Wednesday, February 13, 2008

Differential processing of objects under various viewing conditions in the human LOC

Grill-Spector, K. et al. (September 1999). Differential processing of objects under various viewing conditions in the human lateral occipital complex. Neuron, Vol. 24, 187-203.

This study used fMRIa techniques to investigate the brain's object-selective regions (namely the lateral occipital complex). fMRIa assumes that a group of neurons will respond to repeated presentations of a stimulus with attenuated responses. This signal reduction is presumably the result of neural fatigue from repeated exposure. Thus, "adaptation" data can be used to identify which types of stimulus are effectively treated identically by a certain region of the brain. This is especially interesting in structures further up the processing hierarchy where lower-level transformations have likely given way to more abstract, general representations of the stimuli. Researchers can then explore what properties of objects are preserved and which are transformed to a canonical representation by the time signals converge on a specific region.

First, the study sought to understand how long adaptation effects last. Time durations as long as 8 sec between matched stimuli still elicited amplitude reductions, establishing that adaptation has a fairly long-lasting effect.

Secondly, the study set out to examine which object properties were invariant within the LOC. The results indicate that the LOC is less sensitive to changes in size and position, compared to changes induced by illumination and viewpoint (rotation). In other words, it seems the LOC receives visual input which has been normalized for size and position.

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