Abstract Representations of Object-Directed Action in the Left Inferior Parietal Lobule

Abstract

Prior neuroimaging and neuropsychological research indicates that the left inferior parietal lobule in the human brain is a critical substrate for representing object manipulation knowledge. In the present functional MRI study we used multivoxel pattern analyses to test whether action similarity among objects can be decoded in the inferior parietal lobule independent of the task applied to objects (identification or pantomime) and stimulus format in which stimuli are presented (pictures or printed words). Participants pantomimed the use of objects, cued by printed words, or identified pictures of objects. Classifiers were trained and tested across task (e.g., training data: pantomime; testing data: identification), stimulus format (e.g., training data: word format; testing format: picture) and specific objects (e.g., training data: scissors vs. corkscrew; testing data: pliers vs. screwdriver). The only brain region in which action relations among objects could be decoded across task, stimulus format and objects was the inferior parietal lobule. By contrast, medial aspects of the ventral surface of the left temporal lobe represented object function, albeit not at the same level of abstractness as actions in the inferior parietal lobule. These results suggest compulsory access to abstract action information in the inferior parietal lobe even when simply identifying objects.

Figure 1.

Schematic of cross-item multivoxel pattern analysis. (A) From the perspective of the experimental design, the stimuli are organized into 2 triads, in which 2 of the 3 items (of a triad) were similar by manner of manipulation or function. (B) To decode action, the classifier was trained to discriminate, for example, “scissors” from “corkscrew” and the classifier was then tested on “pliers” versus “screwdriver”. Because the action of using scissors is similar to that of using pliers, and likewise for corkscrew and screwdriver, accurate classification across objects indicates that the voxels being classified differentiate the action representations over and above the specific objects themselves. The same procedure was used to decode object function.

Figure 2.

Searchlight analyses of cross-item classification for action. First column: decoding of action during tool pantomiming. Second column: decoding of action during tool identification. Third and last column: cross-task decoding of action. All results are thresholded at P < 0.01 (cluster corrected), except for the cross-task decoding of action (trained on “identification” and tested on “pantomime,” thresholded at P < 0.05, cluster corrected).

Figure 3.

Direct comparison of tool-preferring regions and searchlight analyses for action. The searchlight results for the within-task decoding of action (red and pink) overlapped tool-preferring regions in the left hemisphere. The overlap of voxels identified as representing actions in both tasks in the searchlight analysis is in yellow. The searchlight results for the cross-task decoding of action were outlined in dark blue and light blue. Tool-preferring regions outlined in black are shown based on group results, thresholded at P < 0.01 (cluster corrected). All results are overlaid on a representative brain.

Figure 4.

Searchlight analyses of cross-item classification for function. First column: decoding of function during tool pantomiming. Second column: decoding of function during tool identification. Third and last column: cross-task decoding of function. All results are thresholded at P < 0.01 (cluster corrected), except for the decoding of function during tool pantomiming (thresholded at P < 0.05, cluster corrected).

Figure 5.

Direct comparison of tool-preferring regions and searchlight analyses for function. The searchlight results for the within-task decoding of function (light blue and green) overlapped tool-preferring regions in the left hemisphere. The overlap of voxels identified as representing function in both tasks in the searchlight analysis is in dark blue. The searchlight results for the cross-task decoding of function are outlined in red and yellow. Tool-preferring regions outlined in black are shown based on group results, thresholded at P < 0.01 (cluster corrected). All results are overlaid on a representative brain.

Figure 6.

ROI analyses of cross-item classification. The bars represent the mean classification accuracy for within- and cross-task decoding (red: trained with “pantomime” data and tested with “pantomime” data; pink: trained with “identification” data and tested with “identification” data; dark blue: trained with “pantomime” data and tested with “identification” data; light blue: trained with “identification” data and tested with “pantomime” data). Black asterisks indicate statistical significance with one-tailed t tests across subjects with respect to 50% (*P < 0.05; **P < 0.01; ***P < 0.001). Solid black lines indicate chance accuracy level (50%). Error bars represent the standard error of the mean across subjects. L-IPL, left inferior parietal lobule; L-PMC, left premotor cortex; L-MTG, left posterior middle temporal gyrus; L-MFG, left medial fusiform gyrus.