Action and perception: Evidence against converging selection processes

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1 VISUAL COGNITION, 2002, 9 (4/5), Action and perception: Evidence against converging selection processes Claudia Bonfiglioli MRC Cognition and Brain Sciences Unit, Cambridge, UK, and Dipartimento di Psicologia, Università degli Studi di Bologna, Italy John Duncan MRC Cognition and Brain Sciences Unit, Cambridge, UK Chris Rorden MRC Cognition and Brain Sciences Unit, Cambridge, UK, and Institute of Cognitive Neuroscience, Department of Psychology, University College London, UK Steffan Kennett Institute of Cognitive Neuroscience, Department of Psychology, University College London, UK In a series of experiments we investigated whether identification of a lateralized visual target would benefit from concurrent execution of a reaching movement on the same side of space. Participants were tested in a dual-task paradigm. In one task, they performed a speeded reach movement towards a lateralized target button. The reach was cued by an auditory stimulus, and performed out of the participant s sight. In the other task, participants identified one of two simultaneous visual stimuli presented to the left and right visual fields, close to movement target locations. If motor activity were effective in modulating Please address all correspondenc e to C. Bonfiglioli, Department of Psychology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK. claudia.bonfiglioli@rhul.ac.u k We would like to thank Jon Driver and Rob Ward for useful suggestion s and discussions throughout the project. This work was supported by the Medical Research Council (UK). Ó 2002 Psychology Press Ltd DOI: /

2 ACTION AND PERCEPTION 459 perceptual processes via a visuo-attentional shift, identification performance should have improved when the visual stimulus appeared at the movement target location. In fact, identification was not affected by the side of reach. Such results suggest substantially independent selection processes in motor and visual domains. One aspect of visual attention is the ability to focus processing on a selected region of space. What factors influence such selection? Recently, many studies have suggested links between the motor and perceptual systems. These studies suggest that spatially directed motor activity could be one of the factors influencing the spatial focus of visual attention. An influential model of this sort has been proposed by Rizzolatti and coworkers (Rizzolatti, Riggio, Dascola, & Umiltà, 1987). According to this premotor model, programming of a movement determines a shift of visual attention towards the region of space where the target for the movement is located. Although premotor effects were originally proposed for the programming of oculomotor responses (see also Deubel & Schneider, 1996), the theory has subsequently been generalized to include movements requiring different effectors (Rizzolatti, Riggio, & Sheliga, 1994), with the idea that attentional shifts depend on the facilitation of neurons within any pragmatic map (effector-specific representation of space) that is activated whenever a goaldirected movement is programmed. A similar proposal has been made in the context of Duncan s (1996) integrated competition hypothesis (see also Desimone & Duncan, 1995; Duncan, Humphreys, & Ward, 1997). According to this hypothesis, different objects in the environment compete for processing in multiple brain systems, including those representing different object properties and their implications for action. As an object assumes dominance within any one system, this dominance has a tendency to spread both to other representations of the same object (Duncan, 1984) and to related objects (Duncan & Humphreys, 1989), allowing integrated processing of related information across multiple components of the sensorimotor network. Again, selecting a particular region of space as the target for motor activity should produce a general enhancement of that region s processing. Several studies have investigated interactions between motor activity and perceptual performance in brain-damaged patients. In particular, studies have focused on two disorders relating to lateral attentional bias: Unilateral neglect, manifest as a tendency to disregard the side of space opposite to a brain lesion in a wide range of activities including, for example, drawing, navigation, or eating from a plate; and sensory extinction, manifest as a failure to detect or identify contralesional inputs in the presence of simultaneous, competing inputs on the unimpaired or ipsilesional side. Halligan, Manning, and Marshall (1991), for example, investigated line bisection in a single left neglect patient. Bisection was carried out with the right hand, which was moved from either a left or a

