The phrase "earliest exponents" refers to the people who were among the first to support or promote a particular idea or concept. They were the ones who introduced and popularized it at its beginning stages. Full definition
Other early exponents of radical change in the Italian visual arts include proto Arte Povera artists: Alberto Burri, Piero Manzoni, and Lucio Fontana and Spatialism. (widewalls.ch)
Other early exponents of radical change in the visual arts include proto Arte Povera artists: Antoni Tàpies and the Dau al Set movement, Alberto Burri, Piero Manzoni, and Lucio Fontana and Spatialism. (en.wikipedia.org)
Nam June Paik was a major early exponent and in his pioneering performance Magnet TV, 1965 he made abstract shapes appear on a collection of TVs by using magnets to distort the images. (fineartmultiple.com)