263
7 “Hearing” as a Physiological Response
While some fi eld studies have assessed hearing in free-fi eld situations (e.g. Chapman
1973 ; Chapman and Hawkins 1973 ; Chapman and Sand 1974 ), the vast majority of
especially physiological work has tested “hearing” in the lab environment. Sound
travel in laboratory tanks is notoriously complex (Akamatsu et al. 2002 ) and likely
contains particle motion information well beyond what is typically thought of as the
nearfi eld limit. A common physiological measure of fi sh “hearing” is the use of
auditory evoked potentials (AEP, previously called auditory brainstem response or
ABR). In this technique a fi sh is restrained under or at the surface of the water and
a recording electrode implanted somewhere over the brainstem (Corwin et al. 1982 ;
Kenyon 1996 ; Kenyon et al. 1998 ). In response to “sound” the waveform of brain
activity will change in defi nable ways and the disappearance of this change as sound
level decreases is taken to represent some sort of “auditory threshold”. This technique is attractive to researchers because it is a fairly simple way to determine what
sounds fi sh can detect and AEP can be quite useful in a comparative context to test
how sound detection changes ontogenetically (e.g. Kenyon 1996 ; Higgs et al. 2002,
2003; Caiger et al. 2013 ), how experimental manipulations can affect detection (e.g.
Yan et al. 2000 ; Radford et al. 2012 , 2013 ; Higgs and Radford 2013 ), and to examine the bandwidth of detection between different species of interest (e.g. Corwin
et al. 1982; Kenyon et al. 1998 ; Niemiller et al. 2013 ). While we have both used this
technique extensively (e.g. Higgs et al. 2002 ; Radford et al. 2012 , 2013 ; Higgs and
Radford 2013 ) and recognize its utility in examination of sound detection, we no
longer feel it can be used as an accurate test of hearing sensu strictu . The purpose of
the present synopsis is not to review all the AEP/ABR papers that have been published, as that has been effectively done elsewhere (Ladich and Fay 2013 ), but rather
to review the limited available evidence for the dual roles of the ear and lateral line
in physiological responses to “acoustic” stimulation. The primary innervation sites
for both auditory and lateral line nerves lie in close proximity in the brainstem
(McCormick 1999 ; Higgs et al. 2006) and the same AEP recording setup that is
commonly used in “hearing” studies has recently been used to measure direct lateral
line stimulation (Brack and Ramcharitar 2012 ), therefore it seems likely that past
AEP/ABR studies have been detecting responses of both systems. While there
exists behavioural evidence that the lateral line can play a role in “acoustic”
responses (see below), to our knowledge the only examination of the role of the
lateral line in AEP responses is Higgs and Radford ( 2013 ). In that paper we showed
that canal neuromasts play a role in “acoustic” thresholds previously attributed
solely to the ear and that detection of sound stimuli in experimental tanks is likely
an integrative response of both the ear and the lateral line, at least at low frequencies
(<400 Hz). Direct recording from lateral line afferents in response to “acoustic”
stimulation also shows that neuromasts can directly detect conspecifi c calls (Weeg
and Bass 2002 ; Radford and Mensinger 2014 ) and can aid in localization of these
calls in free-swimming fi sh (Radford and Mensinger 2014 ). The highlighted evidence clearly demonstrates that what was typically considered “hearing” may often
The Potential Overlapping Roles of the Ear and Lateral Line in Driving “Acoustic”…
7 “Hearing” as a Physiological Response
While some fi eld studies have assessed hearing in free-fi eld situations (e.g. Chapman
1973 ; Chapman and Hawkins 1973 ; Chapman and Sand 1974 ), the vast majority of
especially physiological work has tested “hearing” in the lab environment. Sound
travel in laboratory tanks is notoriously complex (Akamatsu et al. 2002 ) and likely
contains particle motion information well beyond what is typically thought of as the
nearfi eld limit. A common physiological measure of fi sh “hearing” is the use of
auditory evoked potentials (AEP, previously called auditory brainstem response or
ABR). In this technique a fi sh is restrained under or at the surface of the water and
a recording electrode implanted somewhere over the brainstem (Corwin et al. 1982 ;
Kenyon 1996 ; Kenyon et al. 1998 ). In response to “sound” the waveform of brain
activity will change in defi nable ways and the disappearance of this change as sound
level decreases is taken to represent some sort of “auditory threshold”. This technique is attractive to researchers because it is a fairly simple way to determine what
sounds fi sh can detect and AEP can be quite useful in a comparative context to test
how sound detection changes ontogenetically (e.g. Kenyon 1996 ; Higgs et al. 2002,
2003; Caiger et al. 2013 ), how experimental manipulations can affect detection (e.g.
Yan et al. 2000 ; Radford et al. 2012 , 2013 ; Higgs and Radford 2013 ), and to examine the bandwidth of detection between different species of interest (e.g. Corwin
et al. 1982; Kenyon et al. 1998 ; Niemiller et al. 2013 ). While we have both used this
technique extensively (e.g. Higgs et al. 2002 ; Radford et al. 2012 , 2013 ; Higgs and
Radford 2013 ) and recognize its utility in examination of sound detection, we no
longer feel it can be used as an accurate test of hearing sensu strictu . The purpose of
the present synopsis is not to review all the AEP/ABR papers that have been published, as that has been effectively done elsewhere (Ladich and Fay 2013 ), but rather
to review the limited available evidence for the dual roles of the ear and lateral line
in physiological responses to “acoustic” stimulation. The primary innervation sites
for both auditory and lateral line nerves lie in close proximity in the brainstem
(McCormick 1999 ; Higgs et al. 2006) and the same AEP recording setup that is
commonly used in “hearing” studies has recently been used to measure direct lateral
line stimulation (Brack and Ramcharitar 2012 ), therefore it seems likely that past
AEP/ABR studies have been detecting responses of both systems. While there
exists behavioural evidence that the lateral line can play a role in “acoustic”
responses (see below), to our knowledge the only examination of the role of the
lateral line in AEP responses is Higgs and Radford ( 2013 ). In that paper we showed
that canal neuromasts play a role in “acoustic” thresholds previously attributed
solely to the ear and that detection of sound stimuli in experimental tanks is likely
an integrative response of both the ear and the lateral line, at least at low frequencies
(<400 Hz). Direct recording from lateral line afferents in response to “acoustic”
stimulation also shows that neuromasts can directly detect conspecifi c calls (Weeg
and Bass 2002 ; Radford and Mensinger 2014 ) and can aid in localization of these
calls in free-swimming fi sh (Radford and Mensinger 2014 ). The highlighted evidence clearly demonstrates that what was typically considered “hearing” may often
The Potential Overlapping Roles of the Ear and Lateral Line in Driving “Acoustic”…