Figure-Background Perception in Right and Left Hemispheres of Human Commissurotomy Subjects

Abstract

The right and left hemispheres of four complete commissurotomy subjects were tested for the ability to recognize and integrate figure and background elements of composite visual stimuli. In the first experiment the subjects were required to identify from a four-choice array in free vision the stimulus card that matched the briefly lateralized sample stimulus. For all subjects the left hemisphere was proficient at identifying the figure, but performed at near-chance level in recognizing the textured background. In contrast, the right hemisphere was equally adept at identifying figures and backgrounds. Both hemispheres could easily identify the isolated figure or background from a choice array, demonstrating that the observed hemisphere effects were due to figure–background interactions rather than the result of any difficulty in processing specific elements of the composite stimulus. The second experiment involved the determination of the size and position of a dot that appeared against various plain and textured backgrounds. The right hemisphere of two subjects, but not the left, performed with greater accuracy when the background consisted of a ‘natural’ texture gradient rather than a plain white backing. Similar though less consistent results were obtained when an inverted gradient or an evenly spaced grid was used as the background. For each condition, right-hemisphere performance resembled that of normal control subjects. In contrast, the left hemisphere provided a pattern of results dissimilar to that of control subjects for the various figure–background tasks described. It appeared to be generally insensitive to background effects, except when the information provided by the background was highly unusual, as from an inverted texture gradient. The results suggest a preeminent role for the right hemisphere in (i) the recognition of background components of a whole-field stimulus, (ii) sensitivity to the influence of the background on the perception of an object, and (iii) the ability to use natural perspective cues to assist in the accurate perception of an object.