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.Heidbreder, E. (1933). Seven psychologies (pp. 331–340). Appleton Century Crofts.Schultz, D.P. & Schultz, S.E. (2000). A history of modern psychology, 7th ed. (pp. 355–357). Harcourt College Publishers. 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.Swarthmore College Bulletin, March 1998, p. 5
Life 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.Harris, C.S. (2001) "Hans Wallach (1904–1998)", American Psychologist, 56(1), 73–74. doi:10.1037/0003-066X.56.1.73 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.".Swarthmore College Bulletin, December 2002, pp. 38–39
Köhler, who was not Jewish but who strongly resisted the growing Nazi influence,Henle, M. (1978). "One man against the Nazis—Wolfgang Köhler", American Psychologist 939–944Neisser, U. (2002) Wolfgang Köhler 1887–1967. Biographical Memoirs 81. National Academy of Sciences. had decided to emigrate as well. In 1935 he was offered a position at Swarthmore College, in Pennsylvania.Mandler, G. (2007) A history of modern experimental psychology (pp. 152–153). MIT Press. 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. He rose through academic ranks, becoming a full professor in 1953, and chair of the psychology department from 1957 to 1966. In 1971 he was designated Centennial Professor of Psychology. He retired from teaching in 1975, but remained active in research until 1994.
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. Phoebe died in 1968.Saxon, W. (1998, Feb. 15) "Obituary of Hans Wallach", New York Times, 15 February 1998 Hans Wallach died on February 5, 1998. Their son Karl Wallach died in 2001.
Psychological research 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."Wallach, H. (1976) On perception. Quadrangle/New York Times Book Company. . His studies span a broad range of psychological topics, including the following:
Movement of lines behind apertures Wallach's doctoral dissertationWallach, H. (1935). Über visuell wahrgenommene Bewegungsrichtung. Psychologische Forschung, 20, 325–380. 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.Wuerger, S., Shapley, R., & Rubin, N. (1996). On the visually perceived direction of motion by Hans Wallach: 60 years later. Perception-London, 25, 1317-1368.
Because the original paper was in German, this research was not well known to English-speaking psychologists for several decades. In 1976, Wallach published an English summary of his dissertation experiments,. Twenty years later, in 1996, Sophie Wuerger, Robert Shapley, and Nava Rubin 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.
Figural after-effects In a frequently-cited paper published in 1944,Köhler, W. & Wallach, H. (1944) Figural after-effects: An investigation of visual processes. Proceedings of the American Philosophical Society, 88, 269–357. 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.Kaufman, L. (1974). Sight and Mind. Oxford University Press. 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 general Wallach avoided neurophysiological explanations for perceptual phenomena, and the paper on figural after-effects was not included in a collection of his articles that Wallach published in 1976.
Sound localization In a series of papersWallach, H. (1938) Über die Wahrnehmung der Schallrichtung. Psychologische Forschung, 22, 238-266.Wallach, H. (1939). On sound localization. The Journal of the Acoustical Society of America, 10(4), 270–274.Wallach, H. (1940). The role of head movements and vestibular and visual cues in sound localization. Journal of Experimental Psychology, 27, 339-368. Wallach explored the ability of humans to locate sounds in the median plane – that is, to determine whether a sound comes from a source at the same elevation as the ears or from a source that is higher or lower, or even in back of the head. Binaural sound cues, including the phasing or time of the sound's arrival at each ear and the sound's relative intensity at the two ears (known respectively as ITD and ILD) enable a listener to determine a sound's lateral location (whether it is on the left, right, or straight ahead). But two sounds at different elevations can present identical ITD and ILD information to the ears, and so binaural cues to a stationary ear do not suffice to identify a sound's location in the median plane. Monaural cues that depend on the shape of the head and the structure of the external ear help with vertical localization, but binaural cues also play a part if the head is not stationary.
Wallach's research showed that when the human head moves (either by tilting or by rotating around a vertical axis), it creates a dynamic pattern of binaural cues that can, when paired with information about the direction and extent of the head movement, enable a listener to determine the elevation of a sound source. It is not necessary that the listener actively make the head movements; a subsequent paper demonstrated that sounds could be correctly located in the median plane when the observer is passively rotated or when a false sense of bodily rotation is induced by means of visual cues.
In 1949 Wallach, in collaboration with Edwin B. Newman and Mark Rosenzweig, published a seminal paper defining the precedence effect in sound localization.Wallach, H., Newman, E. B., & Rosenzweig, M. R. (1949). The precedence effect in sound localization. The American Journal of Psychology, 62, 315-336. Their experiments demonstrated that when a localizable sound reaches the ears and is immediately followed by an identical sound coming from a different direction, the listener perceives a single sound at the location of the first-arriving stimulus. The delay between the first-arriving and the second-arriving sound can be in the range of 1 to 5 ms for clicks, and as much as 40 ms for complex sounds such as speech and music. At delays above these thresholds, the second sound is heard as an echo. This phenomenon illustrates how the auditory system suppresses local reverberations to enhance the intelligibility of perceived soundsFreyman, R. L., Clifton, R. K., & Litovsky, R. Y. (1991). Dynamic processes in the precedence effect. The Journal of the Acoustical Society of America, 90, 874. and it is a critical factor in acoustical engineering and design of sound reinforcement systems.Zurek, P. M. (1987). The precedence effect. In Yost, W.A. & Gourevitch, G. Directional hearing (pp. 85-105). Springer US. . Wallach et al. also noted that the precedence effect plays an important part in perception of stereophonic sound.
Achromatic color and brightness constancy