Final answer:
Patients typically have a higher threshold for perceiving depth in random dot stereopsis compared to local stereopsis because local stereopsis involves finer details that the brain can use to extract depth information, whereas random dot stereograms lack distinctive patterns and require more sensory acuity to decode depth.
Step-by-step explanation:
You cannot expect a patient to have as low a threshold in random dot stereopsis as local stereopsis because local stereopsis involves finer details. This is due to the fact that local stereopsis is based on the ability to perceive depth based on the presence of distinct features or patterns in the visual field, allowing the brain to align and compare distinct elements.
On the other hand, random dot stereograms contain no distinctive features within the dots, so depth perception must be achieved solely through the perception of disparities in the random dot patterns between the left and right eye images. Therefore, more effort and sensory acuity is required in the visual system to decode the depth information from random dot stereograms. The retinal disparity that occurs due to objects falling on different spots of the two retinae allows the brain to extract depth perception creating the impression of three-dimensionality, even from a two-dimensional image.