211
pare the relative sensitivity of the saccule and the utricle to particle motion at auditory frequencies. In addition, Vasconcelos removed the otolith from the endorgan
(unilaterally or bilaterally) to assess the role of the saccule and the utricle in body
posture and movement following recovery from the surgery. The results of those
behavioral experiments indicated that the utricle functions both as an auditory and
vestibular endorgan in the Lusitanian toadfish (Vasconcelos et al. 2012). Although
the auditory responsiveness for the saccule was consistent with previous work on
auditory processing in this species, the results of the postural experiments were less
clear, and additional research is being done. Therefore, although likely that the toadfish brain receives both auditory and vestibular information from the utricle and the
saccule, more work is required to delineate the specific contributions of each to
orientation, auditory processing, and sound source localization.
The third otolithic endorgan, the lagena, is the smallest of the otolithic endorgans
in toadfishes, difficult to access in vivo, and its association with the posterior semicircular canal indicates a vestibular role. In goldfishes, the lagena is nearly equivalent in area to the saccule (Edds-Walton and Popper 2000), lies directly adjacent to
the saccule (Platt 1977), and the nerve bundle from the lagena joins the saccular
bundle prior to joining the other components of VIII, all of which are consistent
with a similar sensory role for the saccule and lagena in that species (for physiological comparisons, see Coombs et al. 2010; Dailey and Braun 2011). Therefore, clarification of the role of the lagena in toadfishes would be of interest, but it was not
included in any of the studies by Fay and Edds-Walton.
As described above (Sect. 3.2.1), saccular afferents often occur in bundles that
reflect their site of origin on the epithelium: rostral, middle, caudal. Fay and EddsWalton (1997a, b; Edds-Walton et al. 1999) evaluated afferents from those regions
of the saccule and concluded that (1) in general, primary afferents are broadly tuned
with most afferents responding best to the 100 Hz stimulus; (2) there is no evidence
for a frequency map along the rostral-caudal axis of the saccule; (3) the majority of
afferents are directional in that their responses reflect responses from a single hair
cell orientation (producing a cosine response almost identical to that of a single hair
cell; Fig. 1a); (4) the saccule provides information about the axis of stimulation for
all angles in the vertical plane (0–90°), consistent with the hair cell orientation pattern described by Edds-Walton and Popper (1995); in the horizontal plane, responses
are consistent with the orientation of the saccule in the otic capsule (Fig. 2a).
The variations in the saccular data also provided interesting insights into the
auditory system. The best response directions (best stimulus axis) among saccular
afferents that were filled with neurobiotin (to identify the location of their dendritic
arbors on the saccular epithelium) did not always coincide with the predicted best
direction based on the hair cell orientation drawing. Edds-Walton et al. (1999) provided evidence that the epithelium does not lie flat against the otolith, and the unexpected best directions (in particular, low elevations of cells along the edge of the
epithelium) are likely to be due to curvature of the epithelium where it lies along the
sculptured otolith (see Fig. 1c). Fay and Edds-Walton (1997a) also noted that some
afferents (about 20 %) are nearly omnidirectional, lacking a null in the directional
response plot. They hypothesized that those afferents contacted hair cells with two
different orientations, based on a simple model (see Fig. 11 in Fay and Edds-Walton
What the Toadfish Ear Tells the Toadfish Brain About Sound
pare the relative sensitivity of the saccule and the utricle to particle motion at auditory frequencies. In addition, Vasconcelos removed the otolith from the endorgan
(unilaterally or bilaterally) to assess the role of the saccule and the utricle in body
posture and movement following recovery from the surgery. The results of those
behavioral experiments indicated that the utricle functions both as an auditory and
vestibular endorgan in the Lusitanian toadfish (Vasconcelos et al. 2012). Although
the auditory responsiveness for the saccule was consistent with previous work on
auditory processing in this species, the results of the postural experiments were less
clear, and additional research is being done. Therefore, although likely that the toadfish brain receives both auditory and vestibular information from the utricle and the
saccule, more work is required to delineate the specific contributions of each to
orientation, auditory processing, and sound source localization.
The third otolithic endorgan, the lagena, is the smallest of the otolithic endorgans
in toadfishes, difficult to access in vivo, and its association with the posterior semicircular canal indicates a vestibular role. In goldfishes, the lagena is nearly equivalent in area to the saccule (Edds-Walton and Popper 2000), lies directly adjacent to
the saccule (Platt 1977), and the nerve bundle from the lagena joins the saccular
bundle prior to joining the other components of VIII, all of which are consistent
with a similar sensory role for the saccule and lagena in that species (for physiological comparisons, see Coombs et al. 2010; Dailey and Braun 2011). Therefore, clarification of the role of the lagena in toadfishes would be of interest, but it was not
included in any of the studies by Fay and Edds-Walton.
As described above (Sect. 3.2.1), saccular afferents often occur in bundles that
reflect their site of origin on the epithelium: rostral, middle, caudal. Fay and EddsWalton (1997a, b; Edds-Walton et al. 1999) evaluated afferents from those regions
of the saccule and concluded that (1) in general, primary afferents are broadly tuned
with most afferents responding best to the 100 Hz stimulus; (2) there is no evidence
for a frequency map along the rostral-caudal axis of the saccule; (3) the majority of
afferents are directional in that their responses reflect responses from a single hair
cell orientation (producing a cosine response almost identical to that of a single hair
cell; Fig. 1a); (4) the saccule provides information about the axis of stimulation for
all angles in the vertical plane (0–90°), consistent with the hair cell orientation pattern described by Edds-Walton and Popper (1995); in the horizontal plane, responses
are consistent with the orientation of the saccule in the otic capsule (Fig. 2a).
The variations in the saccular data also provided interesting insights into the
auditory system. The best response directions (best stimulus axis) among saccular
afferents that were filled with neurobiotin (to identify the location of their dendritic
arbors on the saccular epithelium) did not always coincide with the predicted best
direction based on the hair cell orientation drawing. Edds-Walton et al. (1999) provided evidence that the epithelium does not lie flat against the otolith, and the unexpected best directions (in particular, low elevations of cells along the edge of the
epithelium) are likely to be due to curvature of the epithelium where it lies along the
sculptured otolith (see Fig. 1c). Fay and Edds-Walton (1997a) also noted that some
afferents (about 20 %) are nearly omnidirectional, lacking a null in the directional
response plot. They hypothesized that those afferents contacted hair cells with two
different orientations, based on a simple model (see Fig. 11 in Fay and Edds-Walton
What the Toadfish Ear Tells the Toadfish Brain About Sound