205
1998a). Therefore, while we have learned much about the contribution of the saccule
to auditory processing, we do not know all that the ear tells the brain about sound
(see Sect. 5). In each of the studies described here, auditory processing was evaluated in sexually mature toadfish of both sexes that were obtained during their spring
and early summer breeding season.
3.1 The Ear of the Toadfish
Each of the otolithic endorgans consists of the sensory epithelium, a calcareous
otolith, and a gelatinous substance that mechanically links the epithelium to the
otolith in a fluid-filled sac within the otic capsule. The saccule is the largest of the
otolithic endorgans in oyster toadfish. The saccular otolith is a dense, highly sculptured structure that is curved in both the vertical and horizontal planes (Figs. 1c and
2a). The sensory epithelium on which the hair cells are located is smaller in area
than the otolith, but both are intricately associated such that the greater relative
motion of the epithelium (e.g., from particle motion in a sound field) causes deflection of the apical structures on the hair cells.
As shown in Fig. 1, maximum excitation occurs along a central axis, toward the
single kinocilium. The hair cell response is inherently directional with a single maximum excitatory direction (solid blue line in Fig. 1a), decreasing responsiveness
along other axes and a null perpendicular to the best axis (modeled as a cosine function of relative activity versus angle of stimulation; Fig. 1a). The hair cell orientations on the sensory epithelium dictate the directional responsiveness of the
endorgan. In toadfish, the saccule is oriented in the dorsal–ventral plane, with a
sweeping hair cell orientation on both the rostral and caudal saccule, and a region of
vertically oriented hair cells in the middle (Fig. 1b). Thus the hair cells on the saccule can respond to a particle motion stimulus at any angle in the vertical plane;
however, the responsiveness in the horizontal plane is restricted by the angle of the
endorgan with respect to the midline of the fish and the orientation of the endorgan
on the curved otolith (Fig. 1c).
Like all components of the ear, the saccule is a paired endorgan. The saccular
otoliths are heavily calcified and are obvious on X-rays, lying on either side of the
midline (Fig. 2a) in the otic capsule. The rostral saccular otoliths lie at approximately ±35° with respect to the midline of the fish, and the caudal end is curved to
lie adjacent to the midline in the otic capsule. This angled orientation is important
because the left and the right saccules “point” into different regions of acoustic
space, and their directional responses will be different, though complimentary. Nonparallel orientation of the saccules is also seen in other fishes that are not closely
related to the toadfish (e.g., perch, Sand 1974; cod, Dale 1976; trout, Schellart and
Buwalda 1990; sleeper goby, Lu and Xu 2002). The functional significance of the
non-parallel orientation may lie in the location-dependent binaural contributions for
encoding the direction of a sound source (see Enger et al. 1973; Schuijf and Buwalda
1975; Schellart and deMunck 1987; Schellart and Buwalda 1990).
What the Toadfish Ear Tells the Toadfish Brain About Sound
1998a). Therefore, while we have learned much about the contribution of the saccule
to auditory processing, we do not know all that the ear tells the brain about sound
(see Sect. 5). In each of the studies described here, auditory processing was evaluated in sexually mature toadfish of both sexes that were obtained during their spring
and early summer breeding season.
3.1 The Ear of the Toadfish
Each of the otolithic endorgans consists of the sensory epithelium, a calcareous
otolith, and a gelatinous substance that mechanically links the epithelium to the
otolith in a fluid-filled sac within the otic capsule. The saccule is the largest of the
otolithic endorgans in oyster toadfish. The saccular otolith is a dense, highly sculptured structure that is curved in both the vertical and horizontal planes (Figs. 1c and
2a). The sensory epithelium on which the hair cells are located is smaller in area
than the otolith, but both are intricately associated such that the greater relative
motion of the epithelium (e.g., from particle motion in a sound field) causes deflection of the apical structures on the hair cells.
As shown in Fig. 1, maximum excitation occurs along a central axis, toward the
single kinocilium. The hair cell response is inherently directional with a single maximum excitatory direction (solid blue line in Fig. 1a), decreasing responsiveness
along other axes and a null perpendicular to the best axis (modeled as a cosine function of relative activity versus angle of stimulation; Fig. 1a). The hair cell orientations on the sensory epithelium dictate the directional responsiveness of the
endorgan. In toadfish, the saccule is oriented in the dorsal–ventral plane, with a
sweeping hair cell orientation on both the rostral and caudal saccule, and a region of
vertically oriented hair cells in the middle (Fig. 1b). Thus the hair cells on the saccule can respond to a particle motion stimulus at any angle in the vertical plane;
however, the responsiveness in the horizontal plane is restricted by the angle of the
endorgan with respect to the midline of the fish and the orientation of the endorgan
on the curved otolith (Fig. 1c).
Like all components of the ear, the saccule is a paired endorgan. The saccular
otoliths are heavily calcified and are obvious on X-rays, lying on either side of the
midline (Fig. 2a) in the otic capsule. The rostral saccular otoliths lie at approximately ±35° with respect to the midline of the fish, and the caudal end is curved to
lie adjacent to the midline in the otic capsule. This angled orientation is important
because the left and the right saccules “point” into different regions of acoustic
space, and their directional responses will be different, though complimentary. Nonparallel orientation of the saccules is also seen in other fishes that are not closely
related to the toadfish (e.g., perch, Sand 1974; cod, Dale 1976; trout, Schellart and
Buwalda 1990; sleeper goby, Lu and Xu 2002). The functional significance of the
non-parallel orientation may lie in the location-dependent binaural contributions for
encoding the direction of a sound source (see Enger et al. 1973; Schuijf and Buwalda
1975; Schellart and deMunck 1987; Schellart and Buwalda 1990).
What the Toadfish Ear Tells the Toadfish Brain About Sound