The overall goal of this procedure is to measure the effects of deep brain stimulation on saccadic eye movements. The combination of deep brain stimulation with saccade measurement provides a tool to study normal and pathological brain function. This method can help answer key questions about the pathophysiology of diseases relating to the basal ganglia.
The main advantage of this technique is that it is quantitative and precise, allowing for the accurate detection of subtle abnormalities. These are some of the types of stimulator implants used in deep brain stimulation. They contain a battery and electronics to generate a small electrical stimulus.
The stimulus pulses are delivered to precise target locations in the brain using contacts on the end of a wire like this one. Begin by explaining the details of the study and providing the participant with a consent form to sign. Set up the saccadometer so that for each trial the central fixation target is displayed for a random period of 1.0 to 2.0 seconds, after which it is extinguished, and one of the peripheral red targets appears randomly to the right or left.
Next, ask the participant to sit 1.5 meters away from a flat matte screen. Place the saccadometer on the participant's head so that it rests on the bridge of the nose and secure with an elastic adjustable strap. Ensure that the ambient lighting is dim so that the stimuli are clearly seen.
Then, instruct the participant that the 40 minute testing session consists of five blocks with a one minute break between each block. Prior to the first block, instruct the participant to move their eyes as quickly and accurately as possible in order to follow the red dot jumping from the middle to one side or the other. Explain that they must do this 60 times.
Set the saccadometer to generate a sequence of 60 trials, then press the button on the device to start the first block of trials. After the completion of the first block wait one minute before starting the second block. Reset the saccadometer to generate a sequence of 40 trials.
Towards the end of the one minute gap, instruct the participant to move their eyes as quickly as possible in the opposite direction of the red dot. Inform them that they will be required to do this 40 times. Then, start the second block of trials.
After completion of the second block, wait one minute, then repeat the instructions from the second block two more times for blocks three and four. Finally, after completing the fourth block, wait one minute, then instruct the participants to move their eyes as quickly and accurately as possible to follow the red dot jumping from the middle to one side to the other, exactly as they did in the first block. Reset the saccadometer to generate a sequence of 60 trials and start the final block of trials.
Note for participants with deep brain stimulators, or DBS systems, conduct saccadic testing two times. Once with the DBS system running as normal, and a second time with the DBS system turned off. To conduct the off stimulation session, have a trained clinical staff switch off the DBS system 30 minutes prior to testing.
Then, complete saccadic testing. Once the session is complete, switch the DBS system back on. To analyze the data, download the raw data from the saccadometer to a computer.
Next, use the saccadometer's software program to exclude saccades that are distorted by blinks and head movements. Calculate variables including saccadic latencies, peak velocities, and amplitudes. Here, prosaccadic latency results are shown for a Parkinson's patient after undergoing DBS of the subthalamic nucleus when the DBS is turned on and off.
The distribution of saccades is altered when the stimulator is switched on, showing a reduction in the number of long latency saccades, and a corresponding decrease in the mean latency. Once mastered, this technique can be done in about 70 minutes if it is performed properly. While attempting this procedure, it is important to remember to ensure that the saccadometer is sitting centrally on the bridge of the nose and the room lights are dim.
Following this procedure, more advanced saccadic tasks can be used in order to answer additional questions, like how the cognitive control of eye movements is affected by deep brain stimulation. After watching this video, you should have a good understanding of how to use psychodometry in subjects with deep brain stimulator systems. Don't forget that working with lasers can be hazardous, and you should take care to ensure that both the participants and the experimentor don't look directly into the laser beams.
