Aug 13 2009

Vision and How the Brain Organizes Itself

Knowledge about brain organization and development is rapidly evolving, partly due to new tools we have for research, such as various MRI techniques. One type of question being investigated is the degree to which brain organization is pre-programmed in the genes vs a result of the process of development.

We know that both processes are involved. Human brains are all grossly the same – I have looked at hundreds of MRI scans of patients and they are all pretty much laid out the same way. Neurologists get very good at predicting exactly where a lesion will be in the brain based upon a patient’s symptoms and exam. This ability to localize a lesion is based upon the assumption that all brains are organized the same, and this assumption turns out to be almost always correct, at least to a level of precision required for a clinical assessment. (There are known variations in terms of which side of the brain will house certain specific functions – for example most people are dominant for language in the left hemisphere, but some left-handed people have language in the right hemisphere.) This suggests that genetics plays a dominant role in determining the large-scale structure of the brain.

But neuroscience researchers often operate at a much finer level of detail in terms of neuroanatomy, often dealing with the micro-structure of the brain. And the question remains, at fine levels of detail, how much of the brain’s structure is pre-determined by genetics, how much results from the process of development, and how much is plastic (determined by later use)?

We know that mapping plays an important role in brain development. This is a process by which neurons will map to sensory input or other feedback. When babies see, for example, their visual cortex will map to the stream of information coming in from the retina. The visual cortex therefore creates a literal map of the retina. Genes do not need to lay out this map, like a blue-print, but only have to set up the rules and then allow the mapping to take place.

Neuroanatomy of Vision

But the organization of vision in the brain is much more complex than a simple map of the retina. Visual information undergoes a great deal of processing to make the information more useful. Edges and contrast are enhanced, for example. Also information is processed in such a way as to create three-dimensionality and depth. Generally speaking, optical illusions are tricks that expose these kinds of visual processing.

But even more than optical processing, what we see also conveys meaning (arguably the ultimate evolutionary utlity of vision). The most obvious is probably pattern recognition: we don’t just see shapes, color, and depth – we see things. The famous eponymous essay in the book The Man Who Mistook His Wife for a Hat by Oliver Sacks desribes a man who had a stroke in the occipital-temporal region on boths sides. As a result he had visual agnosia – he could see but could not put all the pieces together and recognize specific things. So he could see the shape of his wife’s head, which is similar to the shape of his hat, but could not tell the two apart.

There are also specific forms of visual agnosia, or type-specific. Prosopagnosia (lesion of the right fusiform gyrus), for example, results in the inability to recognize faces. Landmark agnosia (right parahippocampal gyrus) results in the inability to identify specific buildings or landmarks.

ventral streamTo back up a bit further – our current models of visual organization in the brain is that there are two streams of visual information: the ventral stream relates to object recognition (and lesions of which result in visual agnosia, as described above), and the dorsal stream is for planning actions. Lesions of the dorsal stream result in what is called optic ataxia.

The dorsal stream of visual information is therefore related to seeing an object heading in your direction and getting out of the way, or to the act of catching a baseball. While the ventral stream will tell you that what you are catching is a baseball and not a rock.

To add a further layer of complexity, the ventral stream is divided into various regions with different specialties. The medial structures in the ventral stream relate to non-living objects, while more lateral structures specifically relate to living objects. There are also identifiable specialized regions, such as for visual word recognition and texture vs form.

But clearly, living vs non-living is an important distinction to make, and it is interesting that the visual pathways would separate out based upon such an abstract concept.

Now Back to Development

This all leads back to the original topic of this post, which is to discuss some new research relating to the development of separate visual pathways in the ventral stream for living vs non-living objects. The question at hand is whether or not this level of brain organization within the ventral stream is determined by genetics or by development and use.

To answer this question Dr. Mahon and colleagues at the Center for Mind/Brain Sciences (CIMeC) at the University of Trento, Italy, and Harvard University looked at the brain of subjects with normal vision and subjects who have been blind since birth. The assumption is that those who have been blind since birth have never had any sensory input to their visual cortex, and therefore no developmental mapping or plasticity with use has taken place. Therefore, whatever organization they find is likely innate and determined by genetics.

From the press release:

“Using functional magnetic resonance imaging, we found that the same regions of the ventral stream that show category preferences for nonliving stimuli and animals in sighted adults, show the same category preferences in adults who are blind since birth,” explains senior study author Dr. Alfonso Caramazza from the CIMeC and Harvard University. “Our findings suggest that the organization of the ventral stream innately anticipates the different types of computations that must be carried out over objects from different conceptual domains.”

Cool. So our brains evolved to inherently distinguish between living and non-living things – this ability is built into the basic structure of our visual cortex.

This reminds me of a book I recently read called Supersense by Bruce Hood, in which he argues that we have an innate sense that there is something fundamentally different between things that are alive and things that are not alive. This results, he argues, in supernatural beliefs in a life force or life essence to explain our strong innate feelings.

It seems that this latest research supports Hood’s premise – the distinction between living and inanimate seems to be fundamental to human brain organization. And if the evolutionary pressures were strong enough to result in these categories of objects being distinct in our vision, it is likely that the same is true about how we emotionally and intellectually categorize the world too.

In fact Hood goes over many psychological experiments that show just that – even infants think very differently about living and non-living things.

Conclusion

This is a fascinating area of neuroscience research, which is really taking off due to fMRI and other techniques. It is interesting, but not surprising, that at this fundamental level of  brain anatomy development appears to be following a genetic blue-print, rather than just reacting to input or use.

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