Image credit: Giuseppe Milo (CC BY 2.0)
As we interact with the world around us, signals flow from neuron to neuron and from one brain area to the next. When we look at an object, for example, signals pass along a pathway of areas in the outermost part of the brain, called the cortex. Each area along this visual pathway performs more complex processing than the one before it. But information also flows in the opposite direction along such cortical pathways. These feedback signals enable areas further along the pathway to influence the activity of those before them.
Studies in animals suggest that much like a highway, information is travelling in opposite directions within the cortex along different lanes. In mammals, these lanes consist of distinct layers of cells. In the visual cortex of monkeys, feedback signals travel via deeper layers of cortex, whereas feedforward signals travel via the upper layers. Brain activity in the upper layers also has a higher frequency than that in the lower layers.
But is this also the case in our own brains? Bonaiuto et al. used a technique called MEG to measure the frequency of brain activity within the upper and lower layers of cortex in healthy volunteers. The volunteers had to look at images on a screen and then respond by pressing a button. Bonaiuto et al. observed that activity in deeper layers of cortex occurred mostly at lower frequencies, while activity in upper layers mostly happened at higher frequencies. This pattern, which matches that seen in monkeys, was found in both visual cortex and in areas of cortex that help plan and execute movements. In visual cortex, the activity in the upper layers appeared to carry feedforward signals. But in movement-related areas, feedback and feedforward signals were less clearly related to cortical layers.
These findings lend support to current theories about how the cortex is organized. They also show that MEG can reveal rapidly changing brain activity at a high spatial resolution. The findings may also provide clues to the origins of brain disorders called oscillopathies. These involve changes in specific frequencies of brain activity, and include schizophrenia and epilepsy, among others.
