.Y~~,cr,,p\~rkok,yru, Vol 26. No 4. pp. 521-531. 1988. Prmlrd I” Great Bnlam.

002X-3Y32/88 $3 OO+O.l~O Pergamon Press plc

HEMISPHERIC ASYMMETRIES IN MEDIATING INTENTION, BUT NOT SELECTIVE ATTENTION*?

MIEKE VERFAELLIE,$ DAWN BOWERS and KENNETH M. HEILMANQ:

Veterans Administration Medical Center and Department of Neurology. College of Medicine, University of Florida, Gainesville, Florida, U.S.A.; and $Memory Disorders Research Center, Veterans Administration

Medical Center, Boston MA. U.S.A.

(Received 7 Oc~toher 1986; accepted 4 December 1987)

Abstract-Heilman and Van Den Abe11 [Nruropsycholoyia 17, 315-321, 19791 reported a right hemispheredominance for cerebral activation. In the present study, we further examined the nature of this finding in an experimental paradigm in which selective attention and intention (response readiness) were manipulated independently. Normal subjects were tested on a choice reaction time task in which they were given preliminary information about where a target stimulus would occur (selective attention) and which hand to use for responding (intention). Our findings indicate that at short foreperiod durations, the right hemisphere is superior for mediating intention. There was no evidence for a right hemisphere superiority for selective attention.

INTRODUCTION

THE MANNER in which incoming information is processed in the brain is modulated by a number of processes, including arousal, selective attention, and intention/) [4]. Arousal is a physiological response that increases neuronal excitability [ 15,421. Selective attention is the guiding selective effect superimposed on the arousal reaction [28] and refers to the mechanism by which one decides what information to process and how far to process it. Intention is also superimposed on the arousal reaction and refers to the mechanisms by which one selectively prepares for action [33].

Although the latter two processes have been studied largely independently by cognitive psychologists and investigators of brain-behavior relationships, similar conceptual distinctions have been made. BROADBENT [7, S] distinguished between stimulus-set and response-set as two types of selection and suggested that the former one is input selective (selective attention) and the latter one output selective (intention). A similar distinction is implicit in more recent studies on selective set, in which the subject is selectively prepared

*This work was supported by the Medical Research Sercice of the Veterans Administration and by the State of Florida Department of Health and Rehabilitative Services. Aging and Adult Services Memory Disorders Clime.

tPrcsentcd in part before the 13th Annual Meeting of the International Neurops)chological Society, San Diego. CA, 1985.

SCorrespondence to be addressed to: Kenneth M. Heilman. Box J-236. Department of Neurology, College of Medicine. University of Florida, Gainesville. FL 32610. U.S.A.

,jln the context of the present formulation, the term intention is used solely to refer to aspects of motor preparation. and in this sense, it is synonymous to the terms motor set or response readiness. The term intention is used here. however. because it has previously been used in the same context.

522 MIEKI VKKFAI LLW. DAWN Bowms and KFNUCTH M. HEILMAN

either for certain stimuli or for certain responses. POSNER et ~1. [30, 311 investigated the effects of preparatory cues on the processing of incoming information in a series of studies examining how attention is directed to a position in space. In an initial study 1301. they presented a midline cue that indicated where in space a lateralized target stimulus would most likely follow. They found that when this information was correct. it significantly decreased reaction times as compared to a situation in which no information was given (benefits). When this information was incorrect, it significantly increased reaction time (costs). The authors subsequently showed that similar attention shifts could be triggered by peripheral cues 1311 and that these costs and benefits cannot be explained by a change in response criterion. Rather. attention shifts are due to an active allocation of attention to a position in space [27. 311. Furthermore, these covert shifts in attention do not depend on saccadic eye movements [27. 31. 341. Not only can attention be shifted with eyes fixed, but also. attention shifts are unaffected by movements of the eyes opposite to the direction of attention.

Using a similar precueing technique, a number of investigators have examined different aspects of response preparation [24,25,35]. In a study by ROSENBAUM and KORNRLLM 1361. for instance, subjects were presented on each trial with an auditory cue indicating which of two alternative responses would be required. It was found that reaction times were faster for cued than for noncued responses, and furthermore, that the reaction time for the cued

response did not depend on the identity of the other possible response. The distinction between selective attention and response readiness (intention) has

important implications on both neurophysiological and behavioral levels. Based on the work of SOICOLOV [38] and ROUTTIINBERC; 1371, a number of investigators have suggested that selective attention and intention are mediated by different neurophysiological systems. PRmRAhl and MCG~JINNIISS 133) postulated that arousal is controlled by a neural system in the amygdala, whereas intention is controlled by a neural system in the basal ganglia. Marc recently, HEII.hlAN et trl. [I31 and WATSON rt trl. [41] have proposed that attention is modulated by a cortical (temporoparietal) limbic reticular system and that intention is mediated by a cortical (frontal) limbic basal ganglia- reticular system.

