156
Horst Bleckmann et al.
physiological experiments and the emergence of highly selective . response
properties at higher brain levels.
Anatomical studies (Alexandre and Ghysen 1999) and studies in which water
surface waves have been used as a stimulus (Zittlau et al. 1986) clearly show that
there are at least some central lateral line maps. In all bony fishes and in amphibians primary lateral line afferents distribute such in the medulla that fibers from
the anterior lateral line nerve are represented ventromedially and fibers from the
posterior lateral line nerve dorsolaterally (e.g., McCormick 1989; Song and Northcutt 1991 ). The projections from the anterior and posterior lines run parallel but do
not mix. Moreover, the position of neuromasts is represented in the MON in that
the projection of anterior neuromast lies ventrolateral to the projections of more
posterior neuromast. The same applies to the projections of the posterior lateral
line (Alexandre and Ghysen 1999). Thus, the primary projections of the lateral
line of fishes are doubly somatotopic. There may also be a crude somatotopy with
the dorsolateral and ventrolateral surfaces of the trunk represented ventrally and
dorsally, respectively, in the MON of some fishes (Song and Northcutt 1991).
There are no indications that superficial and canal neuromasts map differentially in
the MON or that vertically and horizontally oriented neuromasts are mapped
separately (Song and Northcutt 1991). Whether the two hair cell populations in a
neuromast map differentially is also not clear (Fritzsch 1989).
What are the reasons why lateral line research does not match the progress made
in the analysis of information processing in other sensory systems? One possibility
is, of course, that many types of information processing known from other sensory
systems (e.g. central maps, parallel pathways, highly selective central units) are
rare or even do not exist in the fish lateral line system. This is unlikely, however.
For instance, in amphibians a central computed lateral line map has been
demonstrated. Tectallateralline units of the clawed frog, Xenopus laevis, and the
axolotl, Ambystoma mexicanum, encode the direction of propagation of quasi
natural surface wave stimuli (Zittlau et al. 1986; Bartels et al. 1990). Clearly, with
dipole stimuli tecta! directional maps would not have been discovered.
7 Natural Hydrodynamic Stimuli
When we evaluate lateral line research we should keep in mind that background
noise was always kept low and that the applied stimuli were usually quite artificial. In rivers and creeks, as well as along the ocean shoreline, the water constantly moves and even in seemingly quiet lakes some water movements may be
present. Furthermore, even if the water does not move, the fish may move, or both.
Thus, in natural environments the lateral line rarely faces stillwater conditions. In
addition, animate sources of natural hydrodynamic stimuli rarely vibrate with
constant frequency and amplitude.
Horst Bleckmann et al.
physiological experiments and the emergence of highly selective . response
properties at higher brain levels.
Anatomical studies (Alexandre and Ghysen 1999) and studies in which water
surface waves have been used as a stimulus (Zittlau et al. 1986) clearly show that
there are at least some central lateral line maps. In all bony fishes and in amphibians primary lateral line afferents distribute such in the medulla that fibers from
the anterior lateral line nerve are represented ventromedially and fibers from the
posterior lateral line nerve dorsolaterally (e.g., McCormick 1989; Song and Northcutt 1991 ). The projections from the anterior and posterior lines run parallel but do
not mix. Moreover, the position of neuromasts is represented in the MON in that
the projection of anterior neuromast lies ventrolateral to the projections of more
posterior neuromast. The same applies to the projections of the posterior lateral
line (Alexandre and Ghysen 1999). Thus, the primary projections of the lateral
line of fishes are doubly somatotopic. There may also be a crude somatotopy with
the dorsolateral and ventrolateral surfaces of the trunk represented ventrally and
dorsally, respectively, in the MON of some fishes (Song and Northcutt 1991).
There are no indications that superficial and canal neuromasts map differentially in
the MON or that vertically and horizontally oriented neuromasts are mapped
separately (Song and Northcutt 1991). Whether the two hair cell populations in a
neuromast map differentially is also not clear (Fritzsch 1989).
What are the reasons why lateral line research does not match the progress made
in the analysis of information processing in other sensory systems? One possibility
is, of course, that many types of information processing known from other sensory
systems (e.g. central maps, parallel pathways, highly selective central units) are
rare or even do not exist in the fish lateral line system. This is unlikely, however.
For instance, in amphibians a central computed lateral line map has been
demonstrated. Tectallateralline units of the clawed frog, Xenopus laevis, and the
axolotl, Ambystoma mexicanum, encode the direction of propagation of quasi
natural surface wave stimuli (Zittlau et al. 1986; Bartels et al. 1990). Clearly, with
dipole stimuli tecta! directional maps would not have been discovered.
7 Natural Hydrodynamic Stimuli
When we evaluate lateral line research we should keep in mind that background
noise was always kept low and that the applied stimuli were usually quite artificial. In rivers and creeks, as well as along the ocean shoreline, the water constantly moves and even in seemingly quiet lakes some water movements may be
present. Furthermore, even if the water does not move, the fish may move, or both.
Thus, in natural environments the lateral line rarely faces stillwater conditions. In
addition, animate sources of natural hydrodynamic stimuli rarely vibrate with
constant frequency and amplitude.