Exams

• Answer each of the 10 questions on a separate sheet of paper.
• Do not use more than one page per question.
• Answer each question by a short paragraph.
• You are encouraged to use tables and hand-drawn sketches if it helps clarity.
• All questions will be weighted equally for grading.

1. Tonotopic mappings are found throughout the auditory pathway.

a) State a specific nucleus or region that is tonotopically mapped. Be more specific than “brainstem” or “cortex”.

b) For the region you chose in part a, describe the orientation of the tonotopic axis (e.g., medial to lateral; high frequency to low frequency).

c) For the region you chose in part a, state one stimulus property that has been claimed to be mapped along a direction orthogonal to the tonotopic axis.

2. What is meant by the terms resolved and unresolved harmonics? Describe their respective roles in pitch perception. How would you determine physiologically if a given set of harmonics is resolved?

3. Several psychophysical tests were done using (1) a 1000 Hz, 20 ms probe tone at a sound level 15 dB above the subject's threshold in quiet (i.e. 15 dB SL), and (2) a 300-ms masker tone whose level was adjusted until the probe could be heard on 75% of the trials (i.e. the probe was at its "masked threshold"). Both masker and probe were ramped on and off to avoid spectral splatter.

Initially, the masker frequency was set to 1000 Hz. Masked thresholds were measured in two conditions. In one condition, the probe occurred just before the end of the masker. The resulting threshold masker level was L_B1000. In the other condition, the probe occurred just after the end of the masker, giving a threshold masker level L_A1000. The thresholds were found to be within a few dB of each other in both conditions, i.e. L_A1000 ≈ L_B1000.

The masker frequency was then increased to 1060 Hz, and the two threshold measurements repeated with all other parameters remaining the same. The resulting threshold masker levels were L_B1060 and L_A1060 for the probe presented before and after the end of the masker, respectively.

Answer the following two questions:

a) Which is greater, L_A1060 or L_A1000? Explain.
b) Which is greater, L_B1060 or L_A1060? Explain.

4. Coincidence detection is a general neural mechanism which plays a role in binaural hearing and, possibly, in intensity coding and pitch extraction as well. Below are listed some cellular characteristics of coincidence detector neurons. For each of these, state whether (1) it is essential for coincidence detection, (2) it is not a necessary feature, but may help coincidence detection in some cases, (3) it is largely irrelevant. Give a brief explanation for each answer.

a) Their excitatory inputs are subthreshold.
b) Their inhibitory inputs are restricted to the soma and proximal parts of dendrites.
c) Their inputs form delay lines.
d) They possess low-threshold, outward-rectifying potassium conductances.
e) They have bipolar shapes.
f) They have short membrane time constants.

5. It is often stated that the cochlear nucleus is the origin of parallel functional pathways in the auditory system. Write a short paragraph that provides physiological evidence for this view.

6. The Siebert (1968) model for intensity discrimination predicts Weber’s law for pure tones by relying on information from unsaturated auditory nerve fibers located on the edges of the tonotopic excitation pattern produced the stimulus. Give psychophysical evidence against this model. Does spread of excitation play another role in intensity coding or loudness perception?

7. Responses were measured from two auditory-nerve fibers, each stimulated with a tone at its characteristic frequency (CF), 10 dB above threshold. A low-frequency, "bias" tone of 40 Hz, 100 dB SPL was added to the CF tone and period histograms were calculated at the period of the 40 Hz tone. One fiber shows suppression during a single phase of the 40 Hz tone, while the other fiber shows suppression at two phases approximately 180 degrees part. Explain what causes the suppression and give one plausible hypothesis for why there is a difference between the two fibers.

8. Describe two peripheral neural codes that could be used by the auditory system to represent the frequency of pure tones. What psychophysical evidence is there for or against these coding mechanisms?

9. State two neural mechanisms that have been hypothesized to account for the plasticity of auditory cortex mappings.

10. Describe two factors that contribute to binaural advantages in speech intelligibility in noise. Explain how they contribute and in which frequency region they are the most effective.