Psychedelic rock emerged in the mid-'60s, as British Invasion and folk-rock bands began expanding the sonic possibilities of their music. Instead of confining themselves to the brief, concise verse-chorus-verse patterns of rock & roll, they moved toward more free-form, fluid song structures. Just as important -- if not more so -- the groups began incorporating elements of Indian and Eastern music and free-form jazz to their sound, as well as experimenting with electronically altering instruments and voices within the recording studio. Initially, around 1965 and 1966, bands like the Yardbirds and the Byrds broke down the boundaries for psychedelia, creating swirling layers of fuzz-toned guitars, sitars, and chanted vocals. Soon, numerous groups followed their pattern, including the Beatles and the Rolling Stones, both of whom recorded psychedelia in 1966. In no time, groups on both sides of the Atlantic embraced the possibilities of the new genre, and the differences were notable. In Britain, psychedelia tended to be whimsical and surrealistic. Nevertheless, bands -- most notably Pink Floyd and Traffic -- played extended instrumentals that relied on improvisation as much as their American contemporaries the Grateful Dead, the Doors, Love, and Jefferson Airplane. In other corners of America, garage bands began playing psychedelic rock without abandoning their raw, amateurish foundation of three-chord rock -- they just layered in layers of distortion, feedback, and effects. Eventually, psychedelic evolved into acid rock, heavy metal, and art rock, but there continued to be revivals of psychedelia in the decades that followed, most notably in the American underground of the mid-'80s.