4.14 Ventral Stream and Perceptual Constancy

Perceptual Constancy: The ability to maintain a consistent perception of the environment and objects within it despite variations in viewing angles, perspectives, or lighting conditions.

Size Constancy: The perception that an object maintains its size regardless of changes in distance from the observer.
Example presented: Picture of trees.
- Observation: Though the apparent size of trees varies in the image, the brain understands that they are roughly the same size.
- Cognitive processing: The brain uses depth cues to adjust the perception of size.
Impact of background elements:
- Size Illusions: When background items are emphasized, size discrepancies seem more obvious.
- Example: Viewing people in a line where depth cues influence the perception of height. Reducing background clarity allows discrepancies to be revealed, contradicting the consistent perception formed initially.

Ponzo Illusion: A classic visual illusion demonstrating size perception based on context.
Depicted example: Two beavers, one at the top and one at the bottom of the image.
- Observation: The top beaver appears larger though both are identical in size.
- Cognitive Mechanism: The brain interprets objects farther away as larger when they occupy the same amount of visual space on the retina due to inherent assumptions about distance and size.

Various cartoon illustrations demonstrate varying sizes due to perspective, such as:
- A man appearing larger as he moves back in space.
- A small monster perceived as tiny being chased by a much larger creature, although they are the same size.
- These examples reveal how the brain adjusts for depth cues leading to misleading perceptions of size.

Ames Room Illusion: A physical construct demonstrating size constancy through architectural distortion.
Description: A trapezoidal room designed so that it appears normal from a specific viewpoint.
- When viewed from this distinct angle, the woman on the right appears significantly larger than the woman on the left.
- Cognitive Inference: The illusion exploits learned principles about how we perceive height relative to room dimensions.
- Objects appearing larger because they fill vertical space are interpreted as being larger than they truly are.

The brain integrates various depth cues and contextual details to formulate the most coherent size perception of objects.
This integration emphasizes the malleability of human perception based on environment and visual context, reinforcing concepts of constancy in visual processing.