ERPLAB INTRODUTION ERPLAB introduction n n ERPLAB contains

ERPLAB INTRODUTION

ERPLAB introduction n n ERPLAB contains a set of new functions, which are added as plug-ins into EEGLAB, extending the set of operations that a user can perform within EEGLAB additional EEG processing manipulations (e. g. , new functions for marking trials with artifacts) along with functions that provide powerful methods for sorting EEG epochs and averaging them together. a set of routines that operate on the averaged ERP waveforms routines were inspired by ERPSS (the Event-Related Potential Software System)

Basic ERPLAB Processing Steps(1) n n n 1. Import an EEG file into EEGLAB, where it becomes a dataset. Once you have imported the raw EEG, you should view it with Plot > Channel data (scroll) to make sure everything looks OK. The original EEG file can come from one of the many different commercial EEG recording systems supported by EEGLAB (including Neuroscan, EGI, and Biosemi; see the Neuroscan, EGI, and Biosemi EEGLAB documentation for the supported file types. ) The ERPSS RAW format is supported by EEGLAB but apparently does not work well at this time. Any of the EEGLAB functions that operate on continuous data can be applied at this point. 2. There are several EEGLAB routines that you may wish to run at this point (or after epoching). For example, if you wish to filter the EEG, you can select Tools > Filter the data. Alternatively you may wish to use ERPLAB's filter routines (see Tools > Filter the data. Alternatively you may wish to use ERPLAB's ERPLAB > Filter & Frequency Tools), which we think are better for most conventional ERP applications. You may also wish to enter electrode location coordinates at this point This can be accomplished with Edit > Channel locations, which allows you to automatically choose standard locations on the basis of the electrode names (or enter custom coordinates). 3. Create the EVENTLIST structure from the current dataset using ERPLAB > Event. List > Create EEG Event. List 3. Create the EVENTLIST structure from the current dataset using ERPLAB > Event. List > Create EEG (basic or advanced). The EVENTLIST structure will ordinarily be attached to the EEG structure. It stores information about ERPLAB's event processing and bin assignments. It can also be saved as a text file, allowing you to insert, modify, or delete events and then import the new events back into the dataset. At this point, you can assign text labels for each numeric event code (or assign numeric event codes if your datafile contains text labels instead of numeric codes). You can even assign bins for the events if there is a simple 1: 1 mapping between event codes and bins. 4. If you need to specify a more complex mapping between events and bins, you will do this using ERPLAB > Assign Bins (BINLISTER). This involves creating a Bin Descriptor File (BDF), a text file that provides an abstract description of the events that will be averaged together (just as in ERPSS). For example, you can specify that Bin 3 will consist of targets that were preceded by a non-target and followed 200 -1500 ms later by a left-hand button-press response. This step can also be configured to extract reaction times, which are saved in the bdf field of the EVENTLIST structure and can be exported to bdf field of the EVENTLIST a text file with ERPLAB > Event-List > Export Reaction Times to Text from EEG. . 5. Convert the continuous EEG data in the dataset into a set of fixed-length epochs around each event using ERPLAB > Extract Bin-Based Epochs. (Note that this differs from EEGLAB's Tools > Extract epochs function in several ways, so you should not use the EEGLAB epoching function in conjunction with ERPLAB. ) Any of the EEGLAB functions that operate on epoched data can be applied at this point.

