Hand Posture Effects on Handedness Recognition as Revealed by the Simon Effect

We investigated the influence of hand posture in handedness recognition, while varying the spatial correspondence between stimulus and response in a modified Simon task. Drawings of the left and right hands were displayed either in a back or palm view while participants discriminated stimulus handedness by pressing either a left or right key with their hands resting either in a prone or supine posture. As a control, subjects performed a regular Simon task using simple geometric shapes as stimuli. Results showed that when hands were in a prone posture, the spatially corresponding trials (i.e., stimulus and response located on the same side) were faster than the non-corresponding trials (i.e., stimulus and response on opposite sides). In contrast, for the supine posture, there was no difference between corresponding and non-corresponding trials. Control experiments with the regular Simon task showed that the posture of the responding hand had no influence on performance. When the stimulus is the drawing of a hand, however, the posture of the responding hand affects the spatial correspondence effect because response location is coded based on multiple reference points, including the body of the hand.


Silva 
Univer
idade Federal Fluminense
Rio de JaneiroBrazil

Fernanda M Ferreira 
Universidade Federal Fluminense
Rio de JaneiroBrazil

Cláudia D Vargas 
Universidade Federal do Rio de Janeiro
Rio de JaneiroBrazil

Carlo Umiltà 
Università di Padova
PadovaItaly

Antônio Pereira apereira@ufrnet.br 
Universidade Federal do Rio Grande do Norte
NatalBrazil

The Edmond and Lily Safra International Institute of Neuroscience of Natal
NatalBrazil

Silvio Ionta 

University of Geneva
Switzerland


Ecole Polytechnique Fédérale de Lausanne
Switzerland


Cathy Reed
Claremont McKenna College
USA


Universidade Federal do Rio Grande do Norte
Campus Universitário, Caixa-Postal 150659078-970NatalRNBrazil

Hand posture effects on handedness recognitio as revealed by the Simon effect
30 November 2009D25A4A7CE02B891232D48D123D4AA51610.3389/neuro.09.059.2009Received: 13 August 2009; paper pending published: 07 September 2009; accepted: 15 November 2009;handedness recognitionSimon effecthand posturemotor imagery
We investigated the infl uence of hand posture in handedness recognition, while varying the spatial correspondence between stimulus and response in a modifi ed Simon task.Drawings of the left and right hands were displayed either in a back or palm view while participants discriminated stimulus handedness by pressing either a left or right key with their hands resting either in a prone or supine posture.As a control, subjects performed a regular Simon task using simple geometric shapes as stimuli.Results showed that when hands were in a prone posture, the spatially corresponding trials (i.e., stimulus and response located on the same side) were faster than the non-corresponding trials (i.e., stimulus and response on opposite sides).In contrast, for the supine posture, there was no difference between corresponding and non-corresponding trials.Control experiments with the regular Simon task showed that the posture of the responding hand had no infl uence on performance.When the stimulus is the drawing of a hand, however, the posture of the responding hand affects the spatial correspondence effect because response location is coded based on multiple reference points, including the body of the hand.

mental transformations aimed at reorienting them in space.Such strategy, in t e case of body parts, is complicated by the fact that their internal representation is built from visual and somatosensory information (Schendan and Stern, 2007).For instance, a simple task, such as judging the handedness of a drawing of the human hand, requires motor imagery that follows the same rules of the real movement, including compliance to biomechanical and physical constraints (Parsons, 1994;Vargas et al., 2004;De Lange et al., 2006).More specifi cally, handedness judgment also involves a preattentive handedness-recognition process before the mental simulation of one's own hand moving towards the stimulus (Parsons, 1987(Parsons, , 1994;;Parsons and Fox, 1998;Parsons et al., 1998).It is important to stress the distinction between these two sequential phases: the automatic recognition of the hand (fi rst phase) and the subsequent confi rmatory motor imagery (second phase), even though they can share neural substrates (Gentilucci et al., 1998).

