EEG E XPERIMENT

2.2.3.1 PROCEDURE

Prior to participation, written consent was obtained by a form approved by the Duke University Medical Center Institutional Review Board. Furthermore, subjects had to give a short medical history report in order to exclude any history of psychiatric or neurological disorders, complete the Edinburgh Handedness Questionnaire (Oldfield, 1971) and report which languages they spoke.

The Young Controls were tested individually in a dimly lit sound attenuated booth sitting in a comfortable recliner. All other groups were tested in an electromagnetically shielded and sound proofed booth sitting in a comfortable chair. They listened to the sentence scenarios via Sennheiser HD circumaural headphones while their EEG was recorded. The only task was to listen attentively to the sentences; subjects were told that the experimenter would interrupt the presentation in random intervals in order to ask them to repeat the previously heard stimuli. This was performed four times per block and done in order to make sure subjects were indeed listening attentively. Each block was about 11 min long, with a total duration of stimulation of about 45 min. In order to minimize the contamination of the EEG by oculomotor artifacts, subjects were asked to keep their eyes fixated to a white cross which was displayed on a computer screen as long as the sentences were being played and which remained on the screen for another 2500 ms after the offset of the final word. It was then replaced by a green cross for 3000 ms, and subjects were told that this was the time when they could blink. The cross turned white again 1000 ms before the next sentence was played.

2.2.3.2 EEG RECORDING

The EEG was recorded from 32 tin electrodes mounted in an Electrocap with 19 electrode placements according to the 10-20 system (Jasper, 1958) at FP1, FP2, F3, F4, F7, F8, C3, C4, T3, T4, P3, P4, T5, T6, O1, O2, FZ, CZ, and PZ. Three symmetric lateral pairs were added at interspersed locations at FC1, FC2, FC5, FC6, CP1, CP2, CP5, CP6, and two additional midline sites were included as well at AFZ and POZ. Vertical and horizontal eye movements were monitored via a left sub-orbital electrode referred to FP1 and a bilateral external canthus montage, respectively. The right mastoid was used as the recording reference, and the left mastoid was also recorded for later off-line algebraic re-referencing.

Impedances were kept below 5 kΩ. The EOG and EEG recordings (band pass 0.01 – 30 Hz) were amplified with Neuroscan® SynAmps amplifiers and continuously digitized at 250 Hz.

Data were stored on a hard disk along with stimulus codes for off-line averaging and data analysis.

2.2.3.3 SOUND DELIVERY

The young control group was tested at Duke University, where a Creative® Sound Blaster Audio PCI 64 V sound card was used for stimulus delivery. For the presentation of stimulus materials, the software package "Presentation" was used. All other subject groups were tested at the University of California at Davis. There, stimuli were delivered at 65dB above hearing threshold; the digital output of a Creative® Sound Blaster Audigy II card was converted back to analog in order to assure better sound quality by feeding into a Benchmark® DAC 1 Digital to Analog Converter. The output of the DAC 1 was then fed into an UltraCurve Pro® DSP8024 which allowed volume adjustment in 0.5 dB steps separately for each ear. The final delivery of the sound was controlled by a Mackie® 1604 VLZ PRO 16 Chan Mixer. This setup allowed the experimenter to monitor the sound delivered to the subject’s headset as well as talk to the subject both via the headset and via speakers in the subject room.

2.2.3.4 HEARING ASSESSMENT

In order to compensate for potential hearing loss, prior to participation, the hearing of the Elderly Controls and the aphasic patients was examined with a clinical Audiometer (GSI 61 2 channel Clinical Audiometer, VIASYS Healthcare, Inc.). The volume of the stimuli was

differentially adjusted by means of an UltraCurve Pro® DSP8024 to be 65dB above threshold for each ear. The entire sound system was calibrated by means of the Brüel & Kjaer 2238 Mediator sound level meter and the Brüel & Kjaer Artificial Ear 4153.

2.2.3.5 OCULAR ARTEFACT REDUCTION FOR PATIENT GROUPS

Trying to keep aphasic patients from withholding eye blinks for a defined period of time and executing them in another would have invoked a go-nogo task comprising the control of executing and withholding eye blinks at predefined periods. Thus, it was refrained from instructing patients to blink only during the presentation of the green cross. Oculomotor artifacts were reduced using the “Ocular Artefact Reduction” routine implemented in Scan4.3 from Neuroscan®. The algorithm implemented in this routine works by calculating an average artifact waveform in the eye channel, and also the averaged phase-locked artifact transmission in each EEG channel. A covariance value is then calculated in each EEG channel, and this COV value is used in a regression equation where:

corrected chan = (original chan) – (eye chan * original chan’s COV value with eye chan).

2.2.3.6 DATA ANALYSIS AND DATA REDUCTION

Prior to off-line averaging, all single trial waveforms were screened for amplifier blocking, DC drift, muscle and oculomotor artifacts. This was done over an epoch of 1400ms, starting 200 ms before the onset of the final word. Trials contaminated by artifacts were rejected. For each subject, average waveforms were computed across the remaining trials for each condition after normalizing to a 100 ms pre-stimulus baseline.