220
which further indicates that the computations are the result of a variety of excitatory
and inhibitory interactions among the various sources of input to the TS.
Edds-Walton and Fay (2009) demonstrated that binaural cells result from the
convergence of excitatory (EE) and inhibitory (EI or IE) inputs to cells in both DON
and NC (and see Edds-Walton et al. 2010). Therefore, there are potential sites in
fishes for bilateral comparisons that may function similarly to binaural sites in the
ascending auditory pathway of other vertebrates (e.g., nucleus laminaris in birds or
nuclei of the superior olivary complex in mammals).
The otolith tipping experiments described for cells in the DON (Sect. 3.4.1) were
also conducted with cells in the TS. The only otolithic input that was altered during
these experiments was from the saccule. As in the DON, altering saccular input
altered the DRPs and/or spiking activity of cells in the TS (Edds-Walton and Fay
2009). Unlike the DON, spike activity was rarely eliminated in a TS cell during tipping, consistent with complex interactions of converging inputs from various
sources.
Lastly, plotting the characteristic axis for cells in the TS on the flattened globe
(as in Fig. 6) provided confirmation that acoustic space around the fish is well represented in the midbrain as it is in DON (Edds-Walton and Fay 2003). Given that the
directionality of auditory cells improves (becomes sharpened) along the ascending
auditory pathway to the level of the midbrain, Edds-Walton and Fay (2005b) concluded that encoding the location of a sound source is an important component of
auditory processing in the toadfish.
4 Conclusions
Studies in the oyster toadfish combined anatomical tract-tracing and physiological
recordings from identified sites on the saccule, in the DON, and in the torus semicircularis to document auditory processing at each level of the ascending auditory pathway. The toadfish saccule encodes frequency, sound level, and sound source direction
in phase-locked activity conveyed via auditory afferents ipsilaterally to nuclei of the
octaval column. The large DON plays a major role in auditory processing and contributes bilaterally to the ascending auditory circuit. Binaural convergence of auditory information provides the fish with information about sound sources at locations
all around the fish. Furthermore, a series of related physiological studies showed that
the auditory system of the toadfish consistently encodes frequency, temporal pattern,
sound level, and the axis of particle motion for sound sources. The response characteristics in the midbrain indicate that frequency tuning tends to be broad with little
narrowing of the frequency response in the ascending auditory circuit. However,
computations in the medulla and the midbrain narrow directional responses and
should allow the fish to locate a vocalizing conspecific, to determine the locations of
multiple sound sources around the fish based on direction and relative sound levels,
and, in general, evaluate the soundscape. The toadfish ear tells the toadfish brain
“what” and “where”—we are just beginning to understand “how.”
P.L. Edds-Walton
which further indicates that the computations are the result of a variety of excitatory
and inhibitory interactions among the various sources of input to the TS.
Edds-Walton and Fay (2009) demonstrated that binaural cells result from the
convergence of excitatory (EE) and inhibitory (EI or IE) inputs to cells in both DON
and NC (and see Edds-Walton et al. 2010). Therefore, there are potential sites in
fishes for bilateral comparisons that may function similarly to binaural sites in the
ascending auditory pathway of other vertebrates (e.g., nucleus laminaris in birds or
nuclei of the superior olivary complex in mammals).
The otolith tipping experiments described for cells in the DON (Sect. 3.4.1) were
also conducted with cells in the TS. The only otolithic input that was altered during
these experiments was from the saccule. As in the DON, altering saccular input
altered the DRPs and/or spiking activity of cells in the TS (Edds-Walton and Fay
2009). Unlike the DON, spike activity was rarely eliminated in a TS cell during tipping, consistent with complex interactions of converging inputs from various
sources.
Lastly, plotting the characteristic axis for cells in the TS on the flattened globe
(as in Fig. 6) provided confirmation that acoustic space around the fish is well represented in the midbrain as it is in DON (Edds-Walton and Fay 2003). Given that the
directionality of auditory cells improves (becomes sharpened) along the ascending
auditory pathway to the level of the midbrain, Edds-Walton and Fay (2005b) concluded that encoding the location of a sound source is an important component of
auditory processing in the toadfish.
4 Conclusions
Studies in the oyster toadfish combined anatomical tract-tracing and physiological
recordings from identified sites on the saccule, in the DON, and in the torus semicircularis to document auditory processing at each level of the ascending auditory pathway. The toadfish saccule encodes frequency, sound level, and sound source direction
in phase-locked activity conveyed via auditory afferents ipsilaterally to nuclei of the
octaval column. The large DON plays a major role in auditory processing and contributes bilaterally to the ascending auditory circuit. Binaural convergence of auditory information provides the fish with information about sound sources at locations
all around the fish. Furthermore, a series of related physiological studies showed that
the auditory system of the toadfish consistently encodes frequency, temporal pattern,
sound level, and the axis of particle motion for sound sources. The response characteristics in the midbrain indicate that frequency tuning tends to be broad with little
narrowing of the frequency response in the ascending auditory circuit. However,
computations in the medulla and the midbrain narrow directional responses and
should allow the fish to locate a vocalizing conspecific, to determine the locations of
multiple sound sources around the fish based on direction and relative sound levels,
and, in general, evaluate the soundscape. The toadfish ear tells the toadfish brain
“what” and “where”—we are just beginning to understand “how.”
P.L. Edds-Walton