Precisely how we perceive and interpret the three-dimensional world remains a mystery to scientists, but a recent study from the lab of Richard Born, HMS professor of neurobiology, may have brought us a step closer to understanding this process. In the February Nature Neuroscience, Born and colleagues identify a pathway within the primate visual cortex that is specifically responsible for carrying and integrating information regarding depth of moving stimuli.
Depth is an illusion generated by the brain’s visual cortex through a computation known as binocular disparity—in which visual input received separately from each eye is compared. This visual information is initially conveyed to the primary visual cortex (V1) via the thalamus, where it is then relayed to other visual areas (V2, V3, V4) in a hierarchical manner. Although signals of binocular disparity and motion occur throughout the visual cortex, these signals are thought to be integrated within an area known as the middle temporal visual cortex (MT). Here, neurons have been found to signal both direction and speed as well as depth changes of moving stimuli.
Transferral of visual depth and motion information to MT occurs either directly or indirectly (via areas V2 and V3). A common hypothesis has been that the indirect pathway was redundant, but recent observations suggest it plays a complementary role to that of the direct pathway.
Born and his group set about investigating the latter hypothesis by observing the impact of selective inactivation of the indirect neural route on monkeys’ ability to perceive direction and depth of moving stimuli. First author and graduate student Carlos Ponce recorded neurons in the MT of awake rhesus monkeys while the animals observed moving stimuli positioned at various binocular disparities (depths) on a screen. Using this experimental setup, he saw that prior to inactivation, the neurons selectively increased their firing rate to movement, changes in depth, or both. When the indirect neuronal pathway from V1 to MT was inactivated, however, neuronal firing to depth but not to motion was altered significantly.
Ponce also investigated the behavioral impact of this inactivation by measuring the monkeys’ eye movements while they tracked a moving stimulus with changing depth. In line with their neuronal-firing pattern, the monkeys also demonstrated behavioral impairments with tracking visual depth.
“What we are showing here is a very clean experiment demonstrating the role of the indirect pathway from V1 to MT in visual processing,” said Ponce. “Monkeys, like humans, are very good at separating out different surfaces based on motion and depth. So in this experiment, if we could ask the monkey what aspects of a computerized stimulus he could see after we inactivated the indirect pathway, we would predict that he could tell us the direction in which the stimulus was moving but not whether it was nearer or farther on the screen.”
“This is very fundamental neuroscience research,” said Ponce. “Our next step is to further explore these pathways, possibly using sophisticated molecular techniques, which would enable us to understand this process more fully. This is important if we want to know how we make spatial sense of the world.”