3 460 BONFIGLIOLI ET AL. right starting position; rightward bias was reduced when movement started from the left. In a series of experiments, Robertson and North (1992, 1993, 1994) required a left neglect patient to perform letter cancellation and reading tasks while simultaneously executing irrelevant movements with different effectors (left/right hand, left leg) in different hemispaces (on the left or on the right of his body midline). For this patient, visual task performance was dramatically improved while moving a left effector in left hemispace. In a similar study, Mattingley, Robertson, and Driver (1998) investigated a patient showing left visual field extinction. The task was to detect briefly presented targets appearing either on the left, on the right, or on both sides of a computer screen. The patient had to trigger each trial herself by pressing a start key that was placed either on the left or on the right, using either her left or right hand. The number of left-sided targets omitted in the bilateral condition was clearly reduced by use of the left hand in left space. A number of contradictory findings should also be noted. In a group study on patients suffering from left neglect, for example, Cubelli, Paganelli, Achilli, and Pedrizzi (1999) failed to replicate the results obtained by Robertson and North in their patient TD, and Brown, Walker, Gray, and Findlay (1999) reported that limb activation did not reduce the number of left-sided stimuli omitted by left neglect patients in a simple digit detection task. At least in some cases, however, it seems that patients suffering from unilateral neglect or extinction can show a substantial interaction between movement and visual attention, contralesional movements shifting attention towards that side. In this group, the data further suggest that both the hand used and the side of the body on which this hand is placed are important considerations. For various reasons, we should be cautious in generalizing from a patient group of this sort to the normal case. It has often been suggested, for example, that neglect patients may partly overcome their lateral bias by strategic or topdown direction of attention towards the contralateral field (e.g., Humphreys, Boucart, Datar, & Riddoch, 1996). A requirement to direct movements into that field could plausibly act as a reminder or stimulus to such strategic efforts; a form of apparently motor effect, which would not be relevant to the normal case. For reasons like this it is important to complement the study of patients with corresponding investigations of normal participants. One important study of such participants was reported by Deubel, Schneider, and Paprotta (1998). Each trial began with presentation of two horizontal strings of five place-markers, one to the left and one to the right of fixation. Positions 2 4 on each side were potential movement targets; these were marked by coloured ellipses, with a different colour for each position. There followed two events. First, the movement was cued by having the central fixation point turn into a coloured arrow. The direction of the arrow indicated whether to move left or right; the colour indicated which of the three possible target locations on that side to choose. Then, either 150 ms later (Experiment 1)

4 ACTION AND PERCEPTION 461 or at movement onset (Experiment 2), an array of ten alphanumeric characters was presented, one at each location. The task was to identify the middle character (position 3) on one side, this attended position (middle position on left or right) being fixed throughout a trial block. The results suggested tight coupling between movement and identification tasks; identification accuracy was substantially increased when the location to be attended for the identification task was also cued as the movement target. However, it could be argued that the better perceptual performance for targets appearing at movement target location is independent from factors relating to motor preparation and/or execution per se. Rather, it could be due to the fact that, in order to identify the correct movement target, subjects had to perform a visual selection prior to any motor process. Selection of the movement target was based on visual identification of which location contained the appropriately coloured ellipse. Thus, it is likely that better performance in the discrimination task when the visual target appeared in the same position as the movement target simply reflected the fact that visual attention was already allocated to that position, because that position had to be selected as movement target to plan the required action accordingly. In other words, what was supposed to be only a movement cue ended up cueing also a specific visual target location. In the present study, our aim was to extend the work of Deubel et al. (1998) while avoiding such visual factors in the motor task. Participants were tested in a dual-task paradigm. The first, speeded task was to perform a reach movement towards a lateralized target button. The movement was cued by a centrally presented acoustic stimulus, and was made out of the participant s view under a board forming a raised table-top. The second, unspeeded task was visual discrimination of a single, briefly flashed letter. This visual stimulus target was presented either in the left or right visual field, immediately above the board but otherwise close to movement target locations. However, visual stimulation was always bilateral, that is, target appearance on one side was accompanied by the simultaneous appearance of a distractor on the opposite side. In this way, lateral attentional capture due to the abrupt onset of a single bright visual stimulus was prevented. Our simple prediction was that letter identification would be better when movement and visual targets were on the same side (corresponding condition), compared to when they were on different sides (noncorresponding condition). To examine possible evolution of this effect over time, we manipulated the interval between the acoustic movement cue and the subsequent visual target. It is worth mentioning at least two factors, besides motor preparation itself, that may often be associated with lateralized movements. One is proprioceptive input from the moving limb, a potential influence on visual attention through cross-modal integration (Driver & Spence, 1998; Làdavas, Berti, Ruozzi, & Barboni, 1997). A second is generalized activation of the

