The ability to identify letters of different size remains, in relation to the allowed effort, the most informative measure of visual capacity. Numerous letter charts have been produced with varying design, size, and size progression of the letters. Attempts to 'standardize' letter testing have been fruitless. In Sweden, the most commonly used letter charts show deficiencies in letter selection, size, and progression. In the present study, letter legibility was studied. 10 letters of average legibility were selected and a new chart constructed using these letters in sizes corresponding to acuities from 0.1 to 2.0. A geometric size progression was found optimal (factor 1.26).
The fact that a physiological function generally unconsciously controlled can be self-regulated was demonstrated as long as 20 years ago. The biological feedback (BF) is the back return of information to a man or an animal on the function of their viscera and systems. BF is a process that through audible or visual signals transmits long-term routine information on physiological reactions and parameters, such as blood pressure and skin temperature, which are uncontrollable by an individual's consciousness and generally recorded by noninvasive techniques. Biological controlling techniques and procedures are used for the treatment and rehabilitation of a wide range of diseases and nosological entities -- from migraine to AIDS and cancer mainly in the USA, less in Western Europe, and incomparably little in the CIS countries. The Novosibirsk Institute of Medical and Biological Cybernetics, Siberian Branch of the Russian Academy of Medical Sciences is working at biological controlling problems, by making researches, investigations, clinical studies and developments of stimulators for BF.