Event Related Potentials

Division Chief: Colin Sauder Ph.D.

 

Event Related Potentials Laboratory

The focus of the Electrophysiological Imaging Division is spatio-temporal mapping of information processing in the human brain in health and disease. Event-related potentials (ERPs) are the most frequently performed type of electrophysiological study. ERPs are scalp-recorded voltage fluctuations resulting from evoked neural activity. These are extracted from the background EEG by time-locked selective averaging and provide very high temporal resolution reflection of the patterns of neuronal activity evoked by sensory, cognitive or motor events. Roughly 3/4 of the studies are in normal subjects and 1/4 in patients.

Facilities: The ERP lab consists of two audiometric examinations rooms, a preparation room and a workstation room. The examination rooms are custom installed double-wall insulated, sound-attenuating, RF-shielded testing chambers (Industrial Acoustics) measuring 8’8”w x 8’6”l x 6’6”h and 9’0”w x 7’0”l x 6’6”h. The larger testing room can comfortably seat two participants for any studies involving real-time human feedback or interaction. Each testing room is equipped with adjustable interior lighting and a monitoring window (the integrity of the RF shielding is protected in all cases), as well as a participant chair, adjustable table and an intercom system communicating to outside the chamber. Both chambers can be used as auditory recording booths, or for behavioral testing when a sound-attenuated environment is needed. The prep room includes locking storage cabinets for supplies and sensitive materials (e.g., completed consent forms) and a large sink for cleaning and disinfecting recording equipment. The workstation room houses computers for experimental design and testing, and data processing and analysis. In addition, engineers with access to mechanical and electronic workshop facilities for repair of ERP equipment or construction of custom-built equipment are available at the Research Imaging Institute.

 

Data Acquisition: The ERP chamber is equipped with a BioSemi ActiveTwo EEG/ERP system, which uses “active” electrode technology, with a preamplifier located at each recording electrode to increase the signal-to-noise ratio. The electrodes snap into an elastic cap (Electrocap) with preset labeled holes, and are connected directly to a battery-powered analog-to-digital conversion box (AD-box). Additional external electrodes for monitoring eye movement or physiological activity (e.g., GSR) can also be used. Each AD-box channel consists of a low-noise DC-coupled post-amplifier, with a first order anti-aliasing filter, followed by a Delta-Sigma modulator with an over-sampling rate of 64, and decimation filter with a steep fifth order sinc response and high resolution 24-bit output. The digital outputs of all the AD converters (capacity for up to 256 channels of data) are digitally multiplexed and sent to a PC via a single optical fiber without any data compression or reduction. The amplifiers have 4 user-adjustable sampling rates (2, 4, 8 or 16 kHz per channel). A receiver unit converts the optical data coming from the AD-box to an USB2 output. In addition, the USB2 receiver has a trigger port with 16 independent trigger inputs and 16 independent trigger outputs. This setup keeps the complete stimulation setup galvanically isolated from the subject. The trigger output signals can be controlled with an independent LabVIEW thread integrated in the BioSemi acquisition software. The trigger inputs allow easy setup of EP/ERP measurements, and event logging. Digital EEG/ERP data is stored on a PC running Windows XP and ActiView acquisition software (based on Labview). ActiView is a free open-source complete acquisition program designed to display all ActiveTwo channels on screen during recording and save all the data to (network) disk in BioSemi Data Format (BDF).

 

Stimulus Generation and Behavioral Responses: Stimulus generation will be accomplished using Presentation software running on a Windows XP platform PC. Presentation has the capacity to display the stimulus, both to the participant in the chamber and the experimenter outside the chamber using a video splitter, as well as continuous real-time monitoring of performance (e.g., behavioral responses such as button press) and upcoming stimuli (using a script dialog). Presentation reports precise timing for each stimulus and response event (critical for ERP research) to the digitization computer and interfaces with standard low cost input devices (e.g., mouse, joy-stick or serial response box). Presentation has a powerful scripting language that allows ample flexibility for experiment design and stimulus generation. This chamber will be equipped with a 21-inch CRT color monitor with low distortion and high refresh rate for visual-stimulus presentation, headphones for auditory-stimulus presentation, an ergonomic finger-response pad and a table-mounted video camera connected to a monitor outside the chamber. A monitor switch is used to switch between stimulus presentation and real time data acquisition on the participant monitor when necessary. For psychophysical-level control of the stimuli in auditory experiments, the computer-generated acoustic stimuli will be funneled through additional equipment in an auditory rack containing special filters, a mixer and decibel-precision attenuators.

 

Data Processing and Analysis: A Linux platform (Red Hat Enterprise) PC runs EEG/ERP analysis software, including ERPSS (free open source software from the Hillyard lab at UCSD), which has sophisticated programs for artifact rejection, selective averaging to extract the ERPs, data manipulation, calculating topographic contour plots of both the voltage potentials and reference-free current-source densities (CSDs), and plotting of both the ERP waveforms and the various types of contour plots. We are acquiring EEGLab (free open source software from the Makeig Lab at UCSD; Delorme and Makeig, 2004), another powerful software package that provides complimentary analysis tools to ERPSS using Independent Components for source localization, time/frequency for dynamic brain activity analysis and other methods. The lab also owns the Brain Electrical Source Analysis (BESA) software (M. Scherg) for dipole source localization. Additional software and strategies have been developed in-house to integrate the ERP data with data sets from the other brain imaging technologies at the center (i.e., structural MRI, functional MRI, and PET).

 

Scalp 3-D Spatial Digitization: The ERP lab is acquiring a handheld Polhemus Patriot 3D Digitizer system with stylus, for spatial mapping of the scalp electrodes. These spatial coordinates will be used when overlapping data collected from other imaging techniques, such as fMRI, for the same individual. Scalp mapping is currently done using equipment in the TMS laboratory at the Research Imaging Institute.