Name: simultaneous eye tracking and single-neuron recordings in human epilepsy patients
Uploaded: 2019-06-17T13:00:00
Description: Watch this Scientific Journal Video about Simultaneous Eye Tracking and Single-Neuron Recordings in Human Epilepsy Patients at JoVE.com

We thank all patients for their participation. This research was supported by the Rockefeller Neuroscience Institute, the Autism Science Foundation and the Dana Foundation (to S.W.), an NSF CAREER award (1554105 to U.R.), and the NIH (R01MH110831 and U01NS098961 to U.R.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.&#160;We thank James Lee, Erika Quan, and the staff of the Cedars-Sinai Simulation Center for their help in producing the demonstration video.

1. Participants
<ol>
	<li>Recruit neurosurgical patients with intractable epilepsy who are undergoing placement of intracranial electrodes to localize their epileptic seizures.</li>
	<li>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 protocol2.</li>
	<li>Once the patient returns to the...

<ol>
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In this protocol, we described how to employ single-neuron recordings with concurrent eye tracking and described how we used this method to identify target neurons in the human MTL.
The setup involves three computers: one executing the task (stimulus computer), one running the eye tracker, and one running the acquisition system. To synchronize between the three systems, the parallel port is used to send TTL triggers from the stimulus computer to the electrophysiology system (<strong class="xfi...

Human single-neuron recordings are a unique and powerful tool to explore the function of the human brain with extraordinary spatial and temporal resolution1. Recently, single-neuron recordings have gained wide use in the field of cognitive neuroscience because they permit the direct investigation of cognitive processes central to human cognition. These recordings are made possible by the clinical need to determine the position of epileptic foci, for which depth electrodes are temporarily implanted into the brains of patients with suspected focal epilepsy. With this setup, single-neuron recordings can be obtained using microwires protruding from the tip of the hybrid depth electrode (a detailed description of the surgical methodology involved in the insertion of hybrid depth electrodes is provided in the previous protocol2). Among others, this method has been used to study human memory3,4, emotion5,6, and attention7,8.
Eye tracking measures gaze position and eye movements (fixations and saccades) during cognitive tasks. Video-based eye trackers typically use the corneal reflection and the center of the pupil as features to track over time9. Eye tracking is an important method to study visual attention because the gaze location indicates the focus of attention during most natural behaviors10,11,12. Eye tracking has been used extensively to study visual attention in healthy individuals13 and neurological populations14,15,16.
While both single-neuron recordings and eye tracking are individually used extensively in humans, few studies have used both simultaneously. As a result, it still remains largely unknown how neurons in the human brain respond to eye movements and/or whether they are sensitive to the currently fixated stimulus. This is in contrast to studies with macaques, where eye-tracking with simultaneous single-neuron recordings has become a standard tool. In order to directly investigate the neuronal response to eye movements, we combined human single-neuron recordings and eye tracking. Here we describe the protocol to conduct such experiments and then illustrate the results through a concrete example.
Despite the established role of the human medial temporal lobe (MTL) in both object representation17,18 and memory3,19, it remains largely unknown whether MTL neurons are modulated as a function of top-down attention to behaviorally relevant goals. Studying such neurons is important to start to understand how goal-relevant information influences bottom-up visual processes. Here, we demonstrate the utility of eye tracking while recording neurons using guided visual search, a well-known paradigm to study goal-directed behavior20,21,22,23,24,25. Using this method, we recently described a class of neurons called target neurons, which signals whether the currently attended stimulus is the goal of an ongoing search8. In the below, we present the study protocol needed to reproduce this previous scientific study. Note that along this example, the protocol can easily be adjusted to study an arbitrary visual attention task.

