207
More specific projection data were obtained from physiologically characterized
saccular afferents that were injected with neurobiotin (see Physiology below).
Auditory afferents sometimes bifurcated and sent a process rostrally to AON and
caudally to the dorsal subdivision of DON, indicating that the same auditory input
can go to both nuclei (Edds-Walton et al. 1999). However, more afferents went
exclusively to the dDON, and the input had a distinctive lateral to medial organization. Individual primary afferent fibers projected along the length (rostral to caudal)
of the DON, with medial projections bearing bouton-like endings at multiple sites
along the length of DON (Edds-Walton et al. 1999). These anatomical data indicate
that there is redundancy in the input from the saccule that occurs along the length of
the dDON, which may reflect a computational axis for directional analyses.
Additional studies have shown that there are topographic commissural connections
between the left and right DONs that would permit binaural computations (EddsWalton 1998b; and see Physiology below).
Lastly, examination of the afferent branches from the saccular epithelium during
the labeling study reported by Edds-Walton (1998a) revealed interesting variations
in afferent organization that may provide clues to the location(s) of hair cells with
response characteristics consistent with vestibular functions (e.g., tilt perception) on
the saccule. In all toadfish, a small but distinct bundle of afferents (distinguished
from efferents by the presence of somata in the periphery) exited the rostral tip of
the saccule and merged with the anterior and horizontal canal cristae and utricular
inputs, rather than with the rostral bundle of saccular afferents. In some toadfish, a
small bundle of afferents exited vertically from the caudal saccule and joined VIII
with afferents from the lagena and posterior canal crista (see Fig. 2A in Edds-Walton
1998a). These anatomical data suggest that if hair cells with vestibular response
characteristics are present on the saccule, their distribution may be limited to the
most rostral and most caudal sites along the saccular epithelium.
3.2.2 The DON
The organization of the DON is interesting in that the pattern of inputs across species indicates that DON has both vestibular and auditory roles (McCormick 2011).
In toadfish, as in other teleosts, the dorsal regions receive inputs primarily from the
marily from the semicircular canal cristae (Highstein et al. 1992). Utricular afferents project to dorsolateral sites in DON (Highstein et al. 1992; unpublished data)
where they may overlap with saccular inputs, but there is also substantial input from
the utricle to the ventrolateral region (Highstein et al. 1992). The distribution of
utricular inputs in DON is consistent with both auditory and vestibular (orientation)
roles for that endorgan.
As noted previously, Fay (1984) provided physiological evidence that utricular
and lagenar afferents in goldfish responded to 140 Hz with directional response
characteristics consistent with the orientation of each endorgan and its hair cell
orientation pattern, providing evidence that both endorgans could contribute to
What the Toadfish Ear Tells the Toadfish Brain About Sound
More specific projection data were obtained from physiologically characterized
saccular afferents that were injected with neurobiotin (see Physiology below).
Auditory afferents sometimes bifurcated and sent a process rostrally to AON and
caudally to the dorsal subdivision of DON, indicating that the same auditory input
can go to both nuclei (Edds-Walton et al. 1999). However, more afferents went
exclusively to the dDON, and the input had a distinctive lateral to medial organization. Individual primary afferent fibers projected along the length (rostral to caudal)
of the DON, with medial projections bearing bouton-like endings at multiple sites
along the length of DON (Edds-Walton et al. 1999). These anatomical data indicate
that there is redundancy in the input from the saccule that occurs along the length of
the dDON, which may reflect a computational axis for directional analyses.
Additional studies have shown that there are topographic commissural connections
between the left and right DONs that would permit binaural computations (EddsWalton 1998b; and see Physiology below).
Lastly, examination of the afferent branches from the saccular epithelium during
the labeling study reported by Edds-Walton (1998a) revealed interesting variations
in afferent organization that may provide clues to the location(s) of hair cells with
response characteristics consistent with vestibular functions (e.g., tilt perception) on
the saccule. In all toadfish, a small but distinct bundle of afferents (distinguished
from efferents by the presence of somata in the periphery) exited the rostral tip of
the saccule and merged with the anterior and horizontal canal cristae and utricular
inputs, rather than with the rostral bundle of saccular afferents. In some toadfish, a
small bundle of afferents exited vertically from the caudal saccule and joined VIII
with afferents from the lagena and posterior canal crista (see Fig. 2A in Edds-Walton
1998a). These anatomical data suggest that if hair cells with vestibular response
characteristics are present on the saccule, their distribution may be limited to the
most rostral and most caudal sites along the saccular epithelium.
3.2.2 The DON
The organization of the DON is interesting in that the pattern of inputs across species indicates that DON has both vestibular and auditory roles (McCormick 2011).
In toadfish, as in other teleosts, the dorsal regions receive inputs primarily from the
marily from the semicircular canal cristae (Highstein et al. 1992). Utricular afferents project to dorsolateral sites in DON (Highstein et al. 1992; unpublished data)
where they may overlap with saccular inputs, but there is also substantial input from
the utricle to the ventrolateral region (Highstein et al. 1992). The distribution of
utricular inputs in DON is consistent with both auditory and vestibular (orientation)
roles for that endorgan.
As noted previously, Fay (1984) provided physiological evidence that utricular
and lagenar afferents in goldfish responded to 140 Hz with directional response
characteristics consistent with the orientation of each endorgan and its hair cell
orientation pattern, providing evidence that both endorgans could contribute to
What the Toadfish Ear Tells the Toadfish Brain About Sound