مغزنگاری الکتریکی (EEG) در پژوهش های زبان شناختی: طراحی آزمون، اجرا و گزارش نتایج

1- Introduction

Electroencephalography (EEG) is a popular technique that enables to record neuronal activation in real time, with millisecond precision. It has been used effectively to study various aspects of cognition, including language.

Electroencephalography (EEG) reflects the electrical activity (in voltage) of the human brain over time. In the EEG Lab, researchers study how the brain responds to spoken, signed, or written language. EEG can tell us a great deal about the time-course of language processing. While the brain is constantly producing low-frequency electrical activity, researchers are interested in the electrical activity produced as a response to some specific stimuli. This phenomenon can be used to study many types of linguistic research questions.

2- Design

2-1- Experiment Design

Experiment design is the process where the stimuli are defined and it is decided how to present the stimuli to the participants. This can be done using a variety of presentation software.

2-1-1- Objective

Before starting any experiment design in any presentation software, it is necessary that the objective of the experiment is clearly defined. The objective of the experiment in fact fulfills the objective of the study.

2-1-2- Stimulus and Time

The type of stimulus should be defined (e.g., visual, or auditory) and the researcher should design the stimulus to induce the expected changes in the neuronal system of brain that is hypothesized in the study.

2-1-3- Trials, Blocks, and Conditions

The number of trials of each stimulus and total number of trials should be specifically defined in the design. If there are different conditions involved, then the number of blocks can be more than one.

2-1-4- Participants’ Response and Feedback

In general, EEG experiments consist of participant responses against the stimuli where they mentally (e.g., counting the occurrence of certain stimuli) or physically do any action like pressing a button.

2-1-5- Events Synchronization with EEG (Trigger)

2-2- EEG Equipment and Software

2-2-1- EEG Equipment and Data Acquisition Software

EEG data acquisition is a time-consuming activity that needs clear objectives and proper planning to acquire the desired EEG signals.

2-2-2- Presentation Software

2-3- EEG Experimentation: Ethical Issues and Guidelines

2-3-1- Ethical issues: Gaining Consent/ ethics approval

2-3-2- Sample Size Computation

Sample size computation is an important aspect of neuroscience, behavioral, and clinical investigations. Practically, it is not feasible to include the whole population in any research study. There is always a common question, “How many individuals should be included in the research study?” Thus a small set of individuals is selected from the population that is small in size but statistically sufficient to represent the target population.

3- Experiment call

3-1- Subject Qualifications

Age, handedness, mood, education, culture, gender, language, health, …

3-2- Questionnaires

Handedness, GHQ, PANAS, Language History, …

4- Perform

4-1- Gaining Consent and Subject Information

4-2- Facilities and Equipment

4-3- EEG Data Acquisition

4-3-1- General Data Acquisition Setup:

The general acquisition setup should be planned and previous literature should be consulted. The important factors are the number of EEG channels and their configuration (montage, bipolar, or referential), EEG reference selection, sampling rate, filtering, and recording of supplementary data.

4-3-2- Montage (Bipolar, Referential):

Montage is a combination of EEG electrodes during recordings. There are two main types of montage, i.e., bipolar and referential montages.

4-3-3- Sampling Rate:

The sampling rate is the number of quantitative values to be recorded in each channel per second such as 128 samples per second. The higher the sampling rate, the higher the temporal resolution of EEG recordings.

4-4- Perform the routine EEG within a quiet, temperate room with controllable light levels.

Sit or recline the subjects comfortably for the duration of the set up and recording.

4-5- Training requirements/ Instructions to Participant:

In each and every experiment, it is important to define the instructions to the participant in an understandable manner. The instructions may vary from task to task and depend on the target participants. When the researcher finalizes the target participants, the instructions will be prepared accordingly.

4-6- Test Procedure:

First of all, electrodes need to be attached to a subject’s scalp to pick up the EEG.

4-6-1- Electrodes

4-6-1-1- Number and Distribution of Electrodes

4-6-1-2- Dry or Gel-Based Electrodes

4-6-1-3- EEG References

4-6-1-4- Electrode Placement

The 10-20 system

4-6-1-5- Electrode Impedance

4-6-2- Pre-test Checks

4-6-3- Recording

Annotations

Make regular annotations to mark any changes in the EEG or the subject’s state and behavior throughout the recording. This includes any movement of the subject, any external stimuli, artefacts, instructions given etc.

Length of Recording

Machine Settings – Display

Polygraphic recording

During the routine EEG it may be necessary to record from additional polygraphic channels generally with the purpose of distinguishing artefact from cerebral potentials.

4-6-4- End of Task

5- Data Analysis

5-1- Clean EEG data and artifacts

Before you jump into data collection and analysis, there’s one thing you should make your mantra: There is no substitute for clean data. Always make sure your data is as clean as possible, meaning the collected data reflects brain activity only.

5-1-1- Physiological artifacts

5-1-1-1- Muscle activity (EMG, ECG)

5-1-1-2- Eye movements

5-1-1-3- Blinks

5-1-2- External sources of artifacts

5-1-2-1- Movement

5-1-2-2- Line Noise

5-1-2-3- Swaying and Swinging

5-2- EEG analysis (ERP, Frequency Analysis)

Despite its widespread use, there exists a certain degree of uncertainty as to how the EEG signal should be analysed. Although there is a number of EEG signal preprocessing steps that are common to different analysis methods, their parameters need to be properly adjusted so as not to affect the signal in an adverse way, resulting in false representation of the brain signal.

6- How to write an EEG Report?

The EEG report is structured to include demographics of the subject studied and reason for the EEG; specifics of the EEG techniques used; a description of the patterns, frequencies, voltages, and progression of the EEG pattern that were recorded. The interpretation should be concise, clear and to the point.

6-1- An initial outline

6-2- The description

6-3- The summary, interpretation, or classification