How the motor and visual areas of the brain work together to guide reaching and grasping movements
Walking around your house, grasping your coffee cup, or preparing breakfast might be relatively simple but require different brain mechanisms to be correctly done. Humans heavily rely on vision, and one of the main brain structures, known as the visual cortex, processes visual information and is located at the back of the brain. The visual cortex itself is split into different regions, each with their own specialty. For example, the primary visual area is a region of visual cortex that is located the farthest back and is the first point of entry in the brain for visual information arriving from the eye. Interestingly, brain cells in the primary visual cortex form a one-to-one visual field map in such a way that neighboring regions of the visual image captured in our eye are represented by neighboring regions of the primary visual area, forming a “retinotopic map” (see Figure 1). For example, what you tend to see in the lower left part of your vision activates one subregion of the primary visual area, what you see in the lower right activates a different subregion. In some ways, the primary visual area is the brain’s way of redrawing what the eyes see in a way that it can understand.
The primary visual area receives visual information from the retina and sends it to higher cortical areas through the dorsal or ventral pathways. The dorsal pathway sends information to motor areas (dotted outline). The retinotopic map corresponds to the projection of the visual input (here the coffee cup) in the retina to the primary visual area, allowing to redraw the corresponding image although distorted in this area. Brain graphic was modified from Wikipedia images by user Hankem.
From the primary visual cortex, information about what is being seen is processed and relayed to the other parts of the brain through two visual pathways: the dorsal and ventral pathways (see Figure 1). The dorsal pathway is the brain circuit where information travels dorsally (to areas at the top of brain) from the visual cortex and it processes information about location and space. Let us go back to the example of reaching for your coffee cup. As you reach for your coffee cup, the dorsal pathway estimates how far the cup is so that appropriate arm movements can be made to grab it. On the other hand, the ventral pathway processes information about features of the cup itself, including how heavy it is, its shape and its color. Both pathways provide important information about the world around us and how to appropriately interact with it. Recently, a research study published in the journal Cerebral Cortex investigated whether these areas located higher in the visual processing send back, to the primary visual area, information related to the incoming planned actions.
The study was led by Professor Jody Culham from Western University and Jason Gallivan, her former PhD student, now a Professor at Queen’s University. The primary visual area receives feedback projections from a lot of different brain regions. This network of connections was already shown to help improve perceptual representation or maintain information relative to stimulus features of the visual stimulus in the context of visual-perceptual tasks or working memory tasks. However, feedback projections to the primary visual area related to the motor domain was not explored yet. For example, when I am looking my cup of coffee, does the visual information are sent to higher areas, such as motor cortex, only through a passive processing? Or does the primary visual area is influenced by information received from this motor related areas before any action was executed? The second hypothesis, investigated in this study, suggests the importance of the projections network in the brain and that information about planned action can be relay to the visual system before its execution. This will help refine and adjust the movement to perform the action you would like to do, such as grab your cup of coffee or any everyday life actions.
To answer these questions, Dr. Culham and Dr. Gallivan used functional magnetic resonance imaging (fMRI) to study the patterns of brain activity in the primary visual cortex before a movement was performed. fMRI is a brain imaging technique that allows researchers to measure which brain areas are activated during a task. In this study, researchers measured this brain activity using three separate fMRI experiments while participants performed easy tasks. Lying in an MRI scanner, participants were presented with actual objects (cube or cup) and then asked to interact with these objects in different ways. The experiments tested different locations of objects (middle, left or right), different body parts performing the action (left/right hand or eye), different actions (grasping and placing the object down, reaching or looking). Brain activity was measured during a ‘delay’ period where participants received the command for which action they will be requested to perform, and during an ‘execute’ period where they performed the action.
Interestingly, during the delay period before the action was performed, the activity measured in the part of the primary visual area associated with the location of the object (e.g. the cup in the bottom left), was different according to which action was about to be performed and how. When you see a cup you want to grab, your brain’s visual areas respond differently than when you see a cup you just want to reach. But how can this be, if it’s the same cup? Well, because the parts of your brain that have to move your body actually change your brain’s ‘view’ of the object by sending feedback to your visual areas before you even start to grab your cup.
This research finding shows that the visual cortex is not only passively transferring the information towards higher cortical areas (e.g. motor cortex) along the dorsal and ventral pathways, but is also modulated by feedback projections received from these areas. This study validates the second hypothesis stated above. The authors suggest this feedback system allows the visual cortex to rapidly detect potential movement errors so that it can correct and adjust those movements before they are executed by sending updated information back to those motor areas. For example, in order to drink my coffee, I have to locate my coffee cup, reach out and start to adjust my fingers so that they can grasp the cup handle, then grasp, lift and bring the cup to my mouth. By receiving feedback from the motor areas that control these movements before they are performed, the visual area can anticipate the consequences of each action and then do corrections in case of errors or mismatches between what was predicted and what’s actually happening. This process is important for adjusting the orientation of our hands while grasping the cup to avoid spilling the coffee when we see that it’s very full and about to spill if we don’t correctly update our hand orientation.
This study shows how actions and vision are closely related and can influence each other. This close relationship and the continuous back-and-forth communication across brain areas are important in our everyday lives – like when we want to enjoy a nice cup of coffee without making a mess!
Original Research Article:
Gallivan, J.P., Chapman, C.S., Gale, D.J., Flanagan, J.R., Culham, J.C., 2019. Selective Modulation of Early Visual Cortical Activity by Movement Intention. Cereb Cortex 29, 4662–4678. https://doi.org/10.1093/cercor/bhy345