Behavioral studies in humans have suggested that each hemisphere is important for mediating selective attention and intention in contralateral hemispace [4, 51. In addition. several lines of evidence suggest that the right hemisphere may be dominant for these processes. Hemispatial neglect. a syndrome due to a unilateral disorder of arousal and

attentional mechanisms [lo. 131, is more often associated with right than with left hemisphere lesions 16. 10. 231 and is also more severe following right hemisphere lesions [ 1. 171. Testing patients with unilateral lesions on a reaction time task, several investigators 13, 1 1, 161 found that right hemisphere lesions cause greater slowing in reaction times than those of the left hemisphere. Several studies of normal subjects [2, 171 found that simple reaction times were faster Lvhen stimuli were presented in the left visual field than in the right visual field. In a study by HEII.~IAN and VAN DFN AHEI.L [14], neutral warning stimuli were presented to the left or right visual field and followed by midline stimuli to which manual reaction times were obtained. They found that warning stimuli prescntcd in left visual field reduced reaction times more than did those presented in right visual field. Unlike findings in the studies that used lateralized reaction time stimuli, those in the Heilman and Van Den Abel1 study could not be explained by asymmetries in anatomic connections. Instead. these authors interpreted the data as evidence for a right hemispheric dominance for mediating cerebral activation: warning stimuli directed to the right hemisphere produce more

HEMISPHER,(‘ ASYMMETKIES 1Y MEDIATII\;G INTENTION. BUT UOT StLECTIVt ATTFNTIO~ 523

activation than do warning stimuli directed to the left hemisphere. Because activation increases processing speed, this results in faster responses following warning stimuli in left

than in right visual field. This study suggests that there is a right hemisphere dominance for mediating cerebral

activation. It does not, however, address the nature of this activation. A neutral warning stimulus activates the subject and prepares him for a subsequent event [26]. This may involve attentional as well as intentional processes. Hence, the question arises whether the role of the right hemisphere in mediating activation reflects a role in selective attention, intention, or both.

One way to address this question experimentally on a behavioral level is to manipulate selective attentional and intentional processes within the same task and to study their effects on reaction time. In the present study, we used a cueing paradigm similar to that used by POSN‘ER et al. [30], and we manipulated the presence of attentional and intentional cues simultaneously but independently. Cues were presented centrally, because in this situation attention shifts are more under the subject’s control [19] and independent of sensory peripheral changes [27]. The subject’s task was to respond as quickly as possible, with the left or right hand, to one of two target stimuli located in left or right hemispace. Before the onset of a lateralized target, two warning stimuli were presented. One of these warning stimuli could indicate where the target stimulus would occur (attentional warning), and the other one could indicate which hand the subject would have to use for responding (intentional warning).

An assumption underlying the present paradigm is that the attentional and intentional cues influence different processes. If this assumption is correct. attentional and intentional cues should affect reaction times independent ofeach other. That is, information about where a target stimulus will occur (attentional information) should influence reaction times independent of knowledge of which hand to use to respond. Similarly, information about which hand to use (i.e. intentional information) should influence reaction times independent of knowledge of where the target stimulus will occur.

If the effects of selective attention and intention can indeed be separated by the present paradigm, then the question can be addressed as to how both hemispheres contribute to these processes. We defined hemispheric dominance as the differential ability of the two hemispheres to be activated by attentional or intentional cues. Behaviorally, this was inferred by comparing response latencies of the left and right hand to stimuli presented in left and right hemispace. We hypothesized that if the right hemisphere is superior for mediating selective attention to contralateral stimuli, reaction times should be faster when attention is directed to the left hemispace than when directed to the right hemispace. Similarly, WC hypothesized that if the right hemisphere is superior for mediating intention, reaction times should be faster when the left hand is prepared for action than when the right hand is prepared for action.

METHOD Suhjec,t.s

Sixteen right-handed college students from an introductory psychology course participated in thts study. All subjects were right-handed according to self-report.

