Relative disparity processing in the dorsal visual pathway.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Krug and Parker Binocular disparity results from the subtle differences between the two eyes' views, and provides a powerful cue to the depth structure of the world around us. Disparity is widely conceptualized as having two types of signal: absolute disparity, which corresponds to the retinal distance of objects from an observer's current point of fixation, and relative disparity, which corresponds to distances between objects independent of where the eyes are fixating. The calculation of relative disparity therefore relies on absolute disparity information. The processing of these two signals has been suggested to differ between two visual pathways, with absolute disparity processed predominantly in the dorsal stream for visually guided actions (e.g., navigation, grasping, vergence eye movements) and relative disparity processed in the ventral stream for the identification of objects (e.g., 3D shape processing, object recognition) (Neri et al., 2004; Anzai et al., 2011). Previous studies investigating relative disparity computations have used a center-surround stimulus configuration , assuming this reflects receptive field organization (Uka and DeAngelis, 2006; Anzai et al., 2011). However, a failure to detect relative disparity processing in dorsal visual areas by presenting a center-surround stimulus might reflect suboptimal neuronal stimulation rather than different types of disparity computation. In a recent issue of The Journal of Neu-roscience, Krug and Parker (2011) developed a novel stimulus configuration to test whether there is selectivity for relative disparity in area V5/MT, which is located in the dorsal visual pathway. They recorded individual neurons in awake, fixating macaque monkeys when two superimposed transparent planes were presented. These planes had different depths and contained dots moving in opposite directions. The stimulus set typically consisted of eight absolute disparities with varying relative depths. A disparity-tuning curve was generated by plotting firing rate against absolute disparity. Disparities were then added to the entire stimulus set to change its position in depth. For a relative-disparity-selective neuron, this manipulation shifts the position of the tuning curve, because the neu-ron responds to the depth between the two planes. In contrast, the tuning curve of a neuron selective for absolute disparity does not shift when at a …