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ABSTRACT
Five experiments were c a r r i e d out t o d i s c o v e r whether t h e r e i s a s u b j e c t i v e enhanceinent of temporal c o n t o u r s analogous t o t h e Mach band e f f e c t s i n s p a t i a l v i s i o n . A t r a n s i e n t overshoot i n t h e i n c r e m e n t a l t h r e s h o l d o c c u r r e d immediately a f t e r t h e c r e s t of r i s i n g l i n e a r ramps a t p h o t o p i c l e v e l s .
A p s y c h o p h y s i c a l matching experiment v e r i f i e d ths t t h i s p e a k i n g e f f e c t i s p e r c e i v e d a s a b r i g h t p u l s e and can b e c o n s i d e r e d t h e temporal e q u i v a l e n t o f t h e b r i g h t Mach band. No e q u i v a l e n t of t h e 6 a r k Mach band a t t h e f o o t of r i s i n g l i n e a r ramps
was found. R a t h e r t h e r e was a n a n t i c i p a t o r y d e c r e a s e o f
s e n s i t i v i t y ( crawford e f f e c t ) a t t h e bsse of a s c e n d i n g g r a d i e n t s . The magnitude of t h i s e f f e c t was d i r e c t l y p r o p o r t i o n a l t o t h e r a t e o f change of t h e s t i m u l u s a t p h o t o p i c l e v e l s .
Data f'rom descending ramps was l e s s s i m p l e t o i n t e r p r e t . A s n s l l rise i n i n c r e m e n t a l t h r e s h o l d was sometimes observed a t t h e
c r e s t of ramps, b u t t h i s e f f e c t was much l e s s pronounced t h a n t h e peak a t t h e c r e s t o f e q u i v a l e n t ascending? ramps. Katching p r o c e d u r e s l o c a t e d t r o u g h s a t t h e f o o t o f descending ramps which correspond t o t h e d a r k s p a t i a l hlach b a n d , T h i s e f f e c t was l e s s a p p a r e n t when i n c r e m e n t a l t h r e s h o l d t e c h n i q u e s were
employed.
An experiment u s i n g e x p o n e n t i a l r a t h e r t h a n l i n e a r change i n
luminance over time gave r e s u l t s i n e e n e r a l accord w i t h t h e above.
The Crawford e f f e c t was found t o f o l l o w t h e Bunsen-Roscoe Law.
It was found t h a t t h e t r a n s i e n t undershoot and overshoot e f f e c t s were r e l a t e d t o t h e r a t e o f change r a t h e r t h a n t h e a c t u a l form o f t h e ramp when t h e r a t e of change was g r e a t e r t h a n about 300 mL p e r second.
Impulse responses were derived by differentia tin^ s t e p responses. Ascending s t e p s generated a b i p h a s i c impulse response and descending s t e p s give a t r i p h a s i c u n i t impulse
response. This l a c k o f equivalence over conditions i s d i a g n o s t i c of system n o n l i n e a r i t y . Fourier a n a l y s i s o f t h e impulse
responses showed equivalent s p e c t r a l components and lovipass f i l t e r a c t i o n i n each case, i n d i c a t i n g change i n gain and
phasing r a t h e r t h a n time c o n s t a n t s occur between 'on' and ' o f f conditions
.
The system was shown t o be q u a s i l i n e a r w i t h i n conditions s i n c e l i n e a r convolution o f t h e impulse responses with a p p r o p r i a t e ramps generated f u n c t i o n s t h a t were a f a i r approximation t o t h e psychophysical response t o such ramps.
The r e l a t i o n s h i p between the impulse response and t h e s p a t i a l equivalent l i n e spread f u n c t i o n was discussed and the
s i m i l a r i t y o f t h e impulse responses t o dynamic changes i n s e n s i t i v i t y i n f e r r e d from r e s u l t s of masking experiments
pointed out. The r e s u l t s were then considered i n r e l a t i o n t o a multichannel model f o r processing spatiotemporal information.
