The time dimension accessible to MEG/EEG offers some considerable variety in the design of experimental paradigms for testing virtually any basic neuroscience hypothesis. Managing this new dimension is sometimes puzzling for investigators with an fMRI neuroimaging background as MEG/EEG allows to manipulate experimental parameters and presentations in the real time of the brain, not at the much slower pace of hemodynamic responses.
In a nutshell, MEG/EEG experimental design is conditioned on the type of brain responses of foremost interest to the investigator: evoked, induced or sustained. The most common experimental design by far is the interleaved presentation of transient stimuli representing multiple conditions to be tested. In this design, stimuli of various categories and valences (pictures, sounds, somatosensory electric pulses or air puffs, or their combination, etc.) are presented in sequence with various inter stimulus interval (ISI) durations. ISIs are typically much shorter than in fMRI paradigms and range from a few tens of milliseconds to a few seconds.
The benefit of the high temporal resolution of MEG/EEG is twofold in that respect:
- it allows to detect and categorize the chronometry of effects occurring after stimulus presentation (evoked or induced brain responses), and
- it provides leverage to the investigator to manipulate the timing of stimulus presentation to emphasize the very dynamics of brain processes.
The first category of experimental designs is the most typical and has a long history of scientific investigations in the characterization of the specificity of certain brain responses to certain stimulus categories (sounds, faces, words, novelty detection, etc.) as we shall discuss in greater details below. It consists in the serial presentation of stimuli and possibly, subject responses. These experimental events are well-separated in time and the brain activity of interest in related to the presentation of each individual event, hence an 'event-related' paradigm.
Experimental protocol and behavioral results recorded during an event-related session. T1 and T2 are two task-related stimulus objects. In this experiment, each trial consisted in a simple sequence containing five items: T1, followed by a mask (M), and T2 (which could be present or absent) followed by two successive masks. The stimulus onset asynchrony (a sub-type of ISI) between T1 and T2 could be either short (258 ms) or long (688 ms). Presentation of T2 was signaled by 4 surrounding squares. When T2 was absent, the four squares were presented on a blank screen. Each trial ended with a question on T2 (Q2: visibility scale) and, in the dual task condition, a question on T1 (Q1).
Adapted from (Sergent, Baillet & Dehaene, Timing of the brain events underlying access to consciousness during the attentional blink. Nature Neuroscience, 2005, 8, 1391-1400)
The second category of designs aims at pushing the limits of the dynamics of brain processes: a typical situation would consist in better understanding how brain processes unfold and may be conditional to a hierarchy of sequences in the treatment of stimulus information from e.g., primary sensory areas to its cognitive evaluation. This may be well exemplified by paradigms such as oddball rapid serial visual presentation (RSVP, (Kranczioch, Debener, Herrmann, & Engel, 2006)), or when investigating time-related effects such as the attentional blink (Sergent, Baillet, & Dehaene, 2005, Dux & Marois, 2009). Steady-state brain responses triggered by sustained stimulus presentations belong also to this category. Here, a stimulus with specific temporal encoding (e.g., visual pattern reversals or sound modulations at a well-defined frequency) is presented and may trigger brain responses locked to the stimulus presentation rate or some harmonics. This approach is sometimes called ‘frequency-tagging’ (of brain responses). This has lead to a rich literature of steady-state brain responses in the study of multiple brain systems (Ding, Sperling, & Srinivasan, 2006, Bohórquez & Ozdamar, 2008, Parkkonen, Andersson, Hämäläinen, & Hari, 2008, Vialatte, Maurice, Dauwels, & Cichocki, 2009) and new strategies for brain computer interfaces (see e.g., (Mukesh, Jaganathan, & Reddy, 2006)).
A typical event-related paradigm design for MEG/EEG. The experiment consists of the detection of a visual ‘oddball’. Pictures of faces are presented very rapidly to the participants every 100ms, for a duration of 50ms and an ISI of 50ms. In about 15% of the trials, a face known to the participant is presented. This is the target stimulus and the participant needs to count the number of times he/she has seen the target individual among the unknown, distracting faces. Here, the experiment consisted of 4 runs of about 200 trials, hence resulting in a total of 120 target presentations.
As a beneficial rule of thumb for stimulus presentation in MEG/EEG paradigms, it is important to randomize the ISI durations as much as possible for most paradigms, to minimize the effect of stimulus occurrence expectancy from the subjects. Indeed, this latter triggers brain activity patterns that have been well characterized in multiple EEG studies (Clementz, Barber, & Dzau, 2002, Mnatsakanian & Tarkka, 2002) and which may bias both the subsequent MEG/EEG and behavioral responses (e.g., reaction times) to stimulation.