7. Electrophysiological Techniques
313
the auditory system. For instance, there is considerable information available in the temporal envelope of a returned echo, including target size,
spatial orientation, and material composition (e.g., Au and Martin 1988;
Floyd 1988; Hanna and Masakowski 1991). If animals are to extract and use
this envelope information in communication or biosonar tasks, then the
system must be able to extract and follow rapid envelope fluctuations for
single as well as multiple envelope components simultaneously.
Using stimuli consisting of multiple envelope components Dolphin
(1996) demonstrated that the auditory system was capable of extracting and
following multiple envelope components simultaneously. In Figure 7.11 the
spectra are shown of representative averaged AEPs obtained from one
animal during a single experimental session. There are clear peaks in the
response spectra at frequencies corresponding to the envelope components
in the stimulus waveform. Additionally, in the response there is considerable energy at harmonics of the envelope frequencies. (Energy at 60 Hz is
due to the combined contribution of the harmonic component of 2[2.1 and
alternating current power line interference.) The occurrence of spectral
peaks in the evoked response at frequencies coincidental to the stimulus
envelope components indicates that the response is phaselocked to stimulus envelope periodicities.
3.1.2.6 Response Latencies
For sinusoidal stimuli, the phase of the response, relative to the stimulus
phase, can be used to determine the group delay of the response using
a minimum phase angle technique (e.g., Dolphin and Mountain 1992;
Dolphin 1995). The group delay is a measurement of the time of the major
neurophysiological response relative to the arrival of the stimulus at the
cochlea and is, therefore, a measure of the delay through the auditory
system. The group delay, or response latency, is thought to indicate the brain
region in which a major component of the response measured from the
scalp is generated. In plotting the phase values for use in estimation of the
group delay a minimum phase shift was assumed.
Group delays using the MTF approach were obtained from false killer
whales (P. crassidens), beluga whales (D. leucus), and bottlenose dolphins
(T. truncatus) (Dolphin et al. 1995). The group delay ('t) was calculated as
the negative of the slope of the phase response of the system calculated
such that
't = !!.8/21t[mod
where !!.8 was the cumulative phase shift in radians for a change in modulation frequency of [mod Hz.
In Figure 7.12 the response phase for averaged MTFs obtained from T.
truncatus using two-tone signals have been plotted for each of four primary
frequencies (0.5, 1.0, 4.0, and 10.0 kHz). The response phase MTF was
divided into three regions based on group delay. The group delay for the
313
the auditory system. For instance, there is considerable information available in the temporal envelope of a returned echo, including target size,
spatial orientation, and material composition (e.g., Au and Martin 1988;
Floyd 1988; Hanna and Masakowski 1991). If animals are to extract and use
this envelope information in communication or biosonar tasks, then the
system must be able to extract and follow rapid envelope fluctuations for
single as well as multiple envelope components simultaneously.
Using stimuli consisting of multiple envelope components Dolphin
(1996) demonstrated that the auditory system was capable of extracting and
following multiple envelope components simultaneously. In Figure 7.11 the
spectra are shown of representative averaged AEPs obtained from one
animal during a single experimental session. There are clear peaks in the
response spectra at frequencies corresponding to the envelope components
in the stimulus waveform. Additionally, in the response there is considerable energy at harmonics of the envelope frequencies. (Energy at 60 Hz is
due to the combined contribution of the harmonic component of 2[2.1 and
alternating current power line interference.) The occurrence of spectral
peaks in the evoked response at frequencies coincidental to the stimulus
envelope components indicates that the response is phaselocked to stimulus envelope periodicities.
3.1.2.6 Response Latencies
For sinusoidal stimuli, the phase of the response, relative to the stimulus
phase, can be used to determine the group delay of the response using
a minimum phase angle technique (e.g., Dolphin and Mountain 1992;
Dolphin 1995). The group delay is a measurement of the time of the major
neurophysiological response relative to the arrival of the stimulus at the
cochlea and is, therefore, a measure of the delay through the auditory
system. The group delay, or response latency, is thought to indicate the brain
region in which a major component of the response measured from the
scalp is generated. In plotting the phase values for use in estimation of the
group delay a minimum phase shift was assumed.
Group delays using the MTF approach were obtained from false killer
whales (P. crassidens), beluga whales (D. leucus), and bottlenose dolphins
(T. truncatus) (Dolphin et al. 1995). The group delay ('t) was calculated as
the negative of the slope of the phase response of the system calculated
such that
't = !!.8/21t[mod
where !!.8 was the cumulative phase shift in radians for a change in modulation frequency of [mod Hz.
In Figure 7.12 the response phase for averaged MTFs obtained from T.
truncatus using two-tone signals have been plotted for each of four primary
frequencies (0.5, 1.0, 4.0, and 10.0 kHz). The response phase MTF was
divided into three regions based on group delay. The group delay for the