Subjects were given a choice reaction time task in which they manually responded to a lateralized target light to the left or right of a central fixation point. Before onset of the target stimulus. tno midline warning stimuli were

524 MIEKE VEKFACLLIE, DAWN BOWEKS and KENNETH M. HEILMAN

presented that might indicate where in space the target stimulus would occur (attentional warning) and which hand the subjects would have to use for responding (intentional warning).

Wurni~~ stimuli. The warning stimuli consisted of two sets of three vertically aligned lights with white lenses (luminance=0.30 mscp) positioned on a 48.5 x 26.5 cm grey board. One set of warning lights was positioned 2.4. 4.8, and 7.2’ above a centrally placed fixation point. and the other set ofthree Waring lights was positioned at similar angles below the fixation point (see Fig. I). One set conveyed attentional (or no) information and the other set conveyed intentional (or no) information

0

FIG I. Schematic representation of the stimulus display.

Superimposed on the top and bottom hght ofthe LIIILW/IOW/ ~c~rnincq ,stirmdrrr w was an arrow pointing IO the left or right. The arrows indicated whether 21 target stimulus to the left or right of fixation should be expected to occur. The middle warning light of the set was a neutral warning stimulus that gave no information as to where the subsequent target stimulus would occur. These warning stimuli were defined as the attentional learning stimulus set because the information provided by the nxrning stimulus, if any. enabled subjects to correctly shift their attention to the prob;lblc spatial location of the subsequent t;lrget stimulus.

Superimposed on the top and bottom light of the ir~t~utimtrl \vrrrr/irly srimuhs .M was an index finger pointing to the left or right. The direction in which the linger pointed indicated which hand the subject should expect to use to respond to the target stmlulus. The middle warning light of the set was again :I neutral wkirning stimulus. illumination of which provided no information ~1s to % hich hand would have to be used for respondmg. These wnrning stimuli were delined as the intentional \barning stimulus set. because the information they provided. if any. concerned the hand to be used to respond to the target stimulus. For half the subjects. the three upper h.arning stimuli served as the attentional warning stimulus set and the three lower w;irning stimuli served as the intentional warning stimulus set. The remaining subjects had the opposite situation.

T~cqrr sfimuli. The target stimuli consisted of two incandescent lights with white lenses (luminance=0.30 mscp) located seven degrees to the left and right of the lixntion point. Each light could be either bright or dim. (A dim light wits obtxincd by adding 430 Ohms rcaistancc.) For half the subjects. onset of a bright light to the left or right dcslgmtted the right hand should be used and onact of a dim light designnted the left hand should he used. The remaining subJects had the opposite situ:ition. Manual reaction times to the target stimuli \\ere measured by the rcleasc of one of two mlcrns\\ltchca hurlrontally aligned and posItioned 16 cm to the left and right of body midlmc. The right index linger wxs positioned on the right microawitch and the left index linger was posItioned on the left microswitch.

Each trial beg”n with the atimultaneous prescntatlon of two central L\;trning stimuli. one each of the :tttentlonal set nnd the intentional set. for 500 msec. This was folloticd after variable interstimulus intervals (I 500, 1X00. 2100. and 2400 msec) by onset of one of the target btimuli. Multiple interstimulus intervkds were used in order to decrease expectancy. Onset of the target stimulus actl\atcd R digital timer (Gerbrandb Gl270. Gerbrands Corpor;ltion. Arlington. MA) th;lt w;~s stopped bk the mnnunl rcleasc of the appropriate response key. In C:W subjects made ;!n error (l.c. responding with the left [right] h:lnd when the target stimulus indicated ;I right [left] hand response). J tono W;IS given and subjects \\crc rcquircd to correct thclr response before the next trial w:ls InitLrtcd. DIgital (BRSI logic controlled the durlltlvn of the wcarning ;~nd targcr stirnull ;LS well ;I:, the intershmulus mtcrcal between the warning and tarfct stlmulu\.

HEMISPHERIC ASYMMETRIES IN MEDIATING INTENTION, BUT NOT SELECTIVE ATTENTION 525

Procedure

During testing, the subjects sat in a dark room at a table 60 cm in front of the stimulus board with their left and right index finger positioned on the left and right microswitch, respectively. They were instructed to fixate on the fixation point and to respond as quickly as possible to the target stimulus presented to the left or right of fixation. It was stressed that the subjects should use the information conveyed by the warning stimuli, as this would reduce their reaction time. The subjects were told that speed and accuracy of performance were equally important.