<table>
	<tbody>
		<tr>
			<td>Cedrus Response Box</td>
			<td>Cedrus (https://cedrus.com/)</td>
			<td>RB-844</td>
			<td>Button box</td>
		</tr>
		<tr>
			<td>Dell Laptop</td>
			<td>Dell (https://dell.com)</td>
			<td>Precision 7520</td>
			<td>Stimulus Computer</td>
		</tr>
		<tr>
			<td>EyeLink Eye Tracker</td>
			<td>SR Research (https://www.sr-research.com)</td>
			<td>1000 Plus Remote with laptop host computer and LCD arm mount</td>
			<td>Eye tracking</td>
		</tr>
		<tr>
			<td>MATLAB</td>
			<td>MathWorks Inc</td>
			<td>R2016a (RRID: SCR_001622)</td>
			<td>Data analysis</td>
		</tr>
		<tr>
			<td>Neuralynx Neurophysiology System</td>
			<td>Neuralynx (https://neuralynx.com)</td>
			<td>ATLAS 128</td>
			<td>Electrophysiology</td>
		</tr>
		<tr>
			<td>Osort</td>
			<td>Open source</td>
			<td>v4.1 (RRID: SCR_015869)</td>
			<td>Spike sorting algorithm</td>
		</tr>
		<tr>
			<td>Psychophysics Toolbx</td>
			<td>Open source</td>
			<td>PTB3 ( RRID: SCR_002881)</td>
			<td>Matlab toolbox to implement psychophysical experiments</td>
		</tr>
	</tbody>
</table>

simultaneous eye tracking and single-neuron recordings in human epilepsy patients

Intracranial recordings from patients with intractable epilepsy provide a unique opportunity to study the activity of individual human neurons during active behavior. An important tool for quantifying behavior is eye tracking, which is an indispensable tool for studying visual attention. However, eye tracking is challenging to use concurrently with invasive electrophysiology and this approach has consequently been little used. Here, we present a proven experimental protocol to conduct single-neuron recordings with simultaneous eye tracking in humans. We describe how the systems are connected and the optimal settings to record neurons and eye movements. To illustrate the utility of this method, we summarize results that were made possible by this setup. This data shows how using eye tracking in a memory-guided visual search task allowed us to describe a new class of neurons called target neurons, whose response was reflective of top-down attention to the current search target. Lastly, we discuss the significance and solutions to potential problems of this setup. Together, our protocol and results suggest that single-neuron recordings with simultaneous eye tracking in humans are an effective method to study human brain function. It provides a key missing link between animal neurophysiology and human cognitive neuroscience.

To illustrate the usage of the above-mentioned method, we next briefly describe a use-case that we recently published8. We recorded 228 single neurons from the human medial temporal lobe (MTL; amygdala and hippocampus) while the patients were performing a visual search task (Figure 3A, B). During this task, we investigated whether the activity of neurons differentiated between fixations on targets and distractors.
...

Watch this Scientific Journal Video about Simultaneous Eye Tracking and Single-Neuron Recordings in Human Epilepsy Patients at JoVE.com

Simultaneous Eye Tracking and Single-Neuron Recordings in Human Epilepsy Patients

We describe a method to conduct single-neuron recordings with simultaneous eye tracking in humans. We demonstrate the utility of this method and illustrate how we used this approach to obtain neurons in the human medial temporal lobe that encode targets of a visual search.

Intracranial recordings from patients with intractable epilepsy provide a unique opportunity to study the activity of individual human neurons during ...

