Inside the magnet room

Scanning and presenting stimuli

Acquisition schemes

Stimulus presentation

FMRI Facility: Visual stimulus presentation

Visual stimuli are projected inside the magnet bore onto a specialized plexiglass screnn with a non-dispersing optical coating. An Epson 3000 lumens LCD projector, fitted with custom Buhl lens, is housed in a custom-made, grounded and shielded Faraday box to prevent RF noise contamination from the projector's motor and circuitry.

For some studies it is necessary to know where the subject is looking when a stimulus is presented. Eye movements can be recorded using an Eye-Tracker from Applied Science Labs. This device uses long-range optics infrared videography and all movements are recorded digitally to a PC at up to 250 Hz sampling rate. Pattern recognition software uses pupil and corneal reflections to track the eyes' positions, as well as fluctuations in pupil diameter.

Auditory stimulus presentation

Measuring the brain response to auditory stimuli presents a special technical challenge. First, the normal operation of the scanner produces extremely loud (115-130 dB) acoustic noise, which is generated by the electrical current driving the magnetic gradient coils. Second, most hi-fidelity sound delivery systems are electronic devices which will not function correctly in the high magnetic field. (4 T is 80,000 times the strength of the Earth's magnetic field.)

The loud scanner noise heard by the subject can be reduced by layers of acoustic barriers and acoustic absorbers at strategic locations in the scanner, and by sound attenuation devices - including earmuffs and ear plugs. Delivery of sound stimuli to the subject in the scanner is achieved by means of a air conducting tubes, connected to specially designed transducers, and a high quality equalization system (Silent Scan 2100). Despite the limitation of air conduction, we are able to present sounds with a flat frequency response between 200-3500 Hz, including the main part of the speech spectrum. This system is embedded within earmuffs, providing 20dB sound attenuation of the external scanner noise. To simulate spatial location of sounds, we record sounds from within the ear-canals of subjects using very small capsule microphones. This allows the best simulation of acoustic space with earphones, accounting for all cues of sound localization.