Event Related Potentials (ERP) Facility
When brain cells fire, they release tiny amounts of electricity that travel through the brain and scalp to escape into the outside world. This bid for freedom is very convenient because it allows us to measure a person's brain responses from outside their head. We do this using small sensors that are sewn into a soft cap.
At the ERP Facility, we measure two types of brain responses: event-related potentials (ERPs) and fixation-related potentials (FRPs). An ERP is the average brain response to a particular stimulus, such as a high-pitched musical note or the photo of a face. An FRP is the average brain response to one part of a whole visual stimulus, such as the eyes in a face, or a word written in a sentence.
Neuroscan ERP System
We use a 364-channel SYNAMPSII amplifier from Neuroscan, which measures ERPs at all frequencies, including brainstem responses. This system is suitable for high-density, low noise recordings. Our headbox supports 70 channels: 64 monopolar, 4 bipolar, and 2 high-level channels. Participants' electroencephalograms (EEGs) are detected at the scalp using Ag/AgCl sintered electrodes that are held in place by EasyCaps (small, medium, and large). The EEGs are recorded using Curry 7 software.
The stimuli are presented using Presentation®, MATLAB with Psychtoolbox, or PsychoPy software that drives a SoundBlaster Audigy2 ZS soundcard and NVIDIA GEForce FX 5200 videocard. Sounds are presented through speakers or headphones. Images are presented on an LCD Monitor. The resolution of the system (i.e. the time delay between when a stimulus code is recorded on the EEG and when the stimulus is actually presented) is less than 1 ms.
An FRP is the average brain response elicited when natural eye-fixations are made to visual stimuli, such as the mouth or eyes in a face, or a word written in a sentence. We measure FRPs using a method developed by our collaborators in Salzburg. In this system an eye-tracker monitors and records eye-movements, and sends codes to the ERP system at the beginning of a stimulus presentation, or when the eyes fixate on a pre-defined area of a visual scene for more than 35 ms. We use these codes and the record of eye-movements to calculate the average brain activity in response to the presentation of a visual stimulus, as well as subsequent fixations made to one or more areas of the stimulus. For example, we can calculate the average brain response to the eyes in a face and the average brain response to the mouth in the same face, by directing initial fixations to be over specific regions of a stimulus when presented. The full FRP paradigm then allows us to record the brain activity that occurs when participants' eyes then fixate on various parts of the face during natural scanning. This allows us to observe brain activity during dynamic visual processing, and is a very useful technique if you want to measure brain responses to stimuli presented in real-world situations such as words within sentences, eyes in faces, or (perhaps) musical notes in scores.
Emotiv Epoc + EEG/ERP System
We use an Emotiv Epoc + EEG system that was designed as a brain-computer-interface device. We have validated this system against Neuroscan for measuring event related potentials to auditory and visual stimuli and it produces equivalent results (see auditory in adults, auditory in children and visual in adults). The system is limited with respect to the electrode layout and sampling rate but has the benefit of being light-weight, easy to setup, and portable.
The EEG electrodes are housed within a one-sized fits all (we haven’t tested below age 6) headset. The EEG sensors are gold-plated and contact is made using saline soaked cotton rolls (as opposed to the gel we use for Neuroscan). The headset had 16 sites, aligned with the 10-20 system: AF3, F7, F3, FC5, T7, P7, O1, O2, P8, T8, FC6, F4, F8, FC4, M1/P3 and M2/P4. We use the mastoids (M1 and M2) as reference sites (thought P3 and P4 can also be used). M1 acts as a ground reference point for measuring the voltage of the other sensors, while M2 acts as a feed-forward reference point for reducing electrical interference from external sources. Only signals from the other 14 non-reference channels are saved at a sampling rate of 128 or 256 Hz.
Functional Transcranial Doppler Ultrasound System
The lab hosts a Delia EMS-9UA Transcranial Doppler system with bilateral monitoring 2MHz ultrasounds probes, which can be attached to Delica headframe. Delica's software is used for data recording and we use the ‘DOPOSCCI’ MATLAB toolbox for data processing (written by one of our lab managers).
Functional Transcranial Doppler Ultrasound (fTCD) is a non-invasive imaging technique that takes advantage of Doppler physics to determine the rate of blood flow to the brain, often within the middle cerebral arteries. By simultaneously examining the rate of blood flow in response to some cognitive task (e.g., language or spatial processing), fTCD can be used to determine whether the neural substrates for the cognitive process are predominantly located in the left or right hemispheres of the brain; i.e., the lateralization of cognitive processes.
Although the spatial resolution of fTCD is very limited compared with magnetic resonance imaging (MRI), simply reflecting the neuro-metabolic coupling to very general regions of the cortex (i.e., the middle cerebral arteries supply blood to approximately 50% of the brain), the temporal resolution is high, sampling the rate of blood for 125 times per second (125 Hz). Additional advantages of fTCD are that it is inexpensive to use, portable, and it is robust to movements which would spoil MRI recordings. Therefore, different experimental paradigms are feasible with fTCD and it is suitable for use with very young children.