Neurophysiological and Neuropsychological Aspects of Spatial Neglect, M . Jeannerod (editor) O Elsevier Science Publishen B.V. (North-Holland), 1987

UNILATERAL ATTENTION DEFICITS AND HEMISPHERIC ASYMMETRIES IN THE CONTROL OF ATTENTION

Eric A. Roy, Patricia Reuter-Lorenz, Louise G. Roy, Sherrie Copland

and Morris Moscovitch

Hemispheric d i f f e r e n c e s i n the control o f a t t e n t i o n are the focus o f t h i s chapter. D i f f e r e n t i a l hemispheric involvement i n the control o f a t t e n t i o n may be r e f l e c t e d i n the increased incidence and s e v e r i t y o f hemi-inattenti811 associated w i t h r igh t hemisphere damage. Clues t o the bas i s o f hemispheric asymmetries i n the control o f a t t e n t i o n a r e , then , soughtthrough considering how components o f a t t e n t i o n may be a f f e c t e d i n these hemi- attentionaldeficits.Alertingorarousa1, orient ingand capacity components o f a t t e n t i o n are each considered through reviewing workdone i n o u r labora toryandbyothers .

D e f i c i t s i n a t ten t ionhave been a f o c u s o f studyinneuropsychologyfor many years (e.g., Heilman, Watson&Valens te in , 1985;Mesulam, 1985).Oneof the more puzzling and well-known a t t e n t i o n d e f i c i t s i s un i la tera l neglect or hemi-inattention, a disorder i n which pat ients appear unaware o f and f a i l t o respond t o s t imulat ion occuring contralateral t o the damaged , hemisphere. L e f t neglect associated wi th r ight hemisphere damage tends t o . ' be more common and severe than r igh t neglect associated wi th l e f t hemisphere .' damage. Th is d i f f e r e n c e i n t h e incidence o f l e f t versus r ight neglect may r e f l e c t d i f f e r e n t i a l hemispheric involvement i n the control o f a t ten t ion . Clues t o the basis o f t h i s d i f f e r e n c e between the hemispheres may emerge through considering how the components o f a t t e n t i o n contribute t o the varied mani fes ta t ions o f hemi-attentionaldeficits: a d e f i c i t i n a ler t ing or arousal (Heilman & Watson, 1977), a d e f i c i t i n or ien t ing (Kinsbourne, i' 1977; Posner, Cohen & R a f a l , 1982), or i n directed a t t e n t i o n (Mesulam, 1981).

Th is chapter w i l l consider hemispheric d i f f e r e n c e s i n the control o f a t t e n t i o n i n view o f these d i f f e r e n t a t ten t ional processes. A b r i e f overview o f the d i s t i n c t i o n s between arousa l /ac t iva t ion , capaci ty , and the se lec t ion aspects o f a t t e n t i o n begins the discussion. Evidence concerning possible hemispheric asymmetries i n these processes i s then considered through reviewing work done i n our laboratory and by others . Studies on normal subjects and pat ients w i t h l a t e r a l i z e d brain damage, w i t h and without n e g l e c t , are considered i n an e f f o r t t o understand t h e a

contribut ions o f the r igh t and l e f t hemisphere t o various a t t e n t i o n processesandthe ro le oftheseprocessesinneglect.

Caaponentsof Attention I t has long been recognized t h a t a t t e n t i o n comes i n many v a r i e t i e s

(James, 1890). Three bas ic a s p e c t s o f a t ten t ionhave beendescribed over the past two decades. ~ h c s e E ~ ~ r ~ ~ e ~ a ~ o ~ s a 1 ~ , ~ a p ~ c ~ _ t ~ ~ ' ~ ~ ~ n d ~ , ~ e ~ e _ c ~ t ~ i ~ o n (Posner h Boies, 1971). Arousal or a ler tness i s thought t o be c l o s e l y related t o the

25 E. A. Roy et al.

underlying l e v e l o f physiological ac t iva t ion ( D u f f y , 1957). Arousal i s r e f l e c t e d i n performance e f f i c i e n c y or the readiness t o take i n and respond t o information i n the environment (Easterbrook, 1959; Kahneman, 1970). Vigi lance taskswhich require periods o f s u s t a i n e d a t t e n t i o n h a v e t y p i c a l l y been used t o study t h e i n f l u e n c e o f a l e r t n e s s o n i n f o r m a t i o n p r o c e s s i n g . T h e e f f e c t s o f warning s ignals on react ion time and accuracy have a l so been studied t o determine the role o f phasic changes inarousa lonpercep tua l and response readiness (Posner & Boies, 1971). A var ia t ion o f these react ion time experiments involves presenting warning s ignals t o one or the other visual hemif ield (hemisphere) (e .g. , Heilman & Van den Abe l l , 1979) or presenting the react ion signal t o one or the other v i sua l hemi f ie lds (e.g., Berlucchi, 1978), and observing d i f f e r e n c e s i n reaction t imes between the two hands or between signals presented t o the two hemi f ie lds . These comparisons enable a study o f hemispheric asymmetries i n response preparation.

The notion o f capacity or resources r e f e r s t o the a l loca t ion o f mental energy t o a task which, i n t u r n , i n f l u e n c e s the q u a l i t y o f performance (Navon & Gopher, 1979;Wickens, 1 9 8 4 ) . S i n c e t h e p o o l o f a v a i l a b l e resources is f i n i t e , the a b i l i t y t o perform numerous t a s k s a t once i s l imi ted . The d e f i c i t i n performance incurred by doing two t a s k s simultaneously rather than separately may r e f l e c t the capacity requirements o f a given task. Capacity requirements can vary due t o practice and as a r e s u l t o f the nature and d i f f i c u l t y o f a t a s k .

The d i s t i n c t i o n between automatic and contro l led , or e f f o r t f u l , processing has been important i n accounting f o r the decreased need f o r a t ten t ional involvement tha t occurs wi th extended practice (Schneider & S h i f f r i n , 1976; Schneider,Dumais & S h i f f r i n , 1984). The type o f processing associated with seemingly e f f o r t l e s s , well-practiced behaviours has been called automatic. These behaviours can occur invo lun tar i ly w i t h l i t t l e or no conscious in terven t ion and require minimal resources. On t h e otherhand, controlled processing i s e f f o r t f u l , requires resources and i s subject-regulated.

The se lec t ive property o f a t t e n t i o n i s tha t which determines what information w i l l be processed r e l a t i v e t o a l l t h e sources present. Se lec t ive a t ten t ion provides a means f o r choice t o be exercised regarding sensory experience. The se lec t ion o f one stimulus from among many can be done on the bas i s o f any o f a number o f stimulus a t t r i b u t e s , such as colour, s i z e , and loca t ion (Treisman, 1969; Duncan, 1980). In t h e study o f un i la tera l neg lec t , the process o f se lec t ing on the bas i s o f spa t ia l locat ion has been o f most i n t e r e s t (Kinsbourne, 1970a,b, 1974; Posner e t a l . , 1982). Orienting towards the relevant loca t ion i s cen tra l t o spat ial se lec t ion . Orienting may involve overt movements o f the eyes and head or j u s t a c o v e r t s h i f t o f a t t e n t i o n ( P o s n e r , 1978).

In the following sec t ion , we consider evidence f o r hemispheric asymmetries i n each o f these processes: arousa l /ac t iva t ion , capaci ty , and se lec t ion . We f i r s t discuss arousal /act ivationprocesses . Thenwe consider capacity and the spa t ia l a l loca t ion o f a t t e n t i o n i n the context o f v i sua l search. F ina l ly , we examine work on the orient ing o f a t t e n t i o n , a process important i n s e l e c t i v e a t ten t ion;

Arousal and Activation Heilman and h i s colleagues (Bowers & Heilman, 1980; Heilman & Van den

Abel l , 1979, 1980) have suggested tha t there may be d i f f e r e n c e s between the hemispheres i n arousa l /ac t iva t ion processes. They propose t h a t t h e r igh t hemisphere i s dominant f o r arousa l /ac t iva t ion processes and i s capable o f act ivat ing (preparing) responses f o r both hands. The l e f t hemisphere,

Unilateral attention deficits and hemispheric asymmetries

however, i s capable of a c t i v a t i n g responses f o r t h e c o n t r a l a t e r a l r i g h t hand only. I n s t u d i e s by Heilman used t o suppor t t h i s no t ion , s u b j e c t s were r equ i red t o r e l e a s e a response key upon t h e appearance of t h e r e a c t i o n s i g n a l which was preceded by a v i s u a l warning s i g n a l presented t o one hemisphere o r t h e o t h e r . The main focus of t h e s e s t u d i e s was on t h e r e l a t i v e e f f e c t s of p r e s e n t i n g t h e warning s i g n a l t o t h e r i g h t o r l e f t hemisphere on r e a c t i o n time. The p r e d i c t i o n was t h a t r e a c t i o n t ime should be f a s t e r when t h e warning s i g n a l i s d i r e c t e d t o t h e r i g h t hemisphere than t o t h e l e f t hemisphere s i n c e t h e r i g h t hemisphere was thought t o enjoy an advantage i n arousal/activationprocesses.

