F ig u r e 4 .1 4 .
T h re s h o ld s f o r d e t e c t i n g p a r a l l a x d e p th c o r r u g a t i o n s m easured f o r a s t e r e o b l i n d o b s e r v e r (AK).
p a r a l l a x as a so u rc e of d e p th in f o r m a tio n .
I
The e x p e rim e n ts d e s c r ib e d in t h i s c h a p te r have looked a t th e s e n s i t i v i t y of th e v i s u a l sy stem to d epth m o d u la tio n s s p e c i f i e d by r e l a t i v e m otion o r d i s p a r i t i e s . The s e n s i t i v i t y f u n c tio n s have shown t h a t t h e r e a r e c lo s e s i m i l a r i t i e s betw een th e t h r e s h o ld c h a r a c t e r i s t i c s o f th e p a r a l l a x and s t e r e o s c o p i c p r o c e s s in g sy ste m s. They have a l s o su g g e ste d t h a t t h e r e m ight be an o p tim a l a r e a o v er which d i f f e r e n t i a l m otion o r d i s p a r i t y can be p ic k e d up. The e x p e rim e n ts r e p o r te d i n th e
fo llo w in g c h a p te r s d e s c r i b e some s u p r a th r e s h o ld d e p th e f f e c t s which
ti
have p ro v id e d f u r t h e r i n d i c a t i o n s of th e n a tu r e and s p a t i a l e x te n t ofth e mechanisms u n d e r ly in g d e p th p r o c e s s in g . These e f f e c t s have a l s o
"3
confirm ed th e c lo s e s i m i l a r i t y betw een m otion p a r a l l a x and s t e r e o p s i s .
97
%
:'a Chapter 5 Successive Contrast Effects In the Depth Domain
C o n t r a s t e f f e c t s have been e x te n s i v e l y r e p o r te d and i n v e s t i g a t e d th ro u g h o u t th e h i s t o r y o f v i s u a l p e r c e p ti o n . S u c c e ss iv e c o n t r a s t e f f e c t s o r a f t e r e f f e c t s d e s c r ib e p e r c e p tu a l e f f e c t s where th e phenom enal im p r e s s io n of a stim u lu s i s a f f e c t e d by th e s t i m u l i which have been p r e s e n t e d p r e v i o u s l y . S im ultaneous c o n t r a s t e f f e c t s , on th e o t h e r hand, o c c u r when th e p e rc e iv e d n a tu r e of a s tim u lu s i s a f f e c t e d by s t i m u l i a d ja c e n t i n s p a c e , r a t h e r th a n tim e. The two ty p e s of c o n t r a s t e f f e c t a r e , t h e r e f o r e , a r e s u l t of e i t h e r tem p o ral o r s p a t i a l i n t e r a c t i o n s . I t has been known s in c e A r i s t o t l e , f o r exam ple, t h a t p ro lo n g e d v iew in g of a b r i g h t d is k g iv e s r i s e to a com plem entary a f te r im a g e and t h a t a g rey d i s c w ith a l i g h t su rro u n d a p p e ars d a r k e r th a n a com parable d i s c w ith a d ark su rro u n d . S i m i l a r l y , c o lo u re d a f t e r i m a g e s and c o lo u r c o n t r a s t (where a grey d i s c ta k e s on th e com plem entary c o lo u r to i t s su rro u n d ) were n o ted and s tu d i e d by Leonardo da V in c i ( s e e B o rin g , 1942). Over th e l a s t hundred y e a rs i t
h a s become a p p a r e n t t h a t s i m i l a r s u c c e s s iv e and sim u lta n e o u s c o n t r a s t " ■i e f f e c t s o ccu r i n many v i s u a l dim ensions and th e s e have been w id e ly | s t u d i e d (H elm h o ltz, 1925; H ering 1886; Wohlgemuth, 1911; G ibson, 1 9 33).
R e c e n tl y , one of th e m ajor re a so n s f o r stu d y in g s u c c e s s iv e and sim u lta n e o u s c o n t r a s t e f f e c t s has been th e b e l i e f t h a t such e f f e c t s can be used to e l u c i d a t e th e p r o p e r t i e s of th e mechanisms u n d e r ly in g v i s u a l p r o c e s s in g ( S u th e r la n d , 1961; Blakemore and S u tto n , 1969; M ello n , 1974; A n s t i s , 1975; F r is b y 1979). I t has been argued t h a t s u c c e s s iv e c o n t r a s t e f f e c t s p r o v id e e v id en ce f o r th e e x is t e n c e of s p e c i f i c p r o c e s s in g m echanism s, f o r exam ple, mechanisms r e s p o n s iv e to d i f f e r e n t s p a t i a l f r e q u e n c ie s (Blakem ore and S u tto n , 1969). S im u ltan eo u s c o n t r a s t e f f e c t s , on th e o t h e r hand, i n d i c a t e th e s p a t i a l e x t e n t over
which such processes operate (Ratliff, 1965; von Bekesy, 1968).