The basic assumption that motivated the current study was that the confi rmatory motor imagery process (second phase) necessary for handedness recognition might be infl uenced by proprioceptive inputs.Previous studies had already shown the basic infl uence of proprioceptive information on motor imagery.Parsons (1994), for instance, showed that mental rotation of the hand is faster when the subjects' own hands assume a "canonical" posture (palms down on a table) instead of an "awkward" one (palms facing away from the subject's midline).A similar result was obtained with children by Funk et al. (2005), while Ionta et al.

otor imagery depends o
somatotopic correspondence.In a related study, Vargas et al.


INTRODUCTION

choice react
on time (RT) tasks, compatibility between elements of the stimulus and response sets infl uences performance, as measured by the speed and a

uracy of the subject
s motor response (e.g., Kornblum et al., 1990).For example, the manual response to a visual cue is usually faster and more accur te when the response key has the same relative location to the subject's midline as the stimulus.The importance of the spatial location of the stimulus on stimulus-response (S-R) compatibility tasks is underscored by its infl uence on the subject's performanc

even whe
it is not directly relevant to response selection, as shown by the Simon and spatial Stroop tasks (Umiltà and Nicoletti, 1990;Lu and Proctor, 1995).

The stimuli used in S-R compatibility tasks have been of a wide variety.Parts of the body, however, have never been used, even though they would make an interesting type of stimulus because they are processed differently by the brain.There is ample evidence showing that visual information about ordinary objects and visual information about parts of the human b

y are relatively seg
egated in different pathways, with specialized cortical areas devoted to the processing of information related to each category (Downing et al., 2001;Parsons, 2003;Zacks et al., 2003;Urgesi et al., 2007).In particular, information about the body is p

cessed in
reas of the mirror neuron system (MNS), whose neurons are active both during the execution and the observation of a movement (see Gallese et al., 1996;Rizzolatti and Craighero, 2004;Gawryszewski et al., 2007).

To recognize visual objects, their perceptual characteristics must be compared to stored representations.Because visual objects are seen from different viewpoints, they must fi rst be subjected to (2004) investigated, by means of Transcranial Magnetic Stimulation (TMS), how corticospinal excitability, facilitated during the mental simulation of a hand movement, is affected by the actual posture of the hand.Participants were asked to imagine themselves joining the tips of two fi ngers while maintaining a hand posture either compatible or incompatible with the imagined movement.Results indicated that the actual limb posture exerted a mo ulatory effect upon the motor imagery process.

A similar result was obtained by De Lange et al. (2006) in an fMRI study which found that the posture of the participants' own left and right arm infl uenced handedness judgments through modulation of a parietal-frontal network, whose activity increased with increasing biomechanical complexity of the imagined hand movements.That result indicates that motor imagery depends on the current confi guration of the limbs, as determined by proprioceptive inputs.

Here, we further elucidated the role played by proprioceptive inputs on handedness discrimination by using a Simon-like S-R compatibility task, in which handedness of the drawings is the relevant f

ture
or response selection while spatial location is the irrelevant one 1 .We investigated the effects of proprioceptive inputs on the task by asking participants to have their hands assume either a prone or a supine position.The spatial correspondence effect, characteristic of regular S-R compatibility tasks occurs because irr

evant information ab
ut the stimulus location interferes, at the response selection stage, with the learned association between the relevant stimulus feature and the correct response.In our modifi ed task, there is also the possibility for the irrelevant proprioceptive information about the posture of the responding hand to confl ict

ith the se
sorimotor representation normally engaged by motor imagery.Thus, we asked how those two irrelevant sources of information interacted with each other and with the relevant feature to affect the dynamics of the response in a handedness judgment task.


EFFECT OF HAND POSTURE ON HANDEDNESS RECOGNITION AND A REGULAR SIMON TASK MATERIALS AN METHODS

We tested two groups, a handedness-task group and a regular Simon task group.


Participants

Handedness task.Twelve right-handed volunteers (eight male and four female, 18-22 years old, mean = 19.5 years) participated in the study.

Regular Simon task.Sixteen right-handed volunteers (eight male and eight female, 17-29 years old, mean = 21 years) participated in the experiment.

All had normal visual acuity and were naïve as to the purposes of the experiment.All participants provided written informed consent and the study was approved by our Institution's Research Ethics Committee in accordance with the Helsinki Declara ion (2008).


