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Light Perception - Discussion
of the other two, which lingered in the maze very long before making a decision at all. These
two were the reproductive pair, both being wild-captured. Hence the question arises whether
it was really lacking motivation that made them perform so reluctantly or whether their visual
capabilities were worse than those of their offspring reared in a lighted laboratory, and
whether they thus needed more time for preferential decisions. It would be thinkable that the
eye, or better the neuronal pathways of vision, degenerate after birth if not constantly excited
by light cues. A loss of possible function during development has been already shown in
Spalax ehrenbergi, where the lense starts to degenerate soon after ocular development onset
(Sanyal et al. 1990); however, this degenerative process is phylogenetically pre-determined and
independent of later light exposure. The process of neuronal plasticity would explain the use
of vision in the light-reared offspring better: neuronal plasticity refers to changes that occur in
brain organization, particularly to changes in the location of specific information processing
functions. These changes derive from learning and also from experience. As the concept of
plasticity can be applied to environmental events (Schwartz & Begley 2003), the unusual event
of constant light exposure might re-activate the usually rarely used visual brain-centers.
Our study showed that mole-rats can at least discriminate a difference between light
and dark in the intensity order of 0.6 µmol
photons
indeed the light intensity appeared low, but 0.6 µmol
35
−2 −1
⋅ m ⋅ s . This value seems small, and
photons
−2 −1
⋅ m ⋅ s equal 33 lux, and 33
lux belongs to the photopic range of vision (>5 lux; Kelber & Gross 2006). However,
subterranean European moles (Talpa europaea) have been shown to discriminate light at much
higher light intensities (350 lux), and to be unable to perceive light at an intensity of 60 lux
(Johannesson-Gross 1988) – the Zambian mole-rats have yet, in our study, shown a
performance about twice as good as the European mole.
It is nevertheless a pity that both the light sources and the light measuring instrument
did not allow to test visual performance under presence of lower light intensities, even down
to scotopic levels (