MEG system

Whole-head Magnes 3600 WH MEG system with 248 radial magnetometers developed by 4D Neuroimaging

Visual stimulus systen

The magnetically-silent visual stimulus delivery system consists of a high performance commercial projector capable of projecting high-resolution images through an opening in the MSR wall (Figure 1), thereby eliminating electromagnetic interference.

Figure 1. Visual stimulus projector mounted on the outside of the MSR.

The image is reflected off a series of distortion-free, high-reflection, mirrors and back-projected onto a screen located in the subject's visual field (Figure 2). The image can be viewed from either the seated or supine position. Timing accuracy of visual presentation is assured by use of a photo-diode on the display..

Figure 2. Visual stimulus is back-projected onto a screen located in front of the subject.

Concurrent visual-tactile stimulation can be achieved through pneumatic tactile stimulators (see below) and a set of 8 fiber-optic LEDs (Figure 3).

Figure 3: Visual-tactile stimulator placed on the index finger of a research subject

Auditory stimulus system

A non-magnetic auditory amplification and stimulus delivery system is provided by way of an amplifier, attenuator and electromechanical transducer located outside the MSR. The transducer is coupled to a matched set of acoustic transmission tubes, 6 meters in length, leading to proprietary adapters and ear mold fittings (Figure 4). The system is acoustically-compensated and produces a usable frequency response from 200 to 6400 Hz, with a dynamic range of 110 dB (measured with a 2 ccm coupler at the ear piece).

Figure 4. Auditory stimuli are delivered via acoustically-compensated transmission tubes to the subject's ears.

Somatosensory stimulus system

The non-magnetic somatosensory stimulator applies tactile stimuli via pneumatic activation of a membrane that is applied to the skin surface. The stimuli are therefore natural and entirely painless. Clips are provided for application of the membrane surface to the fingers or lip (Figure 5). The timing of the tactile stimuli can be controlled through either external or internal timing signals. Four stimulus-output channels are provided, and can be activated either individually or in pairs.

Figure 5. Finger clips attach to the finger tips for application of tactile somatosensory stimuli.

Presentation software/hardware

A dedicated stimulus presentation computer (Windows XP) is installed with Presentation and E-Prime, and interfaces with all stimulus delivery systems. Triggers are sent through the parallel port. As an option, users may use their own laptop for stimulus presentation.

Analysis software

- 4D Magnes software package

- Curry

- Dynamical Statistic Parametric Mapping

- FreeSurfer

- Fieldtrip

- and various in-house Matlab-based software tools

(most images courtesy of 4D Neuroimaging)