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Abstract:
The visual system consist of several subsystems, which perform their task nearly independent of each other. One visual subsystem is the accessory-optic system, performing the gaze stabilisation by the nystagmus (physiological nystagmus). In humans there are two kinds of nystagmus: the stare nystagmus and the look nystagmus. The best know
example for stare nystagmus in humans can be seen on a train journey. The vis-a-vis - staring out of the window - moves his eyes nystagmic. Without fixating any single item he runs his eyes over the landscape, most of the time following and bouncing back every now and then. This behavior is driven by the accessory-optic system, a set of subcortical nuclei, while cortical structures contribute to the look nystagmus. To know which cones contribute to the stare nystagmus, we measured its spectral sensitivity. Two instructions were given, leading to a stare and a look nystagmus, respectively. The spectral sensitivity of the eye
movement differed for the two instructions. The stare nystagmus is dominated by the long wavelength sensitive cone, though a contribution of the medium wavelength sensitive cone can not be excluded. On the other hand, the contribution of the short wavelength cone to the look nystagmus is obvious. The optomotor system is colour-blind and uses the long wavelength part of the light spectrum in all examined species of insects, fish, amphibia, and reptiles. Man is to be added to this list. It is still unknown why the evolution ofthe visual system took
the same direction in all these classes.
