Monday, November 11, 2013

Is Association Cortex Capable of Holding Mental Imagery?

Antonio Damasio has proposed that early sensory cortices construct “image space” and that association cortices construct “dispositional space.” He believes that association cortex does not hold any imagery itself and that everything we see in our “mind’s eye” is constructed in sensory cortex alone. I, on the other hand, believe that association areas are capable of holding imagery that people consciously experience. In my opinion association cortices hold imagery of higher-order concepts that are disoriented from spatial mapping or retinotopic coordinates. The imagery created in association cortex probably embodies conceptual relationships and perceptually transcendent concerns. Association areas may hold true imagery in the sense that they can invoke high-level perceptions of things of which the person can become conscious. Thus, cortical areas involved in visual processing - from the posterior occipital pole to the anterior frontal pole – lie together on a continuum with coordinate bound maps on one side and abstract, conceptual imagery on the other.

Consistent with Damasio; however, I agree that association areas do not possess all of the information held in the early sensory cortices that converge upon them. In other words, the firing of a grandmother neuron in the anterior temporal cortex alone does not produce a conscious visual depiction of a grandmother in the mind’s eye. One can probably not visualize a spatial, line-bound image of one’s grandmother without early visual cortex. The mental imagery of our grandmothers produced by association cortex would be much more abstract and invariant, and may include behavioral predispositions such as the way we feel, act and compose ourselves when we are around our grandmother.

What would imagery mappings in association areas such as the dlPFC look like? Early visual areas create retinotopic visual information because the inputs to the cortex correspond to the geometric arrays of photoreceptors in the retina. The dlPFC does not contain an objective input geometry that maps directly onto something real in the environment. Instead, the input, and thus the maps correspond to the placement and relative orientation of the lower-order projection inputs that were arranged during the mammalian evolution of the neocortex. Thus, the question regarding the spatial architecture of higher order thought and its imagery can only be answered in the future by neurocartographic investigation of the unique connectional geometry found in higher-order areas.

I believe that the early visual cortex activation creates vibrant, experiential imagery simply because it has become correlated with the appearance of this imagery in the environment. Brain cells create a theatre of the mind because they have “taken on” certain external properties. If this is true, then imagery must be held everywhere because each part of the brain has become correlated with some type of environmentally induced experience. Like the neurons responsible for the sensations in a phantom limb, early visual neurons “hold” the experiential properties of experiences with which they have been correlated with in the past. Surely anterior association areas have also correlated with experiences, albeit abstract ones. Thus, purporting that association areas do not hold true imagery is like saying that imagery is held in the “dots” of primary visual cortex but not in the “contours” of secondary visual cortex. When you imagine things, from simple objects to abstract concepts, you experience them again because you fire the same neurons that fire when it was experienced. This thinking then frames consciousness as a jumbled up reflection of environmental occurrences. It is fascinating that we are able to construct a cohesive percept from a hodgepodge aggregate of previously distinct microrepresentations.

The idea that association areas may hold their own brand of abstractly mapped imagery frames the brain as a system of interacting modules specializing in mapping different topographies that are all trying to generate their best interpretation of what the other modules are doing. Because some of these modules have assemblies that fire for sustained periods, they are better positioned to direct activity through time. The dorsolateral PFC is a good example of a module with the capacity for sustained influence over modules specialized for visual and auditory processing, whereas an area such as the orbital PFC may direct maps generated by emotional and reward-related modules. Further research into the deficits and intact abilities in patients with damaged association cortex may better elucidate this issue.

I have published an article on this issue that you can read here:

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