Simultaneous Eye Tracking and Single-Neuron Recording to Identify Target-Selective Neurons

Take a human participant with depth electrodes embedded in the brain's medial temporal lobe, which responds to visual stimuli.

The electrodes contain microwires that record electrical activity from individual neurons.

Position a display connected to an eye-tracking system in front of the participant. The system tracks eye movements in response to visual stimuli.

Present a target object to the participant, followed by a search array containing multiple objects, including the target. Instruct the participant to identify the target in the array and confirm detection by pressing a button on a response box.

Upon detection, target-selective neurons in the brain modify the rate of firing electrical signals, called action potentials, to transmit visual information.

The electrode microwires record these signals extracellularly.

Correlate the timing of target detection, obtained from button press and eye-tracking data, with the timing of the change in the firing rate to assess the presence of target-selective neurons.

To begin, connect the electrodes to the recording system, and position the patient sitting upright. Then, connect the stimulus computer to the electrophysiology system and eye tracker. Place the remote non-invasive infrared eye-tracking system on a robust mobile cart. Then, attach a flexible arm that holds an LCD display to the cart.

Place a fully charged, uninterrupted power supply on the eye-tracking cart, and connect all devices related to eye-tracking to the power supply rather than to an external power source. Make sure the IV device connected to the patient is running on battery and is not plugged into the wall. Start the eye-tracking software. Adjust the distance between the patient and the LCD screen to between 60 and 70 centimeters.

Also, adjust the angle of the LCD screen so that the surface of the screen is approximately parallel to the patient's face. Place a sticker on the patient's forehead so that the eye tracker can adjust for head movements. Now, 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 and button press are recorded properly before starting the experiment. Start the acquisition software. First, visually inspect the broadband 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 from 300 Hertz to 8 kilohertz.

Select one of the eight microwires as a reference for each microwire bundle. Enable saving the data as an RD file before recording. Provide task instructions to the 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 if they find the target, and press 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 and run the task. Present a target cue for one second, and present the search array using the stimulus presentation software. Record the button presses and provide trial-by-trial feedback to the participants.