Stimuli

Handedness task.Drawings representing the human left and right hand in either back or palm views were presented randomly either to the left or to the right of the central fi xation point.Left and right hands were mirror images of each other.Stimuli were

bout 15.5° high and
.3° wide and had a black profi le set against a gray background.Stimuli were located 7.5° either to the left or to the right of the central fi xation point and were always presented with the fi ngers pointing upwards (see Figure 1).


Regular Simon task.

Drawings of geometric fi gures (squares and circles comprising 1° of visual angle) were presented on a 20-in VGA monitor screen.The geometric center of the stimuli were located 7.5° either to the left or to the right of the central fi xation point.


Apparatus

The experiment was conducted in a quiet and dimly lit room.A personal computer (PC 486) was used both for stimulus presentation and recording the response.The head of the participants was positi ned in a forehead-and-chin rest and the distance between the eyes and FIGURE 1 | Drawings of left and right hands in back and palmar views (modifi ed from Parsons, 1994). 1 In

study, both the relevant and t
e irrelevant stimulus feature overlapped with the relevant response feature (see Kornblum et al., 1990).Therefore, the task resembles a spatial Stroop task (e.g., Lu and Proctor, 1995).However, the relevant right/left stimulus feature is not spatial in nature but rather concerns handedness.Therefore, the most appropriate term would be Simon-like S-R compatibility task.We decided to use, instead, the neutral term Handedness task.

the screen was about 57 cm (see Figure 2).The Micro Experimental Laboratory software (MEL, version 2.0) was used to determine events' sequences and to measure response latency.The stimulus remained on the screen for 1000 ms and the subjects responded by pressing one of two micro-switches, located 25 cm to the left or to the right.Participants employed two hand postures (prone and supine) when responding.In both postures, they executed the same index fi nger movement (fl exion) to press the response key, since the response apparatus allowed an 180° turn (see Figure 2).We used an Eye Track System (Model 210 -Applied Science Laboratories) to control the subject's eye fi xation during the test.


Task

Handedness task.The task was to press the right key to the appearance of a right-hand drawing and the left key to the appearance of a left-hand drawing.The relevant feature for response selection was

imulus' handedness (
eft or right hand drawing) and the irrelevant feature was its location (left or right hemifi eld).


Regular Simon task.

The task was to press a micro-switch (right or left) when a stimulus (circle or square) appeared.Usually, in this regular Simon task, the corresponding condition (stimulus ipsilateral to the response key) elicits faster responses than the non-corresponding condition (stimulus contralateral to the response key).


Procedure

Handedness task.Participants attended two sessions on different days.Each session was divided into four blocks of 68 trials, resulting in 272 trials per session.In two consecutive blocks, the participant responded with his/her palms face down (prone posture) and, in the other two blocks, with his/her palms face up (supine posture).

The participants started with a posture in one session and with another posture in the other sess

r of th
postures was counterbalanced across participants.Thus, the hand posture matched the view of the drawings on 50% of trials.The left or right hand drawings were presented randomly in either the back or palm view.We did not systematically ask the subjects about their strategy, only occasionally.Because hand views (palm or back) were randomly presented, it was not p

sible for the participant
o use a gross asymmetric feature of the drawing, such as the relative position of the thumb, for handedness recognition.

Participants were instructed: (i) to maintain the gaze at the central fi xation point, (ii) to avoid looking at the stimulus and (iii) to respond as fast as possible by pressing the right microswitch to a right-hand stimulus and the left micro-switch to a left-hand stimulus.

Regular Simon task.There were three sessions on different days.The fi rst session was considered practice and was not included in the analysis.Each session was subdivided into four blocks of 80 trials, resulting in 320 trials per session.Participants responded either with their palms face down (prone postur ) in two blocks or with their palms face up (supine posture) in the other two.They were instructed to keep their gaze at the central fi xation point until the stimulus appeared, avoid looking at the stimulus itself and to respond as fast as possible to its appearance.Half of the participants used the right micro-switch for the square and the left micro-switch for the circle.The others had the reverse assignment.

Experimental onditions were counterbalanced across subjects to account for practice or fatigue effects.Participants performed two consecutive blocks with the same posture.


