Vision in echolocating bats - Fladdermus.net

(Heffner & Heffner 1992; Heffner et al. 1999). This reflex is even
quicker than the reaction to a flashlight (Whittington et al. 1981), and
hence suggests that hearing is closely co-ordinated with vision (Heffner
1997).
Sound localisation acuity is related to retinal organisation
and the width of fields of best vision (defined as the portion of the retina
with at least 75% of maximum ganglion cell density, Heffner et al. 2001).
Animals with narrow fields of best vision (foveae) have generally better
localisation acuity than animals with broad or elongated fields of best
vision (visual streaks). The retinas of microchiropteran bats are loosely
arranged in visual streaks and the density of ganglion cells falls
irregularly toward the periphery. The field of best vision is concentrated
in the temporal part of the retina and seems to be broader in frugivores
than in insectivorous species (Heffner et al. 2001). Overall, there is a
higher ganglion cell density in the inferior part of the retina than in the
superior (Marks 1980; Pettigrew et al. 1988; Koay et al. 1998; Heffner et
al. 2001; Eklöf unpublished). This means that the sharpest image on the
retina results from light reaching the eye from above, and consequently,
the bat eyes focus slightly upwards. Without moving their heads, bats are
looking up (Pettigrew 1988). The functional significance of this can be
difficult to establish, but it seems likely that vision and echolocation have
evolved to provide the bat with as little information overlap as possible.
While echolocation call emission and hearing is most effective in the
flight direction and downwards (Schnitzler & Grinnell 1977; b), vision
serves as a complement by being most effective upwards it thus gives
additional information of obstacles and landmarks further away. In
Megachiroptera, which do not echolocate, one would thus expect the
fields of best vision to be above rather than below the optic disk, which in
fact seems to be the case (Pettigrew 1986).
All bats have well developed retinofugal projections
(pathways of information from retina to visual cortex) to the lateral
geniculate nuclei as well as to the superior colliculus (see above), which
are the main targets for retinal projections in mammals (Pentney & Cotter
1976; Suthers & Bradford 1980; Reimer 1989). In the superior colliculus,
different sensory modalities are integrated and transformed, and the
output may be perceived as a “new product” (Stein & Meredith 1993).
The capacity to deal with multisensory information is however developed
first after experience of multimodal inputs (Wallace & Stein 2001). The
superior colliculus controls for example eye movements, which serves to
keep objects of interest in the focal field. Auditory projections to the
superior colliculus are generally sparse in mammals. However, in the
mustache bat Pteronotus parnellii (Mormoopidae), at least three areas in
the brain stem contribute with well-developed auditory projections to the
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