After receiving her DPhil, Treisman worked in the Medical Research Council Psycholinguistics Research Unit conducting research in selective listening. In 1964, Treisman proposed her Attenuation Theory, which modified Broadbent's Filter model by stating that unattended information is attenuated rather than completely filtered out. Treisman used a dichotic listening task during which participants heard multiple languages and different voices (male vs. female). She showed that a difference between two equally known languages allowed no more efficient selection than a difference in subject matter between two messages in the same language. Unknown foreign languages, however, produced less interference. It appeared that complete rejection of one language was almost impossible; with some degree of variability depending on physical characteristics and language of the message received. Treisman concluded that features of multiple incoming messages are successfully analysed, and that selection between messages in the same voice, intensity, and localisation takes place during, rather than before or after, this analysis, which results in the identification of their verbal content. Information-handling capacity is limited following this analysis; the process handles one input at a time, either keeping to one message where possible, or switching between the two. Thus, Broadbent's suggestion that classes of words constitute separate "input channels" could be rejected. Her theory also indicated that physical characteristics are processed early, while semantic processing occurs at a later point. Her work had an enormous impact on her field. For example, In 1967, while working as a visiting scientist at Bell Telephone Laboratories' psychology department, she published a paper in "Psychological Review" that was "central to the development of selective attention as a scientific field of study".