ANALYSES


Central tendency measurements

The mean of correct Manual Reaction Time (MRT) values was entered into an ANOVA with one between-participant factor, task (Handedness task and regular Simon task) and two withinparticipant factors, hand posture (prone and supine) and correspondence (corresponding and non-corresponding conditions).A corresponding trial was the condition in which the stimulus appeared on the same side of the response key.For e

mple, when the right
hand (left-hand) drawing appeared in the right (left) visual hemifi eld and the participant responded by pressing the right (left) key.A non-corresponding trial was the condition in which the stimulus appeared in the opposite side of the response key, for example, when the right-hand (left-hand) drawing appeared in the left (right) visual hemifi eld and the participant responded by pressing the right (left) key.We chose to use only right-handed subjects in order to have an homogeneous sample.Besides, we believe that the eventual existence of a general bias toward faster responses for right stimuli in this sample has o effect on our main goal of verifying the effect of hand posture on handedness recognition The Newman-Keuls method was used for post-hoc analyses, with α < 0.05.When interpreting statistics one should bear in mind, however, that they refer to two groups with different sizes.


Delta-plot analysis

The Delta-plot procedure (Ridderinkhof, 2002) is based on correct MRT distributions for corresponding and non-corresponding trials in both prone and supine postures.These rank-ordered distributions were divided into fi ve proportional bins (quintiles) such that each one contained the same number of trials (onefi fth).Mean MRTs for each bin was calculated and the size of the correspondence effect (mean MRT in non-corresponding trials minus mean MRT in corresponding trials) was plotted as a function of response speed for each hand posture.The correspondence effect amplitudes were entered into an ANOVA with ta k (Handedness task and Simon task) as a between-participant factor and hand posture (prone and supine) and bin (1st bin, 2nd bin, 3rd bin, 4th bin and 5th bin) as within-participant factors.The Newman-Keuls method was used for post-hoc analyses, with α < 0.05.


RESULTS


Errors

Overall errors amounted to 5.81%, of which 4.17% were judgment errors, 1.57% were omission errors and 0.07% were anticipation errors.Errors were entered into an ANOVA with two with

re and correspondence interaction showed
hat, when the hands were in the prone posture, MRT for the corresponding condition (507 ± 6 ms) was signifi cantly faster (p < 0.0002) than MRT for the non-corresponding condition (523 ± 5 ms).However, when hands were in the supine posture, there was no signifi cant difference between corresponding and non-corresponding conditions (528 ± 6 ms vs. 534 ± 4 ms) (p > 0.08).

The three-way interaction showed that in the Handedness task the corresponding condition (636 ± 8 ms) was faster (p < 0.0048) than

he non-corr
sponding (653 ± 8 ms) condition when the participants were in the prone posture, whereas, with the supine posture there was no difference between corresponding (668 ± 7 ms) and non-corresponding (665 ± 5 ms) conditions (see Figure 3) (p > 0.05).For the Simon task, the corresponding condition was signifi cantly faster (p < 0.035) than the non-corresponding condition in both postures (prone 410 ± 4 ms vs. 426 ± 3 ms and supine 423 ± 5 ms vs 435 ± 4 ms) (p < 0.005).


Delta-plot analysis

Both hand posture (F 1,26 = 9.71; p < 0.005) and bin (F 4,104 = 18.32; p < 0.0001) were signifi cant main effects in the Delta-plot analysis.There was also a signifi cant interaction between task and posture (F 1,26 = 7.21; p < 0.015).Overall, there was no signifi cant difference between the correspondence effect amplitude in both prone (11 ± 3 ms) and supine (10 ± 2 ms) postures in the regular Simon task (p > 0.45).However, in the Handedness task, the amplitude of the correspondence effect in the prone posture (17 ± 2 ms) was signifi cantly larger (p < 0.025) than in the supine posture (−3 ± 2 ms; see Figure 4).

For both tasks and both postures, the correspondence effect decreased as a function of bin.For the handedness task, the correspondence effect for both postures was present and signifi cant at the fi rst and second bin, 26 (p < 0.0015) and 17 ms (p < 0.035), respectively, and practically disappeared at the slowest three bins (respectively, 3, −9 a

0 ms).For the regular Simon t
sk, the correspondence effect for both postures was present and signifi cant at the fi rst three bins, 24 (p < 0.001), 20 (p < 0.0213) and 14 ms (p < 0.0263), respectively, and practically disappeared at the two slowest bins (respectively, 3 and −7 ms).The three-way interaction was not signifi cant, indicating that posture did not modulate the trend of the correspondence effect as a function of bin in either task.The size of the correspondence effect for the prone

sture wa
larger than for the supine posture at all bins.