The r e s u l t s from t h e s e s t u d i e s g e n e r a l l y suppor ted t h i s p r e d i c t i o n i n t h a t f a s t e r r e a c t i o n t imes were observed fol lowing warning s i g n a l s presented t o t h e r i g h t hemisphere. I n a d d i t i o n , i t was found t h a t warning s i g n a l s presented t o t h e r i g h t hemisphere produced r educ t ions i n r e a c t i o n t ime f o r both hands r e l a t i v e t o a no warning s i g n a l c o n d i t i o n , wh i l e those presented t o t h e l e f t hemisphere reduced r e a c t i o n t imes on ly i n t h e c o n t r a l a t e r a l ( r i g h t ) hand.

S t u d i e s i n ou r l a b o r a t o r y (Copland, Note 1) a r e f u r t h e r examining Heilman's p r e d i c t i o n s . A l l t h r e e exper iments t o b e d i s c u s s e d below involved a t a s k s i m i l a r t o Heilman's i n which s u b j e c t s were r equ i red t o depres s a response key upon t h e appearance of a r e a c t i o n s i g n a l . E a c h t r i a l b e g a n w i t h t h e appearance of a f i x a t i o n po in t followed 200 msec l a t e r by a warning s i g n a l l a t e r a l i z e d t o one v i s u a l f i e l d . A s i n Heilman's s t u d i e s , t h e r e a c t i o n s i g n a l followed t h e warning s i g n a l a t varying t ime i n t e r v a l s . Also, a s i n Heilman's s tudy , a no-warning s i g n a l cond i t ionwas included. I n experiment one, t h i s no warning cond i t ionwas embeddedinthewarnedtrials, while i n t h e second exper iment , t h e no-warning c o n d i t i o n was run sepa ra t e ly . A s imple r e a c t i o n t ime paradigm was used i n which on ly one hand r e s p o n d e d i n a n y o n e b lockof t r i a l s .

O v e r a l l , t h e r e s u l t s of t h e s e f i r s t two exper iments d i d no t r e p l i c a t e Heilman's f i n d i n g s , t h a t i s , t h e r e w a s no advantage i n r e a c t i o n t ime f o r t h e r i g h t hemisphere. P o s s i b l y , t h i s l a c k of suppor t d e r i v e s f r o m t h e f a c t t h a t a s imple r e a c t i o n t ime t a s k was used i n t h e s e exper iments , a paradigm where t h e s u b j e c t knows i n advance t h e r equ i red response. Bowers and Heilman (1980) q u a l i f i e d t h e i r t heo ry of r i g h t hemisphere dominance f o r a c t i v a t i o n by i n d i c a t i n g t h a t l ' a response-linked d e c i s i o n a l p rocesswas necessa ry f o r inducing a c t i v a t i o n asymmetries". The re fo re , a cho ice r e a c t i o n t ime paradigm i n which t h e s u b j e c t does n o t know which hand w i l l be used p r i o r t o t h e r e a c t i o n s i g n a l might be more s e n s i t i v e t o a c t i v a t i o n asymmetries.The t h i r d experiment was des igned t o examine d i f f e r e n c e s i n r e a c t i o n t ime betweenwarning s i g n a l c o n d i t i o n s , i n a c h o i c e r e a c t i o n t ime paradigm.

I R e s u l t s of t h i s experiment b a s i c a l l y r e p l i c a t e d t h o s e o f t h e f i r s t two I , experiments. There were no d i f f e r e n c e s i n r e a c t i o n t ime between t h e

l a t e r a l i z e d w a r n i n g cond i t ions . I n cons ide r ing t h e even t s on e a c h t r i a l , t h e l a c k o f a r i g h t hemisphere

e f f e c t of t h e l a t e r a l i z e d warning s i g n a l may have r e l a t e d t o t h e appearance of t h e f i x a t i o n do t a t t h e beginning of each t r i a l . That i s , on each t r i a l , a f i x a t i o n d o t appeared 200 m p r i o r t o t h e l a t e r a l i z e d warning s i g n a l . The S f i x a t i o n do t may have ac t ed a a warning s i g n a l , poss ib ly washing o u t t h e p red ic t ed r educ t ion i n r e a c t i o n t ime fol lowing warning s i g n a l s d i r e c t e d t o t h e r i g h t hemisphere. I f t h e appearance of t h e f i x a t i o n po in t was a c t i n g t o a l e r t t h e s u b j e c t , t hen i t would seem reasonable t o remove i t s a l e r t i n g c h a r a c t e r i s t i c s the reby enab l ing t h e l a t e r a l i z e d warning s i g n a l s t o be t h e s o l e source of a c t i v a t i o n . I n a new s e r i e s of exper iments , t hen , s e v e r a l methodological changes a r e being made t o reduce t h i s a l e r t i n g e f f e c t of t h e f i x a t i o n po in t . These changes may permit a c l e a r e r examination of t h e

2 8 E.A. Roy et al.

activatingeffectofthelateralizedwarningsignalsonreactiontime.

Visual Search Kinsbourne (1970a,b) proposed a model o f or ien t ing i n which a common

regulatory principle governs hemispheric control o f a l l d i rec t ional orient ing behaviours. He explained t h a t l i k e head and eye movements, a t t e n t i o n s h i f t s are directed t o the r igh t or l e f t by the contralateral hemisphere. In neg lec t , the d i rec t ional tendency o f the i n t a c t hemisphere dominates when it can no longer be opposed by the damaged hemisphere. There fore , l e f t neglect i s due to t h e unopposed rightward a t ten t ional b ias o f the i n t a c t l e f t hemisphere, whereas r igh t neglect i s a n e x p r e s s i o n o f t h e r ight hemisphere's le f tward bias. Kinsbourne (1974, 1977) hasalsoproposed that the rightward orient ing tendency o f the l e f t hemisphere i s e s s e n t i a l l y stronger than the le f tward tendency o f the r igh t hemisphere. The basic d i f f e r e n c e i n s trength i s r e f l e c t e d i n the asymmetry i n overt or ien t ing i n the formof head turning found i n i n f a n t s ( (Turkewi t z , Gordon&Birch, 1965; Corye l l , 1 9 8 5 ) a n d i n t h e d i f f e r e n t i a l i n c i d e n c e o f r igh t and l e f t neglect .

Heilman's group has also o f f e r e d an account o f neglect which has considered the a l loca t ion o f at tent ion inspace .The irmu1t i - face tedtheory addresses many aspects o f neglect phenomena andcenters o n t h e idea t h a t the r ight hemisphere i s dominant for a t t e n t i o n , although they do not r e f e r s p e c i f i c a l l y t o spat ial o r i e n t i n g i n the sense used byKinsbourne (1970a,b; 1 9 7 4 ; 1977)andPosner (1980; s e e b e l o w ) . O n e a s p e c t o f t h i s d o m i n a n c e i s t h e a b i l i t y o f the r ight hemisphere t o a t t e n d ( 0 r i e n t ) t o b o t h t h e r igh t and l e f t sides o f space, whereas l e f t hemisphere control i s contralateral . Th is proposal i s based primarily on the f inding tha t electroencephalographic measures ( E E G ) i n normal sub jec t s ind ica ted r ighthemispheredesynchronyto e i t h e r r ight v i sua l f i e l d (RVF) or l e f t v i sua l f i e l d (LVF) warning s ignals . L e f t hemisphere desynchrony followed only RVF warning events (Heilman and Van Den Abe l l , 1980).

