Signal transduction and contextual modulation in the human visual system

The transduction of luminance contrast is nonlinear. As contrast increases, responses first accelerate, and then saturate. This acceleration is thought to be responsible for the “dipper effect”: the improvement in stimulus sensitivity at low absolute contrasts. Moreover, a stimulus that cannot provoke a neuronal response can still modulate a neuron’s response to its preferred stimuli. Recent evidence suggests that similar computations exist in more complex visual parameter spaces. Furthermore, there is indication that the divisive inhibitory mechanisms of the brain are affected in aging. In this thesis, I present three experiments that explored these possibilities. My first experiment investigated reports that surround suppression of contrast strengthens with age, implying disruption of excitatory/inhibitory balance. I suspected that previous reports may reflect contributions from overlay masking, a distinct form of suppression. Using stimuli that preclude overlay masking, I found surround suppression to be similar in younger and older observers, suggesting that spatial suppression is stable with age. I also reported a novel finding of untuned suprathreshold suppression in central vision. My second experiment explored whether the “dipper effect” can be found in the mid-level perception of global form. I also expanded on reports of enhanced sensitivity to concentric and radial form. By manipulating the saliency of the global form percept elicited by Glass patterns, I reported a dipper effect that was similar across the pattern axes I examined. In my third experiment, using electroencephalography, I investigated whether this global form “dipper effect” is predicted by neuronal responses. At ventral electrodes, I found evidence for a transducer that would produce a dipper effect, though the level of global form for which it predicted peak sensitivity differed from psychophysical predictions. I reasoned that this discrepancy is likely due to methodological limitations, but also provided an explanation from attentional modulation and gain control