Top down processing


A recent paper by Dima et al, “Understanding why patients with schizophrenia do not perceive the hollow-mask illusion using dynamic causal modeling”, in NeuroImage has been commented on by a number of bloggers. Here is a large part of the abstract:

Patients suffering from schizophrenia are less susceptible to various visual illusions. For example, healthy participants perceive a hollow mask as a normal face, presumably due to the strength of constraining top-down influences, while patients with schizophrenia do not … However the neural mechanisms underpinning this effect remain poorly understood. We used functional magnetic resonance imaging to investigate the hollow-mask illusion in schizophrenic patients and healthy controls. The primary aim of this study was to use measures of effective connectivity arising from dynamic causal modeling (DCM) to explain differences in both the perception of the hollow-mask illusion and associated differences in neural responses between patients with schizophrenia and controls, which we hypothesised would be associated with difference in the influences of top-down and bottom-up processes between the groups. Consistent with this explanation, we identified differences between the two groups in effective connectivity. In particular, there was a strengthening of bottom-up processes, and weakening of top-down ones, during the presentation of ‘hollow’ faces for the patients. In contrast, the controls exhibited a strengthening of top-down processes when perceiving the same stimuli. These findings suggest that schizophrenic patients rely on stimulus-driven processing and are less able to employ conceptually-driven top-down strategies during perception, where incoming sensory data are constrained with reference to a generative model that entails stored information from past experience.

In this illusion, a hollow (concave) mask of a face appears as a normal face (convex). Cannabis users may also be less deceived by the illusion whilst on the drug. Dima’s theory is that perception principally comprises three components: firstly, sensory input (bottom-up); secondly, the internal production of concepts (top-down); and thirdly, a control (a ‘censor’ component), which covers interaction between the two first components. In schizophrenia there is a lack of connectivity between the top-down and bottom-up processes.

Jonah Lehrer gives a clear description of the process. (here)

What happens then? In order to make sense of this visual cacophony, the brain has to do what cameras don’t: interpret the input. It has to parse all those lines and figure out which objects are where. As I’ve noted before, we now know that what we end up seeing is highly influenced by something called “top-down processing,” a term that describes the way cortical brain layers project down and influence (corrupt, some might say) our actual sensation. After the inputs of the eye enter the brain, they are immediately sent along two separate pathways, one of which is fast and one of which is slow. The fast pathway quickly transmits a coarse and blurry picture to our prefrontal cortex. Meanwhile, the slow pathway takes a meandering route through the visual cortex, which begins meticulously analyzing and refining the lines of light. The slow image arrives in the prefrontal cortex about 50 milliseconds after the fast image.

Why does our mind see everything twice? Because our visual cortex needs help. After the prefrontal cortex receives its imprecise picture, the “top” of our brain quickly decides what the “bottom” has seen, and begins slyly doctoring the sensory data. (It’s somewhat akin to tweaking a photo in photoshop…) Form is imposed onto the formless rubble of the V1. If these interpretations are removed, our reality becomes unrecognizable.

And (here).

The polite term for this mental ability is “top-down processing,” a term that describes the way cortical brain layers project down and influence (corrupt, some might say) our actual sensation. After the inputs of the eye enter the brain, they are immediately sent along two separate pathways, one of which is fast and one of which is slow. The fast pathway quickly transmits a coarse and blurry picture to our prefrontal cortex. Meanwhile, the slow pathway takes a meandering route through the visual cortex, which begins meticulously analyzing and refining the lines of light. The slow image arrives in the pre-frontal cortex about 50 milliseconds after the fast image.

I find the ‘top-down’ a misleading name for this process. Maybe ‘gestalt’ would be better. I like ‘fast’ and ‘slow’. There is also fast and slow processes in perceiving sounds – and a theory that a disconnect between them is a cause of dyslexia. The fast process is likely to produce the vague, wide-angle outline in consciousness while the slow process is likely to produce the vividly, detailed focus.

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