Basic ERPLAB Processing Steps(2) n n n n 6. Perform artifact detection on the epoched dataset, marking problematic EEG segments so they can be excluded during the averaging process. Prior to artifact detection, you may wish to create new channels in the dataset (e. g. , a bipolar HEOG channel from separate HEOG-left and HEOG-right channels). This is accomplished with ERPLAB > EEG Channel Operations. After you have performed artifact detection, you can use Plot > Channel data (scroll) to see which epochs have been marked for rejection and to manually mark and unmark specific epochs. You can, of course, use EEGLAB's ICAbased artifact correction instead of, or in addition to, artifact rejection. EEGLAB also contains artifact detection routines, which you can use in conjunction with ERPLAB's routines. 7. Average together the epochs that have been assigned to each bin using ERPLAB > Compute Averaged ERPs. . The averaged data are stored in ERP. bindata structure, which is basically an Electrodes x Time x Bins matrix. averaged data are stored in ERP. bindata field of the ERP structure, which is basically an Electrodes x Time x Bins matrix. Other fields within the ERP structure describe the subject, sampling rate, electrode names, etc. The new ERP structure becomes the current ERPset , and you can save it to disk as a binary file (a ". erp" extension is recommended, but it is really a becomes the current ERPset, and you can save it to disk as a binary file (a ". erp" extension is recommended, but it is really a Matlab ". mat" file). You can also export this to a text file by using ERPLAB > Export ERP to Text (readable by ERPSS) or ERPLAB > Export ERP to Text (universal). Note that the ERP structure also contains a copy of the EVENTLIST or ERPLAB > Export ERP to Text (universal). Note that the structure from the EEG structure that was used to provide the data that were averaged together to create the ERP structure. This can be used to perform behavioral analyses that include only the trials included in the averaged ERPs (e. g. , the trials that did not contain artifacts, etc. ). 8. You can filter your ERP data with ERPLAB > Filter & Frequency Tools. . 9. You can also create new/modified bins (e. g. , difference waves) using ERPLAB > ERP Bin Operations or new/modified channels (e. g. , re-referenced channels) using ERPLAB > ERP Channel Operations. 10. Plot the averaged ERP waveforms using ERPLAB > Plot ERP Waveforms or plot topographic maps of voltage using ERPLAB > Plot 2 D ERP Map. These plots can be exported (in vector format) as PDF files, which can then be imported into and edited by a variety of illustration programs. 11. Once you have done the above steps for each subject, creating a separate ERPset for each subject, you can create a grand average across subjects using ERPLAB > Average Across ERPsets. Note that you may want to process the first 1 -3 average across subjects using ERPLAB > Average Across ERPsets subjects using the GUI and then write a script for processing subsequent subjects. A history of the functions called by your GUI actions can be obtained by typing eegh and erph or ERP. history in the command window, and this can GUI actions can be obtained by typing eegh or EEG. history and provide an excellent starting point for writing a script. 12. Perform ERP measurements, such as mean amplitude and peak latency using ERPLAB > ERP Measurement Tool. The measurements are saved in a text file in a manner that is easily imported by Excel or SPSS.

Important Background Concepts and Data Structures n n n n EEGLAB uses a data structure called EEG to store the EEG data and associated information from a single recording session Likes EEGLAB, ERPLAB to store averaged ERP waveforms an EVENTLIST structure that provides a link between the EEG and ERP structures Datasets and the EEG Structure Bins, ERPsets, the ERP Structure (Bins a set of averaged ERP waveforms, one for each electrode site, which were created by averaging together a specific set of trials, A single bin within an ERP structure is an Electrodes x Time Points matrix, The set of bins within an ERP structure adds a third dimension to this matrix (Electrodes x Time Points x Bins) Most EEGLAB signal processing functions operate on the EEG structure, and the output of these processes is typically a new EEG structure Each processing step typically operates on the current dataset, creates a new dataset based on some set of operations, and then makes the new dataset the current dataset

The EVENTLIST Structure and import data n n An EVENTLIST is a simple and compact structure created by ERPLAB that contains information about all of the events in an EEG structure (typically stimuli and responses, but it can also include eye movements, EMG bursts, etc. ) ERPLAB relies on EEGLAB for loading and displaying the EEG, and this section provides some useful information about using EEGLAB A window will probably appear asking you fill in certain criteria depending on how much of your data you would like to import. These options may vary by data file type. the default settings are appropriate Please refer to EEGLAB documentation for further instructions on importing data, setting import criteria, and accessing old datasets. Eventually, a window will appear that will ask you to name the dataset that you are importing and ask you if you would like to save this dataset as a file. You may choose to save this dataset if you would like to stop and continue working on this dataset at a later point. Not saving a dataset does not mean you will lose your original EEG data, because you will still have the original EEG data file. However, to avoid going through the importing process again, you will usually want to save the dataset you have created.

Hints About File Names, Datasets, Memory, and the Command Window n n Hints about Filenames Hint about datasets Hint about the command window Hints about memory

the EVENTLIST n n the EVENTLIST has a record indicating the event code, its time of occurrence, its duration, a set of 8 binary artifact flags that indicate whether an artifact was present for that event and can be used to code anything that the user would like to represent about an event. a copy of all the event information stored in the type field of the EEG structure, information about events that have been marked for artifact rejection in the reject field of the EEG structure, plus several additional pieces of information that will be described later

The EVENTLIST structure steps n n n Extract a list of the event codes from the EEG data in a dataset and store it in an EVENTLIST structure, this list may then be edited to add, delete, and/or modify the events Using the events stored in the EVENTLIST structure, determine which events will be assigned to each bin. Convert the continuous EEG in the dataset into a set of fixed-duration epochs, time-locked to the relevant events in the EVENTLIST structure Apply artifact detection routines to mark epochs that should be excluded from averaging. Average together the epochs for each bin, creating an ERP structure Perform behavioral analyses on the data stored in the EVENTLIST, possibly excluding events with artifacts so that the behavioral analyses reflect the same trials as the ERP waveforms.