Cerebellar tDCS Effects on Conditioned Eyeblinks using Different Electrode Placements and Stimulation Protocols

Abstract

There is good evidence that the human cerebellum is involved in the acquisition and timing of classically conditioned eyeblink responses (CRs). Animal studies suggest that the cerebellum is also important in CR extinction and savings. Cerebellar transcranial direct current stimulation (tDCS) was reported to modulate CR acquisition and timing in a polarity dependent manner. To extent previous findings three experiments were conducted using standard delay eyeblink conditioning. In a between-group design, effects of tDCS were assessed with stimulation over the right cerebellar hemisphere ipsilaterally to the unconditioned stimulus (US). An extracephalic reference electrode was used in Experiment 1 and a cephalic reference in Experiment 2. In both parts the influence on unconditioned eyeblink responses (UR) was investigated by starting stimulation in the second half of the pseudoconditioning phase lasting throughout the first half of paired trials. In a third experiment, effects of cerebellar tDCS during 40 extinction trials were assessed on extinction and reacquisition on the next day. In each experiment, 30 subjects received anodal, cathodal or sham stimulation in a double-blinded fashion. Using the extracephalic reference electrode, no significant effects on CR incidences comparing stimulation groups were observed. Using the cephalic reference anodal as well as cathodal cerebellar tDCS increased CR acquisition compared to sham only on a trend level. Analysis of timing parameters did not reveal significant effects on CR onset and peaktime latencies nor on UR timing. In the third experiment, cerebellar tDCS during extinction trials had no significant effect on extinction and savings on the next day. The present study did not reveal clear polarity dependent effects of cerebellar tDCS on CR acquisition and timing as previously described. Weaker effects may be explained by start of tDCS before the learning phase i.e., offline, individual thresholds and current flow based on individual anatomy may also play role. Likewise cerebellar tDCS during extinction did not modulate extinction or reacquisition. Further studies are needed in larger subject populations to determine parameters of stimulation and learning paradigms yielding robust cerebellar tDCS effects.

Keywords: associative learning; cerebellum; extinction; eyeblink conditioning; tDCS.

Figure 1

Experimental protocol: Experiment 1 and 2. At the beginning 16 conditioned stimulus (CS) alone trials and 16 unconditioned stimulus (US) alone trials were presented in a random sequence (pseudoconditioning), followed by 100 paired CS-US trials. Thirty CS alone extinction trials were given at the end. Transcranial direct current stimulation (tDCS) was started after 16 pseudoconditioning trials and lasted throughout the first 50 paired trials of the acquisition phase, i.e., across 20 min.

Figure 2

Eyeblink conditioning in an individual subject during anodal tDCS using the cephalic reference electrode. Rectified and filtered EMG-data of the orbicularis oculi muscle are shown from the beginning with the unpaired phase (top) consisting of 16 CS alone and 16 US alone trials in a random sequence followed by 100 CS-US paired trials and 30 CS alone extinction trials at the end of the experiment (bottom). tDCS was started with the second half of unpaired and lasted throughout the first half of paired trials. The vertical lines indicate the onset of the CS and US. Responses occurring within the 150 ms interval after CS onset (dotted line) were considered alpha-responses. See “Subjects and Methods” Section for further details.

Figure 3

Experimental protocol: Experiment 3. Ten CS alone trials and 10 US alone trials were presented at the beginning in a random sequence (pseudoconditioning), followed by the acquisition phase of 84 paired CS-US trials and 36 CS alone trials interspersed. At the end 40 CS alone trials were given as extinction trials. tDCS was applied throughout extinction trials lasting for 12 min. Saving effects were tested on the next day 24 h later using 28 paired CS-US trials and 12 CS alone trials interspersed.

Figure 4

Experiment 1 using an extracephalic reference (A) and Experiment 2 using a cephalic reference (B). Mean percentage conditioned response (CR) incidence and standard errors (SE) in paired trials shown per block of 10 trials during anodal (blue circles), sham (red circles) and cathodal tDCS (gray circles). See “Results” Section for further details.

Figure 5

Experiment 3 using a cephalic reference. Mean percentage CR incidence and SE in acquisition trials (A), extinction (B) and savings on the next day (C), shown per block of 10 trials. Anodal (blue circles), sham (red circles) and cathodal tDCS (gray circles) was applied during extinction trials. CR incidences in block 1 of extinction trials are shown more detailed in blocks of two trials. Decline of CRs across the four extinction blocks was not significantly different comparing stimulation groups. See “Results” Section for further details.