5 462 BONFIGLIOLI ET AL. contralateral hemisphere, sometimes proposed to give a lateralized attentional shift (Reuter-Lorenz, Kinsbourne, & Moscovitch, 1990). In previous work, these potential effects of motor preparation, proprioception, and hemispheric activation have sometimes been confounded (e.g., Halligan et al., 1991), or sometimes clearly separated (e.g., Deubel et al., 1998). Our strategy was to begin by combining all these potential lateral biases, with the view that, if any effect on visual attention was detected, the exact cause could be isolated in subsequent experiments. EXPERIMENT 1 In the first experiment, the manual task called for a simple, hidden reach from an approximately central start position to a lateralized target. The left hand was used for reaches in left space, while the right hand was used for reaches in right space. Potentially, motor preparation, proprioceptive feedback and hemispheric activation were all possible influences on bias in the visual task. In this and subsequent experiments, participants were sampled from an elderly population. Most of the studies that reported enhanced perceptual processes as a function of motor activity were carried out on brain-damaged patients, whose ages varied roughly between 50 and 80 years. Elderly healthy subjects were then chosen, in order to determine whether the same beneficial effect of movement on visual performance described in patients could be found in healthy individuals of a comparable age group. Method Participants. Five paid participants (three men and two women, aged between 60 and 69 years) were recruited from the panel of the MRC Cognition and Brain Sciences Unit. Four were right handed, and all had normal or corrected to normal vision and were naive as to the purpose of the experiment. They attended one experimental session of approximately an hour and a half. Apparatus and materials. Figure 1 shows the experimental set-up for Experiment 1. The experiment took place in a dimly lit, anechoic room. Participants were seated at a table with the head positioned on a chin rest. The chin rest was fixed on the edge of a wooden board that was mounted 24 cm above the table, completely obscuring the table-top from view. The hands were placed in the space beneath this board, so that they were at all times out of view. Two LED displays (7 cm horizontal 4.8 cm vertical) were placed on the board with their centres 28 cm on either side of fixation, 39 cm in front of the chin rest. Each LED consisted of seven segments configured as a rectangular figure of

6 ACTION AND PERCEPTION 463 Figure 1. Schematic view of the experimental set-up used in Experiment 1. eight lying on its side. Different segments could be independently illuminated to form targets, distractors, and masks. Two boxes each containing a movement target button were mounted underneath the board, exactly under the corresponding LED, so that the distance between the centre of the button and the board was 9 cm. The buttons (2.5 cm in diameter) were facing towards the front of the table, and participants could reach them quickly and directly without visual feedback about either their moving arm or target location, and usually without hunting for the final position. Two origin buttons, one for the left and one for the right index finger, were placed immediately in front of the participant, out of sight on the tabletop. They were approximately 2.5 cm apart, centred on the participants midline, and 25 cm from the edge of the table. Two tones (high tone: 3000 Hz; low tone: 400 Hz) were used to cue movement direction. These tones were emitted simultaneously by two loudspeakers placed 19 cm on the left and on the right of participants midline, 45 cm from the edge of the table. Horizontal eye movement was assessed on each trial, with the intention of excluding any trial with a movement above 7 (note that the centre of the LEDs was 36 away from fixation). Eye movements were recorded with an ASL 504 remote eye tracker (sampling frequency 60 Hz). The spatial error between true eye position and computed measurement was less than 1. A grid consisting of 3 3 positions was laid directly on the board in front of the participants and was used to calibrate visual space. The central position was on participants midline, aligned with the two LEDs, and was used as a fixation point throughout the experiment.

