1. ANATOMY AND PHYSIOLOGY O F THE CENTRAL NERVOUS SYSTEM
63
limit found in ostariophysid fishes. For most nonostariophysid fishes the
upper limit for sensitivity is well below 1000 Hz. However, in some
species of nonostariophysids there are specializations of the swim bladder
which cxtrude and contact some part of the labyrinth (Sparidae) or
there are air-filled cavitics around the inner ear (Mormyridae, Anabantidae, and Clupeadae ) . These animals are sensitive to tone frequencies
which can go as high as 1250 and 3000 Hz, respectively. However, the
morphological basis of pitch discrimination is not known. The labyrinth
has no obvious morphological frequency analyzers within the receptive
units. The three organs which would receive sound frequencies are
embedded in cavities which are filled with endolymph and completely
covered by membranes with areas lined by sensory epithelium. It would
appear then that pitch discrimination can be found in the absence of a
niorphological discriminator only by two possible mechanisms: one, a
synchronization between sound frcquency and the frequency of impulse
discharge, and the other, the use of different sensory units for different
sound frequency ranges ( Enger, 1963).
The anatomy and ultrastructure of the lamprey labyrinth have been
described (Lowenstein et al., 1968). Except for a new type of hair cell
and large central labyrinthine ciliated chambers ( Lowenstein et al.,
1968), it conforms with other fish labyrinths (Young, 1962).
Approximate acoustic thresholds have been obtained in the shark by
conditioning to low frequency sounds and using unconditioned cardiac
response as the response to the frequency (Wisby et al., 1964). There
was usually no heart response to frequencies higher than 500 Hz. The
number of sharks responding at each frequency decreased as the frequency increased. Deceleration of heart rate was more pronounced and
less variable at frequencics from i . 5 to 40 Hz than at higher frequencies.
Sharks can be conditioned to sound (Clark, 1963; Kirtzler and Wood,
1961; Wisby et al., 1964). Using conditioned responses, it was found that
the upper detectable frcquency limit for the shark was approximately
500 Hz. These data are in accordancc with unconditioned heart rate
experiments.
The mechanisms of producing sound in marine fishes have been reviewed (Tavolga, 1964) as well as the role of the lateral line in audition
(Dijkgraaf’, 1963, 1964; Flock, 1965) and will not be covered here. Fish
have been trained to a two-way conditioned avoidance response using
pure tone as the conditioned stimulus and shock as the unconditioned
stimulus. Two marine fish, the squirrelfish, Holocentrus ascensionis, and
the blue-striped grunt, Haemulon sciurus, were utilized by Wodinsky
and Tavolga (1964) and Tavolga and Wodinsky ( 1963, 1965). The lowest
thresholds in the squirrelfish were at 800 Hz at pressure levels of approxi-
63
limit found in ostariophysid fishes. For most nonostariophysid fishes the
upper limit for sensitivity is well below 1000 Hz. However, in some
species of nonostariophysids there are specializations of the swim bladder
which cxtrude and contact some part of the labyrinth (Sparidae) or
there are air-filled cavitics around the inner ear (Mormyridae, Anabantidae, and Clupeadae ) . These animals are sensitive to tone frequencies
which can go as high as 1250 and 3000 Hz, respectively. However, the
morphological basis of pitch discrimination is not known. The labyrinth
has no obvious morphological frequency analyzers within the receptive
units. The three organs which would receive sound frequencies are
embedded in cavities which are filled with endolymph and completely
covered by membranes with areas lined by sensory epithelium. It would
appear then that pitch discrimination can be found in the absence of a
niorphological discriminator only by two possible mechanisms: one, a
synchronization between sound frcquency and the frequency of impulse
discharge, and the other, the use of different sensory units for different
sound frequency ranges ( Enger, 1963).
The anatomy and ultrastructure of the lamprey labyrinth have been
described (Lowenstein et al., 1968). Except for a new type of hair cell
and large central labyrinthine ciliated chambers ( Lowenstein et al.,
1968), it conforms with other fish labyrinths (Young, 1962).
Approximate acoustic thresholds have been obtained in the shark by
conditioning to low frequency sounds and using unconditioned cardiac
response as the response to the frequency (Wisby et al., 1964). There
was usually no heart response to frequencies higher than 500 Hz. The
number of sharks responding at each frequency decreased as the frequency increased. Deceleration of heart rate was more pronounced and
less variable at frequencics from i . 5 to 40 Hz than at higher frequencies.
Sharks can be conditioned to sound (Clark, 1963; Kirtzler and Wood,
1961; Wisby et al., 1964). Using conditioned responses, it was found that
the upper detectable frcquency limit for the shark was approximately
500 Hz. These data are in accordancc with unconditioned heart rate
experiments.
The mechanisms of producing sound in marine fishes have been reviewed (Tavolga, 1964) as well as the role of the lateral line in audition
(Dijkgraaf’, 1963, 1964; Flock, 1965) and will not be covered here. Fish
have been trained to a two-way conditioned avoidance response using
pure tone as the conditioned stimulus and shock as the unconditioned
stimulus. Two marine fish, the squirrelfish, Holocentrus ascensionis, and
the blue-striped grunt, Haemulon sciurus, were utilized by Wodinsky
and Tavolga (1964) and Tavolga and Wodinsky ( 1963, 1965). The lowest
thresholds in the squirrelfish were at 800 Hz at pressure levels of approxi-