6. Auditory eNS of Dolphins
285
in humans following presentation of infrequent or surprising stimuli. Three
classes of dolphin ERP components were observed: First, middle-latency
components (P25-N60, positive peak at about 25 ms post stimulus and going
negative at about 60ms post stimulus, Fig. 6.7) showed short refractory
periods and were maximal in amplitude to brief click stimuli similar to
echolocation pulses. Second, long-latency components (N200-P450, Fig. 6.7)
showed comparable amplitudes for click and tone stimuli. When stimuli
were repetitive, the N200-P450 was markedly reduced in amplitude at all
intervals tested up to 6s. This refractory process was stimulus specific
because conditioning tones of one frequency did not reduce N200-P450
amplitudes to probe tones of another frequency. The dolphin N200-P450
showed more marked and specific refractory effects than the hl\man
N1OO-P200 recorded in a comparable paradigm. Woods et al. (1986) suggested that the differences between humans and dolphins in the behavior
of these ERPs may reflect a more precise representation of auditory stimuli
in dolphin short-term acoustic memory. Finally, Woods et al. (1986)
observed that when the same stimuli were presented as deviant stimuli
(10% probability of presentation) they produced an enhanced long-latency
positive component (P550). They noted that the dolphin P550 (Fig. 6.7)
resembled in some respects the "decision-related" P300 wave in humans
(Hillyard and Kutas 1983).
7.3 Studies of Hearing Using Evoked Brain Potentials
Since the 1970s, less invasive methods of observing the central auditory
system have been developed. By insertion of very fine needles or wires
through the skin, or by attaching sensors to the head surface with conductive glue or by suction cups, brain electrical activity can be recorded
from the brain stem, from the midbrain, and even from the cerebral cortex
(Seeley et al. 1976; Ridgway et al. 1981; Woods et al. 1986; Popov and
Supin 1990; Supin and Popov 1995; Szymanski et al. 1995; Dolphin 1997;
Szymanski et al. 1998). Easiest to record and most frequently used is the
auditory brain stem response (ABR). Using the ABR, Dolphin (1997) has
employed complex stimuli to study mechanisms underlying auditory
processing.
7.4 The Dolphin Auditory Brain Stem Response (ABR)
The ABR can be recorded from electrodes placed on the skin surface
(Popov and Supin 1990) or placed subcutaneously over the dolphin cranium
(Ridgway et al. 1981). The click-evoked ABR of T truncatus consists of
seven waves within 1Oms, numbered by the positive peaks at the vertex
(Fig. 6.8). This ABR corresponds well, in respect to number and order
of the peaks, with ABRs in other mammals. The dolphin ABR waves are
consistently large, sometimes reaching an amplitude of 10 fJ-Y. The waves
285
in humans following presentation of infrequent or surprising stimuli. Three
classes of dolphin ERP components were observed: First, middle-latency
components (P25-N60, positive peak at about 25 ms post stimulus and going
negative at about 60ms post stimulus, Fig. 6.7) showed short refractory
periods and were maximal in amplitude to brief click stimuli similar to
echolocation pulses. Second, long-latency components (N200-P450, Fig. 6.7)
showed comparable amplitudes for click and tone stimuli. When stimuli
were repetitive, the N200-P450 was markedly reduced in amplitude at all
intervals tested up to 6s. This refractory process was stimulus specific
because conditioning tones of one frequency did not reduce N200-P450
amplitudes to probe tones of another frequency. The dolphin N200-P450
showed more marked and specific refractory effects than the hl\man
N1OO-P200 recorded in a comparable paradigm. Woods et al. (1986) suggested that the differences between humans and dolphins in the behavior
of these ERPs may reflect a more precise representation of auditory stimuli
in dolphin short-term acoustic memory. Finally, Woods et al. (1986)
observed that when the same stimuli were presented as deviant stimuli
(10% probability of presentation) they produced an enhanced long-latency
positive component (P550). They noted that the dolphin P550 (Fig. 6.7)
resembled in some respects the "decision-related" P300 wave in humans
(Hillyard and Kutas 1983).
7.3 Studies of Hearing Using Evoked Brain Potentials
Since the 1970s, less invasive methods of observing the central auditory
system have been developed. By insertion of very fine needles or wires
through the skin, or by attaching sensors to the head surface with conductive glue or by suction cups, brain electrical activity can be recorded
from the brain stem, from the midbrain, and even from the cerebral cortex
(Seeley et al. 1976; Ridgway et al. 1981; Woods et al. 1986; Popov and
Supin 1990; Supin and Popov 1995; Szymanski et al. 1995; Dolphin 1997;
Szymanski et al. 1998). Easiest to record and most frequently used is the
auditory brain stem response (ABR). Using the ABR, Dolphin (1997) has
employed complex stimuli to study mechanisms underlying auditory
processing.
7.4 The Dolphin Auditory Brain Stem Response (ABR)
The ABR can be recorded from electrodes placed on the skin surface
(Popov and Supin 1990) or placed subcutaneously over the dolphin cranium
(Ridgway et al. 1981). The click-evoked ABR of T truncatus consists of
seven waves within 1Oms, numbered by the positive peaks at the vertex
(Fig. 6.8). This ABR corresponds well, in respect to number and order
of the peaks, with ABRs in other mammals. The dolphin ABR waves are
consistently large, sometimes reaching an amplitude of 10 fJ-Y. The waves