Audio System

The audio system is not especially fancy, but the routings are complex: we must be able to get auditory stimuli to the participant in the MSR, to speak to the participant through the audio system (the MSR is rather strongly sound-dampening), to hear the participant outside the MSR, and, if the experiment calls for a participant voice response, to use the participant’s voice as a trigger.

INPUT

To talk to the subject, we use a standard unidirectional microphone that must be turned off at the start of experiments to prevent feedback. To hear the subject, we have installed an Audio-Technica (Stow, OH) ATM10a Omnidirectional Condensor Microphone using a pass-through in the MSR; since the microphone is functionally inside the MSR, a condenser mike is necessary to eliminate RF noise.

MIXING

Input from the subject mike is fed into a Eurorack (Behringer; Willich, Germany) UB502 mixer preamplifier. Here we adjust the gain, in particular boosting the amplification of the subject mike. The mike we use to communicate to the subject is fed into a MACKIE (Woodinville, WA) 1202-VLZ3 Premium mike/line mixer. There are six inputs and four outputs, and the levels on each can be adjusted separately. Also feeding into the MACKIE mixer are the lines for sound generated by the stimulus computers for auditory experiments and experiments providing auditory feedback.

OUTPUT

Output from the MACKIE mixer is sent to three destinations. The first is a mix of the experimenter’s mike and the stimulus computer’s output that goes to the subject, via E-A-Rtone (AEARO Company; Indianapolis, IN) 3A insert earphones. Two 50 Ohm amplifiers for the E-A-Rtone system located just outside the MSR, and sound is fed to the subject via silicon air tubes approximately 1.5m (5′) in length. The second is the stimulus output that goes to the outside speakers where the experimenter is located. Finally, the auditory stimuli may be sent directly to the trigger box to record stimuli onset on a trigger line of the MEG system. We can use two additional channels to record any combination of the subject, the experimenter, and the computer-generated stimulus, if, for example, we should want to examine off-line the response produced by the subject.

Subject output is also sent directly and uniquely from the Eurorack UB502 mixer preamplifier to a set of close by speakers so that we can monitor the subject without hearing the computer output. We also have an OptoAcoustics (Or Yehuda, Israel) fiber optic microphone that we use to record voice onset triggering, by running it through an amplifier and then directly to the trigger box via a BNC line.