Abstract
In 1979, Gibson first advanced the idea that the sight of graspable objects automatically activates in the observer the repertoire of actions necessary to interact with them, even in the absence of any intention to act (“affordance effect”). The neurophysiological substrate of this effect was later identified in a class of bimodal neurons, the so-called "canonical" neurons, located within monkey premotor cortex. In humans, even if different behavioral studies supported the existence of affordance effect, neurophysiological investigations exploring its neural substrates showed contradictory results. Here, by means of Transcranial Magnetic Stimulation (TMS), we explored the time-course of the “affordance effect” elicited by the observation of everyday-life graspable objects on motor cortex of resting observers. We recorded motor evoked potentials (MEP) from three intrinsic hand muscles (two "synergic" for grasping, OP and FDI and one "neutral", ADM). We found that objects’ vision determined an increased excitability at 120 milliseconds after their presentation. Moreover, this modulation was proved to be specific to the cortical representations of synergic muscles. From an evolutionary perspective, this timing perfectly fits with a fast recruitment of the motor system aimed at rapidly and accurately choosing the appropriate motor plans in a competitive environment filled with different opportunities.
Highlights
These authors contributed to this work. In his ecological approach to vision [1], Gibson argued that, when we look at objects, we directly perceive their physical properties, and the constellation of potential actions we can perform on and with them
Superimposed motor evoked potentials (MEP) traces from the three muscles of a representative subject are shown in Fig. 3 Mean MEP areas (+/ 2standard deviation) from opponens pollicis (OP), first dorsal interosseus (FDI) and abductor digiti minimi (ADM) across all subjects were equal to 10.9 (+/211.8), 11.9 (+/27.5), 7.1 (+/24.8) mV*millisecond respectively
The repeated measure ANOVA reported a significant main effect of the factor Muscle (F1,9 = 3.4 p,0.001) and t-test post-hoc comparison showed that both OP and FDI were significantly more activated than ADM (p,0.05), Figure 3
Summary
In his ecological approach to vision [1], Gibson argued that, when we look at objects, we directly perceive their physical properties, and the constellation of potential actions we can perform on and with them He called this set of potential actions ‘‘affordances’’ and stated that they are generated without the need or the intention to act on the observed object; at the same time, they are highly constrained by the observer’s motor repertoire. The authors argued that motor neurons with different discharge properties code different goal-directed actions and all together they constitute a ‘‘motor vocabulary’’ always accessed by visual information. Among these motor neurons, about 20% showed object-related visual properties. Further studies confirmed the existence of ‘‘canonical’’ neurons within both ventral and dorsal premotor cortex [12,13], and within intraparietal region AIP and posterior parietal cortex [14,15]
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