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The CCAL cognitive electrophysiology lab, with Brainproducts equipment and software, is equipped to record EEGs and ERPs from a dense array of up to 128 electrodes; there is an ultrasound digitiser for accurate localisation of electrodes and co-registration with MRI images.

Electroencephalography (EEG) and event-related potentials (ERP)

What is EEG?

EEG (Electroencephalography) is the measurement on the scalp of electricity originating from the brain. Because this electricity fluctuates over time, the EEG recording is informally referred to as ‘brain waves’. Embedded in these waves are electrical potentials which brain cells generate when they receive and transmit information in large assemblies (networks). It is therefore unsurprising that one can extract from the EEG signals that can give us clues about the current state of the brain (e.g. vigilance, sleep, arousal) and even about the way mental processes are ‘wired’ in the brain.

For several decades now, researchers have employed two complementary approaches to EEG. One is to separate oscillations (waves) of different frequencies. These frequencies, which are commonly labelled using Greek letters (alpha, beta, gamma, delta, etc.) have been associated with different states of the brain or ‘modes’ of information processing.

The second approach is to extract from the EEG stretches time-locked to specific stimuli (or responses) and look for characteristic activity (whatever its frequency) associated with types of stimuli and/or experimental tasks in whose context these stimuli are presented. The latter approach is referred to as Event-Related Potentials (ERPs). Whichever approach one adopts, EEG has very high temporal precision (temporal resolution), because it measures brain cell activity (rather than a metabolic consequence of this activity). The drawback of EEG is that, because the brain conducts electricity in all directions and the measurement is from the scalp (rather than from inside the head), it is usually very difficult to determine where in the brain the signal originates.

Is EEG safe?

Completely. In fact, it is the safest of all brain measurement techniques, because it does not subject the person to radiation, or any kind of stimulation- it simply measures the body’s (brain’s) own electricity. The EEG application has its inconveniences- for instance, one needs to insert conductive gel underneath each EEG sensor to ensure good contact between the sensor and the surface of the head. This gel is completely harmless and easily washed- but it does mean that one needs to wash his/her hair after an EEG session. This is about as ‘invasive’ as EEG gets!

What do we do with EEG?

Our primary tool is the ERP method (see 'What is EEG' box). We try to use the exquisite temporal resolution of EEG to determine the precise sequence of cognitive processes. The temporal precision of the EEG also allows one to look for characteristic time-courses of brain activity (referred to as ‘components’), which are associated with specific experimental manipulations. We have been keen to combine EEG with other techniques. To improve the quality of its spatial inference, we have combined it with fMRI. To relate ERP components to the dynamics of visuo-spatial attention, we have combined EEG with eye-tracking. Finally, we are also combining EEG with TMS to identify the electrophysiological correlates of some of the effects of TMS on behaviour we have been documenting.

The eye-tracking lab has two Eyelink II eye-tracking systems (SR-Research, Toronto) which allows monitoring of eye position at sample rates of up to 500 Hz. We also share an Eyelink 2000 with the clinical group.

What is eye-tracking?

Eye-tracking aims to determine where one directs one’s gaze, or, put simply, it where one looks (for instance, on a computer monitor). Because of the richness and importance of visual information in the environment and because the acuity (sharpness) of human vision rapidly falls off as one departs from the centre of the gaze’s focus, humans have evolved to possess a powerful (fast and effective) oculomotor system: six muscles that move the eye-ball in the eye-socket guided by groups of cells in several regions of the brain, most notably in the superior colliculus and frontal eye fields. The behaviour of the oculomotor system is characterised by two states: the gaze is either (virtually) stationary- referred to as ‘fixation’- or it is on a fast move- referred to as ‘saccade’. Since research to date has convincingly shown that no information is acquired during the eye-movement (saccade), which means we only see during the fixation, psychologists tend to be interested in the latter: the location and duration of fixations.

Researchers interested in perception, attention, reading, face processing, scene perception and other areas of cognitive psychology, use the temporal and spatial distribution of fixations to make inferences about ways in which visual information is selected and processed. Neuroscientists interested in the control of eye-movements are often more interested in saccades than in fixations- their timing, velocity, trajectory and targeting. Technologically, eye-tracking relies on ‘beaming’ infra-red light towards the eye and measuring the reflected light from the retina (the back of the eye). Because this reflection depends on the angle between the eye and the measuring camera, one can track the position of the eye (and therefore the gaze) very precisely.

How do we use eye-tracking?

Because eye-movements are fairly tightly coupled to selective visuo-spatial attention, our primary use of eye-tracking is as a measure of spatial attention. We are also starting to use eye-movements to examine language processing and preferences and sampling in decision making tasks. Recently, we have fruitfully combined eye-tracking with concurrent recordings of event-related potentials.

Is eye-tracking safe?

Eye-tracking is perfectly safe. The levels of infrared radiation directed at the eye are very small indeed, smaller than the ones that would reach the eye when one looks at, say, a hot oven from some distance. There is also no associated sensation because infrared light is invisible to the human eye.

More information on eye-tracking research 

The transcranial magnetic stimulation (TMS) lab has a Magstim Rapid 2 stimulator, a BiStim2, and Brainsight Frameless systems for positioning the coil relative to an MRI image. We also have a TMS-compatible EEG system and a tDCS system.

