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theses

Due to the quantum nature of light, when a population of identical photoreceptors in the retina, or identical pixels in a digital sensor, is exposed repeatedly to uniform light, there is both spatial and temporal variation in absorbed light. We have derived the histogram of the expected number of cells absorbing each specific level of energy, the variance of which quantifies the spatial variation in absorption by cells. Moreover, the actual histogram varies across identical trials of exposure. We quantified this temporal variation by the variance of the number of cells at a given energy level, showing that the temporal variation has a unimodal distribution, and is reciprocally related to the spatial variation. The proposed model predicts that, due, not to higher-order visual mechanisms, but rather to low-level vision and the quantum nature of light, stimulating cone cells with uniform colored light produces a distribution of responses in cone excitation space (rather than a point) and these reproduce MacAdam's (1942) classic measurements of the variability of color matches. We measured the temporal dynamics of human color detection when a target is briefly presented on an equiluminant neutral surround. Our results showed that momentary-produced signals of varying luminance in the adapting field are affected by an instantaneous neuronal mechanism rather than a slow cone adaptation, suggesting that the detection threshold is limited by fluctuations of a Poisson nature in the postreceptoral stage. Our model predicts that the signal to noise ratio in a response channel remains the same, regardless of luminance, so long as the state of equilibrium in this channel is constant. A neutral disk appears slightly greenish when surrounded by a large red field, illustrating chromatic induction. When observers' heads were placed inside a special ganzfeld apparatus, they saw a 3 degree, physically neutral disk surrounded by a saturated red area that filled the remainder of the visual field and they adjusted the color of the disk to make it appear neutral. We found that chromatic induction increases non-linearly with surround saturation but falls-off with highly saturated surrounds, suggesting two antagonistic processes, an adaptation-related component and an additive process exerted by surround color.

ETD_8131

Ph.D.

Includes bibliographical references

by Shahram Peyvandi

doi:10.7282/T3WD43HS

ETD doctoral

The author owns the copyright to this work.