7 464 BONFIGLIOLI ET AL. All stimulus presentation and response recording was controlled by an IBMcompatible computer running custom software. Procedure. Participants performed a dual task involving both manual reaching and visual discrimination. For each trial, the sequence of events was as follows. The subject placed the left and right index fingers on the left and right origin buttons, respectively. On a keypress from the experimenter, a tone (duration 200 ms) cued movement direction. Participants were required to reach for and press the designated button as fast as possible. For three participants, a high tone indicated that the left target button should be pressed (with the left index finger), whereas a low tone indicated that the right target button should be pressed (with the right index finger). For the remaining participants the mapping was reversed. At 50, 200, 400, or 600 ms after tone onset (stimulus onset asynchrony or SOA), one of the two LEDs (the target LED on this trial) displayed either an upright or an inverted T. At the same time, the other LED (non-target) displayed a distractor stimulus, a single central vertical line. Following a predetermined exposure duration, a backward mask consisting of two vertical lines, the leftmost and rightmost display segments, replaced the upright or inverted T on the target LED. This mask, along with the continuing distractor stimulus (single vertical line) on the non-target LED, remained present until the end of the trial. The task was to report, without time pressure, whether the target T had been upright or inverted. The trial ended when the experimenter typed this response into the computer. Each participant was tested individually in one experimental session, which comprised 240 trials divided into three blocks. Each combination of reach side, target side for the visual task, and SOA occurred equally often in each block, in an otherwise random order. Before data acquisition participants were trained for the reaching and the visual discrimination tasks separately (20 trials for each single task), then for the two tasks simultaneously. Dual-task practice was used to establish a suitable exposure duration for the visual task, aiming for accuracy in the region of 70% correct; there were a minimum of 100 dual-task practice trials, with more as needed. Target exposure duration was fixed for the remainder of the experiment; it ranged from 49 to 319 ms. Data analysis. Trials where horizontal eye movements were detected were discarded. Trials where errors occurred in the motor task (i.e., release of the origin button faster than 100 ms or slower than 2000 ms, wrong origin button released, wrong target button pressed) were excluded from the analysis of visual performance, though their number was analysed. The aim of the experiment was to compare performance on the visual discrimination task in the corresponding situation, when the target of the movement and the target of the discrimination were located in the same spatial

8 ACTION AND PERCEPTION 465 hemispace, with the non-corresponding situation, where the target of the movement and the target of the discrimination were located in opposite hemispaces. Data for corresponding and non-corresponding trials were collapsed across target sides (left or right). Results and discussion For each condition, the following dependent variables were considered: Performance on the visual task (% error), response time (time from onset of the acoustic cue to target button press), reaching errors (% summed anticipatory/delayed releases, releases of wrong origin button, and presses of wrong target button). For the purpose of statistical analysis, values expressed as percentages were arcsine transformed. Analysis of variance (ANOVA) on each variable had the within-subjects factors correspondence (two levels: corresponding, non-corresponding) and SOA (four levels: 50, 200, 400, 600 ms). No trial was excluded because of eye movements. Results for the visual task appear in Figure 2A. Across SOAs, there was no evident difference between corresponding and non-corresponding trials. The ANOVA showed no effect of correspondence, F(1, 4) = 0.05, n.s., SOA, F(3, 12) = 0.46, n.s., or their interaction, F(3, 12) = 0.89, n.s. As far as motor performance is concerned, response time was almost homogeneous in all conditions. It ranged between 901 and 1000 ms, with no significant differences as a function of either SOA or correspondence. The only significant difference in motor performance concerned reaching errors, with participants being less accurate in non-corresponding (17% errors) than corresponding trials (11% errors; F(1, 4)=10.06, p <.04). Although the interaction SOA Correspondence was not statistically significant, F(3, 12) = 1.33, most of the errors were made in the non-corresponding trials at the 50 ms SOA. The result suggests a form of stimulus response compatibility effect, whereby the onset of a lateralized visual target facilitates an ipsilateral motor response, which is incorrect on non-corresponding trials. The findings of this first experiment provide no support for the notion that motor processes can affect performance in an independent perceptual task. In particular, similar visual accuracy in corresponding and non-corresponding trials suggests that no attentional shift occurred as a consequence of movement programming and/or execution. In principle, corresponding trials could have yielded a better visual identification for a number of reasons. First, as the premotor theory of attention suggests, pragmatic maps representing the left- or the right-reaching space should have been active, given that the motor response was executed either in the left or in the right hemispace, and was directed towards a lateralized target. Second, given that throughout the experiment participants had to move their left or right hand in a random order, each single trial required the selective activation of a specific cerebral hemisphere to

9 A % Errors Corresponding Non-corresponding SOA (ms) B % Errors Corresponding Non-corresponding SOA (ms) C % Errors Corresponding Non-corresponding SOA (ms) Figure 2. Mean percentage errors as a function of correspondenc e and SOA in (A) Experiment 1; (B) Experiment 2; (C) Experiment 3. Please note that in Experiment 3 SOAs were different from Experiment 1 and 2. Bars indicate standard error.

