Updating your cog-neurosci textbook

 In cognitive science, education, modularity

mtl.jpgTake any textbook on cognitive neuroscience. If you go through the book you wil see that there are chapters on perception (e.g. vision), memory, and language. Each chapter has its own vocabulary, theories and experimental evidence. Each chapter may even have been written by different authors (i.e. authorities).

Once reading such a book you will have knowledge about how visual input is processed from the initial steps in the retina, through the thalamic nuclei, and in the visual cortex, just as well as you will learn that as you perceive something as an object you will make use of areas in the temporal lobe, including the fusiform gyrus. You will have learned that memory — especially episodic and semanic memory — is a result of activity occurring in the medial temporal lobe, and especially hippocampus. You will know that theories of language and semantics point to the temporal lobe as important for its functioning.

All in all, you have a nice impression of how the brain is responsible for different perceptual and cognitive functions. But think now of the three examples: they all seem to imply the temporal lobe as important for their functioning. So does this mean that visual perception, memory and language resides in different, non-overlapping parts of the temporal lobe? If so, how do these areas or modules communicate with each other?. What is the lingua franca of neurons comunicating information fro the visual senses to memory and semantics? Add on top of this that parts of the temporal lobe has been implemented in many other functions, including hearing (e.g. the planum temporale and Heschl’s gyrus) and odour processing (e.g. the entorhinal cortex). How does this combine with the other functions? Should we see the temporal lobe as a patchwork of distinct and neatly segregated functions?

For a long time the predominating view of the temporal lobe has been a strictly modular one: one part of the lobe processes visual input, there are language and memory modules. Non-overlapping parts of a lobe that are tuned to process one kind – but not other kinds – of information.

But this view is changing dramatically. Today, following researchers such as Elisabeth Murray, David Gaffan and others (especially from the universities of Cambridge and Oxford, UK) the standard view of temporal lobe function is changing. Instead of a functionally segregated model of the temporal lobe, these researchers now suggest that the lobe has an entirely different way of functioning. In this area, often referred to as the medial temporal lobe one has now documented not only multiple cognitive functions in a brain area once thought to be dedicated to memory, but also redundancy between the structures. Some examples:

  1. There is a functional specialization within the rhinal cortices beyond the involvement in memory: the entorhinal cortex is involved in odour perception as well as multi-modal conjunct perception, i.e. the perception of the entirety of a scene, including sights, sounds and more. The perirhinal cortex is involved in novelty processing, higher-order visual conjunct perception and discrimination, as well as high-specificity semantic processing.
  2. Specific and small anatomical regions are involved in different cognitive functions. For example, the perirhinal cortex has been shown to be involved in memory processes (particularly visual object encoding, but also other forms), novelty processing, semantic processing and high-order visual perception and discrimination

While point 1 does not conflict with a modular view of the brain-mind, point 2 poses a serious problem to any modularist view of the human mind and brain. In many respects, findings now converge on a view of the brain that stresses functional redundancy and degeneracy. In other words: A) one structure can participate in many different functions; and B) many structures are necessary parts of any given cognitive function. A mapping of a 1:1 relationship between a cognitive function and its wetware is thus unsupported by today’s knowledge.

So take that cog-neurosci textbook again, scroll through its pages and ask yourself: how are these cognitive functions connected? Better still, take the chapter to your supervisor, lecturer or whoever you want and ask: “how does the temporal lobe deal with memory, language, visual perception and other multi-modal operations, and how are these processes tied together?” It would be interesting to hear the replies you get.

-Thomas

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