Is the manual following response an attempt to compensate for inferred self-motion?

Abstract

If the surrounding of a visual target unexpectedly starts to move during a fast goal-directed hand movement, the hand reflexively moves along with it. This is known as the ‘manual following response’. One explanation for this response is that it is a compensation for inferred self-motion in space. Previous studies have shown that background motion gives rise to both postural responses and deviations in goal-directed hand movements. To evaluate whether compensation for inferred self-motion is responsible for the manual responses we examined whether galvanic stimulation of the vestibular system would give rise to similar deviations in hand movements. Standing participants tried to quickly tap on targets that were presented on a horizontal screen. Participants could infer self-motion on some of the trials, either from galvanic vestibular stimulation or from background motion. Both perturbations took place during the hand movement. It took both the head and hand about 45 ms longer to respond to background motion than to respond to galvanic stimulation. The head responded in a similar manner to both types of perturbations. The hand responded about as expected to galvanic stimulation, but much more vigorously to background motion. Thus, the manual response to background motion is probably not a direct consequence of trying to compensate for inferred self-motion. Perhaps the manual following response is a consequence of an error in binding motion information to objects.

The response to background motion: Characteristics of a movement stabilization mechanism

“…It is an automatic response that simply updates the planned endpoint of the movement to match the motion in the vicinity. It does not involve an explicit estimation of self-motion and is therefore compatible with the MFR being more prominent than postural responses to background motion ( Zhang et al, 2018 ) and to it not being found after illusory self-motion induced by vestibular stimulation ( Zhang et al, 2019 ), as well as with the finding that the presence of static structures does not influence the MFR. It can also explain the experimental characteristics of the MFR we report here: The preceding stability of the environment and the presence of static background structures are not considered.…”

“…The MFR describes the response of the hand in the direction of sudden background motion when the hand is moving to a target. Several experimental characteristics of the MFR have already been documented that are inconsistent with the idea that the mechanism underlying this response is an estimation of self-motion based on an instantaneous global optic flow analysis ( Zhang et al, 2019 ). Here, we find additional characteristics of the MFR that are also inconsistent with that idea.…”

“…The experimental characteristics of the MFR that we found in these experiments are also inconsistent with other mechanisms that have been proposed to underlie the MFR. The fact that the response was stronger when the three points that moved were near the interception point than when they were near the target and that not only motion along the target's path drives the MFR shows that the MFR is not the result of background motion near the target being incorrectly attributed to the target ( Zhang et al, 2019 ). Our finding that the MFR still emerged when background motion was presented exclusively in the upper half of the screen clearly refutes the suggestion that the MFR is the result of background motion influencing the apparent motion of the hand ( Grierson & Elliot, 2009 ) because in that case, there is no background motion in the vicinity of the hand.…”

“…Second, the MFR is more prominent than postural responses to background motion ( Zhang, Brenner, Duysens, Verschueren, & Smeets, 2018 ), suggesting that the MFR is not a compensation for the same estimated self-motion that leads to the observed postural response. Finally, galvanic and visual stimulation that lead to a similar postural response (presumably compensating for similar estimates of self-motion) lead to MFRs that differ considerably ( Zhang, Brenner, Duysens, Verschueren, & Smeets, 2019 ). Since the response to inferred self-motion should be independent of the sensory modality that gives rise to the estimate of self-motion, one would not expect this difference in the magnitude of the MFR.…”

Hand movements respond to any motion near the endpoint

“…It is well documented that the moving arm quickly follows target displacements (reviewed by Smeets, Oostwoud Wijdenes, & Brenner, 2016 ). It is also known that the moving arm quickly follows motion onsets in the surrounding area (e.g., Brenner & Smeets, 1997 ; Gomi, Abekawa, & Nishida, 2006 ; Saijo, Murakami, Nishida, & Gomi, 2005 ; Whitney, Westwood, & Goodale, 2003 ; Zhang, Brenner, Duysens, Verschueren, & Smeets, 2018 , Zhang, Brenner, Duysens, Verschueren, & Smeets, 2019 ), in particular if they occur near the anticipated movement endpoint (e.g., Brenner & Smeets, 2015 ; Crowe, Smeets, & Brenner, 2021 ). We propose that this following response is due to shifting the planned movement endpoint in the direction of the motion onset (Crowe et al, 2021 ).…”