in this book will be concerned with the trends in 50% probability of acquisition.
However, before proceeding to such studies it is necessary to survey the factors
which contribute to threshold variability and their extent. For convenience
these may be divided into ‘within observer’ variability, ‘between observer’
variability and influence of environment.
3.1 Within observer variability
3.2 Between observer variability
3.3 The influence of environment
3.4 Attempted modelling of frequency of seeing
4 . BASIC THRESHOLDS FOR DETECTION
The most positive point on the frequency of seeing curve discussed in the
previous chapter is the 50% point. The 50% probability of carrying out any task
is also the most useful measure of performance, especially if the variance is known
as it seems to be for many visual processes. It has thus become almost universal
practice to quote the results of basic experimentation on visual detection in terms
of the 50% probability of correct positive responses. For most of the following
chapters we shall follow this line and discuss only the 50% probability points.
The most basic quantity in study of thresholds must be that of differential
energy available to different parts of the retina. However the visual processes
operate - and this will be discussed in succeeding chapters - some difference of
available energy must be present. It is usual to use as the basic practical