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SUMMARY:The concatenation of factors of change in perception - Donald MacL
 eod\, UCSD
DTSTART:20070223T130000Z
DTEND:20070223T140000Z
UID:TALK6592@talks.cam.ac.uk
CONTACT:Cordula Becker
DESCRIPTION:Perception depends on change\, as is shown by the fading of ar
 tificially stabilized retinal images\, which fade gradually to disappear a
 fter some seconds. Likewise\, retinal signals generally indicate change in
  space or time\, not absolute values of local stimulation.  In Land’s ea
 rly retinex model\, a central integrator provides estimates of lightness a
 nd color by combining retinal signals that specify factors of change.\nAny
  such central integration is evidently imperfect since stabilized images d
 o fade. The physiological basis of this fading remains obscure. Rushton at
 tributed it to sensitivity regulation\, and he supported this by showing t
 hat a faded border remains invisible when the intensity is changed by the 
 same factor on both sides of the border\, implying reciprocal adjustments 
 of sensitivity on the two sides. In experiments with Dirk Beer and other c
 ollaborators\, we make similar observaions using unstabilized stimuli in t
 he form of diffuse blobs (with Gaussian excitation profiles). These readil
 y fade under steady fixation much as artificially stabilized images do. On
 ce faded\, blobs of sufficiently high intensity remain invisible when the 
 image intensity changes by a constant factor across the non-uniform intens
 ity profile presented by blob and background. \nIf steady-state sensitivit
 y is inversely proportional to input intensity\, their product is constant
 \, providing a plausible basis for fading. Yet the time constant of sensit
 ivity regulation is too fast to account for the fading. And the afferent n
 eural signals are either too transient or too sustained to account for it.
  To connect neurophysiology with perception one can either assume a centra
 l leaky integrator for the transient signals\, or an AC coupling of the su
 stained signals. In one form of the integrator model\, the afferent input 
 is used to update a centrally generated model or hypothesis. Lack of input
  does not call for immediate change in the perceptual model\, but the even
 tual fading of a stabilized contour\, or its sudden disappearance (often d
 uring an eye movement)\, might be viewed as the abandonment of the hypothe
 sis that the contour is there\, absent any verification by transient signa
 ls associated with eye movement.\nEqual factors of change preserve percept
 ual inequality after fading\, but they also preserve inequality when fadin
 g has not yet reached completion. Thus the significance of incremental or 
 decremental retinal signals for perception depends upon the preexisting pe
 rceived lightness and color prevailing when the new stimulus arrives. Veri
 dical perception requires that the perceptual significance of afferent sig
 nals be contingent in this way on the pre-existing perceptual baseline. Th
 e appearance of new stimuli applied during the fading of effectively stabl
 e images or afterimages supports this principle of perceptual concatenatio
 n of factors of change\, and provides new support for Land’s proposals. 
 Prior to any fading\, perception is roughly veridical and remains so if th
 e stimulus changes. But when fading alters perception\, the perception of 
 new stimulation is correspondingly modified\, even though incremental sens
 itivity by detection criteria may be almost constant during the fading.\nA
 lthough fading and  sensitivity regulation are distinct processes\, both c
 an be at least roughly accounted for by. independent changes for the diffe
 rent photoreceptor systems. Thus the appearance of stable images and after
 images during fading\, and also of new stimuli that replace them\, need no
 t\, on present evidence\, involve color opponent mechanisms.
LOCATION:Seminar Room (ground floor)\, Craik-Marshall Building
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