Artifact keeps changing during pair-pulse stimulation(50ms interval, 1200x)

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littleno
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Artifact keeps changing during pair-pulse stimulation(50ms interval, 1200x)

 Hi, dear all,
                         I just begin my electrophysiological experiment. I try to record fEPSP in hippocampal CA1 after  pair-pulse

                     stimulation to induce LTD.

                        My problem is that when pair-pulse stimulation(1Hz, 1200 trains) was given,  the artifact keeps

                    changing  during this 20mins. This does not always happened. Sometimes LTD can be induced with this  

                   protocol. But everytime, when artifact changing happens, LTD can not be induced successfully. Baseline or after

                    pair-pulse recordings are quite normal (artifact keeps unchanged). I don't know whether this is because

                    unstale  supply of  electric current or maybe there is very trivial movement that can't be detected by bare

                    eyes(motorized manipulators are used). I changed new bipolar electrodes (FHC Stainless steel electrode). But

                   it doesn't help.
               
                   Anyone has experience in handling pair-pulse stimulation,  give me some suggestions to help my

                   project move on.

                                                                                  Thank you very much!!
                                                                                                                                    littleno