How the camel lost its hump: the impact of object typicality on event-related potential signals in object decision.

Using an object decision task, event-related potentials (ERPs), and minimum norm current source estimates, we investigated early spatiotemporal aspects of cortical activation elicited by line drawings that were manipulated on two dimensions: authenticity and typicality. Authentic objects were those that match real-world experience, whereas nonauthentic objects were "doctored" by deletion or addition of features (e.g., a camel with its hump removed, a hammer with two handles). The main manipulation of interest for both authentic and nonauthentic objects was the degree of typicality in the object's structure: typical items are composed of parts that have tended to co-occur across many different objects in the perceiver's experience. The ERP pattern revealed a significant typicality effect at 116 msec after stimulus onset. Both atypical authentic objects (e.g., a camel with its hump) and atypical nonauthentic objects (e.g., a jackal with a hump) elicited stronger brain activation than did objects with typical structure. A significant effect of authenticity was observed at 480 msec, with stronger activation for the nonauthentic objects. The factors of typicality and authenticity interacted at 160 and 330 msec. The most prominent source of the typicality effect was the bilateral occipitotemporal cortex, whereas the interaction and the authenticity effects were mainly observed in the more anterior bilateral temporal cortex. These findings support the hypothesis that within the first few hundred milliseconds after stimulus presentation onset, visual-form-related perceptual and conceptual processes represent distinct but interacting stages in object recognition.