New Issues in Spatial Neglect

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NEU ISSUES IN SPATIAL NEGLECT Marc J e a n n e r o d

S p a t i a l n e g l e c t is one of t h e few a r e a s i n Neuropsychology where c l i n i c i a n s , p s y c h o l o g i s t s and a n i m a l e x p r i m e n t e r s have succeeded in a d o p t i n g a common l a n g u a g e , w i t h t h e happy consequence of f r u i t f u l i n t e r a c t i o n s between t h e t h r e e a p p r o a c h e s of t h i s s t r i k i n g phenomenon. I m p o r t a n t new developments have r e s u l t e d , which i n t u r n j u s t i f y t h e p u b l i c a t i o n of t h e p r e s e n t book. To my knowledge, no monograph o r m u l t i a u t h o r e d book f u l l y d e v o t e d t o t h i s problem h a s a p p e a r e d s i n c e W e i n s t e i n a n d F r i e d l a n d ( 1 9 7 7 ) a n d D e R e n z i (1982). Recent a d v a n c e s i n Neuropsychology and Neurophysiology which have i n f l u e n c e d t h e u n d e r s t a n d i n g of s p a t i a l n e g l e c t c a n b e summarized under t h r e e main h e a d i n g s . 1. Space is a c o n s t r u c t . The s p a t i a l f i e l d h a s l o n g been c o n s i d e r e d a s c o e x t e n s i v e t o t h e v i s u a l f i e l d , a m i s c o n c e p t i o n which o r i g i n a t e d in t h e c l i n i c a l u s e of t e s t i n g n e g l e c t by imposing f i x a t i o n of p a t i e n t ' s g a z e on t h e o b s e r v e r w h i l e r e s p o n s e s t o s t i m u l i p r e s e n t e d i n e a c h h a l f - f i e l d were explored. The new e x p e r i m e n t a l paradigms c l e a r l y d i s s o c i a t e t h e r e t i n o t o p i c map f r o m t h e body-centeredmap of s p a c e , w h e r e o b j e c t p o s i t i o n s a r e e n c o d e d w i t h r e s p e c t t o a body r e f e r e n c e . A c t i o n - r e l a t e d s i g n a l s , and n o t o n l y s e n s o r y s i g n a l s , are t h o u g h t t o p a r t i c i p a t e i n t h e c o n s t r u c t i o n o f t h e s p a t i a l m a p . The n a t u r e of t h e s e a c t i o n - r e l a t e d s i g n a l s , t h e b r a i n areas where t h e s p a t i a l map might b e e l a b o r a t e d , t h e way normal o r brain-damaged s u b j e c t s p e r c e i v e t h e i r body c o o r d i n a t e s a r e among t h e l i n e s o f r e s e a r c h s u g g e s t e d b y t h e phenomenon of s p a t i a l n e g l e c t . 2 . I n p u t - o u t p u t t r a n s f o r m a t i o n is n o t d i r e c t . C o n s t r a i n t s on s p a t i a l b e h a v i o r i n h i g h e r v e r t e b r a t e s impose t h a t a map o r r e p r e s e n t a t i o n of e x t r a p e r s o n a l s p a c e must be b u i l t b e f o r e a c t i o n s c a n be p r o p e r l y o r i e n t e d . The n o t i o n of r e p r e s e n t a t i o n , o n c e c o n s i d e r e d a s a u s e f u l metaphor i n c o g n i t i v e p s y c h o l o g y , is p r o g r e s s i v e l y g a i n i n g r e s p e c t a b i l i t y among neuroscientists. Simple p s y c h o l o g i c a l t e c h n i q u e s , l i k e t h e c l a s s i c a l r e a c t i o n time measurement, can b e used € o r p r o b i n g r e p r e s e n t a t i o n s . Specific e x p e r i m e n t a l s i t u a t i o n s , l i k e t h o s e i n v o l v i n g p r e s e n t a t i o n of s t i m u l i l i m i t e d t o one h e m i s p h e r e , o r t h o s e i n v o l v i n g c u e i n g of c e r t a i n a s p e c t s of t h e s t i m u l i , a l l o w p r e c i s e i n f e r e n c e on t h e m e n t a l c o n t e n t . S p a t i a l n e g l e c t o f f e r s a paradigm f o r t e s t i n g t h e r e l a t i o n s h i p of s p a t i a l r e p r e s e n t a t i o n t o b r a i n f u n c t i o n , and s p e c i a l l y t o h e m i s p h e r i c specialization. 3. A t t e n t i o n i s a d i s t r i b u t e d f u n c t i o n . Coupling between i n p u t and o u t p u t is i n f l u e n c e d by a t t e n t i o n a l mechanisms. The c o n c e p t of a t t e n t i o n , however, h a s e v o l u e d from t h a t of a s u p r a o r d i n a t e f u n c t i o n c o n t r o l l e d by a s i n g l e b r a i n c e n t e r , t o t h a t of a s e l e c t i v e mechanism o p e r a t i n g a t t h e

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M.Jeannerod

s e n s o r i m o t o r l e v e l i n e a c h modality. T h i s c o n c e p t u a l change owes much t o animal e x p e r i m e n t s showing d i f f e r e n t n e u r o n a l r e s p o n s e s t o s e n s o r y s t i m u l i whenthese responses a r e g o a l s f o r a c t i o n . Human e x p e r i m e n t s have l e d t o s i m i l a r c o n c l u s i o n s i n showing t h a t i n t e n t i o n t o o r i e n t i s s u f f i c i e n t f o r s h i f t i n g a t t e n t i o n , hence d i s s o c i a t i n g t h e a t t e n t i o n a l component f o r t h e motor component of g o a l - d i r e c t e d r e s p o n s e s . S p a t i a l n e g l e c t may r e p r e s e n t a c r i t i c a l condition f o r assessing hemispheric s p e c i a l i z a t i o n f o r d i r e c t i n g a t t e n t i o n t o v a r i o u s r e g i o n s o f space. T h i s book emphasizes t h e s e new i s s u e s i n normal s u b j e c t s , i n p a t i e n t s

withspatialneglectandinanimalmodels.

References DeRenzi, E. D i s o r d e r s of s p a c e e x p l o r a t i o n and c o g n i t i o n . J. Wiley, N e w York,1982. W e i n s t e i n , E.A. & F r i e d l a n d , R.P. (Eds.). H e m i - i n a t t e n t i o n and hemisphere s p e c i a l i z a t i o n . Advances i n N e u r o l o g y , v o l . 18, 1 9 7 7 , R a v e n P r e s s , N e w York.

Acknowledgements M o s t o f t h e m a n u s c r i p t o f t h i s bookwas d i l i g e n t l y t y p e d byM.RouviSre.