Hans Wallach

Hans Wallach (November 28, 1904 – February 5, 1998) was a German-American experimental psychologist whose research focused on perception and learning.

Although he was trained in the Gestalt psychology tradition, much of his later work explored the adaptability of perceptual systems based on the perceiver's experience, whereas most Gestalt theorists emphasized inherent qualities of stimuli and downplayed the role of experience.

Wallach's studies of achromatic surface color laid the groundwork for subsequent theories of lightness constancy, and his work on sound localization elucidated the perceptual processing that underlies stereophonic sound.

He was a member of the National Academy of Sciences, a Guggenheim Fellow, and recipient of the Howard Crosby Warren Medal of the Society of Experimental Psychologists.

Wallach was born in Berlin on November 28, 1904, to a Jewish family.

Following early studies in chemistry, he enrolled at the Berlin Psychological Institute, serving first as assistant to its director, Wolfgang Köhler, and subsequently conducting research of his own.

He completed the work for a PhD degree in 1934, hurrying because his vulnerable position as a Jew in Nazi Germany had led him to decide on emigration.

Jewish professors were being dismissed from universities.

Wallach considered himself poorly prepared when he took his oral exams, relating later, "I shall never forget the kindness of [two professors] who, aware of my precarious situation, allowed me to pass.".

Köhler, who was not Jewish but who strongly resisted the growing Nazi influence, had decided to emigrate as well.

In 1935 he was offered a position at Swarthmore College, in Pennsylvania.

The following year he invited Wallach to join him as a research associate.

Wallach worked at Swarthmore for the rest of his career.

For the first six years he did not teach but only conducted research, but in 1942 the demands of the war effort depleted the psychology department faculty, and Wallach (who was ineligible for military service) was appointed as an instructor.

In a frequently-cited paper published in 1944, Köhler and Wallach presented a series of experiments on figural after-effects.

If, for example, an observer stares for about a minute at a fixation point in the center of a visual field that is white except for a large black rectangle on the left side, and then (with the rectangle removed) looks at the center of an array of four evenly-spaced squares, symmetrically arranged around the fixation point, the two squares on the left side will appear farther apart than the ones on the right.

Many similar observations are discussed in the Köhler and Wallach paper.

Köhler considered that this phenomenon supported his theory of psychophysical isomorphism – that the perception of forms is mediated by electrical fields on the cortex of the brain, fields which he thought were isomorphic to the stimulus but which could be distorted through a process of satiation.

However, Wallach came to doubt this explanation and in subsequent years dissociated himself from this research.

In addition to his work at Swarthmore, Wallach was a visiting professor at the New School for Social Research in New York from 1947 to 1957.

In 1948 he held a Guggenheim Fellowship, and in 1954–55 he was a member of the Institute for Advanced Study at Princeton.

Wallach was married to artist Phoebe Kasper, and they had a son Karl.

He rose through academic ranks, becoming a full professor in 1953, and chair of the psychology department from 1957 to 1966.

Phoebe died in 1968.

In 1971 he was designated Centennial Professor of Psychology.

He retired from teaching in 1975, but remained active in research until 1994.

In 1976, Wallach published an English summary of his dissertation experiments, and in 1996, Wuenger et al. published a complete English translation, adding an introduction in which they state that Wallach's findings "are relevant to contemporary research, and have implications not only for the study of motion perception but also form and color perception. His results provide evidence against a modular scheme of visual processing, where form, color, and motion are computed in isolation. Instead. he found that the perceived direction of motion was linked to the perceptual organization of the scene: when several interpretations of the form exist, and several directions of motion are possible, only certain combinations of form and motion are perceived."

Hans Wallach died on February 5, 1998.

Their son Karl Wallach died in 2001.

Wallach was not a theorist, and he did not organize his research around an overarching theoretical system.

He described his style of working as "pursuing a problem as long as the work yielded worthwhile results, and then shelving it until a new idea promised progress."

His studies span a broad range of psychological topics, including the following:

Wallach's doctoral dissertation examined perception of lines moving behind an opening in a masking surface – a phenomenon known as the aperture problem.

If a slanted line moves behind such an aperture, the physical stimulus presented to the eyes will not distinguish whether the movement is horizontal, vertical, or at some other angle.

Wallach found that the motion the observer perceives is determined by the shape of the aperture.

If the aperture is circular, the line (or lines) will appear to move in a direction perpendicular to their orientation.

But if the aperture is rectangular, the lines will seem to move in a direction parallel to the long axis of the aperture.

An example of this phenomenon is the familiar Barberpole illusion.

Wallach explained this finding by asserting that the perceptual system tends to preserve the individual identity of the line segments defined by the end points created by the aperture, and that this mode of movement best preserves that identity.

Because the original paper was in German, this research was not well known to English-speaking psychologists for several decades.