Eye position was not monitored, because previous work has shown that the control of attention is largely independent of the eye movement system 121,271. Furthermore, it appears that when instructed to do so, subjects can accurately maintain fixation [lS]. In particular, in luminance detection tasks, subjects spontaneously suppress eye movements even if they are allowed to make them, because eye movements actually slow down overall reaction time [3 11.

Each subject participated in two testing sessions that were given a week apart. Each session consisted of one block of 50 practice trials and two blocks of 160experimental trials with a 5 min rest interval between blocks. Within each block. four types of trials were given: (1) trials in which an informative attentional warning stimulus was given, providing information about the probable spatial location of the subsequent target stimulus, as well as an intentional warning stimulus providing information about the probable hand to use when responding (attentional and intentional warning); (2) trials in which an informative attentional warning stimulus was given together with a neutral intentional warning stimulus (attentional warning only); (3) trials in which a neutral attentional warning stimulus was given together with an informative intentional warning stimulus (intentional warning only); and (4) trials in which a neutral attentional warning stimulus and a neutral intentional warning stimulus were given (no selective warning). This latter condition was included so that the effects of generalized arousal could be separated from the more specific attentional and intentional effects. The order of these four warning stimulus conditions was randomized, but occurred equally often within each block. Likewise, hemispace in which the subsequent target stimulus was presented as well as hand of response were randomized but occurred equally often.

To offset anticipatory responses, the information provided by the warning stimulus was valid or correct only X0%, of the time with respect to the subsequent target stimulus. Conversely, it was invalid or incorrect on 2O”/u of the trials. For example, on an invalid trial with attentional information only (and a neutral intentional warning stimulus) the attentional warning stimulus indicated that the target would occur in left (right) hemispace when in actuality a target in right (left) hemispace was presented. Likewise, on an invalid trial with intentional information only (and a neutral attentional warning stimulus), the intentional warning stimulus indicated that a left (right) hand response would be required by the target stimulus, when in actuality a right (left) hand response was required. On invalid trials with both attentional and intentional information, both warning stimuli gave invalid information.

Before beginning each experimental session, the subjects were trained to discriminate between a required left and right hand response. This was followed by 50 practice trials in which the warning stimulus preceded the target light and preliminary information was given.

RESULTS

Preliminary inspection of the data showed no systematic differences between the results of sessions 1 and 2. Thus, these data were combined in subsequent analyses. Mean response accuracy (proportion correct) and response latency (reaction time) data were computed for each subject, as a function of the presence of attentional and intentional information, hemispace in which the target was presented, and hand of response. Mean response accuracy and response latency data for the conditions in which valid (or no) information was provided are shown in Table 1. Response latency data are based only on those trials in which a correct response was made. These data were analyzed using weighted repeated measures analyses of variance (ANOVA) with Attentional warning (yes, no), Intentional warning (yes, no), Hemispace in which the target was presented (left, right), and Hand used for response (left, right) as independent variables.

Response uccuruc~

To correct for the absence of a normal distribution in the data, we performed an arcsin square root transformation [20] on the proportion scores. In what follows, however, untransformed means are presented. The results of the ANOVA revealed a significant main effect for Intentional warning [F (1, 15) = 33.54, PC 0.011, indicating that the proportion of

526 MII tx VEKI:N.LLII . DAWN Bowr~s and K~:NNI,TH M. HFILMAN

Tahlc I. Proportion ot” correct rcaponscs and mean reaction times (mscc) for the conditwns m which valid (or no) information was provided

Attentionalk valid

Left hcmispace Right hemispace

Attcntional~ no

Left hemispace Right hemispacc

Intcntionalk valid

Lcli hand

Right hand

Intcntlonal no

Left hand

Right hand

0.9x 0.97 0.98 0.98 440 msec 464 mbec 470 msec 524 mbec

0.97 0.98 0.97 0.9X 472 msec 455 msec 541 msec 504 mscc

0.96 0.92 0.95 0.96 5s’) mscc 565 mscc 608 msec 621 mscc

0.9 I 0.92 0.93 0.93 570 mbec 562 msec 622 msec 61 I mscc

correct responses on trials with intentional warning (mean=0.97) was higher than on trials without intentional warning (mean=0.94). No other main effects or interactions were significant.

The results of the AN0 VA of the reaction time data showed significant main effects for Attentional warning [F (I. 15) = 33.65, P<O.Ol] and Intentional warning [F (I, 15)= 86.38. P<O.Ol]. As can be seen, reaction times were faster on trials with attentional warning (mean = 5 I I msec) than on trials without attentional warning (mean = 563 msec). Similarly. reaction times on trials with intentional warning (mean = 484 msec) were faster than on trials without intentional information (mean = 590 msec).

