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Equipment

MEG machine

The MEG at Neurospin is an Elekta Neuromag 306 sensors whole-head device. The MEG helmet consists of 102 sensors-triplets (1 triplet = 2 orthogonal planar gradiometers and 1 magnetometer). Two MEG recording positions are possible:

supine position in which the subject is laid down on a bed

upright position in which the subject is seated in a chair

Head Position Indicator (HPI) system

This system is based on four coils positioned on known locations on the scalp. Typically, two are placed as high as possible on the forefront and the other two on the right and left mastoïds. The coils should be placed such that they are as far as each other as possible on the head. The coils are stimulated with currents of different frequencies and above the usual physiological range (i.e. above 150Hz).

3-dimensional fastrak digitizer (Polhemus system)

This device includes a 3-D digitizer unit, a transmitter coil fixed behind the wooden chair on which the subject is seated, a digitizing pen and non magnetic googgles with a separate receiver in order to allow slight movements of the subject/patient during the measurement. This device is used during the preparation phase before MEG measurement and allows to digitalize the positions of the HPI coils as well as the landmarks on the head visible in the MRI scan. These landmarks are used to establish a coordinate transformation between MEG and MRI data.

Trigger Unit box /BNC

The trigger unit box is a stimulus I/O system used to synchronize external stimulators and MEG signal averaging. The stimulus I/O system has two trigger unit boxes connected to the main electronics over optical fibers. The two boxes (one in the stimulus cabinet, one on the table) usually work in 'mirrored mode'i.e. everything that happens on one box will be mirrored on the other. Thus, a button box can be connected to the stimulus computer via these two boxes. Each of the trigger I/O interface boxes has 16 lines, individually configurable as inputs or outputs. Stimulators can be connected using individual BNC connectors or a parallel 37-pin multipole connector.

E-prime interface box

One box makes it possible to connect the parallel port of the stimulus delivery computer (triggers sent from E-prime) to the BNC connectors on the Elekta trigger unit box. This e-prime interface box has a parallel port input and 8 BNC outputs as well as 5 BNC inputs (to record TTL from the response system). All 8 outputs are connected to the acquisition channels 1-8 which are available for recording within the acquisition software as channels STI01 to STI08. Subject's responses are recorded within the acquisition software as channels STI07 to STI16. Inputs lines 7 to 16 on the Elekta trigger unit box are connected on BNC inputs on the three e-prime interface boxes (4 BNC inputs by box). Each of this box send TTL signal related to subject's response to the stim computer via parallel cable.

Ordered: Microprocessor

 USB 1208FS

Visual stimulus system

The video signal is split to the stimulus computer’s screen and to a data projector (PANASONIC DLP projector, model PT-D7700E-K).The data projector beams the optical image via an opening in the MSR wall to a nonmagnetic back-projection screen (Flat panel LDC display). The diameter of the screen is 112 cm (44 inches). The angle, height, and viewing distance of the screen are adjustable. The screen is equipped with wheels. The actual field of view can be changed by moving the screen inside the shielded room.

For more details about the visual display setting, see the section Stimulus display and triggerring and Default setting of MEG equipment.

Auditory stimulus system

The audio system is an HiFi-quality artifact-free headphone system with wide frequency response.

The frequency response measured from the subject headphones is flat within ± 3 dB to about 10 kHz (see Figure below).

Somatosensory stimulus system

Somatosensory stimulators with custom-made non-magnetic electrodes and leads are available. Stimulus parameters are controlled from the stimulator (amplitude) and acquisition software (sequence, timing). The non-magnetic electrodes and leads are designed to minimize artifacts.

Response system

A 10 buttons response fORP system is available in MEG. Thus, all response combinations are available for experiments. This system is composed by two handles with five buttons. This system is connected to the Elekta trigger unit box via the parallel 37-pins multipole connector. Subject's responses are collected via three parallel ports on the stim computer (see E-prime section) from the trigger unit box near to the acquisition workstation. TTL signal is sent on the corresponding STI channel in your MEG acquisition, according to this drawing

Eye glasses with corrective lenses

Two eye glasses with a set of corrective lenses from -7 to +7 dioptries (by 0.5 step) are available for optical correction of the subject.