Small devices

MEG lab accelerometers                                                        

MEB lab has two accelerometers. Both of the accelerometers consist of an accelerometer chip connected with a cable to a box with three BNC connectors. Both of the models have an on/off switch, but only the other one has an “on” indicator LED.

The accelerometers work with 2x1.5V batteries. The LED indicator has one separate 1.5V battery for its power source.

The BNC connectors give out max 3V signal and they represent the X, Y and Z side movement of the accelerometer chip. Normally, the X, Y and Z signals are taken to MEG MISC1-3 channels situated next to the Data Acquisition computer. One should always measure from all the 3 channels to get more real acceleration signal. MISC channels are recorded by turning them on in Acquisition.

accelerometer_schematic_vs1_20160518.png

accelerometer_schematic_vs2_20160518.png

MEG lab GSR                                                                  

MEG lab has one GSR or EDA (skin conductance meter) which is compatible with the Triux MEG. The device consists of Becker Meditec BP-BM-30 (supplier Brain Products) GSR module and Wave-Power-15600 5V USB battery. The GSR is connected to the MEG Bio inputs on the side of the gantry. Because the maximum voltage on these inputs is of the order of +/-15mV, the output voltage of the GSR has to be reduced. This is done with resistors. The output voltage reduction with the current resistor configuration is 200-fold.

 GSR_schematic_vs1_20180206.png

If there is no signal reduction, in the output of the GSR module 25 mV corresponds to 1μS with a constant 5V supply voltage. Now, if the USB battery is full, the resistors are exactly what the markings claim them to be, 25mV/200 = 0.125mV corresponds to 1μS.

The participant can be connected to the GSR module with using Ambu single-use skin electrodes. The electrodes can be glued (sticker) to the hand of the participant for example like in the image below. The hand should be the non-dominant hand.

The GSR module is connected to the bioamplifier (BIO) inputs on the side of the MEG gantry (like EOG or ECG cables). Cable marked red to plus, the other to minus.

 GSR_schematic_hand_vs1_20180206.png

The USB battery will not stay on automatically by just plugging the GSR module in to it. Because the required current is so small, the battery will switch off automatically after fer seconds. This is circumvented so that when the measurement is done, there is a small lamp on the USB battery which should be on during the measurement. The lamp is turned on by pressing the only button there is for a while.

UPDATE April 11th 2019: With a new black box this can be circumvented. The box has a large enough resistor inside so that the battery stays on without turning the light on.

The output signal of the GSR module is of slowly changing, DC voltage. In the data acquisition computer the channel settings should be set to

  • Amplification: 640 (or 2000 if the signal is small)
  • DC input

Because the GSR data is only slowly changing, it is difficult to monitor it on the data acquisition computer. The time scale for the GSR signal change is in the order of seconds and more. The maximum time window which can be monitored in the Dacq computer is only few seconds. Additionally, the signal level can change quite a lot during the measurement. To circumvent these problems, it is possible to monitor the GSR signal using the Linux computer and Fieldtrip buffering of the data on the Dacq computer (see more in separate instructions).

On Linux Stim computer the GSR signal can be monitored using the GUI which can be started from a shell script named "gsr_monitor.sh". Note, that before the shell script can be run, one has to first click open “Data” from the left-hand icon bar. This mounts the disc drive system properly for the system.

To the GUI one can insert min and max Y scale and the channel name, which you want to monitor. This channel has to be activated from Dacq and the name syntax is the same as is in the Dacq data monitor (e.g. MEG0123, BIO012, etc). If “Automatic Y scaling” is ticked, the min and max Y scale don’t have effect anymore. Start and stop monitoring buttons are just that. Decimal separator is the dot.

Breathing sensor

There is one piezo-based breathing sensor model ??? in the MEG lab. It has a sensor part (short rubber band which has a green fabric strip connectd) and a Velcro strap. There are two different lengths of Velcro straps depending on the girth of your participant. You can strap the device around your participant either around stomach or chest area. As long as the girth changes depending on the breathing. You can ask the participant to exhale and only then tighten the strap. It has to be comfortably tight, so that the strap lengthens some during breathing, but not overly tight.

The device has two lead wires. Connect those directly to the MEG BIO input, which you have prepared from the MEG Data Acquisition program. The voltage generated by the breathing sensor is so low, that it is perfectly safe to directly connect it to the BIO inputs. When you prepare the BIO channel, set the high pass filter of the channel to DC-mode, because the signal will be only slowly changing. This means, there is no high pass filtering. The amplification can be left to the default value.

Note that the breathing sensor is based on piezo crystal. This means, that the device generates voltage when work is done to the crystal (band lengthens or shortens). When no work is done, the voltage level goes back toward zero line. If the participant is holding his/her breath, you might not be able to differentiate normal exhale or inhale from it in every situation. For normal breathing rhythm, the device should be okay.