Bi lateral control could be operationalized as the a b i l i t y t o or ien t t o the r ight and l e f t s ides o f space. A l terna t ive ly , it could imply tha t the r ight hemisphere can attend t o a broader region o f space a t a given point i n time whereas the l e f t hemisphere at tends t o a more res t r ic ted region. Heilman explains tha t b i l a t e r a l a t t e n t i o n o f the r ight hemisphere enables i t t o compensate f o r the l o s s o f a t ten t ional control when the l e f t hemisphere i s damaged. The l e f t hemisphere cannot provide the same compensation a f t e r r igh t hemisphere damage. Thus, l e f t neglect i s more common. Kinsbourne's idea o f a stronger rightward than lef tward orient ing tendency predicts t h a t r ight hemisphere damage would lead t o a greater d i rec t ional bias . I n t h i s sec t ion , research on hemispheric control o f s p a t i a l a t t e n t i o n w i l l b e c o n s i d e r e d i n l i g h t o f t h e s e h y p o t h e s e s .

The ro le o f hemispheric mechanisms i n the spa t ia l a l loca t ion o f a t t e n t i o n may be r e f l e c t e d i n t h e search behaviour o f pat ients wi th la tera l i zed les ions . A number o f s tud ies have examined the a b i l i t y o f r ight hemisphere and l e f t hemisphere pat ients t o locate a target amidst an array o f d i s t rac tors . Search time and accuracy as a funct ion o f targe t loca t ion p r o v i d e a n i n d e x o f s e a r c h e f f i c i e n c y i n t h e r igh t and l e f t s i d e s o f space.

DeRenzi, Faglioni and S c o t t i (1970) found t h a t , on a v i s u a l search t a s k , both r ight hemisphere and l e f t hemisphere pat ients took s l i g h t l y longer t o find contralesional than i p s i l e s i o n a l targe t s . On a t a c t i l e search t a s k , a s imilar but even strongerpatternemerged.Righthemisphere patients were more severely impaired, o f t e n f a i l i n g t o f ind t h e targe t within the al loted time period when i t was i n the contralesional f i e l d . The au thor s point out that search requires t h e integrated funct ioning o f motoric, a t t e n t i o n a l , and representational processes. Although they

Unilateral attention deficits and hemispheric asymmetries 29

conclude t h a t t h e multimodal na tu re of t h e d e f i c i t a rgues f o r a d i s o r d e r i n space r e p r e s e n t a t i o n , t h e i r f i n d i n g s do not r u l e ou t t h e p o s s i b i l i t y of a t t e n t i o n a l impairment. It i s conceivable t h a t a n o r i e n t i n g b i a s consequent t o b r a i n damage could c o n s t r a i n not o n l y t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n , b u t a l s o t h e d i r e c t i o n a l c o n t r o l o f h e a d a n d limbmovements.

Although s h i f t s i n a t t e n t i o n and eye movements can be d i s s o c i a t e d , t y p i c a l l y , t h e p o s i t i o n of a t t e n t i o n and gaze co inc ides . Thus, one way t o s tudy a t t e n t i o n a l o r i e n t i n g i s t o examine eye movements. Chsdru, Leblanc and Lhermit te (1973) recorded t h e eyemovement s o f r i g h t hemisphere and l e f t hemisphere p a t i e n t s u s ing a v i s u a l s ea rch task . P a t i e n t s showed an o v e r a l l i n c r e a s e i n s e a r c h t ime i n r e l a t i o n t o c o n t r o l s , and both r i g h t hemisphere and l e f t hemisphere groups took longe r t o f i n d a t a r g e t pos i t i oned c o n t r a l e s i o n a l l y . The re fo re , d i so rde red s e a r c h behaviour was common t o bo th groups. The measure which d i sc r imina ted among p a t i e n t groups was t h e percentage of t ime spen t exp lo r ing t h e r i g h t v e r s u s l e f t s i d e s of t h e d i sp lay . Eye movement r eco rds revealed t h a t t h e r i g h t hemisphere p a t i e n t s spen t a g r e a t e r p ropor t ion of t h e s e a r c h t ime i n t h e r i g h t s i d e of t h e d i s p l a y than i n t h e l e f t . L e f t hemisphere p a t i e n t s d iv ided t h e i r time equa l lybetweenthetwohalves .

Both Ch6dru e t a l . and DeRenzi e t a l . found inc reased sea rch time f o r c o n t r a l e s i o n a l t a r g e t s r e g a r d l e s s of t h e l a t e r a l i t y o f t h e lesion.However, p a t i e n t s w i th r i g h t hemisphere damage seemed t o have t h e i r a t t e n t i o n anchored i n t h e i p s i l e s i o n a l s i d e of space more s o than d i d l e f t hemisphere p a t i e n t s . Th i s sugges t s t h a t wh i l e a s p a t i a l l y s e l e c t i v e d e f i c i t i n sea rch ing behaviour may occur a f t e r r i g h t o r l e f t hemisphere damage, t h e d e f i c i t a s s o c i a t e d wi th r i g h t hemisphere damage may be more seve re and may r e f l e c t a d i f f e r e n t under ly ing d i s t u r b a n c e than t h e l e f t hemisphere d e f i c i t . These f i n d i n g s , however, do no t d i s t i n g u i s h betweenKinsbournets a n d H e i l m a n V s t h e o r i e s .

Some recen t work i n ou r l a b o r a t o r y (Roy, Note 2 ; Roy & Roy, Note 3) has u t i l i z e d t h e v i s u a l s e a r c h paradigm t o e v a l u a t e f u r t h e r the proposal t h a t t h e l e f t hemisphere c o n t r o l s a t t e n t i o n p r imar i ly i n t h e r i g h t hemispace, wh i l e t h e r i g h t hemisphere d i r e c t s a t t e n t i o n t o both f i e l d s . I n t h i s t a s k , p a t i e n t s were r equ i red t o i n d i c a t e whether a t a r g e t l e t t e r appeared on a t e l e v i s i o n m o n i t o r by moving a s m a l l t ogg le switch.Thetargetappearedwith a p r o b a b i l i t y of .75, was presented e q u a l l y o f t e n i n t h e l e f t o r r i g h t hemispace, and appeared a lone o r i n conce r t w i t h 17 o r 35 d i s t r a c t o r s . According t o H e i l m a n V s p roposa l , t h e p r e d i c t i o n w a s t h a t damage t o t h e r i g h t hemisphere should be a s s o c i a t e d w i t h inc reased sea rch time and decreased accuracy i n both f i e l d s , wh i l e damage t o t h e l e f t hemisphere should impa i r s ea rch t imeand a c c u r a c y i n t h e r i g h t h e m i s p a c e o n l y .

The r e s u l t s f o r s e a r c h t ime seemed t o suppor t t hese p red ic t ions . Right hemisphere p a t i e n t s demonstrated no s i g n i f i c a n t d i f f e r e n c e s between t h e hemispa t i a l f i e l d s , wh i l e f o r t he l e f t hemisphere p a t i e n t s , s e a r c h t imewas s i g n i f i c a n t l y longe r i n t h e c o n t r a l e s i o n a l hemispace. Both groups e x h i b i t e d longe r sea rch t imes than t h e c o n t r o l s .

The f i n d i n g t h a t r i g h t hemisphere p a t i e n t s e x h i b i t e d no d i f f e r e n c e i n sea rch t ime between hemispa t i a l f i e l d s sugges t s t h a t damage t o t h i s hemisphere depressed speed of v i s u a l s e a r c h uniformly a c r o s s bo th hemispa t i a l f i e l d s . Such a patternmightbeexpectedifthe r i g h t hemisphere were involved i n b i l a t e r a l a t t e n t i o n a l c o n t r o l . The obse rva t ion t h a t l e f t hemisphere p a t i e n t s e x h i b i t e d inc reased sea rch t ime only i n t h e c o n t r a l e s i o n a l ( r i g h t ) hemispa t i a l f i e l d i s c o n s i s t e n t w i t h t h e i d e a t h a t l e f t hemisphere attentionalcontrolisprimarilycontralateral.

