Foot-related/walking macro-affordances are implicitly activated and preferentially guided by the framing distance of the environmental layout

Animals’ survival is likely guaranteed by a backbone of neural mechanisms developed to support adaptive interactions between animals and their extended natural environment. Within such evolutionary perspective, the brain would have evolved to satisfy the request of a constantly changing environment that presents animals with multiple opportunities and demands for actions.

At the theoretical level, such an intimate and mutual relationship between animals and their surrounding environment has been originally conceptualized in the notion of affordance (Gibson, 1979), defined as a property of the environment providing the viewer with practical opportunities of action, and successively further refined and expanded by embodied and ecological views of perception (Creem-Regehr & Kunz, 2010).

At the experimental level, the notion of affordance has been principally investigated in the realm of the interaction between hand-related actions and manipulable objects/tools. For example, using an incidental object categorization task, such as judging whether an object was upright vs. inverted or natural vs. manufactured, a response-compatibility effect was described in the seminal studies by Tucker and Ellis for the execution of functionally appropriate hand-related actions to visually presented objects (e.g., spatial alignment between the responding hand and the handled object). This effect of motor potentiation, known as “micro-affordance”, refers to a decrease of reaction times for components of hand-related actions afforded by seen objects (Ellis & Tucker, 2000; Tucker & Ellis, 1998, 2001). Crucially, since this motor potentiation or compatibility effect was typically observed in the context of incidental tasks not requiring participants to explicitly pay attention to the affording properties of the presented objects (e.g., upright vs. inverted object orientation), the micro-affordance effect has been typically considered an automatic mechanism, i.e., an automatic activation of representations of actions during object/tool perception.

Converging evidence from experimental work, however, has demonstrated that the micro-affordance effect is rarely or not observed at all during tasks requiring superficial processing of the object perceptual properties, such as during color judgment tasks, or during conditions in which the object is located in the “out of reach” portion of space (Costantini et al., 2010, 2011; Pellicano et al., 2010; Tipper et al., 2006), leading to the hypothesis that affordances activation is flexibly modulated by the task and by the physical and social context [(i.e., both automatic and task-dependent affordances activation, see (Borghi & Riggio, 2015) for a full discussion].

As noticed above, however, the specific focus on the hand–object relationship might have limited, or somehow circumscribed, the discussion on affording properties of objects in the external world and the mechanism of affordance in general. This issue becomes even more critical when considering the emphasis, in the original formulation of affordance, on natural behavior and adaptive interactions between animals and their extended natural environment.

On this basis, a hypothesis was formulated in our previous work (Di Marco et al., 2019) about a potential affordance relationship between the spatial features of the extrapersonal environment and locomotion, a behavior that is essential for survival and ubiquitous among mobile organisms. Specifically, since locomotion appears as the privileged form of action to cover distance and retrieve information contained in the distant environmental/extrapersonal space, a facilitation effect was hypothesized for walking-related actions to distant landmark-objects/locations in the extrapersonal space. More specifically, an affordance effect was hypothesized between the locomotion action and the distant extrapersonal environment, and this effect was expected to take the form of faster initiation times (i.e., release times) for execution of a footstep action, taken as a proxy of walking, in response to far vs. near extrapersonal landmark-objects/locations. To test this hypothesis, a go/no-go incidental priming paradigm was employed requiring the planning/execution of a foot-related action (i.e., simple release of a foot-pedal response button or simple release followed by a footstep ahead) in response to repeated presentations of pictures of an environmental layout containing objects positioned at different distances (far vs. near) from the observer. Consistent with our predictions, a facilitation effect was observed for execution of the footstep action in response to distant (vs. near) landmark-objects/locations in the extrapersonal space and this effect has been referred to as “macro-affordance” to reflect the parallelism with the “micro-affordance” effect observed during the execution of functionally appropriate hand-related actions towards manipulable objects (Altomare et al., 2021; Di Marco et al., 2019; Tosoni et al., 2021).

However, since in this study the manipulation of distance was achieved by moving the camera at different extrapersonal distances along a vector connecting the camera (i.e., point of view) with the objects contained in the environment, the degree to which the macro-affordance effect was guided by the object position (far vs. near) vs. the framing distance of the environmental layout (far/panoramic vs. near/restricted view) could not be determined. Therefore, two (non-mutually exclusive) hypotheses were formulated: one that strongly reflects the analogy with the micro-affordance effect as motor attributes directly included in the representation of visual objects (Tucker & Ellis, 1998) and strictly dependent on the object position/distance, and the other that more generally considers the spatial properties of the general environment and the Gibsonian notion of environmental affordances. Accordingly, while a first basic interpretation of the effect reflects the notion of “object reachability” (i.e., when participants anticipate walking further their action starts faster, “object reachability” hypothesis), a more intriguing interpretation of the effect was also proposed according to which wide and panoramic views of the environmental layout automatically activate mechanisms for walking-related exploration of the distant spatial environment (i.e., “spatial exploration” hypothesis).

