VISUAL FATIGUE, calling for nightly rest, and its 
counterpart: the necessity for daily stimulation, are 
both properties which seem at first sight not to fit 
in the group. There is & lot of physiology involved, 
but emotion may upset the expected decrease in 
performance due to visual fatigue, and attention is 
a second psychological factor which strongly | 
counteracts it. What makes viewing tiresome are such | 
  
factors as: incorrect instrumental adjustment, residual 
y-parallax in stereo-viewing, non-optimum line of sight, an unusual 
illumination (fluorescent, strange colours), unusual poor image quality, 
disturbing patterns, noise, etc. Many of these factors are known and can 
b e be accounted for - ergonomy enters our profession albeit slowly. See 
Nathan 1961, and further journals as "Vision Research" and "Ergonomics". 
A photogrammetric ergonomic study was published in Japanese by Usami 
(Usami 1961). 
[Visual fatigue caused by monotony is more dangerous than tiredness 
after prolonged execution of an interesting task. The latter can be 
reduced by training, the first can hardly be overcome. But, where 
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monotony has to be avoided, it is nevertheless wise to keep vision 
in good condition by daily training: human beings need to see 
everyday in order not to loose even the ability to see straight 
lines straight and to recognize simple shapes - five days of arti- 
ficial blindness is enough to disturb vision temporarily (Vernon 
1962, p. 189).] 
The CAPACITY of the neural network and the FLOW of 
INFORMATION are terms which lead us to a communi- 
cation theory. From this point of view it is not 
strange to ask questions on the "hows" and "what's" 
of the visual information flow, the input to the 
brain. 
To state it frankly: human capacity of information 
flow is rather limited. Both eyes cannot see two 
  
different pictures at the same moment; if such an 
experiment is set up - the eyes tend to alternate, 
and each sends the information during the inactive phase of the other. 
Listening and seeing does not go together either, there should be at 
least 1/5 of a sec. time interval between the stimuli for the eyes and 
ears, or the signals should contain sufficient redundancy.