=17- 
Mixed Aspects - Responses to Complex Visual Tasks. 
The yellow heraldic diamond with its tight packing 
of cards contains those properties which' cannot simply be 
classified under the pure physiological or psychological 
aspects. These are indeed mixed aspects, responses to 
complex visual tasks, in which both physio- and 
psychological properties play a role. In some of them, 
the first surely dominates the latter, notzonly in 
U Visual Acuity, Pointing, Fusion, and Optical 
Illusions. These four have in common that the presented object is mostly 
well defined, but they differ in the visual activities needed and in the 
output or goal. See fig. 3 
Visual Acuity is the family name for the visual tasks which can be 
fulfilled without thinking. It is a "yes or no visibility" 
of simple configurations, as à single dot, line or edge; 
double stars, double lines and unsharp edges; Landolt 
rings, multi-line patterns, resolution targets, and the 
like. 
The responses to all these tasks can be "yes" or "no" 
— the performance of the eye is expressed by the angle 
subtended by the smallest characteristic dimension of the object which 
just can be seen, and is usually expressed in minutes of arc. (Between 
#' and 20'; an order smaller for long lines). 
  
G = 
  
It has long been assumed that this acuity testing 
. / J reveals the most characteristic property of the eye, 
namely the ability to resolve small detail. But since a 
/ decade, the physiologists know that both Imaging Quality 
and Contrast Discrimination are active in this and in 
= I 
  
the next task, and the research goals now on 
  
these more basic properties. 
[1 is indeed possible to derive the response to these and similar 
objects when knowing the object configuration and the object 
contrast, by convolution with the eye's spread function and evaluation 
of the derived intensity profiles with respect to the rules of 
contrast discrimination, see for instance the early paper by 
R8hler (1962) and Hempenius (1968). The better understanding enables 
to predict the response of medium or low contrast targets, and can 
also be applied on unsharp details. 
Visual Acuity is by no means a dead subject, scientifically speaking. 
New data and theories try to connect the response to simple tasks 
with the physiological characteristics of the nervous system. As an 
example we mention the thesis by Van Nes (1968) which is based solely 
on the visibility of sine-wave targets, the so-called Modulation 
Sensitivity.)