In addition, since our macro-affordance effect was observed in the context of a priming paradigm based on an incidental go/no-go task (i.e., go when the target picture is identical to the prime picture), it was unclear whether it was specifically associated with an implicit coding of environmental distance for walking-related actions or whether it could be also observed during a task requiring an explicit distance categorization. Lastly, although a specificity of the macro-affordance effect was originally observed (Di Marco et al., 2019) for motor actions that mimic the initiation of a walking, the degree to which the effect was specifically associated with a particular response type/effector, e.g., foot- vs. hand-related motor actions, has remained an open question from our previous investigations.

Here, three studies are presented that directly address the questions that have remained open from our original study (Di Marco et al., 2019): (i) whether the macro-affordance effect more strongly reflects the encoding of the spatial position/distance of objects in the environment or the encoding of distance of the environment itself, (ii) the degree to which it is associated with the activation of a system operating at an implicit vs. controlled level of information processing, (iii) the degree to which it reflects a specificity for a particular response type/effector (e.g., foot- vs. hand-related motor actions).

Experiment 1 addressed the question of whether the macro-affordance effect was preferentially guided by the framing distance of the general environment vs. the distance of isolated landmark-objects (i.e., beach umbrella). As outlined in the Introduction section, a stronger effect of distance was hypothesized for the layout vs. the landmark-object session within the framework of the “spatial exploration” hypothesis, while a stronger modulation of distance was expected for the landmark-object session within the framework of the “object reachability” hypothesis. Since the macro-affordance effect was interpreted on the basis of an automatic activation of mechanisms for spatial exploration/navigation of the environment in our previous studies (Altomare et al., 2021; Di Marco et al., 2019; Tosoni et al., 2021), a stronger effect of distance was predicted for the layout compared to the landmark-object session. In addition, a stronger effect of distance (either in the layout or in the landmark-object session) was hypothesized for footstep actions that mimic the initiation of a walking (vs. simple release) based on our original findings (Di Marco et al., 2019).

Experiment 2 addressed the question of whether the macro-affordance effect was specifically observed in the context of an incidental priming paradigm as in Experiment 1 in which the distance dimension was elaborated at an implicit level or whether it could also be observed during a task requiring an explicit categorization of distance. To this aim, an explicit version of the go/no-go paradigm employed in Experiment 1 was developed in which the instructed response movements (simple release, footstep) were performed on the basis of an explicit distance categorization of the environmental layout (only pictures of the layout session were employed). A non-significant effect of distance in this explicit task would suggest that the macro-affordance effect is task-dependent and specifically related to an implicit coding of extrapersonal distance for the execution of foot-related actions. In contrast, a significant effect of distance during both the implicit and the explicit task would indicate that the effect is automatically activated independently from the task at hand.

Experiment 3 addressed the question of the relationship between the macro-affordance effect, in terms of response time modulation by framing distance of the environmental layout, and a specific response type/effector. In particular, by examining data of a manual version of the incidental priming paradigm employed in Experiment 1, the degree to which the macro-affordance effect is specifically associated with the execution of foot- vs. hand-related motor actions was examined in Experiment 3. At the level of the hypotheses, since a preferential affordance relationship was originally hypothesized between the distant extrapersonal environment and the locomotion action, no distance modulation was expected during the manual version of the incidental priming task. In addition, since the macro-affordance effect was interpreted on the basis of an automatic activation of mechanisms for spatial exploration/navigation of the environment in our previous studies (Altomare et al., 2021; Di Marco et al., 2019; Tosoni et al., 2021), a selectivity of the effect for the planning/execution of foot-related motor actions was hypothesized in the current experiment.

The brain has evolved to satisfy the request of a constantly changing environment that presents animals with multiple opportunities and demands for actions. As formalized by the classic notion of affordance (Gibson, 1979) and by the more recent neurophysiological model of the “affordance competition hypothesis” (Cisek, 2007; Cisek & Kalaska, 2010), one adaptive strategy to survive in such a complex environment is to specify in advance the parameters of all possible actions. Consistent with this view, a facilitation effect was described in our recent works for the execution of foot-related actions in response to distant vs. near objects/locations in the extrapersonal space (Altomare et al., 2021; Di Marco et al., 2019; Tosoni et al., 2021).

In the present study, we examined the degree to which this effect, which we have referred to as “macro-affordance” reflects an (i) implicit vs. controlled coding of (ii) environmental vs. landmark-objects’ distance for the planning/execution of (iii) specific response actions (e.g., foot- vs. hand-related motor actions).

