ERP analysis

Measurement examples.

Opening data for analysis
The original record may be continuous or in blocks, may contain up to 32 channels, may be entirely EEG, or may include other peripheral and cardiovascular measures. If the record is continuous, PSYLAB analysis will section data using chosen criteria, creating block files of identical format to the type that are generated by the measuring system in direct block mode. Otherwise, block mode data may be directly opened from Toolbar menu, File Data, Open block data for analysis. Continuous data are shown below, which are then ‘blocked’ to create a ‘multi-block Review’.

A 500 second continuous recording, with events shown in the top two channels. These events are used to extract blocks from the whole record that are synchronous with stimuli.

Multi-block raw data Review showing blocks 53,55, etc. excluded

Screening data for ERP analysis
Before averaging, it is important to exclude blocks which are contaminated. Two major sources of contamination, movement artefact and eye blink, may be automatically detected. The include status for each block is shown by the color of  the block number, seen at the top of the Multi-block Review (see Review#2). Black means it is included, red means it is excluded. Status may be switched by clicking on the number. In the example below, block 53 is excluded, 54 included, etc.

Blocks containing eye blinks are excluded from the average using menu selection Include, Screen blocks.

Bad blocks may be automatically excluded by selecting Review menu, Include, Screen blocks. Channels used in the screening process are selected by clicking the indicator number seen to the left of the axis of each trace in the Review. Frontal channels or eye channels, for example, may be used if these were included in the electrode array. In the example shown, all channels except the top two event channels are set to 1 (on), but the event channels must be switched off before screening is performed, as they do not contain EEG data and would cause every block to be

excluded. When channels are correctly set, automatic screening is then performed. Channel settings and screening parameter settings are stored in a template file so that processing of subsequent records may automatically use the same settings.

Averaging
Data may now be averaged, by selecting Review menu, Process, Average. Only included blocks are used in the

Zoom in to raw data block 53,  shown as excluded because of a blink.

The entire process of loading, screening, manipulating and extracting topical information may be automated using the Macro record and play features, so that multiple records may be processed leaving the computer unattended.

Average of 28 included blocks. Micro Voltage levels for any chosen points in the wave may be seen in the Coordinates window.