10 ACTION AND PERCEPTION 467 programme and execute the reach. Being asymmetrical, such an activation could have induced an attentional shift towards the contralateral visual field. Finally, the proprioceptive feedback arising from the moving limb could have attracted attention to the area of space where the movement was taking place. All the previously mentioned elements might be supposed to enhance selective attention to the general region of the visual stimulus on corresponding trials. Despite this number of potentially contributing factors, however, the data suggested no such spatial enhancement of visual processing. EXPERIMENT 2 The motor task used in Experiment 1 failed to induce any lateralized attentional shift. Experiment 2 was designed to test whether a motor task more spatially segregated could be more effective in determining such a shift. To that end, two major modifications with respect to Experiment 1 were introduced. First, the origin buttons were moved from their central position to the left and right, in order to stress potential effects of starting position. Second, the experiment was divided into two parts, in each of which motor activity was confined to one hemispace. That is to say, in one half of the experiment the right hand (RH) moved in right space (RS) towards one of two buttons, both located in right hemispace, from a right starting position (RSP). The reverse happened in the other half of the experiment, where the left hand (LH) moved in left space (LS) towards one of two buttons, both located in left hemispace, from a left starting position (LSP). This modification was introduced to test the possibility that movement related enhancement of spatial areas needs time to build up. Specifically, if facilitation induced by a movement towards a lateralized target is to some extent offset by recent movements on the opposite side, then enhancement could be easier to observe when the spatial features of the movement (i.e., effector, starting position, end position, and hemispace where the action takes place) are kept constant across a whole trial block. As in Experiment 1, the visual target could appear either in the left or in the right visual field. For the left visual target, performance at the discrimination task should be better in LH-LS-LSP blocks, whereas for the right visual target it should be better in RH-RS-RSP blocks. Method Participants. Six right-handed men aged years were recruited as before. Three had taken part in the previous experiment. Apparatus and materials. Figure 3 shows the experimental set-up for Experiment 2. The visual task was much as before, with a slight change to

11 468 BONFIGLIOLI ET AL. Figure 3. Schematic view of the experimental set-up used in Experiment 2. the positioning of LEDs. This time, LEDs were mounted on a raised support on the board covering the table-top, with their centres 25 cm on either side of fixation, 25 cm in front of the chin rest (i.e., 45 away from fixation), and 8.5 cm above the level of the board. They were turned somewhat to face the participant, at an angle of 65 to the frontoparallel plane. For the motor task, two boxes each containing two target buttons were mounted underneath the board, each box centred exactly 11.5 cm under the corresponding LED and aligned perpendicular to the front edge of the table. The two buttons in each box were positioned 15 cm apart, on the inner vertical face of the box, i.e., facing towards the participant. Again, participants could reach these targets without visual feedback about either their moving arm or target location, and usually without hunting for the final position. Two origin buttons were placed on the table, each one below one of the two boxes, 25 cm on either side of participant s midline, and 25 cm in front of the chin rest. In this way, the two target buttons on each side were equidistant from the starting position on that side. Tone cues for the motor task were as before. Eye movements were recorded much as before, except that the 3 3 calibration grid stood upright on the board. The middle position on the bottom row was used as fixation point throughout the experiment. It was at the same height as the centre of the two LEDs, but displaced towards the rear of the table so that it did not interfere with the eye movement recording. Procedure. The experiment was divided into two parts. In each part the motor performance was confined to one side of space, that is, it involved only

12 ACTION AND PERCEPTION 469 one effector (for example, the left hand), with start- and end-points of all reaches on the side of that effector. Each trial began with the participant placing the index finger of the designated hand on the designated origin button. Tones as before were used to cue speeded reaches: If the tone was high the participant had to press the button further away from himself, whereas if the tone was low the participant had to press the button closer to himself. In other respects, details of procedure for each trial were unchanged from Experiment 1. In particular, visual targets again appeared randomly to left and right within each block, resulting in half corresponding and half non-corresponding trials. Each participant was tested individually in one experimental session, which comprised 256 trials divided into four blocks, two of LH-LS-LSP followed by two of RH-RS-RSP, or vice versa (counterbalanced across participants). Practice was given as before at the start of each half of the experiment. Target exposure duration was based on performance in the first set of dual-task practice trials, i.e., those preceding the first condition, then fixed for the remainder of the experiment. It ranged from 21 to 350 ms. Results and discussion Data were analysed as before, again collapsing over left and right sides. No trial was excluded because of eye movements. Once again, visual performance was not affected by the concurrent execution of lateralized reaching movements (Figure 2B). ANOVA showed no significant effect of correspondence, F(1, 5) = 0.17, n.s., SOA, F(3, 15) = 0.71, n.s., or their interaction, F(3, 15) = 0.52, n.s. Similarly, motor responses were highly homogeneous; response times ranged from 1224 to 1294 ms, reaching error percentages from 3 to 6%, and ANOVA showed no significant effects for either measure. As described for Experiment 1, several factors could have contributed to a better performance in the corresponding conditions in the present experiment, namely movement planning, proprioceptive cues, and hemispheric activation. It should be pointed out that in this case any premotor facilitation would have depended not on planning of movement direction, but on the hemispace within which the action was taking place (and within which the movement target was located). In fact, the direction of reaches was neither towards the left nor towards the right with respect to starting position; instead, the paradigm adopted in this experiment stressed the potential effects of hand starting, movement, and ending position. Dividing the experiment into two parts, in each of which the motor task was confined to one hemispace, might in principle have enhanced any visual bias effect. Again, however, there was no suggestion of such a bias in the data.

