Purpose: The purpose of the study was to investigate the role of subcortical processing in human visual attention. The midbrain contribution to visual attention is unclear. Although evidence exists for a subcortical attentional advantage in ocular motor tasks, such an advantage has not been shown in perceptual tasks. Because retinotectal projections arise predominantly from nasal retina (i.e., temporal hemifield), subcortical attention should be distributed asymmetrically for monocular viewing conditions with an advantage to the temporal hemifield.
Methods: To test for a subcortical attentional effect, the authors compared the results of binocular and monocular viewing conditions using the split priming motion induction paradigm. In this perceptual attention paradigm, priming cues are presented to the left and right of fixation followed by an instantaneously presented horizontal bar. As a result of attention to the priming cues, motion is perceived within the bar as it appears to draw in from the two lateral cues toward a central collision point. Asymmetrically distributed attention results in an asymmetry in the perception of motion within the bar, and thus the perceived collision point will be shifted away from the center.
Results: In two separate studies, one with and one without control of eye movements, the authors found significant differences between the results for monocular and binocular presentation. When the stimulus configuration is presented to the left eye, the perceived collision point is shifted toward the center consistent with a subcortical attentional effect. However, presentation of the stimulus configuration to the right eye yields the same results as those of binocular presentation.
Conclusions: This pattern of results can be explained by a separate and additive interaction between cortical and subcortical attentional effects in the visual field. Dominance of the left visual field for cortical attention and dominance of the temporal visual field for subcortical attention act together when the initial priming cue occurs in the temporal (left) visual field of the left eye. However, these influences compete when the same stimulus configuration is presented to the right eye, where cortical attention predominates in the left visual field and subcortical attention predominates in the temporal (right) visual field.