While ou r s e a r c h t ime d a t a seem t o provide some suppor t f o r t h e n o t i o n t h a t t h e r i g h t hemisphere may d i r e c t a t t e n t i o n t o b o t h h e m i s p a t i a l f i e l d s , a

3 0 E.A. Roy et al

c a r e f u l c o n s i d e r a t i o n of t h e s e d a t a i n con junc t ion w i t h t hose f o r accuracy r a i s e s some concerns about t h e e x t e n t of suppor t f o r t h i s no t ion . One might p r e d i c t t h a t t h e p a t t e r n f o r accuracy d a t a should conform t o t h a t f o r t h e s ea rch time d a t a . Such a cons i s t ency i n p a t t e r n was n o t observed, however. Right hemisphere p a t i e n t s were s i g n i f i c a n t l y l e s s a c u r a t e i n response t o t a r g e t s i n t h e l e f t hemispa t i a l f i e l d wh i l e a t t h e same t ime e x h i b i t i n g a b i l a t e r a l i n c r e a s e i n s e a r c h time. For l e f t hemispher ic p a t i e n t s , accuracy Was e q u i v a l e n t a c r o s s h e m i s p a t i a l f i e l d s , whereas s e a r c h t ime was s i g n i E i c a n t l y l onge r f o r t a r g e t s i n t h e r i g h t h e m i s p a t i a l f i e l d .

The unexpected d i s s o c i a t i o n of s e a r c h t ime and accuracy measures compl ica te t h e i n t e r p r e t a t i o n of t h e s e r e s u l t s and c a u t i o n s a g a i n s t making a t t r i b u t i o n s of hemispher ic dominance o r s u p e r i o r i t y i n t h e c o n t r o l of search . The d i s s o c i a t i o n a l s o unde r sco res t h e importance of u s i n g s e v e r a l lneasures of t a s k performance i n o r d e r t o de termine how s e a r c h i s conducted. As i n p rev ious i n v e s t i g a t i o n s of s e a r c h , both p a t i e n t groups show some s p a t i a l l y s p e c i f i c d e f i c i t . However, t h e s t y l e s o r s t r a t e g i e s of s e a r c h used by t h e two p a t i e n t groups seem t o d i f f e r . Right hemisphere p a t i e n t s mainta in a c o n s t a n t s e a r c h t ime a c r o s s h e m i s p a t i a l f i e l d s a t t h e c o s t of more e r r o r s i n t h e c o n t r a l e s i o n a l f i e l d . L e f t hemisphere p a t i e n t s , on t h e o t h e r hand, ma in t a in accuracy s c o r e s a c r o s s h e m i s p a t i a l f i e l d s a t t h e c o s t of i nc reased s e a r c h t ime i n t h e c o n t r a l e s i o n a l h e m i s p a t i a l f i e l d . Damage t o one o r t h e o t h e r hemisphere may cause d i f f e r e n t impairments i n a t t e n t i o n a l c o n t r o l which i n t u r n seem t o i n f l u e n c e t h e way t h e s e a r c h t a s k i s c a r r i e d ou t . These f i n d i n g s do no t c l e a r l y suppor t p r e d i c t i o n s from Heilman's o r Kinsbourne's models.

To s tudy t h e c a p a c i t y demands of v i s u a l s ea rch , two s e a r c h c o n d i t i o n s were used, a s i n g l e f e a t u r e and a conjoined f e a t u r e c o n d i t i o n ( s e e Treisman h Gelade, 1980). I n t h e s i n g l e f e a t u r e c o n d i t i o n , t h e t a r g e t d i f f e r e d from Lhe d i s t r a c t o r s on a s i n g l e f e a t u r e ( c o l o u r o r l e t t e r shape) . I n t h e conjoined f e a t u r e c o n d i t i o n , t h e t a r g e t d i f f e r e d from t h e d i s t r a c t o r s on the two dimensions, s h a r i n g c o l o u r w i t h some d i s t r a c t o r s and s h a p e w i t h t h e o t h e r s .

Consider ing t h e no t ion of c a p a c i t y demands i n t h i s t y p e of v i s u a l s ea rch t a s k , Treisman and Gelade (1980) have shown t h a t t h e conjoined f e a t u r e c o n d i t i o n i n v o l v e s a s e r i a l s e a r c h wh i l e t h e s i n g l e f e a t u r e cond i t i on i n v o l v e s a p a r a l l e l s ea rch . T h i s d i f f e r e n c e i n t h e n a t u r e of s e a r c h i s r e f l e c t e d i n t h e r e l a t i o n s h i p between s e a r c h t ime and number of d i s t r a c t o r s . I n t h e s i n g l e f e a t u r e c o n d i t i o n , t hey found t h a t s e a r c h t ime d id not i n c r e a s e w i t h number of d i s t r a c t o r s , wh i l e t h e r e was a s i g n i f i c a n t i n c r e a s e i n s e a r c h t ime w i t h t h e number of d i s t r a c t o r s i n t h e conjoined Eeature cond i t i on . One could inferfromthesedifferencesinthe s e a r c h t i m e func t ion , t h a t t h e conjoined f e a t u r e c o n d i t i o n involved a more a t t e n t i o n - demanding s e a r c h process . To a s s e s s whether t h e r e was any d i f f e r e n c e i n a t t e n t i o n demands of v i s u a l s e a r c h fo l lowing l e f t o r r i g h t hemisphere damage, comparisons of t h e s e a r c h t ime f u n c t i o n s i n t h e c o n j o i n e d and s i n g l e f e a t u r e c o n d i t i o n s were made between t h e two brain-damaged groups and t h e c o n t r o l s .

Examination of t h e d a t a r evea l ed a t r e n d f o r l e f t hemisphere p a t i e n t s t o e x h i b i t l onge r s e a r c h t imes over both f e a t u r e cond i t i ons . T h i s f i n d i n g may be due t o t h e f a c t t h a t v e r b a l s t i m u l i w e r e u s e d i n t h i s t a s k . T h e r e w e r e , however, no d i f f e r e n c e s among t h e groups i n t h e s l o p e s of t h e s e a r c h t ime f u n c t i o n s i n e i t h e r f e a t u r e cond i t i on . A c l o s e r examinat ion of t h e s e a r c h time f u n c t i o n s i n a l l t h r e e g r o u p s r evea l ed a s i g n i f i c a n t i n c r e a s e i n s e a r c h time wi th number of d i s t r a c t o r s i n bo th t h e s i n g l e and conjoined f e a t u r e c o n d i t i o n s , sugges t ing t h a t a s e r i a l , a t tent ion-demanding s e a r c h was involved i n b o t h c o n d i t i o n s .

Unilateral attention deficits and hemispheric asymmetries 3 1

Taken t o g e t h e r , o u r d a t a sugges t t h a t t h e r e a r e no d i f f e r e n c e s i n t h e e f f e c t s of va ry ing a t t e n t i o n demands on t h e manner i n which s e a r c h is c a r r i e d o u t by l e f t and r i g h t h e m i s p h e r e p a t i e n t s . D a m a g e t o o n e o r t h e o t h e r hemisphere, t hen , does n o t appea r t o a l t e r s e l e c t i v e l y t h e r e s o u r c e s a v a i l a b l e f o r p roces s ing in fo rma t ion i n t h i s t y p e o f v i s u a l s e a r c h t a s k . H a d damage t o one hemisphere l i m i t e d p roces s ing r e s o u r c e s , one might have expected a g r e a t e r s l o p e t o t h e s e a r c h t ime f u n c t i o n p a r t i c u l a r l y i n t h e conjoined f e a t u r e c o n d i t i o n i n t h a t brain-damaged group. T h i s f i n d i n g s u g g e s t s t h a t t h e d i f f e r e n c e between p a t i e n t groups on s e a r c h t ime and accuracy measures i s no t due t o t h e e f f e c t s of l a t e r a l i z e d damage on r e sou rce a v a i l a b i l i t y . Other r e sou rce demanding t a s k s , however, need t o be examinedbefore one c a n a c c e p t t h i s c o n c l u s i o n w i t h c o n f i d e n c e .

The unexpected ev idence f o r s e r i a l s e a r c h even i n t h e s i n g l e f e a t u r e cond i t i on s u g g e s t s t h a t t h i s t y p e o f attention-demanding s e a r c h p r o c e s s was employed by a l l t h e p a t i e n t s , e v e n t h e contro1,non-brain-damaged p a t i e n t s . Such was no t t h e c a s e i n Treisman and Ge lade ' s (1980) s t u d y i n t h a t s e r i a l s e a r c h was found on ly i n t h e conjoined cond i t i on . T h i s d i sc repancy wi th

: Treisman's work could be due t o t h e f a c t t h a t a l l t h e s u b j e c t s used i n t h i s s t u d y were a t l e a s t f o r t y t o f i f t y y e a r s o l d e r t han Tre isman 's s u b j e c t s . Perhaps , t h e g e n e r a l s lowing a s s o c i a t e d w i t h advancing age (Sa l thouse & Somberg, 1982; Smith, 1984) p l a c e s c o n s t r a i n t s o n p a r a l l e l p r o c e s s i n g w h i c h s e l e c t i v e l y a f f e c t s au toma t i c p roces s ing and f o r c e s t h e u s e of more c o n t r o l l e d p r o c e s s i n g a s r e f l e c t e d i n t h e s e r i a l s e a r c h p a t t e r n s .