13 470 BONFIGLIOLI ET AL. EXPERIMENT 3 One possible reason why the paradigm adopted in Experiment 1 did not evidence any attentional shift is that having only two targets for the reach could result in movements that are relatively automatic. If the planning of an action (particularly of its direction) plays a crucial role in determining a shift of attention, it could be argued that the more a movement becomes automatic, the less planning is required, with a correspondingly reduced attentional shift. In Experiment 3 we tested the hypothesis that visual bias might accompany a more difficult motor choice. Difficulty was increased by increasing the number of movement targets on each side. Method Participants. Five right-handed participants (two men, three women, aged years) were recruited as before. None had taken part in the previous experiments. Apparatus and materials. Figure 4 shows the experimental set-up for Experiment 3. For this experiment, LEDs were placed with their centres 15 cm on either side of fixation, 30 cm in front of the chin rest (i.e., 27 away from fixation), centres raised 8.5 cm above the board covering the table-top. As in Experiment 2, each LED was turned somewhat to face the participant, but this time at an angle of 45 to the frontoparallel plane. Instead of an upright or Figure 4. Schematic view of the experimental set-up used in Experiment 3.

14 inverted T, visual targets were a schematic S (bottom left horizontal segment; central vertical segment; top right horizontal segment) or Z (the mirror image). Backward masks and distractors were as before. Two boxes each containing two target buttons were mounted underneath the table, one centred 11.5 cm under each LED. The two buttons on each box were 8 cm apart, mounted on the inner vertical face of the box. The whole box was fixed under the board at 45 to the frontoparallel plane, like the LED above it. Again, participants could reach bottons without visual feedback about either their moving arm or target location, and usually without hunting for the final position. Two origin buttons, one for each index finger, were placed side by side on the table at the participant s midline, 19 cm in front of the chin rest and approximately 2.5 cm apart. In this way, the four target buttons were almost equidistant from the starting position. Four different tones were used to cue movement direction. These were the four combinations of two pitches (high tone: 3000 Hz; low tone: 400 Hz) and two sound qualities (pure tone or rattle). Eye movements were recorded as before, with the same calibration as in Experiment 2. Procedure. Trials proceeded as in previous experiments. For the motor task, a high tone cued left, whereas a low tone cued right. Within each side, a rattling tone indicated that the more peripheral button was the target, whereas a continuous tone indicated the more central button. In this way, the four possible combinations of pitch and sound quality defined which of the four buttons the participant had to reach for. Given the increased complexity of the required motor response, movement planning was likely to take longer than in previous experiments, resulting in an overall longer response time. Because we were interested in probing visual performance at different phases of motor planning and execution, and because reach times were expected to be longer in this experiment, the SOA between tone and visual target was increased; the four possible SOAs were 200, 500, 800, and 1100 ms. Each participant was tested individually in one experimental session, which comprised 320 trials divided into four blocks. Before data acquisition, participants were trained as before, and target exposure duration then fixed for the remainder of the experiment. It ranged between 56 and 163 ms. Results and discussion ACTION AND PERCEPTION 471 Data were analysed as before, again collapsing over left and right sides, and now also across more peripheral and more central reach targets on each side. Eye movements never occurred. Despite its increased complexity, the motor task did not induce any attentional shift. Once again, participants performance at the discrimination task was not better when the visual target and the movement target were located in the same hemispace (Figure 2C). ANOVA

