Papers by Nikhilesh Natraj
Neuroscience, 2018
The ability to recognize a tool's affordances (how a spoon should be appropriately grasped and us... more The ability to recognize a tool's affordances (how a spoon should be appropriately grasped and used), is vital for daily life. Prior research has identified parietofrontal circuits, including mirror neurons, to be critical in understanding affordances. However, parietofrontal action-encoding regions receive extensive visual input and are adjacent to parietofrontal attention control networks. It is unclear how eye movements and attention modulate parietofrontal encoding of affordances. To address this issue, scenes depicting tools in different use-contexts and grasp-postures were presented to healthy subjects across two experiments, with stimuli durations of 100 ms or 500 ms. The 100-ms experiment automatically restricted saccades and required covert attention, while the 500-ms experiment allowed overt attention. The two experiments elicited similar behavioral decisions on tool-use correctness and isolated the influence of attention on parietofrontal activity. Parietofrontal ERPs (P600) distinguishing tool-use contexts (e.g., spoon-yogurt vs. spoon-ball) were similar in both experiments. Conversely, parietofrontal ERPs distinguishing tool-grasps were characterized by posterior to frontal N130-N200 ERPs in the 100-ms experiment and by saccade-perturbed N130-N200 ERPs, frontal N400 and parietal P500 in the 500-ms experiment. Particularly, only overt gaze toward the hand-tool interaction engaged mirror neurons (frontal N400) when discerning grasps that manipulate but not functionally use a tool-(grasp bowl rather than stem of spoon). Results here detail the first human electrophysiological evidence on how attention selectively modulates multiple parietofrontal grasp-perception circuits, especially the mirror neuron system, while unaffecting pari-etofrontal encoding of tool-use contexts. These results are pertinent to neurophysiological models of affordances that typically neglect the role of attention in action perception.
The perception of tool-object pairs involves understanding their action-relationships (affordance... more The perception of tool-object pairs involves understanding their action-relationships (affordances). Here, we sought to evaluate how an observer visually encodes tool-object affordances. To this end, eye-movements were recorded as right-handed participants freely viewed static, right-handed, egocentric tool-object images across 3 contexts: correct (hammer-nail), incorrect (hammer-paper), spatial/ambiguous (hammer-wood), and 3 grasp-types: no hand, functional grasp-posture (grasp hammer-handle), non-functional/manipulative grasp-posture (grasp hammer-head). There were three Areas of Interests (AOI): the object (nail), the operant tool-end (hammer-head), the graspable tool-end (hammer-handle). Participants passively evaluated whether tool-object pairs were functionally correct/incorrect. Clustering of gaze scanpaths and AOI weightings grouped conditions into three distinct grasp-specific clusters, especially across correct and spatial tool-object contexts and to a lesser extent within the incorrect tool-object context. Permutation tests revealed that the grasp-specific gaze scanpath clusters were reasonably robust to the temporal order of gaze scanpaths. Gaze was therefore automatically primed to grasp-affordances though the task required evaluating tool-object context. Participants also primarily focused on the object and the operant tool-end and sparsely attended to the graspable tool-end, even in images with functional grasp-postures. In fact, in the absence of a grasp, the object was foveally weighted the most, indicative of a possible object-oriented action priming effect wherein the observer may be evaluating how the tool engages on the object. Unlike the functional grasp-posture, the manipulative grasp-posture was a gaze attractor and caused the greatest disruption in the object-oriented priming effect, ostensibly as it does not afford tool-object action due to its non-functional interaction with the operant tool-end that actually engages with the object (e.g. hammer-head to nail). The enhanced attention may therefore serve to encode the intent of the manipulative grasp-posture. Results here show how contextual and grasp-specific affordances directly modulate how an observer gathers action-information when evaluating static tool-object scenes.
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Papers by Nikhilesh Natraj