The Senses 15
Movement detection
Rapid eye movements known as ‘saccades’ prevent moving images from blurring and
enable cats to detect and accurately track fast movements, even if the movement is
slight or to the side of the cat’s direct line of vision. Sensory information regarding
the distance and angle of the object is sent to the muscles surrounding the lens (the
ciliary muscles) but, if this calculation is incorrect or if the object moves in an unex-
pected way, a second corrective saccade follows the first to get the line of vision back
on target. This corrective imaging can occur around 60 times per second, at least
twice as fast as we are capable of (Bradshaw et al., 2012).
Slow movements, however, are not so well detected. Humans can detect move-
ments that are ten times slower than those that can be seen by cats (Pasternak and
Merigan, 1980).
Visual focusing
Visual focusing is one way in which our eyes perform much better than those of cats.
In human eyes, the lens is flexible and, depending on the overall health of the eye, is
able to focus fairly accurately by distortion of the lens. But the lens within a cat’s
eye is inflexible and focuses by moving backward and forward rather than distorting.
The result is that cats are a little slower at transferring their visual focus from near to
distance and vice versa and are unable to focus clearly on anything that is less than
25 cm away from their face.
Colour vision
Perception of colour is another way that human vision is superior. Humans have three
types of colour sensitive cone photoreceptors (red, blue and yellow) and 16 times
more colour-comparing nerves than cats, which allow us to see a far wider range of
different colours.
Rod photoreceptors only allow for monochromatic vision and it was once
thought that cats can only see in black and white. But neurophysiological evidence
Fig. 2.1. Elliptical pupils can dilate more than round pupils, allowing more light to enter the
eye. They can also constrict to very narrow slits to avoid damage to the retina from bright light.