A principle advantage of our technique is that it allows the simultaneous recording of case position, pupil size, and single neurons in patients implanted with depth electrodes at the bedside. This allows us to determine whether there are neurons in the human brain whose activity is sensitive to eye movements and to the identity of the currently fixated stimulus. While this technique is not designed to study a particular disease, it is well-suited to study neurological diseases that result in abnormal eye movements or other ocular motor abnormalities.Demonstrating the procedure will be Nand Chandravadia and James Lee, research associates from my laboratory. And Erika Quan, a neurodiagnostic technician at Cedars-Sinai. 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 a 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 eight kilohertz. Select one of the eight microwires as a reference for each microwire bundle. Enable saving the data as an nrd file before recording.Adjust the distance and angle between the eye tracker and the patient so that the target marker, head distance, pupil, and corneal reflection are marked as ready. This is shown in green in the eye tracking software. Click on the eye to be recorded and set the sampling rate to 500 hertz.Use the auto adjustment of the pupil and corneal reflection threshold. Calibrate the eye tracker with the built-in grid method at the beginning of each block. Confirm the eye positions register nicely as a grid.Otherwise, redo the calibration. Accept the calibration and do the validation. Accept the validation if the maximal validation error is less than two degrees and the average validation error is less than one degree.Otherwise, redo the validation. Then, perform drift correction and proceed to the actual experiment. In this visual search task, use the stimuli from a previous study performed by this group and follow the task procedure as described before.Provide task instructions to the participants. Instruct the participants to find a 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. To illustrate the usage of this method, 228 single neurons were recorded from the human medial temporal lobe while patients were performing a visual search task as shown here. During this task, it was investigated whether the activity of neurons differentiated between fixations on targets and distractors.When responses were aligned at the button press, neurons were found that showed differential activity between target-present trials and target-absent trials. Black lines represent the onset and offset of the search queue. This neuron increased its firing rate for target-present trials but not for target-absent trials.Conversely, this neuron decreased its firing rate for target-present trials but not for target-absent trials. A subset of medial temporal lobe neurons showed significantly different activities between fixations on targets versus distractors. Furthermore, one type of such target neuron had a greater response to targets relative to distractors whereas the other had a greater response to distractors relative to targets.Together, this result demonstrates that a subset of medial temporal lobe neurons encode whether the present fixation landed on a target or not. It is critical to make sure that the eye tracker does not introduce noise into the recording. Any noise must be removed before the experiments are performed.If noise persists, ensure the screen used to display the stimulus to the subject has an external power supply. Exchange power supplies for more high quality power supplies typically used for high-end audio applications. Following this procedure, analysis of spikes and eye movements are performed to extract information from the data that will allow researcher to investigate the neuronal responses to eye movements.This method paves the way to explore many unanswered questions that require analysis of fixations, such as how neurons collectively drive fixations to interesting objects in a scene.

Research

JoVE Journal

Behavior

A Method to Quantify Visual Information Processing in Children Using Eye Tracking

Visual problems that occur early in life can have major impact on a child's development. Without verbal communication and only based on observational ...

Eye-Tracking Control to Assess Cognitive Functions in Patients with Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder with pathological involvement of upper and lower motoneurons, subsequently leading to ...

Eye Tracking During Visually Situated Language Comprehension: Flexibility and Limitations in Uncovering Visual Context Effects

The present work is a description and an assessment of a methodology designed to quantify different aspects of the interaction between language processing ...

Eye-tracking to Distinguish Comprehension-based and Oculomotor-based Regressive Eye Movements During Reading

Regressive eye movements are eye movements that move backwards through the text and comprise approximately 10-25% of eye movements during reading. As ...

Gaze in Action: Head-mounted Eye Tracking of Children's Dynamic Visual Attention During Naturalistic Behavior

Young children's visual environments are dynamic, changing moment-by-moment as children physically and visually explore spaces and objects and interact ...

Simultaneous Recordings of Cortical Local Field Potentials and Electrocorticograms in Response to Nociceptive Laser Stimuli from Freely Moving Rats

Electrocortical responses, elicited by laser heat pulses that selectively activate nociceptive free nerve endings, are widely used in many animal and ...

Investigating the Deployment of Visual Attention Before Accurate and Averaging Saccades via Eye Tracking and Assessment of Visual Sensitivity

This experimental protocol was designed to investigate whether visual attention is obligatorily deployed at the endpoint of saccades. To this end, we ...

Defining the Role Of Language in Infants' Object Categorization with Eye-tracking Paradigms

Assessing infant category learning is a challenging but vital aspect of studying infant cognition. By employing a familiarization-test paradigm, we ...

Exploring Infant Sensitivity to Visual Language using Eye Tracking and the Preferential Looking Paradigm

We discuss the use of the preferential looking paradigm in eye tracking studies in order to study how infants develop, understand, and attend to the world ...

Loneliness Assuaged: Eye-Tracking an Audience Watching Barrage Videos

Researchers usually theorize media exposure based on assumptions of legacy media. However, a new interactive video viewing format, in this case barrage ...