Superior colliculus (SC), which is a centre in the brain that helps orient the head and eyes towards something seen or heard, may also play a role in analysing the environment, scientists at the Salk Institute for Biological Studies have discovered.
Research leader Dr. Richard Krauzlis, an associate professor in the Systems Neurobiology Laboratory, has revealed two complementary studies have shown that the SC performs supervisory functions in addition to the motor control it has long been known for.
"Beyond its classic role in motor control, the primate superior colliculus signals to other brain areas the location of behaviourally relevant visual objects by providing a 'neural pointer' to these objects," he says.
Historically, physiological studies of eye movement control have relied on individual spots of light representing visual targets, but the real world is much more complex than a single dot on a computer screen.
"For example, we can smoothly track a large airplane, with all its intricate visual details, by directing our gaze at its centre. At night, we might only be able to see the strobe lights on the wing tips, but we are still able to track the object's invisible centre," explains Dr. Ziad Hafed, Sloan-Swartz Fellow in the Systems Neurobiology Laboratory and lead author on both studies.
Hafed designed a series of experiments in which the subjects had to infer the invisible centre of a visual target consisting of two peripheral features, much like the above airplane's strobe lights in the night sky.
The subjects tracked it for several seconds or fixated on a stationary dot while the peripheral features were moving back and forth.
In one study, the researchers recorded the activity of single neurons in the superior colliculus while the subjects either fixated on the stationary dot or tracked the invisible centre of the moving object.
"The SC contains a topographic map of the visual space around us just as conventional maps mirror geographical areas. This allowed us to record either from peripheral neurons, representing one of the 'wing tips', or central neurons, representing the foveal location of the invisible centre that was tracked," said Hafed.
The fovea, which is responsible for sharp, central vision, is located in the centre of the macular region of the retina, while peripheral vision occurs outside the centre of our gaze.
The researchers observed that the central neurons were the most active during this tracking behaviour, despite the lack of a visual stimulus in the centre of gaze. When the subjects ignored the invisible centre, the same neurons were significantly less active.
"These neurons highlighted the behavioural importance of the location of the invisible center, because it is this location that was the most important for the subjects to successfully track the object," says Krauzlis.
In another study, the researchers temporarily inactivated a subset of superior colliculus neurons, and analysed the resulting changes in tracking performance.
While the subjects still tracked well, their gaze consistently and predictably shifted away from the centre, demonstrating clearly that the superior colliculus is essential for defining the object location.
"By showing that the SC is not just a motor map, but also a map of behaviourally relevant object locations, our results provide a conceptual framework for understanding the role of the SC in non-motor functions such as visual attention and the functional links between motor control and sensory processing," says Hafed.
The findings have been described in two research articles, separately published in two editions of the Journal of Neuroscience.