Which one of the following is NOT considered a physiological factor affecting visual illusions?

Visual illusions can be greatly influenced by various physiological and psychological factors. Within the context of this question, it is important to understand what constitutes a physiological factor versus other contextual influences.

Expectancy refers to an individual's anticipation based on prior knowledge or experiences, which can shape perception and lead to illusions. Self-imposed stresses, including anxiety or pressure, can also alter visual perception, potentially resulting in misinterpretations of stimuli. Experience, in this case, relates to how previous encounters with visual stimuli can impact one’s ability to correctly interpret what is seen, further contributing to the perception of illusions.

On the other hand, weather conditions, while they can affect visibility and overall environmental perception, are not classified as physiological factors. They relate more to external environmental conditions than to the internal physiological state or processes influencing visual perception. Thus, weather conditions do not fit within the realm of conscious mental processes or physiological responses that directly impact visual illusions, making it the correct answer to the question.