early saccades on the SHIMP responses, we used our published automatic method to detect and identify the presence of early saccades on the obtained SHIMP responses . This open source software algorithm detects and identifies these early saccades for each impulse and its further elimination (Fig.…”

“…For this, after a head impulse, the explorer was trained to perform a slow return to 0 position and to ensure a random duration pause of at least 5 seconds before the next impulse was performed. 8 Written informed consent was obtained from all the participants. If the participant's age was between 15 to 18 years, the signature of a legal representative was also required.…”

“…It has also been described that the presence of early saccadic responses observed in some subjects explored with the SHIMP paradigm could output the lower gain VOR values in SHIMP paradigm due to subjects' anticipatory mechanisms. 8 We hypothesized that other factors could be contributing to the observed differences between HIMP and SHIMP gain values in addition to the actual described de-saccade algorithm and early saccades mechanisms.…”