Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Hayley A. Colman, Roger W. Remington, Ada Kritikos, Giuseppe di Pellegrino.
We examined how factors related to the internal representation of the hands (handedness and grasping affordances) influence the distribution of visuospatial attention near the body. Left and right handed participants completed a covert visual cueing task, discriminating between two target shapes. In Experiment 1, participants responded with either their dominant or non-dominant hand. In Experiment 2, the non-responding hand was positioned below one of two target placeholders, aligned with the shoulder. In Experiment 3 the near-monitor hand was positioned under the placeholder in the opposite region of hemispace, crossed over the body midline. For Experiments 2 & 3, in blocked trials the palmar and back-of hand surfaces were directed towards the target placeholder such that targets appeared towards either the graspable or non-graspable space of the hand respectively. In Experiment 2, both left and right handers displayed larger accuracy cueing effects for targets near versus distant from the graspable space of the right hand. Right handers also displayed larger response time cueing effects for objects near the graspable versus non-graspable region of their dominant hand but not for their non-dominant hands. These effects were not evident for left-handers. In Experiment 3, for right handers, accuracy biases for near hand targets were still evident when the hand was crossed over the body midline, and reflected hand proximity but not functional orientation biases. These findings suggest that biased visuospatial attention enhances object identity discrimination near hands and that these effects are particularly enhanced for right-handers.
A substantial body of research has shown changes to the distribution of visuospatial attention towards objects when they are near hands. Specifically, the location of our hands, their posture, and individual differences such as handedness all have demonstrable impacts on how visual attention is distributed to objects near the body [1, 2, 3, 4, 5, 6, 7, 8]. In addition, how we plan to use our hands to act upon objects in our environment (action goals) impacts near-body visual processing [9, 10]. Typically, measuring near-hand attention confounds the goal of the action with the proximity of the hand. For example, one hand often is adjacent to the display with the distant hand responding to the target [5, 6, 7, 8], or both hands adjacent to the display responding to targets [2, 3, 4]. By contrast, studies with non-manual response measures (i.e. saccade / foot-pedal) disambiguate the action goal from manual responses, but still do not assess directly the impact of the action goal on the effects found. As a result, it is difficult to determine which elements of the action system contribute to near-hand visuospatial attention in each case. The aim of the present research was to investigate systematically how the relationship between the goal of the action and two internal states of the action system: handedness and graspability, influence the distribution of attention to objects near the body.
The aim of Experiment 1 was to provide a measure of baseline visuospatial biases relative to handedness. We examined how the relationship between the laterality of the response hand (left versus right) and handedness (left-handed versus right-handed) influenced the distribution of attention within a visual display, to ascertain whether any response-biases were present. Left and right-handed participants completed a Posner  cueing task with predictable lateral cues responding to targets with either their dominant or non-dominant hand with both hands distant from the display. Because both hands were distant from the display, hand proximity and posture should not impact the pattern of results. Thus any resultant biases in visuospatial attention may be attributable to changes in visual attention based on handedness. If handedness influences the overall distribution of visuospatial attention, we expect responses to be faster to targets when responding with their dominant versus non-dominant hand irrespective of cue validity (main effect of response hand). Alternatively, if handedness influences shifts in visuospatial attention, we expect a greater cueing effect for targets aligned with the dominant hand, when participants responding with their dominant hand. Due to the greater degree of laterality exhibited by right handers it is also possible that right-handers will show more of an effect of response hand (dominant versus non-dominant) either via faster responses overall compared with left-handers when responding to targets with the dominant hand, or interacting with cueing to alter cueing effects.
Experiment 1 rules out any effect of handedness on the distribution of visuospatial attention in a covert orienting paradigm. Following from this, the aim of the second experiment was to systematically the combined influence of handedness, hand proximity and functional orientation of the hand (i.e. whether the palm or back-of-hand was oriented towards the display) on the distribution of visuospatial attention. We adopted posture elements from the methodologies of Lloyd et al., and Reed et al.,  presenting the palmar (grasping) and back-of-hand (non-grasping) surfaces towards one of two target placeholders (in blocked trials) whilst keeping the hand location constant across postures. The participant’s hand was positioned below the monitor directly under one of the two target placeholders with either the palmar or back-of-hand surface oriented upwards (towards the target placeholder). Thus when targets appeared in the hand-adjacent location they were either in graspable or non-graspable space of the hand. Participants held either their dominant or non-dominant hand directly under the placeholder that corresponded with the hand side (e.g., right hand place under the right placeholder) and we compared performance between left and right-handed participants.
In Experiment 2, we found that the functional representation of the right hand, when proximal to the display, influenced the distribution of visuospatial attention. Accuracy of object identification was enhanced near the palm of the right hand and right handers also displayed faster engagement and delayed disengagements of attention to the grasping space of the dominant hand. The aim of Experiment 3 was to examine whether such visuospatial biases remain when the hand is crossed over the body midline, and thus are specific to functional representation of the limb.
The aim of the present research was to investigate how grasping affordances and handedness work in combination with action goals to influence the distribution of visuospatial attention near hands. Experiment 1 found that when completing a distal task, exogenous cues modulated attention. Handedness or the laterality of the response hand, conversely, do not influence attention systematically. Experiment 2 found that the grasping affordances of the right hand, when proximal to a visual display, biased shifts in visuospatial attention. Both left and right handers displayed greater accuracy costs when detecting objects near versus distant from the palm of the right hand. In addition, right handers displayed more rapid engagement slower disengagement of visuospatial attention to hand-adjacent targets near the graspable region of their dominant hand. In Experiment 3, when hands were crossed over the body midline, only right-handers retained dominant hand biases in the accuracy of target identification, and grasping space biases were no longer apparent. Taken together, these findings provide evidence that visuospatial attention is distributed to objects near the hands, relative to grasping affordances, and the strength of the underlying representation.