Lateral Line Research: the Importance of Using Natural Stimuli
153
Using a dipole stimulus one can learn, for instance, that the working range of
lateral line neuromasts covers frequencies from less than 1 Hz up to about 15 0 Hz
(Kalmijn 1989), that primary lateral line afferents have small receptive fields, that
displacement thresholds are as low as 0.01 mm, that superficial neuromasts rank
between velocity and acceleration detectors, and that the lateral line canal system
functions as an acceleration detector (e.g., Schellart and Wubbels 1998; Coombs
and Janssen 1989). In addition we can learn (1) that primary afferents phase lock
to sinusoidal stimuli, (2) that their dynamic amplitude range covers up to 90 dB,
(3) that their sustained responses encode stimulus duration (Fig. 1) and (4) that the
responses of primary afferents change cosine-like with stimulus direction and
quickly diminish with source distance (Bleckmann 1994).
4.2 Central Physiology
Compared with primary afferents, the physiological properties of central lateral
line units are different in many respects. Ongoing activity is low or absent and
responses to constant-amplitude dipole stimuli are usually rapidly adapting (e.g.
Bleckmann and Bullock 1989). Receptive fields are generally larger but their size
may also be similar to receptive fields of primary afferents (Schellart and Wubbels
1998; Fig. 2). If stimulated with a vibrating sphere many central units are remarkably insensitive, 26-30 % of all MON units (Krother, unpubl.; Mogdans and
Goenechea 1999) and I 0% of all units recorded in the torus semicircularis
(Plachta et al. 1999) do not respond to a dipole stimulus. Central units which do
respond to a dipole stimulus show no or only a weak phase locking (Bleckmann
and Bullock 1989; Plachta et al. 1999). Their frequency response usually is similar
to that of primary afferents and barely changes along the neuraxis. Sharply tuned
toral lateral line units have been found, however (MUller et al. 1996). Threshold
sensitivity and dynamic amplitude range of central lateral line units decreases from
medulla to telencephalon (Bleckmann and Bullock 1989) but the amount of data is
not sufficient to make this a general statement.
Experiments with dipole stimuli, no doubt, have uncovered many important
physiological properties of the lateral line. This approach has failed, however, to
fmd a species-specific form-function relationship of the peripheral lateral line
(Coombs et al. 1988). Dipole stimuli did reveal some specific response properties
of central units as, for instance, in the degree of phase coupling, dynamic amplitude range, or receptive field organization (e.g., Bleckmann and Bullock 1989;
Coombs et al. 1998). However, the shape of threshold curves obtained from central units recorded at different levels of the neuraxis (Bleckmann and Bullock
1989; Bleckmann and Mlinz 1990) is similar to the uniform shape of primary
afferent threshold curves. Thus, using vibrating-sphere stimuli, neither specialized
peripheral form-function relationships nor the emergence of highly specific
response properties of units recorded along the neuraxis have been found.
153
Using a dipole stimulus one can learn, for instance, that the working range of
lateral line neuromasts covers frequencies from less than 1 Hz up to about 15 0 Hz
(Kalmijn 1989), that primary lateral line afferents have small receptive fields, that
displacement thresholds are as low as 0.01 mm, that superficial neuromasts rank
between velocity and acceleration detectors, and that the lateral line canal system
functions as an acceleration detector (e.g., Schellart and Wubbels 1998; Coombs
and Janssen 1989). In addition we can learn (1) that primary afferents phase lock
to sinusoidal stimuli, (2) that their dynamic amplitude range covers up to 90 dB,
(3) that their sustained responses encode stimulus duration (Fig. 1) and (4) that the
responses of primary afferents change cosine-like with stimulus direction and
quickly diminish with source distance (Bleckmann 1994).
4.2 Central Physiology
Compared with primary afferents, the physiological properties of central lateral
line units are different in many respects. Ongoing activity is low or absent and
responses to constant-amplitude dipole stimuli are usually rapidly adapting (e.g.
Bleckmann and Bullock 1989). Receptive fields are generally larger but their size
may also be similar to receptive fields of primary afferents (Schellart and Wubbels
1998; Fig. 2). If stimulated with a vibrating sphere many central units are remarkably insensitive, 26-30 % of all MON units (Krother, unpubl.; Mogdans and
Goenechea 1999) and I 0% of all units recorded in the torus semicircularis
(Plachta et al. 1999) do not respond to a dipole stimulus. Central units which do
respond to a dipole stimulus show no or only a weak phase locking (Bleckmann
and Bullock 1989; Plachta et al. 1999). Their frequency response usually is similar
to that of primary afferents and barely changes along the neuraxis. Sharply tuned
toral lateral line units have been found, however (MUller et al. 1996). Threshold
sensitivity and dynamic amplitude range of central lateral line units decreases from
medulla to telencephalon (Bleckmann and Bullock 1989) but the amount of data is
not sufficient to make this a general statement.
Experiments with dipole stimuli, no doubt, have uncovered many important
physiological properties of the lateral line. This approach has failed, however, to
fmd a species-specific form-function relationship of the peripheral lateral line
(Coombs et al. 1988). Dipole stimuli did reveal some specific response properties
of central units as, for instance, in the degree of phase coupling, dynamic amplitude range, or receptive field organization (e.g., Bleckmann and Bullock 1989;
Coombs et al. 1998). However, the shape of threshold curves obtained from central units recorded at different levels of the neuraxis (Bleckmann and Bullock
1989; Bleckmann and Mlinz 1990) is similar to the uniform shape of primary
afferent threshold curves. Thus, using vibrating-sphere stimuli, neither specialized
peripheral form-function relationships nor the emergence of highly specific
response properties of units recorded along the neuraxis have been found.