While we have five wonderful senses, humans rely most on our sense of sight. The allocation of real estate in the brain reflects this hegemony; a far greater chunk of your cerebral cortex is dedicated to vision than to any other sense. So when you encounter people, objects, and animals in the world, you typically use visual information to tell your lover from a toothbrush from your cat. And while it would be reasonable to expect your brain to process all of these items in the same way, it does nothing of the sort. Instead, the visual cortex segregates and plays favorites.
The most dramatic examples of this segregation occur whenever you look at other people. Within the large chunk of visual cortex dedicated to object recognition, two areas in each hemisphere specifically process faces (the FFA and OFA) and two areas in each hemisphere specifically process bodies (the FBA and EBA). In each case, one of these areas is located on the side of the brain (near the back) while the other is tucked away on the bottom surface of the temporal lobe. It’s clear that these areas are important for recognizing faces and bodies. Damage to the face area FFA can profoundly impair one’s ability to recognize faces, while direct electrical stimulation of the same area can temporarily distort perception of a face. And when scientists used a magnetic pulse to momentarily disrupt activity in either the face area OFA or the body area EBA of healthy adults, their participants had difficulty discriminating between similar faces or similar bodies, respectively.
Yet the segregation of objects in your visual cortex doesn’t end there. Scientists have long known that visual information about scenes – including the landmarks and buildings that often define them – is processed separately as well. In fact we have at least two scene areas per hemisphere in classic visual cortex: one on the side of the brain (TOS) and one on the lower surface (PPA).*
But what about other types of objects? If you looked at pictures of a trampoline, a screwdriver, a lamppost, and a toad, would they follow the same path through your visual cortex? The answer is no. In a recent study, Talia Konkle and Alfonso Caramazza at Harvard showed people pictures of a wide range of animals and objects while scanning them with fMRI. They studied the activations in visual cortex for each image and used them to compute something they called preference maps. The preference maps indicated whether each bit of cortex preferred animals or objects and, separately, small or large things. When they combined these maps they found zones of visual cortex that preferred large objects, small objects, or animals of any size.** For large objects and animals (with two zones each), one zone was located on the side of the brain and the other on the lower surface. The only zone that preferred small objects over both large ones and animals lay right at the edge of the brain, smack dab between the side of the brain and its lower surface. The face and body areas fit almost entirely within the animal zones, while the scene areas lay within the large-object zones.
Figure from Konkle & Caramazza, 2013 showing where the face areas, body areas, scene areas, and ‘preference zones’ were in one participant. Each gray blob represents the right hemisphere of the brain, with the left side of each blob representing the back of the brain. The top two brains show a side view while the bottom two show the bottom surface of the cortex.
It may seem odd that object representation in visual cortex is organized based on such arbitrary dimensions. Why should it matter whether the thing you see is big or small, made of cotton or has a cottontail? The study’s authors argue that these divisions make sense if one considers the various ways we use different types of objects. For instance, small objects are generally useful because you can manipulate and interact with them. Recognizing an apple, axe, or comb allows you to eat, chop, or fix your ‘do, respectively – so long as the visual information about these objects gets passed along to brain areas involved in reaching and grasping movements.
Objects like buildings, trees, or couches are obviously too large to be lifted or manipulated. Since they stay put, you’re more likely to use them as landmarks to help you navigate through a neighborhood, park, or room. But you can only use these objects this way if you send the visual information about them to brain regions involved in navigation.
Finally, we have living things, which can move, bite, and behave unpredictably. While a large animal like an elephant might trample you, a small one like a venomous spider or snake could be more lethal still. And don’t even get me started on people! In short, an animal’s size doesn’t determine how you will or won’t interact with it; you need to be ready to predict any animal’s behavior and react accordingly. Should you pet that dog or run from it? Communication between the animal-preferring zones of visual cortex and the social prediction centers in your brain might help you reach the right answer before it’s too late.
What’s the upshot of all this using, manipulating, predicting and fleeing? A wonderful and miraculous map of all the stuff in your world. It’s a modest little map – no larger than a napkin and half the thickness of an iPhone 5 – that represents a vast array of creatures, things, and people based on what they mean to you. How frickin’ amazing is that?
* I’ll get back to this mysterious pattern in a future post.
** I find it interesting that people generally approach the game Twenty Questions with the same category distinctions. The first two questions are almost invariably: Is it alive? And is it bigger than a breadbox?
Konkle T, & Caramazza A (2013). Tripartite organization of the ventral stream by animacy and object size. The Journal of neuroscience : the official journal of the Society for Neuroscience, 33 (25), 10235-42 PMID: 23785139
Konkle T, & Caramazza A (2013). Tripartite organization of the ventral stream by animacy and object size The Journal of Neuroscience DOI: 10.1523/JNEUROSCI.0983-13.2013