Changes in visual spatial organization: response frequency equalization versus adaptation level.

Abstract

Two experiments tested alternative explanations for the apparent change in visual spatia l organizatio n produced by exposure to or anchor stimuli. Th e alternative positions evaluated were that the change was percep tual (adaptation-level [AL] theory ) or that it was the result of response bias (response-frequency-equalization theory) . I n Experimen t I, judgments of spatial organization were obtained for 11 different matrices of dots in a series ranging from columna r organization to rowlike organization. Th e 5s were exposed o n alternate trials to dot pattern s o f extreme horizontal (rowlike ) organization, to extreme vertical (columnar) organization, or to neither (contro l group). A s predicted by both theories, judgments wer e systematically affected by the adapting stimuli. I n Experiment II, the influence of response bias was nullified by a forced-choice procedure. Contrar y to AL theory, once response bias was eliminated group differences vanished. Adaptation i s a chang e in sensor y re sponse which follow s th e presentatio n of some stimulus. Th e study of visual adaptation provides a powerful analytic tool for identifying th e natur e an d characte r of sensory mechanisms which support various perceptual experience s (Weisstein , 1969) . Adaptation-level (AL) theory provides one general framewor k which integrates, i n a limited set of equations, many apparentl y unrelated phenomena connected with adaptation (Helson, 1964). While adaptation may potentially follow exposure to any stimulus which differs from the curren t base-lin e leve l o r pattern of stimulation (Andrews , 1964; Held, 1962), it is importan t to distinguish actual changes in sensor y respons e fro m ersat z form s of adaptation. Onl y the former changes are likely to illuminate sensor y processes. Sek uler and Erlebache r (1971 ) identifie d on e possible artifact which could produce ersatz adaptation, th e tendenc y o f 5 s t o us e available response s wit h equal frequency. Among th e experiment s single d ou t a s demonstrations of ersatz, rather than genuine, adaptation are several reported by Bell 1 Preparation of this article was aided by National Institute of Health Grants EY-00321 and NS-10094. 2 Requests for reprints should be sent to Robert Sekuler, Cresap Laboratory of Neuroscience and Behavior, Northwestern University , Evanston , Illinois 60201. and Seva n (1968) . I n Bel l and x Bevan's first experiment, which may be considered as the prototype for all their experiments, 5s judge d th e spatia l organizatio n o f a series of dots arranged on the vertices of an imaginary matrix. Whe n the vertical distances betwee n dots are greater than th e horizontal distances , the dots seem t o be organized in rows. Whe n the vertical distances betwee n dot s ar e les s tha n th e horizontal distances , the dots seem t o be organized in columns. Thes e observation s demonstrate th e operatio n o f the gestal t principle of proximity. Bel l and Beva n used a graded series of dot matrices ranging from extreme rowlike organization through neutral organizatio n (vertica l distance s equal t o horizontal distances) t o extrem e columnlike organization . Eac h stimulu s was judged , usin g th e metho d o f single stimuli, a s rowlik e o r columnlike . Th e point of subjective equality (PSE ) ca n be defined a s th e stimulu s whic h appeare d most nearl y neutra l (i.e. , judged rowlik e with P = .5). Th e PS E was determined for each of three groups. Fo r the column group, every other stimulu s to be judged was extremely columnar in its organization —such a stimulus was virtually certain to elicit a colum n judgment . Fo r th e row group, every other stimulus was extremely rowlike in its organization—being virtually certain t o elici t ro w responses . A fina l