We have recently added Powerlab and Biopac GSR and Electrodermal stimulation equipment to our technical armoury.

There are a number of test rooms equipped with computers for behavioural research; the School has an E-prime license. There are high performance workstations for computational modelling and analysis of neuroscience data. A super-computer for simulation work has been installed under the direction of Andy Wills.

Transranial Magnetic Stimulation

What is TMS?

Transcranial Magnetic Stimulation (TMS) is a neurophysiological technique that induces a current in a small area of the brain, using a magnetic field to pass the scalp safely and painlessly. In TMS, a current passes through a coil of copper wire that is wound inside a plastic-insulated casing and held over the participant’s head. This coil resembles a paddle or large spoon, and is held in place either by the investigator, or by a mechanical stabilisation device similar to a microphone stand or metal frame. As the current passes through the coil it generates a magnetic field that can pass through the scalp and skull, and in turn induces a current in a small area of the participant’s brain.

Why do we use TMS?

TMS is used in many laboratories to study the effects of localised brain stimulation on perception, attention, movement control and higher thought processes (such as executive control and memory): The magnetic field induces currents stimulating the neurones in a small area beneath the stimulation coil, and temporarily altering their normal function. By measuring the behavioural consequences of TMS, we can infer which brain regions are required for specific behaviours.

Is TMS safe?

The technique is considered to be generally safe for use in neurologically healthy individuals.

Which TMS protocols do we use?

In our lab, we primarily use 'online' single-pulse TMS and 'offline' continous theta burst stimulation (cTBS).

Single-pulse TMS. As the name suggests, one TMS pulse is delivered per trial, at one of several possible times after the onset of task-relevant stimulus. By comparing the behavioural effects of TMS at different times, experimenters can thus infer the time-course of neural processing in the stimulated region.

cTBS: In this protocol, behavioural performance is compared before and after a sustained period of repetitive TMS. A series of TMS pulses is delivered for <1 min before we test performance. Rather than interrupting time-critical processing in the stimulated cortex, the logic of this approach is to modulate cortical excitability for a period of 15-60 minutes following the offset of the TMS. Behaviour is measured during the period of the TMS-induced aftereffect.


TMS-EEG is now possible due to recent technical developments in EEG amplifiers, and opens new avenues in cognitive neuroscience. One of the main limitations of TMS is that it cannot distinguish between effects due to interfering directly with the function of the stimulated brain area (e.g. a frontal area), and indirect effects on functionally connected brain areas (e.g. an occipital area). By combining TMS with EEG, we can examine such indirect effects. Furthermore, with ‘offline’ TMS protocols (i.e. a series of pulses delivered before a task), it is difficult to determine which processing stages are influenced; EEG can clarify this.

Would you like to know more?

Hallett, M. (2007). Transcranial magnetic stimulation: A primer. Neuron, 55(2), 187-99. doi:10.1016/j.neuron.2007.06.026

O'Shea, J., & Walsh, V. (2007). Transcranial magnetic stimulation. Current Biology, 17(6), R196-R199. doi:10.1016/j.cub.2007.01.030

Exeter's MR scanner is a 1.5T Phillips Gyroscan Magnetic with SENSE technology capable of high speed whole-volume acquisitions for event and epoch fMRI designs. Button-box, joystick and trackball response manipulanda with optic-fibre connections are available. The scanner is equipped with an Applied Science Labs MRI-compatible eye tracking system. It is a research-dedicated machine, housed in the Peninsula Medical School's research building, and operated by the medical physicists in the Department of Physics; the School of Psychology is part of a consortium of users in several departments of the university and beyond. Concurrent ERP and fMRI recording is available.

Functional magnetic resonance imaging (fMRI)

What is fMRI?

Functional Magnetic Resonance Imaging (fMRI) is a technique that measures brain activity when people are performing some kind of task. The scanner is a large, open ended, cylindrical tube that contains a very powerful magnet. When a part of the brain is being used it consumes more oxygen which results in extra oxygen being supplied to that part of the brain. It is these changes in blood oxygenation level that fMRI measures. This approach allows us to see very precisely (within about 3-5mm) what parts of the brain are being used when performing a particular task which provides great insight into mental processes.

What is MRI?

MRI works in a very similar way to fMRI and provides extremely detailed pictures of internal regions of the body (such as the brain). During MRI you typically do not have to perform any task, but can just relax while the scan (which usually takes between 5-10 minutes) is taking place.

(© Peninsula MR Research Centre, Exeter)

Is it safe?

fMRI is a completely safe technique, and there are no side-effects. You are not injected with any nasty substances and all you have to do is lie inside a scanner and perform a simple task. The machine may appear imposing to some, as does the idea of having your brain scanned, but the environment is perfectly safe and for most people it is surprisingly relaxing.

The only restriction on volunteers is that, because of the powerful magnet, it is important that you do not have any metal implants or have not undergone surgery in the last 6 months. Normal or gold fillings in your teeth are fine. If you have any body piercings, they will have to be removed on the day. If you wear glasses then you will have to wear contact lenses while you are in the scanner if you have them.

Where is the scanner?

Exeter's scanner is on St. Luke’s campus.

Would you like to know more?