Researchers at the Salk Institute for Biological Studies have discovered a neural circuit that is likely to play an important role in the visual perception of moving objects. Their finding, published in the April issue of the journal Neuron, forces neurobiologists to rethink the neural pathways that our brain relies on to detect motion.
It had long been assumed that sensory information about color and fine detail is relatively unimportant for the perception of moving objects. Mainly, because the neural pathways in the brain carrying color and fine detail information seemed to be completely separate from areas of the brain previously associated with motion processing.
In an elegant anatomical study, Salk researchers now show that a neural pathway carrying color and fine detail does connect to the motion processing areas of the cortex (the outer layer of the brain), and this information most likely helps the brain detect moving objects.
"There are many different kinds of cues in the visual environment that can be used to detect motion - basically anything that is moving," says Edward M. Callaway, Ph.D., senior author of the study and a professor in the Systems Neurobiology Laboratory. "We asked the question, 'Is motion processing taking advantage of the full range of possible cues?' "
This study demonstrates, for the first time, that it is.
Our eyes take in the visual environment and break the incoming images down into three main components: color, position, and brightness. These pieces of information are channeled from the eye to the brain along separate, specialized pathways. The parvocellular (P) pathway carries information about color and fine spatial detail. The magnocellular (M) pathway, on the other hand, is colorblind and has poor spatial resolution; instead, it is sensitive to low contrast and rapid changes. The visual cortex uses the information from these pathways to compute further details about motion, shape, and color.
Until now, it was thought that only the M pathway connected to the cortical motion processing area called MT. This is because the M and P pathways remain separate as they extend through the brain to the primary visual cortex (V1). And the cells in V1 that provide input to MT appeared to receive input from only the M pathway. The new results show that these cells also receive input from the P pathway.
Callaway and his colleagues used a system based on the rabies virus, whose unique infectious properties allowed them to trace neural circuits in reverse, from MT back to the distinct M and P cells that connect to V1. This technique, known as trans-synaptic tracing, showed that the M and P pathways merge before they enter the MT area, on a specialized population of neurons in an area of V1 known as layer 6. These layer 6 neurons, in turn, connect directly with neurons in the MT region, carrying the merged M and P signal onward for further processing.
As graduate student and co-first author Jonathan J. Nassi put it, "We are really pioneering the use of trans-synaptic viral tracing to study the visual system. Already, with our first study and experiments, we're having to rethink how the visual system is wired-up."
Part of the reason why scientists had overlooked this circuit was because the M pathway is known to be more sensitive to rapid changes. Historically, according to Callaway, "people tended to think about detection of fast motion changes. But we also need to detect the motion of things that are moving more slowly. The addition of the P pathway to the motion system helps us to see movement of things to which the M pathway is blind."
An example where the P pathway would be important for motion detection is a colored, slowly moving lizard camouflaged against a background of sand. While the M pathway would be blind to the lizard, the P pathway would detect its color, fine detail and slow movement.