W ith in t h i s g e n e ra l fram ew ork, b o th s u c c e s s iv e and s im u lta n e o u s 1 c o n t r a s t e f f e c t s have been s tu d ie d i n th e dep th dom ain. The
e x p e rim e n ts d e s c r ib e d i n t h i s c h a p te r were d esig n ed to i n v e s t i g a t e s u c c e s s iv e c o n t r a s t e f f e c t s f o r depth s u r f a c e s s p e c i f i e d by m otion p a r a l l a x in f o r m a tio n . For co m p ariso n , a f t e r e f f e c t s f o r s te r e o s c o p i c d e p th were a ls o i n v e s t i g a t e d . S tu d ie s of sim u lta n e o u s d ep th c o n t r a s t a r e d e s c r ib e d i n th e fo llo w in g c h a p te r .
5 .1 S u c c e ss iv e c o n t r a s t e f f e c t s .
S u c c e ss iv e c o n t r a s t e f f e c t s f o llo w in g p ro lo n g ed s t i m u l a t i o n , can be d em o n strated i n many s e n so ry d im en sio n s. The b e s t known example i s p ro b a b ly th e w a t e r f a l l i l l u s i o n o r movement a f t e r e f f e c t (Wohlgemuth, 1911) where a f t e r p ro lo n g ed v iew in g of a s u r f a c e moving i n one
\
d i r e c t i o n , a s t a t i o n a r y p a t t e r n a p p e ars to move i n th e o p p o s ite d i r e c t i o n . T h is ty p e of c o n t r a s t e f f e c t i s an example of a n e g a tiv e a f t e r e f f e c t where a n e u t r a l t e s t s tim u lu s ta k e s on p r o p e r t i e s com plem entary to th o se of th e a d a p tin g stim u lu s (F av reau and C o r b a l l i s , 1 9 7 6 ). . A nother ty p e of c o n t r a s t e f f e c t o ccu rs when p ro lo n g e d view ing c a u se s s h i f t s in th e p e rc e iv e d c h a r a c t e r i s t i c s of s u b s e q u e n tly viewed s t i m u l i . For exam ple, a f t e r p ro lo n g ed view ing of a lum inance g r a t i n g a t one s p a t i a l fre q u e n c y , th e fre q u e n c y of a t e s t g r a t i n g a t a n e ig h b o u r in g s p a t i a l fre q u e n c y i s m lsp e rc e iv e d (Blakem ore and S u tto n , 1 9 6 9 ). I n b o th c a se s th e a f t e r e f f e c t i s th o u g h t to a r i s e from th e a d a p t a t i o n of mechanisms w ith in th e v i s u a l system which become f a t i g u e d th ro u g h p ro lo n g ed s t i m u l a t i o n ( S u th e r la n d , 1961; Blakemore and S u tto n ,
;î
99 ' g
I
1969; 'C o l t h e a r t , 1 971). A l t e r n a t i v e l y , some a u th o r s s u g g e st t h a t #
I
a f t e r e f f e c t s a r i s e from p rolonged i n h i b i t i o n betw een mechanisms f o llo w in g a d a p t a t i o n (D ealy and T o l h u r s t , 1974). I t i s not c l e a r w h eth er n e g a ti v e a f t e r e f f e c t s and th o se which p roduce s h i f t s in p e rc e iv e d s tim u lu s v a lu e s can be e x p la in e d by th e same u n d e r ly in g o r g a n i s a t i o n of p r o c e s s in g mechanism s. R e c e n tly , how ever, b o th ty p e s o f m otion a f t e r e f f e c t have been a t t r i b u t e d to changes i n th e o v e r a l l re s p o n se d i s t r i b u t i o n of in d ep en d en t tuned c h a n n e ls (M ather and M oulden, 1 9 8 0 ).
W ith in th e f i e l d of depth p e r c e p ti o n , t h e r e have been s e v e r a l r e p o r t s of i l l u s o r y d ep th e f f e c t s which fo llo w p ro lo n g e d v iew ing of t h r e e - d i m e n s i o n a l s t i m u l i . For exam ple, K ohler and Emery (1947) r e p o r t e d t h a t i n s p e c t i o n of a stim u lu s which slo p e d away from th e o b s e r v e r in d e p th , a f f e c t e d th e p e r c e p tio n of a s u b s e q u e n tly viewed