15 472 BONFIGLIOLI ET AL. showed no significant effect of correspondence, F(1, 4) = 3.21, n.s., SOA, F(3, 12) = 1.35, n.s., or their interaction, F(3, 12) = 0.24, n.s. Motor performance was extremely homogeneous across conditions; response times ranged from 1545 to 1667 ms, reaching error percentages from 8 to 19%, and ANOVA showed no significant effects for either measure. To confirm that increasing the number of reach targets was effective in making the motor task more complex, an additional analysis of variance was performed. Response time in this experiment was compared to that of Experiment 1, where only two reach targets were used. Although the two experiments are not completely comparable, given that both visual targets and SOAs were different, a longer average movement duration in Experiment 3 (1582 ms) compared to Experiment 1 (956 ms), F(1, 8) = 16.04, p <.004, strongly suggests that the manipulation effectively increased movement complexity. EXPERIMENT 4 A final experiment was designed to ensure that the visual task used in these studies was indeed sensitive to attentional manipulation. In this experiment, attention was oriented by means of an exogenous informative peripheral visual cue towards one of the two LEDs. Once again, participants were required to report the identity of a letter, and accuracy of their performance was taken as a measure of attentional allocation. On the basis of previously reported results (Posner, 1980), performance was expected to improve when the visual target appeared in the same location as the cue. Method Participants. Six right-handed participants (one man, five women, aged years) were recruited as before. None had taken part in the previous experiments. Apparatus and materials. Materials for the visual task were as described for Experiment 3. Given that no movement was required, the loudspeakers and boxes containing reach buttons were removed. Procedure. As before, each trial was initiated by a keypress from the experimenter. After a delay of 500 ms a visual cue, consisting of the lightening of the vertical middle bar of one of the two LEDs, flashed for 100 ms. At an SOA of 600 ms following cue onset, the target S or Z was shown on one LED, then masked as before, while the usual distractor was shown on the other LED. The target appeared on the cued side on 70% of trials ( valid ), and on the other side on the remaining 30% ( invalid ). Other details, including eye monitoring, were as before.

16 ACTION AND PERCEPTION 473 Each participant was tested individually in one experimental session, which comprised 320 trials divided into four blocks. Before data acquisition, participants were trained in the discrimination task (minimum 60 trials), with more as required to establish a suitable target exposure duration. Exposure duration was fixed for the remainder of the experiment, ranging between 25 and 160 ms. Results and discussion No trial was discarded because of eye movements. The experiment confirmed an effect of attentional cueing. There were a mean of 23% errors on valid trials, as compared to 35% on invalid trials, F(1, 5) = 10.36, p <.03. GENERAL DISCUSSION A number of studies have suggested that lateralized motor activity can induce a shift of visual attention towards movement target location or, more generally, towards the side of space where the action is taking place (Halligan et al., 1991; Mattingley et al., 1998; Robertson & North, 1992, 1993, 1994). As we pointed out, however, the majority of these studies have involved patients with unilateral neglect or extinction, raising the possibility of influences not relevant to the normal case. The present study aimed to replicate the same findings in a normal population. In particular, we addressed the issue of whether the identification of a lateralized visual target would benefit from the concurrent execution of a reaching movement in the same side of space, when all aspects of reach cueing and execution were outside visual control. Across three experiments, the results show that participants performance in the identification task did not differ in the corresponding condition, where the reach target was located in the same hemispace as the visual target, compared to the non-corresponding condition, where reach target and visual target were located in different hemispaces. As already pointed out in the introduction, different factors associated with motor activity could contribute to induce a shift of visual attention, including proprioceptive cues, movement starting position, planning the direction of the forthcoming movement, and hemispheric activation. Our results are particularly striking because they suggest that none of these factors produces a strong visual bias. The approach we followed was to design an initial task in which all factors were present at once, giving maximum sensitivity for detecting any visual effect. In the subsequent experiments the effects of starting position (Experiment 2) and movement planning (Experiment 3) were emphasized. However, despite the different experimental manipulations, we did not obtain any result in favour of the notion that motor activity can exert facilitatory effects on visual identification.

