218
nerve. Note also the relatively small number of cells with best directions directly
above the fish, which is consistent with the relatively small and inaccessible area of
the saccule, the middle, where purely vertical cells are located (Fig. 1b). The best
directions for afferents from the caudal saccule overlap the locations of afferents
from the rostral saccule (see Edds-Walton et al. 1999 for details on the caudal afferents). In contrast, the directional plot for DON cells reflects the acoustic space all
around the fish. There are two potential ways to achieve this difference: convergence
of inputs from the left and right saccules or convergence of input from other ipsilateral otolithic endorgans. At present, anatomical data can be used to support either
circuit, but there are physiological data to support the convergence of left and right
saccular data in the DON (and the TS, see Sect. 3.4.3).
Edds-Walton and Fay (2009) argued that the distribution of responses in DON
could be the result of convergence of left and right saccular inputs based on the following data. Although saccular afferents do not cross the midline to the contralateral DON, there is a topographic tract that connects the dorsal divisions of the left
and right DONs (Edds-Walton 1998b). In a unique study, Edds-Walton and Fay
(2009) altered saccular inputs (by tipping one of the two saccular otoliths) while
recording from directional auditory cells in DON. Data were difficult to obtain
because three complete sets of frequency and directional data were needed: Pretipping, Tipping, and Post-tipping data (with results consistent with the pre-tipping
data). Ipsilateral tipping confirmed that the method worked and was reversible.
Often tipping eliminated spike activity in an ipsilateral or contralateral DON cell,
which was consistent with removal of essential excitatory input from the saccule.
Most importantly, Edds-Walton and Fay (2009) showed that altering contralateral
inputs can alter the DRP (and sometimes frequency response) of a cell in DON. As
was apparent in the sharpening analyses, there were a range of differences in the
DRP during tipping, consistent with a variety of computations. The data clearly
show that contralateral saccular input contributes to the computations that occur in
DON (Edds-Walton and Fay 2009), and therefore, at least some of the computations
in DON are binaural.
3.4.2 Other Targets of the Saccule in the Octaval Column
As noted earlier, MON and TON do not appear to be involved in the ascending auditory circuit in toadfish (consistent with other teleosts, McCormick 1999, 2011), but
both receive input from the saccule as well as the utricle, lagena, and canal cristae.
The saccular input to MON and TON is of interest because of the potential for dual
function of the saccule as an auditory and vestibular (gravistatic) endorgan (Platt
and Popper 1981). Highstein et al. (1992) suggested the MON as the origin of the
ipsilateral descending vestibulospinal tract, which is consistent with a role in gravistatic orientation. Physiological recordings in MON confirmed that a subset of cells
(in two of the three subdivisions designated by Highstein et al. 1992) respond well
and phase-lock to auditory frequencies (Edds-Walton et al. 2013). The TON receives
heavy input from all known vestibular structures and very little input from the
P.L. Edds-Walton
nerve. Note also the relatively small number of cells with best directions directly
above the fish, which is consistent with the relatively small and inaccessible area of
the saccule, the middle, where purely vertical cells are located (Fig. 1b). The best
directions for afferents from the caudal saccule overlap the locations of afferents
from the rostral saccule (see Edds-Walton et al. 1999 for details on the caudal afferents). In contrast, the directional plot for DON cells reflects the acoustic space all
around the fish. There are two potential ways to achieve this difference: convergence
of inputs from the left and right saccules or convergence of input from other ipsilateral otolithic endorgans. At present, anatomical data can be used to support either
circuit, but there are physiological data to support the convergence of left and right
saccular data in the DON (and the TS, see Sect. 3.4.3).
Edds-Walton and Fay (2009) argued that the distribution of responses in DON
could be the result of convergence of left and right saccular inputs based on the following data. Although saccular afferents do not cross the midline to the contralateral DON, there is a topographic tract that connects the dorsal divisions of the left
and right DONs (Edds-Walton 1998b). In a unique study, Edds-Walton and Fay
(2009) altered saccular inputs (by tipping one of the two saccular otoliths) while
recording from directional auditory cells in DON. Data were difficult to obtain
because three complete sets of frequency and directional data were needed: Pretipping, Tipping, and Post-tipping data (with results consistent with the pre-tipping
data). Ipsilateral tipping confirmed that the method worked and was reversible.
Often tipping eliminated spike activity in an ipsilateral or contralateral DON cell,
which was consistent with removal of essential excitatory input from the saccule.
Most importantly, Edds-Walton and Fay (2009) showed that altering contralateral
inputs can alter the DRP (and sometimes frequency response) of a cell in DON. As
was apparent in the sharpening analyses, there were a range of differences in the
DRP during tipping, consistent with a variety of computations. The data clearly
show that contralateral saccular input contributes to the computations that occur in
DON (Edds-Walton and Fay 2009), and therefore, at least some of the computations
in DON are binaural.
3.4.2 Other Targets of the Saccule in the Octaval Column
As noted earlier, MON and TON do not appear to be involved in the ascending auditory circuit in toadfish (consistent with other teleosts, McCormick 1999, 2011), but
both receive input from the saccule as well as the utricle, lagena, and canal cristae.
The saccular input to MON and TON is of interest because of the potential for dual
function of the saccule as an auditory and vestibular (gravistatic) endorgan (Platt
and Popper 1981). Highstein et al. (1992) suggested the MON as the origin of the
ipsilateral descending vestibulospinal tract, which is consistent with a role in gravistatic orientation. Physiological recordings in MON confirmed that a subset of cells
(in two of the three subdivisions designated by Highstein et al. 1992) respond well
and phase-lock to auditory frequencies (Edds-Walton et al. 2013). The TON receives
heavy input from all known vestibular structures and very little input from the
P.L. Edds-Walton