Steady-state and dynamic response properties of the Mandelbaum effect

The Mandelbaum effect refers to an inadvertent accommodation response to an intervening surface (e.g. a window screen) while attempting to focus a distant object of interest. Previous research has attributed the Mandelbaum effect to a tendency for the accommodation mechanism to preferentially focus the stimulus nearest the dark (resting) focus position. The present study extends this work by examining dynamic accommodation response properties for different distances and stimulus separations. Accommodation measurements of nine young, emmetropic subjects were obtained with an infrared optometer while they viewed superimposed horizontal and vertical square-wave gratings at various dioptric separations. Three subjects demonstrated a strong focussing preference for the target located nearest their dark focus position for all stimulus conditions, while two other subjects showed a similar, but weaker, preference. Conversely, two subjects primarily focussed the nearer of the two targets, and two subjects tended to focus the vertical target (regardless of whether it was near or far, or was closet to the dark focus). For all subjects, accommodation rarely fluctuated from one target to the other, and was seldom focussed midway between the two targets. Our findings indicate that the influence of the dark focus on the Mandelbaum effect varies among individuals, and plays little or no role in some individuals. Thus, accommodation responses to competing stimuli are more complex than previous findings would suggest.