Lateral Line Research: the Importance of Using Natural Stimuli
155
5 What Can We Learn from Other Sensory Systems?
If we look for commonalities in the processing by other sensory systems we
repeatedly fmd (I) a separation of sensory channels, (2) a systematic representation of stimulus parameters along central maps and (3) the emergence of more
and more selective responses to specific, biologically relevant stimuli as one
ascends to higher brain levels. Examples for the separation of sensory channels include the amplitude and phase channels in auditory (e.g., Konishi 1986) and
electrosensory systems (e.g., Heiligenberg 1991), the parvocellu1ar and magnocellular channels in the primate visual system (e.g., DeYoe and van Essen 1988),
the cold and warmth channels in the somatosensory system (Hensel1974) and the
pheromonal and ordinary smell channels in the brains of insects and vertebrates
(Hildebrand 1995). Sensory maps often show a relation to the spatial order of the
receptors. We know topographic projections from the somatosensory (Catania and
Kaas 1995), electrosensory (Bodznick and Schmidt 1984; Heiligenberg 1991),
visual (Vanegas et al. 1984), and auditory (Heil and Scheich 1992) system. In
chemotopic maps of invertebrates and vertebrates sensory cells expressing a
certain receptor protein project to the same glomerulus (Hildebrand 1995;
Friedrich and Korsching 1998). In this way, identity and concentration of odorants
are encoded by glomerular activity patterns.
Examples of highly selective units include sign-selective neurons in the diencephalon of weakly electric fish (Keller 1988), song-selective neurons in the telencephalon of birds (Whaling et al. 1997) and face-selective units in the temporal
cortex of primates (Rolls 1984). Central maps may be constructed and ordered by
computations (Heiligenberg 1991 ). Examples include auditory space maps in owls
(Konishi 1986), echo-delay maps in bats (Suga 1990), visual orientationpreference maps in primates (Chapman et al. 1996), and auditory frequencymodulation maps in birds (Heil et al. 1992).
6 Peripheral and Central Sensory Channels
Up to now parallel central lateral line channels have not been found. However, the
peripheral lateral line consists of at least two channels. One processes superficial,
the other canal neuromast input. Each neuromast contains two sets of hair cells
which are oriented in opposite directions (Coombs et al. 1988). Due to the directional sensitivity of hair cells, each neuromast gives rise to two populations of
afferent fibers with opposite directional sensitivity. Thus peripheral sensory
channels separate information about direction of water motion with respect to the
long axis of a neuromast. Using vibrating-sphere stimuli, researchers failed to
demonstrate that the above sensory channels continue into the CNS. With one
exception (see below) they also failed to uncover central lateral line maps in
155
5 What Can We Learn from Other Sensory Systems?
If we look for commonalities in the processing by other sensory systems we
repeatedly fmd (I) a separation of sensory channels, (2) a systematic representation of stimulus parameters along central maps and (3) the emergence of more
and more selective responses to specific, biologically relevant stimuli as one
ascends to higher brain levels. Examples for the separation of sensory channels include the amplitude and phase channels in auditory (e.g., Konishi 1986) and
electrosensory systems (e.g., Heiligenberg 1991), the parvocellu1ar and magnocellular channels in the primate visual system (e.g., DeYoe and van Essen 1988),
the cold and warmth channels in the somatosensory system (Hensel1974) and the
pheromonal and ordinary smell channels in the brains of insects and vertebrates
(Hildebrand 1995). Sensory maps often show a relation to the spatial order of the
receptors. We know topographic projections from the somatosensory (Catania and
Kaas 1995), electrosensory (Bodznick and Schmidt 1984; Heiligenberg 1991),
visual (Vanegas et al. 1984), and auditory (Heil and Scheich 1992) system. In
chemotopic maps of invertebrates and vertebrates sensory cells expressing a
certain receptor protein project to the same glomerulus (Hildebrand 1995;
Friedrich and Korsching 1998). In this way, identity and concentration of odorants
are encoded by glomerular activity patterns.
Examples of highly selective units include sign-selective neurons in the diencephalon of weakly electric fish (Keller 1988), song-selective neurons in the telencephalon of birds (Whaling et al. 1997) and face-selective units in the temporal
cortex of primates (Rolls 1984). Central maps may be constructed and ordered by
computations (Heiligenberg 1991 ). Examples include auditory space maps in owls
(Konishi 1986), echo-delay maps in bats (Suga 1990), visual orientationpreference maps in primates (Chapman et al. 1996), and auditory frequencymodulation maps in birds (Heil et al. 1992).
6 Peripheral and Central Sensory Channels
Up to now parallel central lateral line channels have not been found. However, the
peripheral lateral line consists of at least two channels. One processes superficial,
the other canal neuromast input. Each neuromast contains two sets of hair cells
which are oriented in opposite directions (Coombs et al. 1988). Due to the directional sensitivity of hair cells, each neuromast gives rise to two populations of
afferent fibers with opposite directional sensitivity. Thus peripheral sensory
channels separate information about direction of water motion with respect to the
long axis of a neuromast. Using vibrating-sphere stimuli, researchers failed to
demonstrate that the above sensory channels continue into the CNS. With one
exception (see below) they also failed to uncover central lateral line maps in