Representation of object weight in ventral visual cortex
According to the dominant neural framework for understanding visual processing in the primate brain, the ventral visual stream in occipitotemporal cortex (OTC) supports visual processing for perception and object recognition but not for action, which is the purview of the dorsal visual stream in frontoparietal cortex (Goodale and Milner, 1992, see figure below).
However, to date, this division of labour has only been tested for visually guided actions like reaching which are directed towards objects, and not for actions involving object manipulation. Skilled manipulation requires knowledge of an object’s dynamics, like weight, which, unlike size, shape and texture, cannot be directly appreciated through vision alone. In this study we explored the novel hypothesis that object weight, a motor-relevant property, is represented in object-sensitive visual areas of OTC, to date thought to only represent the surface properties of objects (e.g., texture), directly and immediately available to the visual system.
We demonstrate that knowledge of object weight, used to plan and execute lifting movements, is represented in object-sensitive regions of OTC. This novel finding shows, for the first time, that non-visual object properties are represented in the ventral visual pathway, and challenges the widely held view that processing for action is restricted to the dorsal visual stream.
Using human fMRI, pattern classification methods, and an event-related task in which participants first prepared and then executed lifting actions with objects of varying weight, we found that object weight could be reliably decoded from activity patterns in object-sensitive OTC areas prior to the object being lifted (i.e., during action planning). In a first study, we show that the coding of object weight in OTC occurred despite the differently weighted objects being visually identical. In a second study, we show that once an object visual property (e.g., surface texture) becomes reliably predictive of an object’s weight, OTC is involved in establishing this linkage.
Significance of results
Our paper demonstrates a previously undocumented role for the ventral visual stream in action, by showing that object weight, a non-visual object property critical for object manipulation, is represented in OTC. These results indicate that the ventral visual stream directly supports both perception and action and, therefore, the functions performed by areas in OTC are significantly broader than previously thought. Our results should contribute to a significant reshaping of the dominant neural framework used for understanding how objects are visually processed for the purposes of guiding action.
For more information see Gallivan JP, Cant JS, Goodale MA, Flanagan JR (2014) Representation of object weight in human ventral visual cortex Current Biology 24: 1866-1873.