2.2.1 Design
Although the subjects were told that they participated in a frequency-discrimination-experiment where they had to discriminate between two tones, there was only
one tone in the real task that never changed.
Therefore, the subjects heard the same tone for the whole task but were given random feedback, whether they had just heard the high or the low tone. Feedback consisted of a loud noise for the high tone or silence for the low tone.
The feedback was not actually totally random but was organized in strings following Clark et. al.[6].
String Length
High Low
1 18 18
2 9 9
3 6 6
4 4 4
Table 2.3: Number of Strings for the two conditions and the 4 possible string-lengths
String length is thus dened as the number of equal feedbacks (high/low) in a row. Figure 2.1 shows an example of three runs consisting of the string length 2, 3 and 1.
Figure 2.1: This gure shows a possible course of the experiment to clarify the concept of string length. Only the feedbacks are depicted here. In this gure there is two times feedback for the high tone, followed by three times feedback for the low tone and one feedback for the high tone. So the corresponding string lengths are 2 3 1.
Altogether, the real task consisted of 140 trials.
2.2.2 Stimuli
Two kinds of stimuli were needed for the experiment.
1. Three steady-state tones.
• Two are of dierent frequencies and are needed for the practice.
• The frequency of the third lies in the middle of the two practice-tones. Additionally it is mixed with a high bandpass noise.
2. One bandpass noise for the feedback.
All Stimuli were created using a matlab-script.
Steady-State Tone
Three Steady-State Tones each lasting 10 seconds were used for the experi-ment. For details refer to table 2.4.
Task
Carrier-Frequency
Modulation-Frequency
Noise
Practice Low 975Hz 39.0625Hz NO
Practice High 1025Hz 39.0625Hz NO
Real 1000Hz 39.0625Hz YES
Table 2.4: Carrier-Frequencies, Modulation-Frequencies and whether noise was added for the 3 Steady-State Tones
The third stimulus was composed of the steady-state tone and narrow-band noise (7000-10000Hz) at 3dB above the peak-amplitude of the steady-state tone.
The steady-state tones were created by rst generating a pure tone (puretone) with a maximum amplitude "peakamp" and the carrier-frequency "f". Then the envelope for the amplitude-modulation (am) envelope was generated with the modulation-frequency "ampf". Finally, the two resulting vectors were multiplied to form the steady-state tone. The following functions were used for the generation:
puretone(x) =peakamp∗sin(2∗Π∗f ∗x/44100) am(x) =sin(2∗Π∗(ampf /2)∗x/44100)
sst(x) =puretone(x)∗am(x)
Noise was created by using random values multiplied by the amplitude. Af-terwards the noise was rst low-pass ltered and then high-pass ltered and added to the tone.
To simplify the combination of the steady-state tone and the noise will be called "auditory stimulus" for the rest of this diploma-thesis.
The feedback-noise
The noise was generated using random numbers. Afterwards it was bandpass ltered (3000-6000Hz). The duration was 1 second.
For the loudness-tuning the subjects were instructed that they would hear a noise by clicking on a certain button on the screen. Two other buttons controled the loudness. They were asked to make the noise as loud as possible.
It should be uncomfortable but not painful.
2.2.3 Procedure
After giving written consent, the subjects were seated in the EEG-cabin and were prepared for the EEG-recording.
After the preparations were nished, the subjects were told by the inves-tigator that it was the purpose of this experiment to assess the ability of normal-hearing people to discriminate two tones while being disturbed by a noise.
The subjects were told that after a few preparations they would have to solve the real task that consisted of hearing one of the two noise-disturbed tones for about 6 seconds and then having 2 seconds to choose which tone they heard. The subjects had to report this by clicking one of two buttons on the screen. Feedback would be given so that the higher tone was always followed by a loud noise while the lower one was followed by silence. The investigator also tried to make clear that the two tones are chosen to make the task rather easy when there is no noise but rather hard when the tones were played together with the noise.
Then the investigator explained the procedure to the subject:
1. Have the subject "tune" the loudness of the "feedback-noise".
2. Passively listen to the two tones for 3 times each.
3. Practice the task without the disturbing noise.
4. Do the real task.
Getting familiar with the tones
The Subjects then passively listened to the two practice-tones (975Hz and 1025Hz without noise) 3 times. They were told that this was for "`getting to know the tones"`.
The Practice
The practice was like a short version (only 10 trials) of the real task (2.2) except that there really were two dierent tones with consistent feedback.
Although it was not easy for some subjects to discriminate these tones, all of them managed the task after about 5 trials without problems.
The real task
The design of the real-task is shown in gure 2.2.
First the auditory stimulus was presented concurrently. After six seconds, the subject was asked which tone he heard while the auditory stimulus was still played. After 2 seconds the buttons disappeared, the auditory stimulus stopped and the feedback-noise was played in 50% of the trials.
Figure 2.2: Timeline of the task
2.2.4 Stimulus Presentation and Data Acquisition
Stimulus Presentation and Response Acquisition
Stimulus-presentation, presentation of the instructions and rating-scheme and behavioral response acquisition were done using Psyscope X [24] run-ning on an Apple Macintosh iBook (900MHz PowerPC G3) using Mac OS X version 10.3.9.
The instructions and the rating-scheme were shown on a 17 inch CRT-monitor situated about 1.5 meters from the subject.
Auditory stimulation was delivered to the left ear through stereo-headphones (Sennheiser HD pro 180).
To respond subjects used a mouse to move the cursor to the corresponding button on the screen and press the mouse button to deliver the response.
EEG-data recording
EEG recording was carried out in a dimly lit, sound-attenuated room us-ing a 64-channel EEG-system (Neuroscan Synamps). Samplus-ing-rate was 250Hz. Online ltering with a low-pass of 100Hz and DC ltering was ap-plied. Impedance between the electrodes and the skull was kept under 5kΩ.
After data-acquisition the locations of the electrodes was recorded using a 3Space Isotrak II system (Polhemus) to be able to do source-projection af-terwards.