: Orienting

Posner and h i s c o l l e a g u e s a r e p ionee r s i n t h e i n v e s t i g a t i o n of t h e e f f e c t s o f l a t e r a l i z e d b r a i n damage on c o v e r t a t t e n t i o n a l s h i f t s . Posner , Walker, F r i e d r i c h and R a f a l (1984) had p a r i e t a l p a t i e n t s perform a d e t e c t i o n t a s k i n which an advance cue i n d i c a t e d which of two l o c a t i o n s v o u l d m o s t l i k e l y c o n t a i n a t a r g e t . L ike normal s u b j e c t s , p a r i e t a l p a t i e n t s were f a s t e r a t d e t e c t i n g a t a r g e t a t t h e expected l o c a t i o n than a t t h e unexpected l o c a t i o n . T h i s p a t t e r n emerged f o r bo th t h e i p s i l e s i o n a l and c o n t r a l e s i o n a l t a r g e t s , a l t hough r e a c t i o n t ime t o c o n t r a l e s i o n a l t a r g e t s was c o n s i s t e n t l y s lower . T h i s f i n d i n g i n d i c a t e s t h a t t h e s e p a t i e n t s can v o l u n t a r i l y s h i f t t h e i r a t t e n t i o n i n r e s p o n s e t o a c u e .

The most s t r i k i n g d e f i c i t s emerged when p a t i e n t s were misinformed about t h e subsequent t a r g e t l o c a t i o n ( i n v a l i d t r i a l s ) . When they expected t h e t a r g e t c o n t r a l e s i o n a l l y and i t occu r red on t h e o p p o s i t e s i d e , t hey showed t h e normal i n c r e a s e i n r e a c t i o n t ime. However ,whenthey moved t h e i r a t t e n t i o n t o t h e i p s i l e s i o n a l o r "good" s i d e and t h e t a r g e t appeared on t h e o p p o s i t e s i d e , i t took t h e p a t i e n t s s i g n i f i c a n t l y l onge r t o respond t o t h e t a r g e t . T h i s was t r u e f o r bo th r i g h t and l e f t p a r i e t a l p a t i e n t s , bu t t h e e f f e c t s were s i g n i f i c a n t l y g r e a t e r f o r r i g h t p a r i e t a l p a t i e n t s . Posner e t a l . proposed t h a t t h e d e f i c i t on i n v a l i d t r i a l s when t h e t a r g e t i s p re sen ted c o n t r a l e s i o n a l l y i s due t o t h e i n a b i l i t y t o d i sengage a t t e n t i o n i n o r d e r t o s h i f t c o n t r a l e s i o n a l l y . According t o Kinsbourne ' s account of u n i l a t e r a l n e g l e c t , t h e b i a s of theintacthemispheredominateswhenitcanno l o n g e r b e opposed by thedamaged hemisphere. A l s o , t h e r i g h t w a r d o r i e n t i n g b i a s o f t h e l e f t hemisphere i s s t r o n g e r t h a n t h e l e f t w a r d b i a s of t h e r i g h t hemisphere. The re fo re , p a t i e n t s w i t h r i g h t hemisphere damage should have more d i f f i c u l t y d i sengag ing from t h e r i g h t t han l e f t hemisphere p a t i e n t s should have d i sengag ing from t h e l e f t . I n f a c t , t h e f i n d i n g s of Posner e t a l . a r e c o n s i s t e n t w i t h t h i s p r e d i c t i o n . Moving a t t e n t i o n e i t h e r i p s i l e s i o n a l l y o r c o n t r a l e s i o n a l l y d i d n o t seem t o be t h e problem f o r t h e s e p a t i e n t s s i n c e a t t e n t i o n s h i f t s i n e i t h e r d i r e c t i o n w e r e e v i d e n t o n v a l i d t r i a l s . T h u s , a n y d i f f e r e n c e s between r i g h t hemisphere and l e f t hemisphere p a t i e n t s on t h i s

E. A. Roy et al.

t a s k cannot be r e a d i l y accounted f o r by t h e hypo thes i s t h a t t h e r i g h t hemisphere c a n d i r e c t a t t e n t i o n t o e i t h e r s i d e o f space.

I n a s tudy of p a t i e n t s w i th r i g h t hemisphere damage, Riddoch and llumphrey (1983) found s i m i l a r resultsusinglateralcuesonalinebisection t a sk . P a t i e n t s w i t h l a t e r a l i z e d damage t y p i c a l l y draw t h e i n t e r s e c t towards t h e i p s i l e s i o n a l endpoint r a t h e r than a t t h e midpoint , i n d i c a t i n g t h e i r tendency t o underes t imate t h e c o n t r a l e s i o n a l e x t e n t of t h e l i n e . Riddoch and Humphreys placed s i n g l e l e t t e r cues a t e i t h e r t h e r i g h t o r l e f t endpoint o r b i l a t e r a l l y . P a t i e n t s w i th l e f t neg lec t were asked t o name any cue t h a t they saw and then t o b i s e c t t h e l i n e . I t was found t h a t a u n i l a t e r a l l e f t cue was c o n s i s t e n t l y named and s i g n i f i c a n t l y reduced t h e amount of neg lec t . When t h e cues were b i l a t e r a l , s u b j e c t s o f t e n f a i l e d t o name t h e l e f t cue and showed no s i g n i f i c a n t dec rease i n n e g l e c t r e l a t i v e t o t h e no cue cond i t ion . Neglect was the greatestwhenonlytherightcuewas present .

Three important p o i n t s a r e r a i s e d by Riddoch and Humphreys' and Posner ' s r e s u l t s . F i r s t , when cued, p a t i e n t s candeliberatelyorienttheir a t t e n t i o n t o both t h e i p s i l e s i o n a l and c o n t r a l e s i o n a l hemispaces. Second, i n t h e presence of a competing i p s i l e s i o n a l s t i m u l u s , t h e tendency f o r c o n t r a l e s i o n a l o r i e n t i n g i s minimized. Furthermore, Posne r ' s f i n d i n g s sugges t t h a t t h e r e is g r e a t e r d i f f i c u l t y i n d isengaging a t t e n t i o n from t h e r i g h t hemispace f o r r i g h t hemisphere p a t i e n t s t han disengaging from t h e 1 e E t h e m i s p a c e f o r l e f t h e m i s p h e r e p a t i e n t s .

A p o s s i b l e b a s i s f o r t h e e f f e c t s of l a t e r a l i z e d b r a i n damage on o r i e n t i n g has been suggested by our own work (Reuter-Lorenz, Note 4 ; Reuter-Lorenz, Moscovitch & Kinsbourne, Note 5) on t h e h e m i s p h e r i c c o n t r o l o f o r i e n t i n g i n normal s u b j e c t s . A t a c h i s t o s c o p i c l i n e b i s e c t i o n t a s k was used t o a s s e s s t h e d i s t r i b u t i o n of a t t e n t i o n i n space. S u b j e c t s viewed a s e r i e s of b r i e f l y presented ( l e s s than 120 msec) h o r i z o n t a l l i n e s each of which had an i n t e r s e c t pos i t i oned a t midpoint o r s l i g h t l y t o t h e l e f t o r r i g h t of cen te r . The s u b j e c t ' s t a s k was t o judge whether t h e i n t e r s e c t was loca ted a t t h e midpoint o r t o the l e f t o r r i g h t of c e n t e r . The tendency t o underes t imate t h e l e f t o r r i g h t e x t e n t s of t h e l i n e was r e f l e c t e d i n t h e p a t t e r n o f e r r o r s associatedwithidentifyingtheintersect's l o c a t i o n .

