If you have ever wondered at the expert hand-eye coordination of a professional juggler, you are not alone. Neuroscientists have long been puzzled by how our brains work out the three-dimensional (3D) shape and position of objects from 2D retinal images, a talent needed to plan precise hand movements such as catching a ball midflight. Now a research team has identified the brain region responsible for 3D visual processing.
In order to guess the depth and 3D shape of objects, primates rely on two principal visual cues: first, the slightly different image seen by each eye, a phenomenon known as spatial binocular disparity, and second, the way the perceived shape of an object changes as it moves. It has been unclear how our brains integrate these two pieces of information, however. In a study in the 2 August issue of Neuron, neuroscientist Guy Orban of the Catholic University of Leuven in Belgium and his colleagues show that a brain region known as the anterior intraparietal cortex (AIP) is uniquely sensitive to both these visual cues.
The researchers conducted two separate experiments in which the brains of monkeys were scanned using functional magnetic resonance imaging (fMRI) while the animals viewed images of 3D objects. In the first experiment, Orban and his colleagues studied the influence of motion on 3D perception. The monkeys viewed rotating images of connected lines, such as partially unfolded paper clips, that only appeared in the field of view of one eye at a time. In the second experiment, the monkeys used both eyes to view computer simulations of small complex objects, a task that required them to rely upon binocular disparity to perceive depth structure in the images. "We found that these different bits of processing converged on one brain region: the AIP," says Orban, who notes that the AIP lit up in fMRI during both tasks.
The AIP had previously been associated with the visual control of hand movements, for example, picking up a pen. The need to guide "reach and grasp" movements is unique to primates, so Orban believes the AIP may have arisen after primates split off from other animals during evolution. "I'd expect this brain region to be unique to primates and to have evolved in parallel with hands and frontal eyes," he says.
Andrew Parker, a neuroscientist at the University of Oxford, U.K., who specializes in binocular perception, describes the finding as provocative and of significant interest to the field at large. "This study considerably broadens the processes we think the AIP is involved in," he says. Parker cautions that the monkeys were free to move their hands during the study, which may have activated the AIP region somewhat as well, but he found the experiments persuasive nonetheless: "If this research holds up, it will cause me to change my views on depth perception."