The BRAIN CONTROLLED VISUAL ACTIVITY is a definite characteristic
of the active mechanism of human vision. The eye does
not simply record everything. Its movements (scanning
over the scenery, fixation on points of interest,
Physiological focussing, convergence) and even the messages which
Aspects pass up to the brain are guided and controlled by the
brain. We see practically only what we want to see, we
manage to use the eyes effectively for the tasks at
hand, leaving out unimportant detaile (How the brain
"knows", before the message arrives at the brain, which signal is impor-
tant, is discussed by Vernon (Vernon, 1962, p. 213).
e
Important here is that this close cooperation between brain and eye
has to be trained and exercised. For instance, accommodation is normally
coupled via the brain with convergence. Looking through a stereoscope
without magnifying glasses calls for a decoupling of accommodation and
convergence — a task which can only be performed by trying and training,
not by thinking. The initial difficulty is experienced by everyone who
observes for the first time in life a pair of prints through a stereos-
cope.
[4 typical property is the fixation precision: the eyes can be directed
and kept fixed on a point within 2 mrad standard deviation in horizontal
direction, and 6 mrad for vertical stability. (Fender 1964)
Another simple visual activity: when we expect to see long lines, the
eye will scan accordingly; the same attitude will be of profit if
circles are to be expected, on the condition that the eye has suffi-
cient training to scan circularly. (Neisser 1964)
In à grainy print, the graininess can be seen ai will and studied © l|
visually, but it is also possible to suppress it and to discover the
hidden details, if large enough, by de-accommodating and course
scanni nge]
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