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All procedures involving human participants have been performed in compliance with the institutional, national, and international guidelines for human welfare and have been reviewed by the local institutional review board.
1. Participants
- Recruit neurosurgical patients with intractable epilepsy who are undergoing placement of intracranial electrodes to localize their epileptic seizures.
- Insert depth electrodes with embedded microwires into all clinically indicated target locations, which typically include a subset of amygdala, hippocampus, anterior cingulate cortex and pre-supplementary motor area. See details for implantation in our previous protocol.
- Once the patient returns to the epilepsy monitoring unit, connect the recording equipment for both macro- and micro- recordings. This includes carefully preparing a head-wrap that includes head stages. Then, wait for the patient to recover from the surgery and conduct testing when the patient is fully awake (typically at least 36 to 48 h after surgery).
2. Experimental setup
- Connect the stimulus computer to the electrophysiology system and eye tracker following the diagram in Figure 1.
- Use the remote non-invasive infrared eye tracking system (see Table of Materials). Place the eye tracking system on a robust mobile cart (Figure 1A, B). To the same cart, attach a flexible arm that holds an liquid crystal display (LCD). Use the remote mode to track the patient's head and eyes.
- Place a fully charged uninterrupted power supply (UPS) on the eye tracking cart and connect all devices related to eye tracking (i.e., LCD in front of patient, eye tracker camera and light source, and eye tracker host computer) to the UPS rather than to an external power source.
- Adjust the distance between the patient and the LCD screen to 60-70 cm and adjust the angle of the LCD screen so that the surface of the screen is approximately parallel to the patient's face. Adjust the height of the screen relative to the patient's head such that the camera of the eye tracker is approximately at the height of the patient's nose.
- Provide the patient with the button box or keyboard. Verify that triggers (transistor-transistor logic, TTL) and button press are recorded properly before starting the experiment.
3. Single-neuron recording
- Start the acquisition software. First, visually inspect the broadband (0.1 Hz - 8 kHz) local field potentials and make sure they are not contaminated by line noise. Otherwise, follow standard procedures to remove noise.
- To identify single neurons, band-pass filter the signal (300 Hz - 8 KHz). Select one of the eight microwires as a reference for each microwire bundle. Test different references and adjust the reference so that (1) the other 7 channels show clear neurons, and (2) the reference does not contain neurons. Set the input range to be ± 2,000 µV.
- Enable saving the data as an NRD file (i.e., the broadband raw data file that will be used for subsequent off-line spike sorting) before recording data. Set the sampling rate to 32 kHz.
4. Eye tracking
- Start the eye tracking software. Because it is a head-fixation free system, place the sticker on the patient's forehead so that the eye tracker can adjust for head movements.
- Adjust the distance and angle between the eye tracker and patient so that the target marker, head distance, pupil, and corneal reflection (CR) are marked as ready (as shown in green in the eye tracking software; Figure 2 shows a good example camera setup screen). Click on the eye to be recorded and set the sampling rate to 500 Hz.
- Use the auto-adjustment of pupil and CR threshold. For patients wearing glasses, adjust the position and/or angle of the illuminator and camera so that reflections from the glass will not interfere with pupil acquisition.
- Calibrate the eye tracker with the built-in 9-point grid method at the beginning of each block. Confirm that eye positions (shown as "+") register nicely as a 9-point grid. Otherwise, redo calibration.
- Accept the calibration and do validation. Accept the validation if the maximal validation error is < 2° and the average validation error is < 1°. Otherwise, redo validation.
- Do drift correction and proceed to the actual experiment.
5. Task
- In this visual search task, use the stimuli from our previous study and follow the task procedure as described before.
- Provide task instructions to participants. Instruct the participants to find the target item in the search array and respond as soon as possible. Instruct the participants to press the left button of a response box (see Table of Materials) if they find the target and the right button if they think the target is absent. Explicitly instruct the participants that there will be target-present and target-absent trials.
- Start stimulus presentation software (see Table of Materials) and run the task: Present a target cue for 1 s and present the search array using the stimulus presentation software. Record button presses and provide trial-by-trial feedback (Correct, Incorrect, or Time Out) to participants.