The results indicate that the effect is preferentially guided by the framing distance of the environment layout (far/wide/panoramic vs. near/close/restricted view) rather than by the position of isolated objects in the environment (far vs. near) and that it only emerges when the distance dimension is processed at an implicit level (i.e., within the incidental priming paradigm).

The results additionally showed that the effect was specifically associated with execution of foot-related/walking (vs. hand-related) motor actions. Taken together, these findings suggest that the macro-affordance effect specifically reflects an implicit coding of environmental distance for the execution of foot-related actions that mimic the initiation of a walking.

Notably, the results of an explicit rating on the locomotion-related affording properties of our stimulation set complement these findings by indicating that far/panoramic views of the environmental layout preferentially elicit motor intentions and viewer–environment interactions associated with spatial exploration of the environmental layout.

These findings support our original interpretation of the macro-affordance effect in terms of an automatic trigger of walking by positions that preferentially guide spatial exploration (Di Marco et al., 2019). In particular, we propose that the observed preference of the distance effect for the “layout” session and the preferential association between pictures of the far/panoramic environmental layout and the “spatial exploration” interaction/intention support the hypothesis of a general and evolutionary-based mechanism associated with spatial exploration of the extended spatial environment.

The results presented in the current manuscript, however, also support the notion that the “object reachability” and the “spatial exploration” accounts can be thought of as two complementary, or at max hierarchically operating, mechanisms that together fulfill the purpose of adaptive interactions with the environment. Ultimately, also at the evolutionary level, when animals cover distance and reach distant (new) locations for food-hunting they probability also exploit the journey to explore the new environment for a different aim (e.g., colonize, settle down), thereby maximizing their survival chances.

An additional question addressed in the current study was the degree of automaticity of the macro-affordance effect and specifically whether the far vs. near distance facilitation effect observed during execution of foot-related actions was peculiarly associated with an implicit coding of environmental affordances or whether it was independent from the task at hand (Experiment 2). Albeit affordances, and in particular micro-affordances, have been classically considered as an automatic mechanism activated during object perception, mounting evidence from experimental psychology has indicated that affordances are not invariably activated during the presentation of pictures or words referring to manipulable objects: their activation is, therefore, both task- and context-dependent (Borghi & Riggio, 2015). For example, a series of studies have shown that affordance activation closely depends on the specific task employed in the study design and, in particular, that an affordance effect is observed during shape categorization but not during color categorization tasks (Pellicano et al., 2010; Tipper et al., 2006).

Here, we addressed the question of automaticity and contextual dependence of macro-affordances by comparing the modulation of performance by environmental distance during an incidental priming paradigm task in which the distance dimension was implicitly processed versus a task requiring an explicit categorization of the same dimension. The results indicated that the macro-affordance effect was significantly observed during the incidental priming task (Exp.1) but not during the distance categorization task (Exp. 2). Therefore, in accordance with previous experimental evidence and current theoretical views on affordance effects for manipulable objects, our results suggest that also affordance effects for environmental stimuli or macro-affordances are implicitly activated in a context-dependent manner.

A final question addressed in the present study is the degree to which macro-affordances are specifically associated with execution of foot-related/walking (i.e., footstep vs. simple release) and foot- vs. hand-related motor actions (Exp.3). In particular, although a specificity of the effect for the footstep action was observed in our original study (Di Marco et al., 2019), a modulation of distance for both the footstep and the simple release action was observed in the current study (i.e., larger effect of distance in the layout vs. the landmark-object session independently of the instructed response movement). In this respect, a considerable notation concerns the theoretical and methodological aspects of the experimental design. As reported in the methodological section of Experiment 1, in particular, although only the footstep action was taken as proxy of walking in our original work (Di Marco et al., 2019), also the simple release action was included in the design as control for basic aspect of the more complex footstep action. On this basis, therefore, a worthwhile theoretical consideration/specification is that both foot-related response actions can be thought of as constitutive elements of a walking incipit (i.e., consecutive steps of a sensory-motor sequence for the initiation of a walking action). At a more methodological level, moreover, it has to be noted that in the present study the two instructed movements were collected within the same session (see Experiment 1) which might have minimized the distinction/differentiation between the planning and execution components of the two response movements. Importantly, moreover, concerning the issue of the specificity of the macro-affordance effect for a particular response effector (foot- vs. hand-related motor actions), the results of an additional experiment indicated that the macro-affordance effect was not observed during a manual version of the incidental priming paradigm in which foot-related response responses were replaced with spatially directed hand-pointing movements. These findings suggested a foot-effector specificity of the macro-affordance effect.