When l i n e s were presented u n i l a t e r a l l y , s u b j e c t s c o n s i s t e n t l y underes t imated t h e i p s i l a t e r a l e x t e n t . That i s , when t h e l i n e was i n t h e RVF, i t s l e f t e x t e n t was underes t imated, whereas, when in theLVF, t h e r i g h t e x t e n t was underes t imated. Th i s p a t t e r n sugges t s t h a t t h e l e f t hemisphere has rightward a t t e n t i o n a l b i a s and t h a t t h e r i g h t hemisphere h a s a l e f tward b i a s .

The same at tern of r e s u l t s e m e r g e d i n a f u r t h e r e x p e r i m e n t i n w h i c h t h e l i n e s themselves were presented fovea l ly . The l i n e was f lanked by a box whichwas s l i g h t l y d i s p l a c e d f r o m e i t h e r t h e r i g h t o r l e f t e n d p o i n t . O n h a 1 f t h e t r i a l s , t h e box conta ined a d o t a n d o n h a l f , i t d i d n o t . I n o n e c o n d i t i o n , t h e s u b j e c t s were t o l d t o ignore t h e boxes and simply t o r e p o r t where t h e i n t e r s e c t s occurred. I n a second c o n d i t i o n , they had t o a t t e n d t o t h e u n i l a t e r a l boxes, r e p o r t whether they were empty o r f u l l , and, t hen , i n d i c a t e t h e i n t e r s e c t pos i t i on . S u b j e c t s were i n s t r u c t e d t o keep t h e i r eyes f i x a t e d c e n t r a l l y . Cond i t ions were blocked s o t h a t t h e box was presented i n t h e same v i s u a l f i e l d f o r a s e r i e s of t r i a l s . Regardless of whether t h e boxes were a t t ended o r ignored , t hey s y s t e m a t i c a l l y b i a sed a t t e n t i o n . TheRVF box produced a r ightward b i a s o r r e l a t i v e l e f t n e g l e c t o n t h e l i n e b i s e c t i o n t a s k , whereas t h e LVF box produced a l e f tward b i a s o r r i g h t neg lec t . These b i a s e s were o p p o s i t e i n d i r e c t i o n but e q u i v a l e n t i n magni tude.

An i n t e r e s t i n g asymmetry emerged i n f u r t h e r exper iments when c o n f l i c t i n g o r i e n t i n g demands were produced by t h e viewing cond i t ions . I n

Unilateral attention deficits and hemispheric asymmetries

one s e t o f condi t ions , s t imul i were randomized so tha t sub jec t s could not an t ic ipa te i n which v i sua l f i e l d the box would appear. In another, boxes were presented i n both v i sua l f i e l d s and subjects had t o attend se lec t ive lyo t o o n e or theotherwhilemaking the b i s e c t i o n judgement.

In both types o f c o n f l i c t s i t u a t i o n , there were no overal l d i f f e r e n c e s i n b i sec t ion accuracy forRVFversusLVFcondi t ions .However , the r igh t b ias assoc ia tedwi th theRVF condit ions proved t o b e r o b u s t , w h e r e a s t h e l e f t w a r d bias associated wi th LVF condit ions was s i g n i f i c a n t l y diminished i n the presence o f or ien t ing c o n f l i c t . In other words, i n the presence o f l a t e r a l orient ing c o n f l i c t , normal subjects showed a stronger tendency t o or ien t t o :he r igh t and neglect the l e f t ex ten t o f the l i n e than t o or ien t t o the l e f t ~nd neglect the r igh t ex ten t . Th is pattern i s cons i s ten t wi th Kinsbourne's ~roposa l t h a t the rightward l a t e r a l or ien t ing tendency i s s t r o n g e r t h a n t h e !eftward tendency. Furthermore, these f ind ings may lend support t o one in terpre ta t ion o f Heilman's not ion o f r igh t hemisphere b i l a t e r a l a t ten t ion . D i f f e r e n t i a l or ien t ing s t rength may underl ie hemispheric d i f f e r e n c e s i n the spa t ia l a l loca t ion o f a t ten t ion . A strong d i rec t ional bias i n associat ion wi th l e f t hemisphere control may be related t o a highly se lec t ive , focal a l loca t ion policy. A weaker d i rec t ional or ien t ing b ias ~ssoc ia ted wi th the r igh t hemisphere may allow a t t e n t i o n t o be allocated less s e l e c t i v e l y i n space. A weaker d i rec t ional bias may enable the r igh t temisphere t o d i s t r i b u t e a t t e n t i o n o v e r a broaderspat ia l reg ion .

~ S C U S S ~ O U

The evidence reviewed i n t h i s chapter suggests tha t there may be iemispheric d i f f e r e n c e s i n some aspects o f a t ten t ional control . Our own rorkwith normal sub jec t s h a s i n d i c a t e d l i t t l e s u p p o r t f o r t h e p r o p o s a l t h a t the r igh t hemisphere has an advantage i n a r o u s a l / a c t i v i t a t i o n processes. Warning s ignals presented t o the r igh t hemisphere did not serve t o decrease che time t o react t o the react ion signal r e l a t i v e t o warning s ignals presented t o the l e f t hemisphere. The e f f e c t o f the la tera l i zed warning s ignals on react ion time may have been reduced due t o the a ler t ing e f f e c t associated wi th the appearance o f the f i x a t i o n point. Methodological changes are being made t o remove t h i s a ler t ing e f f e c t so as t o assess more c l e a r l y t h e e f f e c t o f the lateral izedwarning s ignalon react ion time.

Wi th regard t o orient ingand se lec t ion p r o c e s s e s , w o r k b y P o s n e r e t a l . ( 1 9 8 2 , 1 9 8 4 ) suggests tha t damage t o the parietal regions o f e i t h e r hemispheres leads t o an impairment i n disengaging a t t e n t i o n from one loca t ion , par t icu lar ly loca t ions i n the i p s i l a t e r a l hemispace, i n order t o d i r e c t it t o another locat ion. Th is disengage component o f or ien t ing seems t o be more a f f e c t e d by r igh t parietal damage and i s part icularly e x e m p l i f i e d i n a t e n d e n c y t o m a i n t a i n o r i e n t a t i o n t o w a r d t h e r i g h t .

The f ind ings o f Reuter-Lorenz e t a l . reviewed above, f i t w e l l w i t h t h i s pattern. Evidence tha t each hemisphere d i r e c t s a t t e n t i o n contra la tera l ly was obtained i n normal subjects . Furthermore, t h e rightward or ien t ing tendency was found t o be more robust than the le f tward tendency. These f indings suggest tha t the or ien t ing behavior o f pat ients wi th la tera l i zed brain damage may r e f l e c t the bias o f the i n t a c t hemisphere. As Kinsbourne has proposed, r i g h t hemisphere damage leaves the contralateral orient ing bias o f the l e f t hemisphere unopposed, whereas l e f t hemisphere damage leaves t h e r igh t hemisphere unopposed. Thus, the i n a b i l i t y t o disengage a t t e n t i o n f romthe i p s i l e s i o n a l f o c u s m a y b e d u e t o t h e d o m i n a t i n g i n f l u e n c e o f the i n t a c t hemisphere. Furthermore, a stronger rightward than lef tward bias should lead t o g r e a t e r d i f f i c u l t y i n thedisengage operation f o r r igh t thanlefthemispherepatients,whichis the pattern found byposner 'sgroup.

These f ind ings suggest tha t t h e h e m i s p h e r e s m a y d i f f e r i n t h e i r c o n t r o l

34 E.A. Roy et al

of t he s p a t i a l a l l o c a t i o n of a t t e n t i o n . A weaker l a t e r a l o r i e n t i n g b i a s i n a s s o c i a t i o n wi th r i g h t hemisphere c o n t r o l may enab le a b roade r s p a t i a l d i s t r i b u t i o n of a t t e n t i o n , whereas a s t r o n g e r o r i e n t i n g b i a s may permit a f o c a l , h igh ly s e l e c t i v e a t t e n t i o n a l mode i n a s s o c i a t i o n w i t h l e f t hemisphere c o n t r o l .

