Practicalities of EEG Measurement and Experiment Design YATING LIU

Transcription

1 Practicalities of EEG Measurement and Experiment Design YATING LIU

2 Content Designing Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

3 Designing Experiments: Discuss, Pilot, Discuss, Pilot Why we need good experiment design? Good experiment design is half done! What we should do before collecting data? Discuss experiment design with colleagues and get their feedback Pilot test experiment behaviorally before collecting brain data Collect EEG data from few subjects and analyze the dataset completely Why we have to do these? Optimize experiment design Save money! Save time!

4 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

5 Event Markers What are experiment event markers? Square-wave pulses that are sent form the stimulus-delivering computer to the EEG amplifier and are usually recorded as a separate channel in the raw data file (or some times, multiple channels)

6 Event Markers The importance of event markers Time-lock the EEG data offline Reconstruct different conditions and responses.

7 EEGLAB Demo

8 Event Markers Tips of using event markers: Multiple markers are not allowed to be sent simultaneously The temporal duration of each marker should be at least a few samples. (5 ms should be a sufficient duration generally) Too short? Cannot be recorded. Too long? Overlap with other markers Confirm accuracy of the timing and the values of the event markers

9 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

10 Intra- and Intertrial Timing Why should we separate experiment events within a trial? Temporal smoothing introduced by time-frequency decomposition Time-frequency response may linger for hundreds of milliseconds after an experiment event has ended. We want to avoid the overlapping of brain responses But sometimes it is difficult to achieve this goal

11 Intra- and Intertrial Timing How to estimate the approximate duration of the electrophysiological response to a stimulus? Pilot the task using long interevent durations The factors that affect the duration of brain response The kind of task The elicited cognitive process

12 Intra- and Intertrial Timing What is intertrial interval? The duration of time between the end of one trial and the start of the next trial Something you should consider before choosing appropriate intertrial interval for your experiment what period of time you will use for baseline normalization of task-related data What frequency you want to analyze (Generally, intertrial interval of at least 1000 ms should be sufficient)

13 Intra- and Intertrial Timing Constant or variable? Constant intertrial intervals: Subjects can predict the temporal onset of upcoming trial and the preparation can be reflected in brain data Variable intertrial intervals: Subjects may try to guess the next trial onset, the temporal expectation, surprise, and variable task preparation can also affect EEG and behavioral performance.

14 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

15 How Many Trials You Will Need Before choosing the number of trials The signal-to-noise characteristics of the data How big the effects is The type of analysis you will perform The more, the better? Large number of trials may lead to tiredness and anxiety of subjects The practical problem of clinical application (patients) or younger children.

16 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

17 How Many Electrodes You Will Need The number of EEG electrodes you need depends on: The analyses you plan to perform Preparation time Gel-filled EEG cap Gel-free EEG cap Data storage and processing time

18 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

19 Which Sampling Rate to Use When Recording Data What is sampling rate? The number of times per second that data are acquired from all electrodes. Defines the temporal resolution of the data Before choosing proper sampling rate Kinds of analyses you plan on performing The frequencies you plan on analyzing Available disk space and processor speed/type

20 Which Sampling Rate to Use When Recording Data Nyquist theorem Nyquist sampling theorem can be stated simply as follows: The sampling frequency should be at least twice the highest frequency contained in the signal. f s 2f c where f s is the sampling frequency and f c is the highest frequency contained in the signal. Aliasing arises when a signal is discretely sampled at a rate that is insufficient to capture the changes in signal

21 Which Sampling Rate to Use When Recording Data Actually, we should sample more than twice the highest frequency

22 Which Sampling Rate to Use When Recording Data Tips of choosing sampling rate: The higher, the better? Sampling rates between 500 Hz and 2000 Hz may be sufficient for all analyses. It is better to record the data using a high sampling rate and then downsample the data prior to analyses. Choosing sampling data from a convenience-of-analysis perspective (For example: 1000 Hz)

23 Content Design Experiments: Discuss & Pilot Event Markers Intra- and Intertrial Timing How Many Trials You Will Need How Many Electrodes You Will Need Choosing Proper Sampling Rate Other Optional Equipment to Consider

24 Other Optional Equipment to Consider Response EMG or force grips Examine cortical-muscular connectivity Eye tracker Facilitate preprocessing and cleaning the dataset (remove oculomotor artifacts) Provide dependent measures (saccades, looking time, changes in pupil dilation)

25 Other Optional Equipment to Consider Electrode localization equipment Increase the spatial precision of EEG Comfortable chair Avoid subjects become distracted and decrease the chances of introducing movement artifacts Good response device Good timing & intuitive layout of buttons

26 Other Optional Equipment to Consider Other environment factors Electromagnetic signals (shielded room) Noise (acoustically shielded room) Light Temperature Questions?

27 Thank You!