Fitzpatrick et al. 1998). Different neural mechanisms may underlie processing of echolocation signals in different species of CF-FM bats and in
different classes of bats (e.g., FM bats), but this is not reviewed here (see
O’Neill 1995).
Spectral combination-sensitive neurons are also present in the ascending
auditory system of mustached bats at the level of the medial geniculate
body of the thalamus (MGB) (Olsen and Suga 1991a; Wenstrup 1999) but
have not been found in the inferior colliculus (O’Neill 1985). This led to the
hypothesis that the spectral combination sensitivity originated at the level
of the MGB as a result of converging inputs from inferior collicular (IC)
neurons tuned to specific frequencies in the sonar pulse and higher harmonics in the returning echoes. Recent findings, however, have challenged
the idea that the IC is the primary source of converging input to the MGB
(Wenstrup and Grose 1995), so additional study is clearly needed.
One potential source of combination-sensitive responses in the MGB is
descending cortical projections. Corticofugal projections originating in
layers V and VI of the auditory cortex project to both the IC and the MGB
as well as subcollicular auditory nuclei. These projections appear to maintain tonotopic organizations such that high-frequency tuned regions of the
cortex project to high-frequency tuned regions of the MGB and IC, for
example. Although it is clear that these projections enhance the tuning of
cortical cells through feedforward stimulation of frequency-matched subcortical units and the reduction of responses in unmatched subcortical units
(Suga et al. 1997; Zhang and Suga 2000), their role in the generation of combination-sensitive responses has only recently come under investigation.
The complexity of this corticofugal system is compounded by the presence
of both direct excitatory connections and inhibitory connections via the
thalamic reticular formation (see Wenstrup 1999).
2.1.3.2. Temporal Combination Sensitivity
Temporal combination-sensitive (TCS) neurons have been well-described
in bats and birds, and there is some evidence for TCS neurons in other
systems (primates: Olsen 1994; cats: Brosch and Schreiner 2000; Gehr et al.
2000). The behavioral requirements for temporal combination sensitivity
differ dramatically for songbirds and bats, and the observed TCS response
properties are tightly linked to the behaviors. This suggests that in systems
where the animal’s behavioral requirements are not sufficiently understood,
it might be possible to overlook neurons with a TCS response for lack of
use of the appropriate stimulus.
In mustached bats, TCS neurons have been described in the ascending
auditory system, starting at the level of the inferior colliculus (Mittmann
and Wenstrup 1995; Leroy and Wenstrup 2000) and continuing in the medial
geniculate nucleus (Olsen and Suga 1991b; Wenstrup 1999) and in multiple
cortical areas (e.g., Suga et al. 1978; Suga and Horikawa 1986; Taniguchi et
338
T.Q. Gentner and D. Margoliash
different classes of bats (e.g., FM bats), but this is not reviewed here (see
O’Neill 1995).
Spectral combination-sensitive neurons are also present in the ascending
auditory system of mustached bats at the level of the medial geniculate
body of the thalamus (MGB) (Olsen and Suga 1991a; Wenstrup 1999) but
have not been found in the inferior colliculus (O’Neill 1985). This led to the
hypothesis that the spectral combination sensitivity originated at the level
of the MGB as a result of converging inputs from inferior collicular (IC)
neurons tuned to specific frequencies in the sonar pulse and higher harmonics in the returning echoes. Recent findings, however, have challenged
the idea that the IC is the primary source of converging input to the MGB
(Wenstrup and Grose 1995), so additional study is clearly needed.
One potential source of combination-sensitive responses in the MGB is
descending cortical projections. Corticofugal projections originating in
layers V and VI of the auditory cortex project to both the IC and the MGB
as well as subcollicular auditory nuclei. These projections appear to maintain tonotopic organizations such that high-frequency tuned regions of the
cortex project to high-frequency tuned regions of the MGB and IC, for
example. Although it is clear that these projections enhance the tuning of
cortical cells through feedforward stimulation of frequency-matched subcortical units and the reduction of responses in unmatched subcortical units
(Suga et al. 1997; Zhang and Suga 2000), their role in the generation of combination-sensitive responses has only recently come under investigation.
The complexity of this corticofugal system is compounded by the presence
of both direct excitatory connections and inhibitory connections via the
thalamic reticular formation (see Wenstrup 1999).
2.1.3.2. Temporal Combination Sensitivity
Temporal combination-sensitive (TCS) neurons have been well-described
in bats and birds, and there is some evidence for TCS neurons in other
systems (primates: Olsen 1994; cats: Brosch and Schreiner 2000; Gehr et al.
2000). The behavioral requirements for temporal combination sensitivity
differ dramatically for songbirds and bats, and the observed TCS response
properties are tightly linked to the behaviors. This suggests that in systems
where the animal’s behavioral requirements are not sufficiently understood,
it might be possible to overlook neurons with a TCS response for lack of
use of the appropriate stimulus.
In mustached bats, TCS neurons have been described in the ascending
auditory system, starting at the level of the inferior colliculus (Mittmann
and Wenstrup 1995; Leroy and Wenstrup 2000) and continuing in the medial
geniculate nucleus (Olsen and Suga 1991b; Wenstrup 1999) and in multiple
cortical areas (e.g., Suga et al. 1978; Suga and Horikawa 1986; Taniguchi et
338
T.Q. Gentner and D. Margoliash