The s e l e c t i o n a s p e c t s of a t t e n t i o n have a l s o been examined i n t h e con tex t of s e a r c h t a s k s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. Gene ra l ly , t h e s e s t u d i e s have shown a g r e a t e r impairment i n te rms of more e r r o r s and s lower s e a r c h t ime i n r i g h t hemisphere p a t i e n t s , p a r t i c u l a r l y those wi th v i s u a l f i e l d d e f e c t s . A s tudy i n o u r l a b o r a t o r y (Roy &Roy , Note 7 ) examined both t h e s e l e c t i o n and c a p a c i t y a s p e c t s of a t t e n t i o n i n a v i s u a l s ea rch t a sk . While t h e r e were no c l e a r d i f f e r e n c e s between t h e l e f t and right-hemisphere p a t i e n t s i n terms of o v e r a l l accuracy o r s e a r c h t ime , snmewha td i f f e r en t s e a r c h p a t t e r n s were observed i n t h e two p a t i e n t g r o u p s . Right hemisphere p a t i e n t s seemed t o ma in t a in s e a r c h t ime c o n s t a n t a c r o s s hemispa t i a l f i e l d s a t t h e c o s t of i nc reased e r r o r s i n t h e c o n t r a l e s i o n a l ( l e f t ) hemispace, wh i l e t h e l e f t hemisphere p a t i e n t s seemed t o op t f o r ma in t a in ing accuracy a c r o s s s p a t i a l f i e l d s a t t h e c o s t of i n c r e a s e d time t o f i nd t a r g e t s i n t h e c o n t r a l e s i o n a l ( r i g h t ) h e m i s p a c e .

These d i f f e r i n g s e a r c h p a t t e r n s may r e f l e c t d i f f e r e n t s t r a t e g i e s . The r i g h t hemisphere p a t i e n t s may be focus ing on s e a r c h t ime, wh i l e t h e l e f t hemisphere p a t i e n t s may be focus ingonaccuracy .The immedia t e impl i ca t ions of t h e s e a p p a r e n t l y d i f f e r e n t s t r a t e g i e s f o r unde r s t and ing s e a r c h perEormance i s no t c l e a r . What i s c l e a r , however, i s t h a t we need t o u se t a s k s and measures of performance which a f f o r d t h e o p p o r t u n i t y t o i d e n t i f y d i f f e r e n t s t r a t e g i e s i n p e r f o r m a n c e .

Consider ing t h e c a p a c i t y a s p e c t of a t t e n t i o n , t h e r e were no c l e a r d i f f e r e n c e s between t h e hemispher ic groups i n t h e e f f e c t s of va ry ing capac i tydemandsonv i sua l sea rchpe r fo rmance .

These f i n d i n g s t h a t have been reviewed provide some i n i t i a l c l u e s t o t h e neurobehavioura l bases of a t t e n t i o n , p a r t i c u l a r l y a s t o hemispher ic asymmetries. Many i s s u e s remain t o b e cons ide red and examined, however. F i r s t , wh i l e we have viewed a t t e n t i o n no t a s a u n i t a r y concep t , bu t a s one which i n v o l v e s a number of componentprocesses , i t i s impor t an t t o r e c o g n i z e t h a t t h e s e components themselves may invo lve subproces ses of t h e i r own. O r i e n t a t i o n , f o r example, seems t o i nvo lve a t l e a s t t h r e e a s p e c t s : d i s engag ing a t t e n t i o n from t h e c u r r e n t focus , moving a t t e n t i o n , and engaging a t t e n t i o n a t a new l o c a t i o n (Posner & Cohen, 1984). Arousal and a c t i v a t i o n , l i k e w i s e , seem t o i nvo lve a t l e a s t two a s p e c t s , a s enso ry ( i n p u t ) and a motor ( o u t p u t ) component. Given t h i s i d e a o f subproces ses , i t behooves u s t o c a r e f u l l y s tudy each of t h e s e w i t h a view t o unde r s t and ing t h e i r n e u r o b e h a v i o u r a l b a s i s . P o s n e r e t a l . (1982, 1984)have b e g u n t o show t h a t t h e components of o r i e n t i n g may have d i f f e r e n t n e u r a l s u b s t r a t e s . Disengaging a t t e n t i o n , a s we have seen , seems t o depend on p a r i e t a l a r e a s . ?loving a t t e n t i o n , on t h e o t h e r hand, seems more dependent on midbra in and c o l l i c u l a r s t r u c t u r e s . I n t h e same v e i n , g iven t h a t t h e r e appea r t o be both sensory and motor components t o a c t i v a t i o n , i t would be impor t an t t o de termine , f o r example, whether t h e r i g h t hemisphere advantage f o r a c t i v a t i o n proposed by Heilman i s r e l a t e d t o an advantage i n p roces s ing inpu t ( s enso ry a s p e c t s ) o r p repa r ing a response (motor a s p e c t s ) .

Another p o i n t he re r e l a t e s t o t h e c a p a c i t y component of a t t e n t i o n . One s tudy (Roy, Note 2 ; Roy &Roy, Note 3 ) c a r r i e d o u t i n o u r l a b o r a t o r y s u g g e s t s t h e r e may not be d i f f e r e n c e s between t h e hemispheres i n t h e e f f e c t s of varying c a p a c i t y demands, a t l e a s t a s measured by s e a r c h t ime f u n c t i o n s i n t he con tex t of v i s u a l s ea rch . Heilman's argument t h a t t h e r i g h t hemisphere i s capable of c o n t r o l l i n g a t t e n t i o n i n both h e m i s p a t i a l f i e l d s , however,

Unilateral attention deficits and hemispheric asymmetries 3 5

suggests t h a t the r igh t hemisphere may engage i n a processingmodewhichis l e s s demanding o f resources and, t h u s , enjoys the capacity o f d i s t r i b u t i n g those resources t o bothhemispatialfields.UsingSchneiderandSchiffrin's (1977) concepts o f automatic versus control led processing, the advantage conferred on t h e r igh t hemisphere may then ar i se because t h i s hemisphere i s more capable o f automatic processing than i s the l e f t hemisphere. Work wi th normals indeed suggests tha t the r igh t hemisphere may be more capable o f processing information i n paral lel (Bryden, 1982). Th is notion o f capacity demand could be fur ther examined using a dual task paradigm i n which pa t ien t smus tper forma secondarytaskwhile engaging i n v i s u a l search.

A second related i s sue concerns the i n t e r f a c e between psychological and neural processes o f a t t e n t i o n . In t h i s chapter , we have been part icularly in teres ted i n hemispheric asymmetries i n a t ten t ional processes. These hemispheric aspects form only a small part o f a larger network o f neural processes underlying a t t e n t i o n . A t ten t ion l i k e other aspects o f human behaviour can be viewed as involving a complex system o f func t ions , a so-called funct ional system. T h i s idea o f a funct ional system r e f l e c t s t h e current view o f brain-behaviour re la t ions termed funct ional pluripotent ial ism ( L u r i a , 1974) and has been applied t o praxis (Roy, 1978, 1983) and t o processes o f a t t e n t i o n (Mesulam, 1981). I n t h i s v i e w , a t t e n t i o n invo lves a number o f funct ional components each o f which i s subserved by a particular brain area. These brain areas, comprised o f r e t i c u l a r s t r u c t u r e s , c ingulate cor tex , and parietal and f ron ta l c o r t i c a l regions, form a neural network. Damage t o any o f these areas seems t o disrupt a t ten t ional processes i n a charac ter i s t i c way depending on which component o f a t t e n t i o n has been compromised ( s e e Mesulam, 1981). Given t h i s v iew, a clearer understanding o f a t t e n t i o n would seem t o depend on paral lel advances i n psychological and neurological perspectives o f a t ten t ion . Developing concepts o f the psychological processes underlying a t t e n t i o n may be mapped on t o neural s t r u c t u r e s , thus fos ter ing a descr ip t ion o f a t t e n t i o n based on an emerging i n t e r f a c e between behavioural and neural processes. Th is approach i s exempli f ied wel l i n Posner's work (e.g., P o s n e r , i n p r e s s ) .

