Final answer:
Alberto Ginastera's third stylistic period was described as neo-expressionist, marked by the use of dissonance, complex rhythms, and a more universal approach to composition.
Step-by-step explanation:
Alberto Ginastera described his third stylistic period as neo-expressionist, in which his compositions integrated an innovative approach influenced by both European avant-garde and prior periods such as expressionism and abstract expressionism. This period is characterized by the use of dissonance, complex rhythms, and often dramatic texture that express an intense, sometimes violent emotional landscape which evolved from the traditional foundations of folk music elements present in his earlier works. During this time, Ginastera's music also made use of 12-tone techniques, though not strictly adhering to the total serialism rules, reflecting his personal evolution in musical language and embracing a more universal rather than solely nationalist approach.