Advances in Photoreception Proceedings of a Symposium on Frontiers of Visual Science (1990) / Chapter Skim
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Partitioning Visual Processes
Partitioning Visual Processes
Pages 78-102
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From page 78... ...
So nonlinear processes are what allow us to pare down the alternative representations of the system and, when they have been pared down enough, to know what goes on inside. The way nonlinearities constrain the alternatives can be seen from a simple example.
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From page 79... ...
, I shall not describe it here except to say that it allows precise and facile control over the contrast, orientation, and spatial frequency of gratings created by optical interference, which can exceed 200 c degas. As the changes of contrast are rapid and afford no extraneous cues, the instrument supports forced choice observations.
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From page 80... ...
1~ 1 ~ / \ 1_ . - ~ Vow - - ~ ~ 1 11 0 200 400 600 BOO tOOD SPACE FIGURE 1 Formation of a difference frequency distortion product.
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From page 81... ...
Thus, there is no change of spatial frequency preceding the nonlinear process, and if the stages preceding the nonlinear process are reasonably isotropic, this procedure affects nothing except those processes that depend on the spatial frequency of the distortion product.
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From page 82... ...
A subsequent low-pass filter blocks the input signals as well as all the distortion products except the lowest frequency, the difference frequency. The entering sine waves create the signal that is detected (the difference frequency)
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From page 83... ...
PARTITIONING VISUAL PROCESSES S , 1 1 1 R 83 Optics First Spatial Filter (Receptor Aperture) Nonlinearity Second Spatial Filter FIGURE 3 The four compartments into which the visual system is partitioned by the interferometer and the nonlinearity.
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From page 84... ...
. Losses Preceding the Nonlinear Stage When distortion products of fixed frequency are created by superimposed interference gratings of variable frequency, the detectability of the distortion products decreases with increasing spatial frequency of the gratings (MacLeod et al., 1985; Chen et al., 1988~.
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From page 85... ...
The octagons show the negative log contrast of a 60-c deg 1 interference fringe superimposed on another 60-e deg 1 fringe of 100 percent contrast. The angle between the two fringes was adjusted to produce distortion products at the frequencies shown on the horizontal axis.
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From page 86... ...
The negative sign represents mutual antagonism without specifically denoting algebraic subtraction. A Test for Serial Filter Model According to the logic expressed by Figure 3, the signals elicited by the gratings are attenuated by two serial filters, one before the nonlinear stage and one after.
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From page 87... ...
Spatial Antagonism Preceding the Nonlinearity Our technique does not allow direct measurement of the attenuation preceding the nonlinear stage at low spatial frequencies. However, if the sum of the effects represented by triangles and octagons equals the total effects determining the shape of the contrast sensitivity curve represented by the squares, the difference between the squares and octagons should resect the shape of the curve represented by the triangles.
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From page 88... ...
Summary of Results The preceding section shows how the interferometer and nonlinear process can be used. Figure 7 summarizes the results this approach has produced so far.
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From page 89... ...
-9 ~ ~ to lo 89 ; : /' ~ // / // /// By/ ~ / it \\ \\ `\ ~ \\ \\ \ \ ~ ~ -3 -2 -1 0 1 2 Minutes FIGURE 6 Poinl~pread ranchos aher lbc in~aUon has pawed 1b~ugb 1be eyed opUo Amid Hne) , lb~ugb the blledng preceding 1be nonlinear stage (dashed Une)
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From page 90... ...
R ,............. Ocular Media Speckle Noise Cone Aperture Pigment Screen Escaped Light Pigment Screen Recaptured Light Stiles-Crfd.
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From page 91... ...
Such light that escapes from one cone only to be recaptured by its neighbor must decrease the contrast of interference fringes cast on the retina by oblique light. The loss of contrast sensitivity to such fringes, then, provides a way to measure the recaptured light.
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From page 92... ...
Finally, the light captured by the first cone it strikes and ultimately absorbed by the same cone may pass through some pigment that neither escaped light nor recaptured light sees; stage 6 is introduced to cover that possibility. None of these individual stages of self-screening is definitely established, but the phenomenon of self-screening in cones is generally accepted, and present evidence is consistent with its occurrence at any or all of these stages.
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From page 93... ...
However, when two homogeneous superimposed fields flicker at slightly different rates, no Dicker at the difference frequency can be detected unless the flicker of the fields also is detectable (Makous and Mandler, unpublished observations, 1984~. This means that temporal filtering after the nonlinear stage does not attenuate the flicker of the superimposed fields below that of the distortion product.
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From page 94... ...
. If one of the two interference fringes used to create these distortion products is presented as an intense 10-msec flash, subsequent presentation of the other fringe alone produces distortions similar to those observed when the two are presented simultaneously (MacLeod et al., 1985~.
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From page 95... ...
. As no higher-order distortion products can be detected, no higher-order terms are required in the polynomial.
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From page 96... ...
Generalizing the Approach The results summarized by Figure 7 suggest that use of the Rochester interferometer to exploit a nonlinear process has been rewarding, and working out the remaining unknowns in this system promises continuing rewards, but these successes also stimulate an interest in testing the power of analogous techniques elsewhere in the visual system. This would require a method for bypassing the present nonlinear process (stage 18)
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From page 97... ...
fails to constrain the locus of the nonlinear process, it does limit applicability of observations on processes preceding the nonlinear stage to individual cones and the parvocellular system. Even the parvocellular system, however, is heterogeneous, including, for example, both on and off pathways.
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From page 98... ...
from subsequent processing, such as those that transform continuous physiological processes into discrete behavioral responses. Multiplicity of processes.
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From page 99... ...
So responses to stimuli that are both at threshold may differ, according to any measure, before the final stage of temporal filtering (cf. Zacks, 1970)
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From page 100... ...
To some extent the schematic diagrams we use arise from the idea that individual neurons are the appropriate units of analysis in the visual system. However, the ubiquitous gap junctions spread electrical potentials through the barriers between cells that guard their independence, so that the cooperative actions of these extensively interconnected neurons generate fields that tend to vary continuously in space, perhaps more in accordance with the rules of field theory than those of the neuron doctrine.
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From page 101... ...
Makous 1985 Difference frequency gratings above the resolution limit. Investigative Ophthalmology and Visual Science 26(Suppl.~:11.
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From page 102... ...
Vision Research 25:195-205. 1985b Visibility of interference fringes near the resolution limit.
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