Electrodes are the sensors which form the “point of contact” between the acquisition system and the brain (Holdgraf et al, 2019). See the section on electrodes for more information on the specific types of electrodes used in iEEG.
Channels are the digital signals recorded by the amplifiers. It is important to distinguish them from the sensors. Channels consist of two electrodes whose activity is referenced to another more distant electrode to form the signal (referential montages).
Signals are voltages at a given site at a given timepoint. The signal consists of different frequencies that can be derived/calculated via a timefrequency analysis. Delta waves range from 1-3 cycles per second (Hz), theta are 4-7 Hz, alpha are 8-12 Hz, and beta waves include those 13 Hz and higher. These can reveal the state that the participant is in, for example, wakefulness or sleep (St. Louis & Frey, 2016).
A session is a grouping of neuroimaging and behavioral data consistent across participants. A session includes the time involved in completing all experimental tasks. This begins when a participant enters the research environment and continues until they leave it. One session would typically start with informed consent procedures followed by participant preparation (i.e., electrodes placement procedure) and ends when the electrodes are removed but can also include a number of pre- or post-EGG observations and measurements (e.g., additional behavioral or clinical testing, questionnaires, structural MRI). Defining multiple sessions is appropriate when several identical or similar data acquisitions are planned and performed on all (or most) participants, often in the case of some intervention between sessions or for longitudinal studies.
A run is an uninterrupted period of continuous data acquisition without operator involvement.
An event is an isolated occurrence of a stimulus being presented, or a response being made. It is essential to have exact timing information in addition to the identity of the events, synchronized to the iEEG signals. For this, a digital trigger channel with specific marker values and timing information is used.
The term epoch designates the outcome of a data segmentation process. Typically, epochs in event-related designs (for analysis of event related potentials - ERPs) are time locked to a particular event (such as a stimulus or a response). Epochs can also include an entire trial, made up of multiple events, if the data analysis plan calls for it.
A trial is a period of time that includes a sequence of one or more events with a prescribed order and timing, which is the basic, repeating element of an experiment. For example, a trial may consist of a cue followed after some time by a stimulus, followed by a response, followed by feedback. Trials of the same type belong to the same condition. Critical events within trials are usually represented as “triggers” stored in the iEEG data file, or documented in a marker file.
Source space refers to iEEG expressed at the level of estimated neural sources that gave rise to the measured signals. Each signal maps onto a spatial location that is readily interpretable in relation to individual or template-based brain anatomy.
Anatomical landmarks are well-known, easily identifiable physical locations on the head (e.g., nasion at the bridge of the nose; inion at the bony protrusion on the midline occipital scalp) that have been acknowledged to be of practical use in the field.
Acquisition parameters are the reported reference and ground electrodes used in data acquisition. Similarly, reference electrode(s) used in data analysis are also reported. Additional electrodes are sometimes applied to the face to measure eye movement, and their exact spatial positions are specified, preferably with reference to well-known anatomical landmarks (e.g., outer canthus of the eye).