On a more general level and with respect to previous empirical evidence, the findings presented in the current study are consistent with the findings of some follow-up studies in the field of micro-affordance showing that spatial alignment effects for simulated reach-to-grasp motor acts are computed online through behavior rather than reflected in explicit estimates or conscious representations of distance (Ambrosini & Costantini, 2013; Ambrosini et al., 2012). Here we found a similar pattern for the macro-affordance effect as directly estimated from a perceptual matching task in which the distance dimension was implicitly computed. This not only supports the conclusion of an online computation of affordance during task execution but also raises the hypothesis that the type of task might directly affect the relevance/salience of the distance dimension for action. More specifically, if one starts from the assumption that stimulus location is computed online for the purpose of the current behavioral goal, then one can formulate the hypothesis that the distance dimension is action-relevant for a foot-related response only during the implicit task. On the other hand, the distance categorization task might be primarily dominated by memory-recollection mechanisms associated with the memory encoding of the condition templates shown during the training phase of the task. According to our hypothesis, therefore, the role of perception (i.e., environmental distance) for action (foot-related action) would be largely preserved during the implicit task but rather impoverished by memory-recollection biases, and thus weekly available to mechanisms of perception for action, during the distance categorization task.

A further task-comparative aspect that might be worth discussing is the potential association between the observed macro-affordance effect and the incidental priming paradigm in which the effect was specifically observed. This is because the implicit but not the explicit distance categorization task was based on a priming paradigm and one might arise the concern that the observed facilitation effect might directly derive from a mechanism of pre-activation of the instructed response movement during the presentation of the prime stimulus. This potential priming explanation, however, would equally apply to the near and far conditions, for which we instead observed a differential modulation of the behavioral response. Therefore, the findings presented in the current study again support the notion that the macro-affordance effect particularly relies on the specific affording properties of the far condition, and in particular of the far and panoramic environmental condition, for execution of foot-related/walking actions.

Future studies, however, would hopefully further clarify the potential implication of the specific paradigm for the emergence of the macro-affordance effect and a set of questions that has remained unaddressed from the present and previous investigation on the “macro-affordance” effect.

One of these questions, for example, is the degree to which the perceptually constitutional aspects/features of the stimulation set employed in this study might affect the performance [see also (Witt et al., 2007) for relevant findings on this topic]. More specifically, a still unaddressed question is how the macro-affordance effect is modulated by the different ratio of depth cues (e.g., linear perspective, texture gradient) and stimulation of the visual field periphery in the two experimental conditions in which an isolated landmark-object or a distributed environmental layout was presented on the widescreen (also note that our studies were always conducted using the same 42″ widescreen covering 70° of visual angle).

An additionally relevant open question concerns the motivational basis of the effect and, in particular, whether within the evolutionary perspective outlined above, the macro-affordance effect can be conceived as associated with the activation of the so-called “seeking” disposition, a behavioral function of the mesolimbic dopaminergic system that is highly conserved during evolution and associated with exploratory and approaching behaviors such as forward locomotion (Panksepp, 2011; Panksepp & Biven, 2012). Within this broader perspective, in particular, we suggest that the proposed interpretation of the effect based on the unique role of locomotion in distance coverage and access to the information contained in distant extrapersonal locations is consistent with the definition of the seeking disposition in terms of “specific types of locomotor activities and other responses directed to attain perceptual information and to progressively orient the organism toward affectively enticing and eventually desired sources of stimulation” (Alcaro et al., 2007).

Finally, at a more general theoretical level, one relevant future direction would be to clarify the effect of automaticity and contextual dependence, which have been so far only addressed in the field of the affordance relationship between manipulable objects and upper limbs actions, i.e., micro-affordances, with specific reference to the distinction between stable and variable affordances [see (Borghi & Riggio, 2015) for a full discussion]. How the distinction between stable and variable affordances applies and translates to environmental affordances for lower limb actions, i.e., macro-affordances, remains still unexplored and requires future investigation.

In summary, future studies will hopefully further clarify the perceptual basis and the bottom-up factors driving the macro-affordance effect, its motivational significance and its contextualization in the affordance theory and ecological theories of perception (Richardson et al., 2008).

Here, we showed that the facilitation effect observed in the current and previous studies for the execution of foot-related actions to environmental layouts framed from a far/wide/panoramic vs. near/close/restricted distance in the extrapersonal space (“macro-affordance”) is based on implicit coding of environmental affordances. At the interpretational level, based on the neurophysiological model of the “affordance competition hypothesis” (Cisek, 2007; Cisek & Kalaska, 2010) and the theorization of primary-process motivational systems, we propose that the macro-affordance effect might be thought of as a fingerprint of the evolutionary-based seeking disposition finalized to the exploration and navigation of our surrounding natural environment.