Figure 5.

Stimulus encoding and processing in the lateral line system. A. Schematic representation of the boundary layer effect between an oscillating sphere and superficial neuromasts (SN) on the skin of a fish. Sphere displacement is shown by the sine wave. Note that the cupula movement results in hair cell deflection. Depending on the hair cell polarity (grey and black hair cells), the corresponding afferents respond 180° out of phase (raster plot in grey and black). B. Schematic of the flow field over a fish’s body. Arrows represent the mean local flow direction and magnitude obtained in small interrogation areas. As shown for five of these areas located along the dorsal trunk of a fish being held stationary in flow, these local fluctuations of the velocity are strongly correlated over the sensory surface of the fish. Data redrawn and modified after [Chagnaud et al. 2008b]. C. Overview of the features of hydrodynamic stimuli encoded in the periphery: The hair-cells and their primary afferents respond in a directionally sensitive manner such that the response of a given afferent changes in a cosine-like fashion with stimulus angle. Neuromasts are typically oriented with their long axis either parallel to the anterior-posterior or the dorso-ventral axis of the animals, leading to a neuronal representation of local flow direction. Given that the precision at which the afferents respond, this separation in two groups of neuromasts with their hair cells being sensitive in a mirror-symmetric fashion leads to a precise encoding of the minute spatiotemporal differences characteristic for global flow-fields. The separation in a superficial and a canal system results in two complementary channels of information with different filter characteristics (see blue and pink input-output functions). The highly sensitive SNs convey low-pass filtered information of the flow properties whereas the less sensitive canal neuromasts (CNs) convey high-pass filtered information. As the flow inside the canals will depend on the placement of the pores, CNs typically sample more local aspects of the otherwise highly correlated global flow field. This suggests that SNs encode the global flow, whereas CNs encode local fluctuations. D. Afferents of the anterior and posterior lateral line nerve project to the medial octavolateral nucleus in the medulla in a topographical manner. Here afferents bifurcate and make synaptic contacts along the length of the MON. The projection of afferents of the anterior nerve course more medially and more ventrally within the MON than afferents of the posterior lateral line nerve. A parallel path of fibers contacts the Mauthner cell; it is presently unknown if this projection is topographically ordered. E. Schematic overview of the major ascending and descending pathways of the mechanosensory lateral line. Abbreviations: Cc: Crista cerebellaris; EG: eminentia granularis; OB olfactory bulb; MON: medial octavolateral nucleus; PE: preeminential nucleus; Tel: Telencephalon; TeO Tectum opticum; TSvl: ventrolateral nucleus of the torus semicircularis.