Index of content:
Volume 104, Issue 5, November 1998
- PHYSIOLOGICAL ACOUSTICS 
104(1998); http://dx.doi.org/10.1121/1.423876View Description Hide Description
In-the-ear calibration of sound pressure level may be problematic at frequencies above 2 kHz, because the pressure can vary significantly along the length of the ear canal, due to reflection of sound waves at the eardrum. This issue has been investigated by measuring behavioral thresholds to tones in a group of human subjects for two different insertion depths of an insert earphone. The change in insertion depth was intended to alter the distribution of pressure in the ear canal, shifting the frequency at which spectral notches occur. The inset earphone or “probe” (Etymotic ER-10C) also contained a calibrated microphone, allowing the recording of sound pressure levels in the ear canal. Prior to the threshold measurements in each subject, the Thevenin acoustic source characteristics of the probe were determined by a special calibration procedure. This calibration allowed the expression of the sound level at threshold in terms of acoustic intensity ( The impact of changes in insertion depth was determined by measuring behavioral threshold at each depth. Because cochlear sensitivity remained constant, the level of sound entering the ear at threshold should have been the same (within measurement error) for both insertions. The difference in sound pressure level (SPL) at threshold between the two probe insertions was greatest at the notch frequency of the first insertion. At this notch frequency, the SPL at threshold increased by an average of 11.4 dB. The change in sound intensity level (SIL) at threshold was almost always less than the change in SPL. At the notch frequency, the SIL decreased, on average, by only 0.5 dB. These results suggest that SIL may be a better indicator than SPL of the sound level entering the ear, especially for frequencies in the 4–8 kHz range.
Click train encoding in primary auditory cortex of the awake monkey: Evidence for two mechanisms subserving pitch perception104(1998); http://dx.doi.org/10.1121/1.423877View Description Hide Description
Multiunit activity (MUA) and current source density (CSD) patterns evoked by click trains are examined in primary auditory cortex (A1) of three awake monkeys. Temporal and spectral features of click trains are differentially encoded in A1. Encoding of temporal features occurs at rates of 100–200 Hz through phase-locked activity in the MUA and CSD, is independent of pulse polarity pattern, and occurs in high best frequency (BF) regions of A1. The upper limit of ensemble-wide phase-locking is about 400 Hz in the input to A1, as manifested in the cortical middle laminae CSD and MUA of thalamocortical fibers. In contrast, encoding of spectral features occurs in low BF regions, and resolves both the and harmonics of the stimuli through local maxima of activity determined by the tonotopic organization of the recording sites. High-pass filtered click trains decrease spectral encoding in low BF regions without modifying phase-locked responses in high BF regions. These physiological responses parallel features of human pitch perception for click trains, and support the existence of two distinct physiological mechanisms involved in pitch perception: the first using resolved harmonic components and the second utilizing unresolved harmonics that is based on encoding stimulus waveform periodicity.