For Alvin M. Liberman

by Björn Lindblom, Stockholm, February 2000

A central theme of Alvin Liberman's scientific contributions is the idea that we listen to speech in a manner that differs from how we hear other sounds. Apparently he was struck by this insight during the early attempts at Haskins Laboratories to construct reading machines for the blind. It soon became clear that a text-to-sound system based on converting texts into strings of letter-based noises was unlikely to ever produce a perceptually useful signal. This experience prompted him to try to understand why human listeners perceive phonetic segments much faster and more reliably than they process other sound sequences. As is well-known, the explanation he proposed now forms part of the motor theory of speech perception.

This theory is an evolutionary account whose starting point is the limited temporal resolving power of the hominid ear. It claims that selection pressures arose from this constraint and gave rise to the 'phonetic module', an innate mechanism linking perception and production and specialized to do two things: (i) to allow the talker to produce speech as a sequence of temporally overlapping, co-articulated units of movement ('phonetic gestures') and (ii) to allow the listener to recover those elements from the co-articulated signal. Thus, speakers and listeners could conduct their linguistic business using the common currency of phonetic gestures. As Al would put it, parity was guaranteed. He saw phonetic representations as primary and not requiring a stage of translation from auditory primitives to discrete segments. In his view, that direct process is what makes it possible for phonetic segments to be perceived much faster and more reliably than other sequences of arbitrary sounds.

In a sense, Al came full circle as he returned to where he had started: reading. The framework he had developed carried the strong implication that it would take the child a special cognitive effort to bring the phonetic gestures of speech to conscious awareness and link them to the visual percepts of reading. He suggested that special difficulty in learning to read might be caused by a weakness of the phonetic module leading to more fragile phonetic representations.

It is understandable that, on the international scene of phonetics, the name of Al Liberman is often primarily associated with speech perception research, partly because of the name of the theory, partly because of the numerous perceptual experiments it has inspired. It is true that his views did not go unchallenged. However, Al seemed to emerge from such controversies unshaken and with new experimental initiatives, no doubt often taken to liberate his opponents from the bondage of their more 'conventional' thinking.

The strong basis of his position becomes apparent when we consider the full scope of his theory. We realize that the motor theory of speech perception deals with much more than on-line listening. It is in fact a theory of the biology of speech. Today when students of spoken language begin to show a growing interest in evolution, we should note that Al's contribution is the first of its kind, a pioneering effort unique in breadth and content and its insistence on biologically based explanation.

I have many fond personal memories of Al. Let me just mention one episode. It happened a few years ago during one of his visits to Sweden.

Al had come to Stockholm to present a paper at a Rodin Academy conference on reading. He had also kindly accepted to give a talk after that meeting at Stockholm University. During the sessions it became clear that - to put it mildly - Al was "coming down with something" and was in no shape to lecture. When I suggested that we cancel the event he would not hear of it. (Later on his much delayed arrival in the U.S., he found out that he had in fact contracted a severe case of pneumonia).

Al heroically appeared on campus. The big lecture room was packed. His voice was barely audible but nonetheless he had everybody's undivided attention. He gave a passionate and eloquent summary of how speech works and how it got that way. A riveting performance brilliantly and compellingly argued and, in view of his condition, paradoxically full of contagious vitality.

For many in the audience this was one of those rare occasions. It was the first time they had seen Al in person. For those of us fortunate to have known Al and heard him speak many times over the years, this was the epitome of the man and his research.

Now that his voice is silent, we take comfort in reflecting on the lasting impact of his work and the memory of his warm and generous presence among us.