7. Electrophysiological Techniques
309
sented with short (20/ls) clicks, there is very little change in ABR amplitude with increasing click rates up approximately 200 to 300 Hz. At higher
rates the responses, being 10 to 15 ms in duration, begin to overlap; however,
a strong following response has been reported to stimulus rates approaching 2 kHz (Szymanski et al. 1995). In contrast, in the cat a click stimulus rate
of 80 Hz reduces all ABR components to approximately 50% of maximum
amplitude, with cessation of a response at approximately 400 Hz. In humans
the ABR fails to follow stimulus rates beyond approximately 200 Hz.
Furthermore, similar to the finding with stimulus intensity, stimulus repetition rate has only minimal effect on ABR latencies in cetaceans. In contrast, in humans there is a marked increase in latency with increased click
rate (e.g., Burkard and Hecox 1987). Similarly, in gerbils (Burkard and Voigt
1989) a mean I to V interval increase of 0.31 ms was found in varying the
click rate from 10 to 90Hz.
3.1.2.4 Modulation Rate Transfer Functions
Thus far in our discussion we have only considered responses to transient
(i.e., click, toneburst, or noise burst) stimuli. However, it is also possible to
assess the temporal resolving power of the auditory system using long duration continuous or pulsed stimuli.
The mammalian auditory system acts as an envelope extractor due to
inherent nonlinearities in the transduction of acoustic, mechanical energy
to neuroelectrical energy that takes place in the cochlea; further nonlinearities are introduced at higher stages within the auditory pathway.
Consequently, in studies using continuous amplitude-modulated stimuli
consisting of a single envelope component, for example, in two-tone
(frequently referred to in the engineering literature as double-sideband
suppressed carrier signals) or sinusoidally amplitude-modulated signals,
it has been demonstrated that AEPs follow the envelope of the stimulating waveform. As this scalp-recorded response appears to be a generalized
following of low-frequency envelope components of the stimulating
waveform, this phenomenon has been termed the envelope following
response or EFR (Dolphin and Mountain 1991). Masking studies (e.g.,
Dolphin an Mountain 1993; Dolphin et al. 1994) have demonstrated that
the EFR arises from populations of neurons tuned to the stimulus carrier
or primary frequencies in sinusoidally amplitude-modulated and two-tone
stimuli, respectively.
The time and frequency domain representation of amplitude-modulated
stimulus signal and the corresponding EFR obtained from an adult T. truncatus is shown in Figure 7.9. In this instance the stimulus consisted of a pair
of tones of frequencies F, and Fl. Due to phase cancellations, combination
of pairs of tones results in an amplitude modulated signal that waxes
and wanes with a periodicity corresponding to the difference in frequency
between the two tones, that is, F l - F, = F l . 1 • It should be noted that there
is no energy in the stimulus at this difference frequency. However, a major
component in the response to these periodic, single-envelope component
309
sented with short (20/ls) clicks, there is very little change in ABR amplitude with increasing click rates up approximately 200 to 300 Hz. At higher
rates the responses, being 10 to 15 ms in duration, begin to overlap; however,
a strong following response has been reported to stimulus rates approaching 2 kHz (Szymanski et al. 1995). In contrast, in the cat a click stimulus rate
of 80 Hz reduces all ABR components to approximately 50% of maximum
amplitude, with cessation of a response at approximately 400 Hz. In humans
the ABR fails to follow stimulus rates beyond approximately 200 Hz.
Furthermore, similar to the finding with stimulus intensity, stimulus repetition rate has only minimal effect on ABR latencies in cetaceans. In contrast, in humans there is a marked increase in latency with increased click
rate (e.g., Burkard and Hecox 1987). Similarly, in gerbils (Burkard and Voigt
1989) a mean I to V interval increase of 0.31 ms was found in varying the
click rate from 10 to 90Hz.
3.1.2.4 Modulation Rate Transfer Functions
Thus far in our discussion we have only considered responses to transient
(i.e., click, toneburst, or noise burst) stimuli. However, it is also possible to
assess the temporal resolving power of the auditory system using long duration continuous or pulsed stimuli.
The mammalian auditory system acts as an envelope extractor due to
inherent nonlinearities in the transduction of acoustic, mechanical energy
to neuroelectrical energy that takes place in the cochlea; further nonlinearities are introduced at higher stages within the auditory pathway.
Consequently, in studies using continuous amplitude-modulated stimuli
consisting of a single envelope component, for example, in two-tone
(frequently referred to in the engineering literature as double-sideband
suppressed carrier signals) or sinusoidally amplitude-modulated signals,
it has been demonstrated that AEPs follow the envelope of the stimulating waveform. As this scalp-recorded response appears to be a generalized
following of low-frequency envelope components of the stimulating
waveform, this phenomenon has been termed the envelope following
response or EFR (Dolphin and Mountain 1991). Masking studies (e.g.,
Dolphin an Mountain 1993; Dolphin et al. 1994) have demonstrated that
the EFR arises from populations of neurons tuned to the stimulus carrier
or primary frequencies in sinusoidally amplitude-modulated and two-tone
stimuli, respectively.
The time and frequency domain representation of amplitude-modulated
stimulus signal and the corresponding EFR obtained from an adult T. truncatus is shown in Figure 7.9. In this instance the stimulus consisted of a pair
of tones of frequencies F, and Fl. Due to phase cancellations, combination
of pairs of tones results in an amplitude modulated signal that waxes
and wanes with a periodicity corresponding to the difference in frequency
between the two tones, that is, F l - F, = F l . 1 • It should be noted that there
is no energy in the stimulus at this difference frequency. However, a major
component in the response to these periodic, single-envelope component