17 474 BONFIGLIOLI ET AL. From a theoretical point of view, it is particularly worth noting that our data do not support the premotor theory of attention, according to which spatial attention results from the activation of a set of neurons in the pragmatic map involved in the planning of a specific goal-directed movement. In its original version, the theory stated that planning a directional movement should shift attention towards the area of space where a movement target is located. We also did not find support for a weaker version of the theory (supported by Mattingley et al., 1998), according to which the crucial aspect of motor planning is not only the direction of the forthcoming movement, but the general area of space within which action occurs. Neither do our data support the general proposal that different components of the sensorimotor network have a strong tendency to converge and work on the same, selected spatial region (Duncan, 1996). In the present experiments, the motor system was favouring a particular side of space (i.e., left vs right hemispace), yet no corresponding visual bias was observed. One obvious reason why our study did not evidence any improvement in the corresponding versus the non-corresponding conditions could be that the visual task adopted was not sufficiently sensitive to attentional bias. However, the results of Experiment 4 argue strongly against this possibility. When a visual cue was used to orient attention, performance in the letter identification task changed according to whether the cue was valid or invalid. These results confirm that the consequences of an effective cueing mechanism could be detected in the visual task we adopted. In our experiments we have investigated the possibility of a simple, general bias of visual attention towards the spatial region of motor activity. Of course, there are many other possibilities for interaction between motor and visual processes. According to the integrated competition hypothesis (Duncan, 1996), for example, convergence on the same dominant location is just one aspect of integration between different parts of the sensorimotor network; as for other object-based models, it is all those grouping cues combining the parts and properties of one object that cause them to be attended together (Duncan, 1984). One important possibility, correspondingly, is that crosstalk may be stronger when action is directed not just to the same spatial region as visual input, but to exactly the same object. 1 Another possibility derives from our use of a strict dual-task format. Possibly, crosstalk between motor and visual processes might be stronger when the visual input is relevant to the same task as motor output, as in a recent study suggesting crosstalk between prepared grip orientation and visual detection of an oriented bar (Craighero, Fadiga, Rizzolatti, & Umiltà, 1999). Finally we might consider interactions beyond a simple attentional bias towards the region of motor activity. In another paper in this issue, Hommel and 1 We thank Steve Tipper for this suggestion.

18 ACTION AND PERCEPTION 475 Schneider (this issue) used a design in some ways similar to ours, but with rather different results. A tone cued a speeded keypress response with index or middle fingers of the right hand; during this auditory-manual response, a rectangular array of four letters was briefly presented, with a bar-marker adjacent to one letter indicating that it should be attended and identified. Though the lateralization of the response was far more modest than in our experiments, the results showed clear evidence of spatial crosstalk: When the tone required an index finger response (relatively leftward), visual performance was better if the cued letter was to the left of the array, whereas if the tone required a middle finger response (relatively rightward), visual performance was better if the cued letter was to the right. Contrasting these results with ours, the likely conclusion is that their crosstalk arose directly in the process of identifying and using the bar-marker to direct attention to the specified location; in other words, in an explicitly spatial control process not required in our tasks. Meanwhile, our data suggest that, at least in the absence of brain damage, a simple bias of visual attention towards the region of motor activity is surprisingly hard to find. To this degree, the results suggest relative encapsulation of selection processes in the two domains. REFERENCES Brown, V., Walker, R., Gray, C., & Findlay, J.M. (1999). Limb activation and the rehabilitatio n of unilateral neglect: Evidence of task specific effects. Neurocase, 5, Craighero, L., Fadiga, L., Rizzolatti, G., & Umiltà, C. (1999). Action for perception: A motorvisual attentional effect. Journal of Experimental Psychology: Human Perception and Performance, 25, Cubelli, R., Paganelli, N., Achilli, D., & Pedrizzi, S. (1999). Is one hand always better than two? A replication study. Neurocase, 5, Desimone, R., & Duncan, J. (1995). Neural mechanisms of selective visual attention. Annual Review of Neuroscienc e, 18, Deubel, H., & Schneider, W.X. (1996). Saccade target selection and object recognition : Evidence for a common attentional mechanism. Vision Research, 36, Deubel, H., Schneider, W.X., & Paprotta, I. (1998). Selective dorsal and ventral processing : Evidence for a common attentional mechanism in reaching and perception. Visual Cognition, 5, Driver, J., & Spence, C. (1998). Crossmodal attention. Current Opinion in Neurobiology, 8, Duncan, J. (1984). Selective attention and the organizatio n of visual information. Journal of Experimental Psychology: General, 113, Duncan, J. (1996). Cooperating brain systems in selective perception and action. In T. Inui & J.L. McClelland (Eds.), Attention and performance XVI. Information integration in perception and communication (pp ). Cambridge, MA: MIT Press. Duncan, J., & Humphreys, G. (1989). Visual search and stimulus similarity. Psychological Review, 96, Duncan, J., Humphreys, G., & Ward, R. (1997). Competitive brain activity in visual attention. Current Opinion in Neurobiolog y, 7,

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