New research has identified a wide range of brain areas that help us recognize objects we see – and it’s also revealed some surprises about how the brain distributes processing power.
A recent study published in the journal Neuron focuses on a patient – known as “SM” – who suffered a lesion in the right lateral fusiform gyrus (LFG), an area known to be involved in recognition of objects and faces. This has created a disorder known as “visual agnosia,” in which the patient can see just fine, but has serious trouble identifying objects. Decades of research on similar cases have shown that this isn’t a language difficulty – it’s a difficulty associating familiar objects with their names.
Visual agnosia is a pretty fascinating disorder, but it’s actually not the big news here – the unexpected part is that this new study has revealed two surprises about how object recognition works. First, it turns out that many more brain regions than scientists expected are involved in object recognition. And second, it seems that the recognition process involves symmetrical activation patterns across both brain hemispheres, one of which can “fill in” when its partner suffers damage.
By correlating fMRI scans with behavioral observations, a team led by Princeton psychologist Dr. Christina Konen discovered that the functional connections involved in object recognition extend throughout the temporal and visual cortices of both the left and right hemispheres:
Visual responses, object-related, and -selective responses were reduced in regions immediately surrounding the lesion in the right hemisphere, and also, surprisingly, in corresponding locations in the structurally intact left hemisphere. In contrast, hV4 of the right hemisphere showed expanded response properties.
Carnegie Mellon University’s Dr. Marlene Behrmann, one of the study’s other leaders, was pretty stunned by this new data. As she puts it:
These results will force us in the field to step back a little and rethink the way we understand the relationship between brain and behavior. We now need to take into account that there are multiple parts of the brain that underlie object recognition, and damage to any one of those parts can essentially impair or decrease the ability to normally recognize objects.
It’ll probably help to back up a bit, and explain exactly why neuroscientists are so amazed by this information.
See, even though the two hemispheres of our brain might look symmetrical, there are also plenty of asymmetries between them – both in structure and in function. One of the most obvious examples is handedness: the majority of people are right-handed, even though there doesn’t seem to be any particular anatomical reason why this should be so – and in fact, plenty of other animals have about a 50/50 distribution between right- and left-handedness.
And then there’s language: it definitely seems to be localized in the left hemisphere. This is fairly easy to verify – the Wada Test involves injecting an anesthetic into one hemisphere of a patient whose corpus callosum, which acts as a communication bridge between the two hemispheres, has been severed. When the two hemispheres can’t communicate, anesthetizing the left hemisphere seriously impairs speech and language comprehension.
In short, the idea that certain functions are localized in one hemisphere is largely accepted today. Even so, fMRI scans show that plenty of tasks activate our brains in symmetrical patterns. What exactly this means is still up for debate – some neuroscientists think it might reflect a “safety net” of functional redundancy, while others say the two hemispheres may complement each others’ processing.
Whatever the case, experiments like the one above demonstrate that even when one hemisphere is damaged, the brain does its best to work around the problem:
There … appeared to be some functional reorganization in intact regions of SM’s damaged right hemisphere, suggesting that neural plasticity is possible even when the brain is damaged in adulthood.
Now, this ties into a much more exciting set of implications about our brains. For example, what about children who’ve had one of their brain hemispheres surgically removed, yet go on to complete college and lead productive lives? Or people who – as fMRI scans have confirmed – learn to literally see via sound or touch?
As long as the brain in question is young enough, or is given enough time to adapt, many areas can take over the functions of others. Perhaps our functional processing is distributed more widely in childhood, and “solidifies” more as we age – but even grown-ups can teach an old brain region new tricks.
This is why it’s not very accurate to say there’s “a” human connectome – each of our brains is wired in a unique way, and is constantly rewiring itself every second. New neurons are being born all the time – and existing ones are always forming new connections and being co-opted for new tasks.
In the unfortunate case of “SM,” not all the IFG’s functionality could be preserved; but even so, the case is a striking example of how versatile our brains can be – and how much we still have to learn about the ways they process information.