The most important fi nding was no doubt the interaction between task (regular Simon task or Handedness task), hand posture (prone or supine) and correspondence (corresponding or non-corresponding conditions).In the Handedness task, when the hands assumed a prone posture, MRTs for the corresponding condition were faster t an MRTs for the non-corresponding condition.This correspondence effect was no longer present, however, when the hands assumed a supine Notice the infl uence of the posture on S-R compatibility only in the handedness recognition.This correspondence effect disappears when the hands assumed a supine posture.


FIGURE 4 | Infl uence of hand posture on amplitude of the correspondence effect for each task.

There is no difference between the correspondence effect amplitude in both postures in the regular Simon task.However, in the Handedness task, the amplitude of the correspondence effect in the prone posture i larger than in the supine posture.

posture.In contrast, in the regular Simon task, for both postures, MRTs for the corresponding condition were faster than MRTs for the non-corresponding condition.The Delta-plot analysis confi rmed this pattern and showed it was not affected by response


SEPARATE THE HANDEDNESS TASK

Because in the previous analyses the view of the hand stimulus was not taken into consideration, we decided to conduct a separate analysis of the Handedness task.

was no difference between the back and palm views.Interestingly, as had happened with the correspondence effect, the match effect disappeared when the responses were executed with the hands in supine posture.Apparently, the supine posture has something "special" that contributes to annul effects that would otherwise manifest themselves.


DISCUSSION

In this paper, we have used a modifi ed Simon task to investigate the infl uence of hand posture on handedness recognition of drawings of the human hand presented either on back or palm view.The participants had to ascertain stimulus handedness by pressing either left or right keys with their hands resting either in a prone or supine posture.As a control, subjects performed a regular Simon task using simple geometric shapes as stimuli.

We have shown that there is no infl uence of proprioceptive input from the responding hand on a Simon-like S-R compatibility task, with drawings of the human hand acting as the relevant stimulus, when the responding hand is in a supine posture.In contrast, with the responding hand in a prone position, not only there was a spatial correspondence effect, but proprioceptive information from the responding hand was also shown to interact with motor imagery used to perfor

the handedness reco
nition task.In control experiments, when geometric shapes were used as relevant stimuli, we found results typical of a regular Simon task, without interference from proprioceptive inputs: MRTs for the corresponding condition were faster than MRTs for the non-corresponding condition, regardless of the posture of the responding hand.The understanding of why and how stimulus properties affect performance in tasks like the present one is important because it may clarify how these properties are used for action selection.


CENTRAL TENDENCY MEASUREMENTS

Correct MRTs were entered into an ANOVA with the following factors: hand posture (prone or supine), match (matching or nonmatching condition) and correspondence (corresponding or noncorresponding condition).A matching trial was the condition in which the participant's hand was in the prone (supine) posture and the stimulus appeared in back (palm) view and vice-versa for the non-matching trial.The Newman-Keuls method was used for post-hoc analyses, with α < 0.05.


RESULTS

Only the signifi cant sources of variance involving the factor match will be reported here.The others were of course a replication of those already reported above.The main effect of match was significant (F 1,11 = 15.12;p < 0.0025).The average MRT for the matching condition (641 ± 8 ms) was faster (p < 0.025) than the MRT for the non-matching condition (669 ± 6 ms).

There was also a signifi cant interaction between hand posture and match (F 1,11 = 13.04,p < 0.0045).When the hands were in the prone posture, the average MRT for the matching condition (back view -613 ± 7 ms) was faster (p < 0.0045) than the average MRT for the non-matching condition (palm view 675 ± 5 ms).In contrast, when the hands were in the supine posture, there was no signifi cant difference between matching and non-matching conditions (back view 670 ± 5 ms vs. palm view 663 ± 5 ms; see Figure 5).Errors were also analyzed with the ANOVA, but no sources of variance were found to be signifi cant.

The novel fi nding with respect to the previous analyses was the interaction b