Abstract
The cerebellum is involved in a wide number of integrative functions. We evaluated the role of cerebellum in peripersonal defensive behavior, as assessed by the so‐called hand blink reflex (HBR), modulating cerebellar activity with transcranial direct current stimulation (tDCS). Healthy subjects underwent cerebellar (sham, anodal, and cathodal tcDCS) and motor cortex tDCS (anodal or cathodal; 20′, 2 mA). For the recording of HBR, electrical stimuli were delivered using a surface bipolar electrode placed on the median nerve at the wrist and EMG activity recorded from the orbicularis oculi muscle bilaterally. Depending on the hand position respective to the face, HBR was assessed in four different conditions: “hand‐far,” “hand‐near” (eyes open), “side hand,” and “hand‐patched” (eyes closed). While sham and cathodal cerebellar stimulation had no significant effect, anodal tcDCS dramatically dampened the magnitude of the HBR, as measured by the area under the curve (AUC), in the “hand‐patched” and “side hand” conditions only, for ipsilateral (F (4,171) = 15.08, P < 0.0001; F (4,171) = 8.95, P < 0.0001) as well as contralateral recordings (F (4,171) = 17.96, P < 0.0001); F 4,171) = 5.35, P = 0.0004). Cerebellar polarization did not modify AUC in the “hand‐far” and “hand‐near” sessions. tDCS applied over the motor area did not affect HBR. These results seem to support a role of the cerebellum in the defensive responses within the peripersonal space surrounding the face, thus suggesting a possible cerebellar involvement in visual‐independent defensive behavior.
Highlights
In the Sherringtonian model reflex responses provide a rapid and stereotyped first line of defense, by adequately reacting to aversive stimuli and optimizing the chances of survival (Sherrington 1906)
Cerebellar transcranial direct current stimulation (tDCS) is able to modulate hand blink reflex (HBR) when the stimulated hand is located inside the peripersonal space surrounding the face, suggesting a possible cerebellar involvement in the defensive peripersonal behavior in humans: whereas cathodal and sham stimulation have no significant effect, anodal polarization reduces area under the curve (AUC)
As anodal tDCS modifies reflex responses in the “near-side” and “eyes patched” conditions, cerebellum seems to interfere with defensive behavior selectively when the visual feedback is missing
Summary
In the Sherringtonian model reflex responses provide a rapid and stereotyped first line of defense, by adequately reacting to aversive stimuli and optimizing the chances of survival (Sherrington 1906). This model has been recently improved (Castegnetti et al 2016; Khemka et al 2017), in accordance with Bayesian theories posing that the brain uses probabilistic inference and stores forward models and prior probabilities to compute optimal behavior (Bach 2015).
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