The Intentional warning x Hand interaction was also significant [F (I. 15)=4.60. P~O.051 and is depicted in Fig. 2. Post hoc comparisons (Duncan’s) indicated that reaction times for the left and right hand were no diKerent when no intentional information was provided (left = 589 msec: right = 591 msec). When intentional information was provided, however, reaction times for the left hand were faster than for the right hand (left ~475 mscc; right =493 mscc: P<O.Ol). suggesting that the left hand did benefit more from intentional information than the right hand.

HEMISPHERIC ASYMMETRIES IN MEDIATING INTENTIOii, HUT NOT SELECTIVE ATTENTION 527

Finally, a signficant Hemispace x Hand interaction was obtained [F (1, 15) = 19.69, P<O.Ol]. Post hoc comparisons (Duncan’s) showed that responses are faster in compatible stimulussresponse conditions (left hemispaceeleft hand and right hemispace-right hand) than in incompatible conditions (left hemispace-right hand and right hemispace-left hand). This interaction was further modified by a significant Attentional warning x Hemispace x Hand interaction [F (1, 15)=6.16, P<O.O5] as well as a significant Intentional

warning x Hemispace x Hand interaction [F (1, 15) = 18.29, P < 0.011. Post hoc compari- sons (Duncan’s) on the former interaction indicated that compatibility effects were present when no attentional information was provided, but partly disappeared when attentional information was provided. Post hoc comparisons (Duncan’s) on the latter interaction indicated that compatibility effects were present when intentional information was given, but disappeared when no intentional information was given. However, because these com- patibility effects are not directly related to the main theme of this report, they are not further considered here. For a more detailed discussion of these findings, see VERFAELLIE et ul. [40].

As previously indicated, cues providing invalid information were presented randomly on 20% of the trials, as a method to offset anticipatory responses. Accuracy data obtained on trials with invalid (or no) information are presented in Table 2. As expected, response accuracy was significantly attenuated after invalid intentional information [F (1, 15) = 26.74, P < O.Ol]. Furthermore, a significant Intentional warning x Hand interac- tion [F (1, 15) =4.64, P< 0.051 was obtained, indicating response accuracy for the left and right hand were no different when no intentional information was provided. Performance of the left hand was more accurate than the right hand, however, when invalid intentional information was provided.

Table 2. Proportion of correct responses for the conditions in which invalid (or no) information was provided

Intentional--valid Left hand Right hand

Intcntionalkno Left hand Right hand

Attentionalkinvalid AttentionalL -+o

Left hemispace Right hemispace Left hemispace Right hemispace

0.79 0.74 0.80 0.69 0.69 0.69 0.69 0.69

0.95 0.91 0.95 0.96 0.93 0.92 O.Y3 0.93

Reaction time data for trials with invalid intentional informa*‘on were not analyzed because the proportion of correct responses on these trials was significantly lower than on trials without intentional information. These data could not be meaningfully interpreted because reaction time data were lacking for those trials in which the invalid intentional information was most detrimental and consequently resulted in incorrect responses. Again, as expected. invalid attentional information significantly increased reaction times [I; ( 1. 15) = 8.2 I, P < 0.05: invalid attentional warning = 635 msec; no attentional warn- ing = 555 msec].

DISCIJSSION

In the present study, we examined the role of both hemispheres in the mediation of selective attention and intention in the context of a behavioral paradigm allowing for the

528 MIEKE VEKFACLLIE, DAWN BOWERS and KENNETH M. HEILMAN

independent manipulation of these processes. Before addressing the issue of hemispheric asymmetries, it is important to examine to what extent the present findings can be explained in the context of the attentional and intentional manipulations that were used.

Both the accuracy and response latency data indicate that manipulating selective attention and intention has different effects on subsequent behavior. Consistent with the findings of POSNER et al. 127, 30, 311, we found that knowledge about where in space a stimulus will occur affects response speed both by facilitating responses at the expected position and retarding them at the unexpected position. This information, however, does not affect response accuracy. Significant costs and benefits were also associated with intentional cues, but these were apparent in both response accuracy and response speed. Information about which hand to use to respond to a subsequent target stimulus increases both response accuracy and response speed of the expected hand and considerably decreases response accuracy of the unexpected hand.