A f i n a l c o n s i d e r a t i o n d e a l s w i t h t h e re la t ionsh ip o f t h e components o f a t t e n t i o n discussed here t o an account o f neglect . The evidence reviewed suggests t h a t there may be hemispheric d i f f e r e n c e s i n cer ta in aspects o f a t ten t ional processes. Our own work on the e f f e c t s o f la tera l i zed warning s ignals on react ion time provided no clear ind ica t ion t h a t t h e r igh t hemisphere i s dominant f o r ac t iva t ion i n normal subjects . Th is r e s u l t does not rule out t h e p o s s i b i l i t y t h a t t h e r igh t hemisphere has the a b i l i t y t o assume control o f ac t iva t ional processes once the l e f t hemisphere i s damaged, whereas t h e l e f t hemisphere cannot do so i n the advent o f r igh t hemisphere damage. Such an a b i l i t y may exp la in the b i l a t e r a l impairment i n search time found by Roy and Roy (Note 3 ) and, as others have suggested, could exp la in t h e grea ter increase i n react ion time a f t e r r i g h t , as opposed t o l e f t , hemisphere damage. Y e t , how could an ac t iva t ion problem o f t h i s kind produce the s p a t i a l l y s e l e c t i v e ( i . e . , con tra les ional ) disturbance found i n neglect? The a n s w e r t o t h i s q u e s t i o n i s n o t l i k e l y t o b e a simpleone because it invo lves the i s sue o f the re la t ionsh ip among the d i f f e r e n t componentsof a t t e n t i o n .

A d e f i c i t i n d i rec t ional or ien t ing would provide a straightforward explanation o f the spa t ia l f ea tures of hemi-neglect. But can i t alone f u l l y account for t h e epidemiological f a c t o f greater l e f t than r igh t neglect? According t o Posner's r e s u l t s , p a r i e t a l d a m a g e t o e i t h e r h e m i s p h e r e i m p a i r s t h i s disengage operation associated w i t h or ien t ing . The greater impairment on t h i s task found i n r igh t than l e f t hemisphere pat ients may be related t o

36 E.A. Roy et al.

t h e s t r o n g e r r ightward b i a s i n normal s u b j e c t s (Reuter-Lorenz, Note 4 ) and may c o n t r i b u t e t o t h e g r e a t e r i nc idence of l e f t n e g l e c t (Kinsbourne, 1974, 1977). However, wh i l e t h e o r i e n t i n g d e f i c i t found by Posne r ' s group was r e l i a b l y a s s o c i a t e d wi th p a r i e t a l damage, i t was e v i d e n t i n p a t i e n t s w i t h o r wi thout s i g n s of n e g l e c t . I t i s p o s s i b l e t h a t t h e magni tudeof t h e o r i e n t i n g d e f i c i t on t h i s t a s k may c o r r e l a t e w i th o t h e r i n d i c e s of hemi - ina t t en t ion ; however, t h i s has y e t t o b e e s t a b l i s h e d .

I t seems r easonab le t o hypo thes i ze t h a t a d i r e c t i o n a l o r i e n t i n g d e f i c i t could form t h e c o r e o f t h e n e g l e c t s y n d r o m e a n d , a s such , provide t h e b a s i s f o r t h e h e m i s p a t i a l o r u n i l a t e r a l n a t u r e of t h e d i s o r d e r . A s t r o n g e r r ightward than l e f t w a r d a t t e n t i o n a l b i a s could c o n t r i b u t e t o t h e d i f f e r e n t i a l i n c i d e n c e of r i g h t and l e f t n e g l e c t . I f a c t i v a t i o n o r a r o u s a l p roces ses a r e a l s o d i s t u r b e d , t h e d i f f i c u l t i e s a s s o c i a t e d w i t h a n o r i e n t i n g impairment may be exacerbated . However ,d i s tu rbances o f a c t i v a t i o n / a r o u s a l a lone may be i n s u f f i c i e n t t o produce a u n i l a t e r a l impairment i n s p a t i a l a t t e n t i o n .

Decrements i n r e sou rces may a l s o be i n s u f f i c i e n t t o p r o d u c e u n i l a t e r a l a t t e n t i o n impairment. As noted above, Roy and Roy (Note 3 ) found t h a t t h e magnitude of c o n t r a l e s i o n a l s e a r c h d e f i c i t w a s n o t i n f l u e n c e d b y t h e t y p e o r t h e number of d i s t r a c t o r s . I f r e sou rce decrements u n d e r l i e n e g l e c t , t h e n inc reased c a p a c i t y demands should have exace rba t ed t h e u n i l a t e r a l s e a r c h d e f i c i t . I n t e r a c t i o n s between t h e c a p a c i t y and s e l e c t i o n a s p e c t s of a t t e n t i o n may emerge i f a h e a v i e r a t t e n t i o n a l load i s imposed and /o r t h e t a s k u s e s m a t e r i a l s (e .g . , shapes ) which t h e p a t i e n t f i n d s d i f f i c u l t t o i d e n t i f y ( c f . L e i c e s t e r , Sitman, Stoddard & Mohr, 1969). Our f i n d i n g s sugges t t h a t decrements i n a t t e n t i o n a l r e s o u r c e s a lone s e e m i n s u f f i c e n t t o produce u n i l a t e r a l a t t e n t i o n a l d i s t u r b a n c e .

T h i s a n a l y s i s s u g g e s t s t h a t a n impairment i n l a t e r a l o r i e n t i n g m a y b e a neces sa ry c o n d i t i o n f o r hemi-attentionaldisturbances of any kind. I n c a s e s where on ly o r i e n t i n g a s p e c t s of a t t e n t i o n a r e a f f e c t e d , on ly s u b t l e f e a t u r e s of t h e n e g l e c t syndrome, such a s e x t i n c t i o n , may be ev iden t . Tncreas ingly s e v e r e forms of n e g l e c t may invo lve a d d i t i o n a l impairment i n o t h e r components of a t t e n t i o n ( i . e . , a r o u s a l / a c t i v a t i o n ) i n con junc t ion w i t h a n u n d e r l y i n g d e f i c i t i n o r i e n t i n g .

Our aim i n t h i s paper has been t o move toward a more a c c u r a t e c h a r a c t e r i z a t i o n of t h e n a t u r e of t h e a t t e n t i o n a l impairments a s s o c i a t e d w i t h l a t e r a l i z e d c o r t i c a l l e s i o n s i n g e n e r a l and n e g l e c t i n p a r t i c u l a r . It i s ou r b e l i e f t h a t t h i s t ype of approach w i l l he lp t o d e f i n e t h e n a t u r e o f hemispher ic d i f f e r e n c e s i n a t t e n t i o n and w i l l a i d i n t h e e l a b o r a t i o n of neu robehav io ra l a t t e n t i o n theory . Moreover, i t may a l l o w f o r t h e development of a taxonomy of h e m i a t t e n t i o n a l d i s t u r b a n c e s which can g u i d e p a t i e n t c l a s s i f i c a t i o n and p o s s i b l y p a t i e n t t r ea tmen t .

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Acknowledgements : P r e p a r a t i o n of t h i s manuscr ip t was funded through g r a n t s t o E . Roy, f r o m t h e N a t u r a 1 Sc i ences andEng inee r ingResea rchCounc i1 and t h e Na t iona l Hea l th Research Development Program, Hea l th & Welfare , Canada.

Unilateral attention deficits and hemispheric myrnrnetries 39

Footnotes

1. Copland, S. Hemispheric d i f f e r e n c e s i n a t t e n t i o n and response p repa ra t ion . Unpublished Mas te r ' s t h e s i s , Department of Psychology, Unive r s i tyofWate r loo ,May , 1985.

2. Roy, L. A t t e n t i o n d e f i c i t s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. Unpublished Mas te r ' s t h e s i s , DepartmentofKinesiology,University

, of Waterloo, May, 1985.

3 . Roy, L. & Roy, E.A. A t t e n t i o n d e f i c i t s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. P o s t e r p r e s e n t a t i o n a t annual meeting of North American Soc ie ty f o r p s y c h o l o g y o f S p o r t & P h y s i c a l A c t i v i t y , M a y , 1985.

4. Reuter-Lorenz, P.A. Hemispheric c o n t r o l of s p a t i a l a t t e n t i o n . Unpublished D o c t o r a l d i s s e r a t i o n , Department of Psychology, Un ive r s i ty of Toronto , 1986.

5. Reuter-Lorenz, P.A., Moscovitch, M., & Kinsbourne, M. L a t e r a l ' a t t e n t i o n b i a s i n a v i s u a l l i n e b i s e c t i o n t a sk : S i m i l a r i t i e s between t h e I p e r f o r m a n c e s o f n e g l e c t p a t i e n t s a n d n o r m a 1 s u b j e c t s . P a p e r r e a d a t N o r t h i American Conference, I n t e r n a t i o n a l Neuropsychological S o c i e t y , San Diego,

C a l i f o r n i a , F e b r u a r y , 1985.