Evolutionarily, brains developed to interact with the outside world, developing out of senses to orient the organism for food and defense. The basic senses came first, and then the ever more complex networks / computations to deduce what their signals mean and ultimately to build an entire world model, against which to evaluate changes in perception. While perception is now heavily influenced by such mental models, it did not begin there.
This history informs the question of where concepts reside in the brain. The wetness of water is something we feel, and our conception of it is naturally tied to our perception / sensation of it. Our language makes unending use of metaphors that extend this concrete perception-based ontology to the highest abstractions. If someone is a dirty scoundrel, they are only metaphorically dirty, and the visual and tactile aspects of dirtiness are evoked to enrich and clarify the abstraction. While one might imagine that, like computers, our brains store this conceptual data of definitions, categories, properties, etc. just about anywhere, whether in a distributed holographic engram or in dedicated storage areas, the fact is that data storage is localized, such that lesions in various parts of the brain can disrupt recall of specific classes of skills, properties, facts, faces, experiences, etc.
It is also apparent that much of this storage is coincident with our perceptual apparatus (and motor and emotional apparatus). Thus, a brain scan of someone asked to think about and apple and how it differs from a grape will light up areas in the later visual system where incoming perceptions of apples and grapes are decoded into the shape, color, size, etc., which we understand as being their respective properties. Perhaps olfactory and tactile areas come into play as well. Likewise, thinking about a guitar might generate activity in the motor planning areas, providing concepts about playing one. So, analogous to a spotlight of attention, we also have back-casting mini-spotlights of classification and conceptualization that connect "higher" areas in the frontal cortex (which are perhaps asking the questions) to motor, emotional, and perceptual areas, allowing the system to encode complex conceptual schemes once only, where they are first established, in the grounded areas such as in perceptual processing. Once there, they can serve both the immediate perceptual needs of classification, and the evidently related needs of rendering the same classes as concepts.
"Not only is object property information distributed across different locations, but also, these locations are highly predictable on the basis of our knowledge of the spatial organization of the perceptual, action, and affective processing systems. Conceptual information is not spread across the cortex in a seemingly random, arbitrary fashion, but rather follows a systematic plan."
This is the subject of a review, and of a recent paper which attempts to show that it is white matter tracts that constitute the property recognizing units of the brain, not the grey matter targets of their communication. They call this model "representation by connection", and their case is not well made. Yet the overall topic is extremely interesing, so here goes. The fact that this characteristic distribution of property storage in the peripheral systems exists is not in dispute. What is still uncertain is how it is all knit together and what, exactly, is being communicated back and forth. And, of course, what out of these ingredients constitutes the resulting gestalt of consciousness.
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The new paper tries to make a big distinction between grey matter (the dense cellular areas of the brain) and white matter (the tracts of myelinated connecting axons, which have recently been so beautifully traced by DTI MRI. They collected 80 people who have suffered lesions in various localized areas of the brain, due to strokes, hemorrhage, or trauma, and correlate their deficits in object property recall / naming, which are specific and diverse, with separately derived maps of white matter tracts, and semantic / notional distance between the same concepts, as provided by a sampling of normal college students. The latter was taken in several dimensions, including properties of color, manipulability, motion, phonological, shape, usage, and others, but only set up a conceptual space field- it was not explicitly correlated with brain anatomy.
"Distributed GM (grey matter) regions that represent different attribute dimensions (e.g., shape, color, manner of interaction) of the same object are connected by WM (white matter). The WM linking pattern itself would then contain multiple dimensions of information in these GM regions and, importantly, additional information about the manner of mapping among various attributes. The incorporation of these elements has been argued to be necessary for the “higher-order” semantic similarity relationships, which are not explained by attribute-specific spaces, to emerge."
The authors developed anatomical maps of normal white matter tracts, and then developed a map of semantic correlations given the defects in the brain damaged patients, over-laying their cognitive defects, on the same object classification tasks as above, over their individual brain defect regions. The question was then, once the semantic defects were anatomically mapped, are their relative positions correlated with the mapping in conceptual space done by normal volunteers? One would naturally expect that yes, cognition of similar objects (scissors, knives) would happen in spatially close areas of the brain. This was indeed found. What it means, however, especially for the author's theory, is extremely hard to say. It is especially hard as the paper is not well written, (the group is from China), leaving their core ideas rather murky.
"In conclusion, using a structural-property-pattern-based RSA (representational similarity analysis) approach, we found that the WM (white matter) structures mainly connecting occipital/middle temporal regions and anterior temporal regions represent fine-grained higher-order semantic information. Such semantic relatedness effects were not attributable to modality-specific attributes (shape, manipula- tion, color, and motion) or to the representation contents of the cortical regions that they connected and were above and beyond the broad categorical distinctions. By connecting multiple modality-specific attributes, higher-order semantic space can be formed through patterns of these connections."
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| Example of one finding. Two brain areas of gray matter in the left hemisphere (red, superior temporal gyrus; and green, the calcarine sulcus) are linked by a mapped white matter tract. The graph shows a significant r-value for the correlation between the neural similarity (map of white matter lesions by properties reported to be defective) and the author's custom mapping of semantic similarity. |
They do claim explicitly that the corresponding gray matter maps from the brain-damaged patients did not correlate as well with the semantic space patterns as did the white matter maps. But this is not so surprising, as gray matter regions (which could be imagined as the CPUs of the brain) are all connected via the white matter (the networking cables). So for a process that depends on the collation of numerous pieces of information, (the concept property units harvested from each percept / action planning/ emotion region), it would be natural to expect that cutting the cables might be a cleaner way to cause particular property type deficits. That doesn't mean, however, that it is the cables that are conscious, or that are doing the most important processing.
So I think, aside from the operational issues, which look rather daunting in this paper and whose resulting correlation maps are only marginally compelling, there are serious theoretical issues that indicate that while this field is ripe for advancement, this does not seem to be the paper to get us there.
A much more through and positive review than this one.
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- Unfit to serve on a sewer board, II.
- Nature -and- nurture.