The aforementioned results may in part be a function of differences in the two underlying systems. Intentional systems are presumably more closely linked with motor systems: invalid intentional information activates the incorrect motor systems that, once activated, have a propensity to respond to the target stimulus, irrespective of the correctness of the response. Attentional systems, however, are presumably not as closely linked to the motor systems. Invalid attentional information directs attention to the incorrect hemispace so that a re- orienting of attention is necessary to detect the stimulus and to decide on the appropriate response. Consequently, this would not affect response accuracy, only response speed. The present findings are consistent with this interpretation.

Additionally, the results indicate the effects of manipulating attentional and intentional cues cannot be explained on the basis of generalized arousal. LANSING et al. 1221 found that warning stimuli reduce reaction times and suggested that neutral warning stimuli serve to activate the individual, and prepare them for action. By using neutral as well as informative warning stimuli in the present study, we could separate the effects of general arousal and superimposed attentional and intentional information. The finding that attentional and intentional manipulations each produce significant main effects, but no interaction is consistent with our initial assumption that both types of cues affect different underlying processes.

In the context of these manipulations, we examined whether the right hemisphere superiority previously observed for activation [ 141 reflects a right hemisphere superiority for selective attention, intention, or both. In the present study, no asymmetries were found for the processing of attentional information. Valid attentional information resulted in a reduction in reaction time, but did so equally for stimuli in either hemispace. The question arises whether hemispheric differences in the control of selective attention might exist at an earlier stage of processing [9], relating to differences between the visual fields. Given the long foreperiod durations and the lack of fixation control in the present study, our paradigm cannot adequately address this question. This appears unlikely, however, in light of the evidence that selective attention is hemispatially mediated. Furthermore, in a peripheral cueing study by POWER et d. [29], no significant visual field effect was found, suggesting that both hemispheres benefit equally from cues directing attention to the corresponding visual field.

In contrast, asymmetries were found for the processing of intentional information. On trials with valid intentional information, reaction times were faster with the left hand than with the right hand. No ditferences between the hands were found on trials without

HEMISPHERIC ASYMMETRIES IN MEDIATING INTENTION, BUT NOT SELECTIVE ATTENTION 529

intentional information. This suggests that the observed left hand advantage is due to the

intentional manipulation and implies a right hemispheric superiority for intention.

Additionally, on trials with invalid intentional information, preparation of the left hand

(when a right hand response was required) resulted in more errors than did preparation of the right hand (when a left hand response was required). This further suggests that activation of

the right hemisphere may affect readiness to respond with the left hand more than activation

of the left hemisphere affects readiness to respond with the right hand.

In a pilot study [39] using the same experimental manipulations but foreperiod dura- tions varying between 2500 and 3400 msec, no hemispheric asymmetries were found in the

processing of intentional information. Thus, it appears that the right hemisphere superiority

for intention may be time locked to the onset of the warning stimulus. A similar effect of

foreperiod duration on differential hemispheric activation has been reported by HEILMAN

and VAN DEN ABELL [14]. They interpreted their findings in terms of an inability of the left hemisphere to sustain activation. According to this hypothesis, however, the advantage of the left hand should become more pronounced as the foreperiod duration increased. Clearly,

this was not the case in the present study. Instead, we suggest that the present data may reflect

the left hemisphere being activated somewhat more slowly by the intentional cue. Once fully activated. however, both hemispheres execute motor responses equally efficiently. Further

studies comparing more directly the effects of differential foreperiod durations on hemispheric asymmetries in activation are needed, however, to clarify this question.

In light of the present findings, a careful distinction between selective attention and intention becomes even more important. These findings suggest that the asymmetric

activation induced by a warning stimulus in the HEILMAN and VAN DEN ABELL [ 141 study

may be due to its intentional properties, rather than to its attentional properties. Recent clinical observations in patients with unilateral lesions are consistent with this

view. POWER et rrl. 1321 studied the effects of spatial cues on performance of patients with

unilateral parietal lesions. They found that patients with left- and right-sided lesions were

equally impaired when an invalid cue was presented on the side ipsilateral to the lesion.

Based on these findings, it was suggested that both hemispheres contribute equally to the

selective aspects of attention. Patients with right-sided lesions, however, performed

significantly worse than patients with left-sided lesions when no cue was provided. This latter finding was related to findings of HEILMAN and VAN DEN ABELL [ 141 and interpreted in the

context of a right hemisphere dominance for arousal. In the context of the present study, it appears that the poor performance of patients with right-sided lesion may be related

specifically to the intentional properties of the warning stimulus and the role of the right hemisphere in mediating this process.

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