250 ms to code for action affordance during observation of manipulable objects

Abstract

It is well known that viewing graspable tools (but not other objects) activates motor-related brain regions, but the time course of affordance processing has remained relatively unexplored. In this study, EEG was continuously recorded from 128 scalp sites in 15 right-handed university students while they received stimuli in the form of 150 pictures of familiar non-tool objects and 150 pictures of manipulable tools, matched for size, luminance and perceptual familiarity. To select the 300 images for the study, a wider set of preliminary stimuli was screened for motoric content by 20 judges using a 3-point scale (0=absent; 2=strong); pictures that scored below 1.5 or above 0.6 were excluded from the tool and non-tool categories, respectively. Tools and non-tools were presented in random order, interspersed with 25 photos of live plants. Each slide was presented for 1000ms, with an interval ranging from 1500 to 1900ms. The task consisted of responding to the photos of plants while ignoring the other stimuli. Both an anterior negativity (210–270ms) and a centroparietal P300 (550–600ms) were larger in response to tools than objects, particularly in the left hemisphere. swLORETA inverse solution identified the occipito-temporal cortex (BA19 and BA37) as the most significant source of activity (in the 210–270-ms time window) for both types of visual objects and the left postcentral gyrus (BA3) and the left and right premotor cortex (BA6) as the most significant source of activity for tools only. These data hint at an automatic access to motoric object properties even under conditions in which attention is devoted to other stimulus categories.