Pattern integration in the normal and abnormal human visual system

Doctoral thesis English OPEN
Baldwin, Alexander

The processing conducted by the visual system requires the combination of signals that are detected at different locations in the visual field. The processes by which these signals are combined are explored here using psychophysical experiments and computer modelling. Most of the work presented in this thesis is concerned with the summation of contrast over space at detection threshold. Previous investigations of this sort have been confounded by the inhomogeneity in contrast sensitivity across the visual field. Experiments performed in this thesis find that the decline in log contrast sensitivity with eccentricity is bilinear, with an initial steep fall-off followed by a shallower decline. This decline is scale-invariant for spatial frequencies of 0.7 to 4 c/deg. A detailed map of the inhomogeneity is developed, and applied to area summation experiments both by incorporating it into models of the visual system and by using it to compensate stimuli in order to factor out the effects of the inhomogeneity. The results of these area summation experiments show that the summation of contrast over area is spatially extensive (occurring over 33 stimulus carrier cycles), and that summation behaviour is the same in the fovea, parafovea, and periphery. Summation occurs according to a fourth-root summation rule, consistent with a “noisy energy” model. This work is extended to investigate the visual deficit in amblyopia, finding that area summation is normal in amblyopic observers. Finally, the methods used to study the summation of threshold contrast over area are adapted to investigate the integration of coherent orientation signals in a texture. The results of this study are described by a two-stage model, with a mandatory local combination stage followed by flexible global pooling of these local outputs. In each study, the results suggest a more extensive combination of signals in vision than has been previously understood.
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    10 Discussion 190 10.1 Conclusions from the work presented here . . . . . . . . . . . . . . . . . . . . . . 190 10.1.1 The visual field inhomogeneity in log contrast sensitivity is bilinear . . . 190 10.1.2 Area summation is spatially extensive and occurs according to a single rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 10.1.3 Summation of threshold contrast over area is normal in amblyopia . . . . 192 10.1.4 The summation of orientation signals is a noisy two-stage process . . . . 193 10.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 10.2.1 Summation of contrast over space . . . . . . . . . . . . . . . . . . . . . . . 194 10.2.2 Integration of orientation signals . . . . . . . . . . . . . . . . . . . . . . . . 194 10.2.3 Extending the orientation Battenberg work to the motion domain . . . . 195 10.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

    A Birdsall's theorem 210 A.1 Early noise and nonlinear transduction . . . . . . . . . . . . . . . . . . . . . . . . . 210 A.1.1 Single-channel systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 A.1.2 Multi-channel systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 A.1.3 Area summation with early noise . . . . . . . . . . . . . . . . . . . . . . . . 212

    B MATLAB code 214 B.1 Log-Gabors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 B.1.1 MATLAB code to produce log-Gabor patches . . . . . . . . . . . . . . . . . 214 B.1.2 loggabor.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 B.2 The witch's hat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 B.2.1 MATLAB code to produce a witch's hat attenuation surface . . . . . . . . 216 B.2.2 witchhat.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

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    Baker, D. H., Meese, T. S., 2011. Contrast integration over area is extensive: A three-stage model of spatial summation. Journal of Vision 11:14 (14), 1-16.

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