VIS Lecture 3/4 - The psychology of visual perception/When perception goes wrong 2

• Perception can be erroneous; visual illusions are examples of when perception goes wrong.
• Visual perception relies on more than retinal images.
• Ambiguous figures exemplify equivocal perception.

Bottom-Up Accounts of Visual Perception

• Structuralism, associated with Wundt and Titchener, describes sensory elements.
• Psychophysics, with Fechner, Weber, and Stevens, is based on measurement of sensory elements.
• Ecological theory of perception, linked to Gibson, posits that perception is offered by the real world through object affordance.
• Bottom-up accounts suggest perception and sensation are equal.
• Perception is purely stimulus-guided.
• Sensory receptors determine perception completely and automatically in bottom-up processing.

Erroneous Perception: Visual Illusions

• Fraser spiral illusion (James Fraser, 1908) involves misperception of the shape of lines.
• The vertical-horizontal illusion causes misjudgment of line length.
• Jastrow illusion (Joseph Jastrow, 1892) leads the viewer to perceive different lengths even when equal.
• Irradiation illusion (Hermann von Helmholtz, 1867) leads to misjudgment of the size of inner areas, with black and white squares.
• The Greeks adjusted architecture like the Parthenon to compensate for optical illusions.

Equivocal Perception: Ambiguous Figures

• Figure/Ground ambiguity is where the vase and two faces can be seen.
• When looking at the figure or ground it reveals multiple items.
• Feature ambiguity is present in figures like the duck/rabbit illusion.
• Depth ambiguity can be seen a Necker cube (Louis Albert Necker, 1832), with front and back interpretations.
• Both the legs and dog example show that 3D perception changes even with the same retinal image, with the blue and red sides changing.

Real-World Visual Illusions and Ambiguity

• Visual illusions mean perception is not always what is represented on the retina.
• Visual illusions can't be explained by bottom-up.
• Equivocal perception occurs when we perceive different objects from the same retinal image.
• Illusions and figures are constructed carefully to mislead and do not exist in the real world, lacking ecological validity.
• Moon illusion makes the moon appear larger near the horizon.
• Waterfall illusion causes stationary objects to appear to move in the opposite direction after observing motion in one direction for 30-60 seconds; due to neuronal adaptation to motion.
• Wagon wheel effect is when a moving wheel appears to stand still or move in the opposite direction due to stroboscopic effect; where the neurons try to make sense of fast motions by interpolating frames.
• Inverse projection problem: 3D objects are represented on a 2D retinal surface leading to multiple objects causing the same pattern of light on the retina, where a real-world object cannot be derived from the retinal image.; examples being untypical angles and superimposed objects.

Theoretical Accounts of Perception

• Experiences of perceptual illusions and ambiguity indicates perception is more than sensation.
• Sensory information from the retina is not enough; perception is an interpretation.
• Top-down accounts describe where perception is an interaction between sensation and cognition.
• Stored object knowledge, context, personal expectations, and motivation are used to interpret sensory stimulation.

Constructive Theory of Perception

• Richard Gregory (1970) states the perception is processed through constructive theory.
• Perception is indirect (interpreted) and involves hypothesis testing, using internal factors to interpret sensory input.
• With ambiguous figures individual factors can cause two equally plausible hypotheses to be established.
• Incorrect hypotheses can be formed, leading to perceptual errors based on personal factors.
• The goal is to explain how meaning is attached to sensory input.
• Perception = Interpretation of sensation.
• The Coffee mug is said to be the sensory input (round, cup-shaped, etc.) matching the perceptual hypothesis, which is based on object representations, context, expectations, and motivation.
• Theoretical is where it tries to explain the function of object recognition etc.
• Ambiguity and perceptual failure are explained as the is is capable of being explained.
• Illusions existing even when it is known are not explained
• Continuous hypothesis testing, which doesn't account for personal experience, suggests that perception is not effortless.

Gestalt Psychology

• Max Wertheimer, Wolfgang Köhler and, Kurt Koffka created the ideology of Gestalt psychology in 1910.
• Illusory movement created when two light flashes presented in a rapid alternating fashion.
• When it comes to illusory movement the sensory experience is insufficient to explain the perceptual experience.
• Gestalt argues that "the whole is more than the sum of its parts".
• The basic question is how we acheive the object.
• PErceptual organisation answers the question by grouping (putting together elements that form an object)
• and segmenting (separating elements from a different group.
• Center-bound receptive fields in ganglian cells detect luminance discontinuities.
• Different areas in the visual feild have similar color of texture properties where V4 neurons enable the detection.

Principles of Perceptual Organisation

• Proximity creates groups of elements that are together.
• Similarity forms groups of elements that look similar.
• Common fate describes where elements appear move together.
• Good continuation is formed with the elements that continue.
• Closure if formed together when elements close a figure.
• Relative size, surroundedness, orientation, and symmetry describe elements that are relatively smaller, are horizontally/vertically oriented, symmetrical or in a surrounded area.
• The law of Prägnanz (good Gestalt) states that out of several geometrically possible organisations, the one that will occur possesses the best, simplest, and most stable shape.

Testing your perception

• Bottom-up accounts fail to explain real world illusions.
• Top-down accounts can explain why they occour.
• Richard Gregory's consrtructive theory explain that senseory input is interpreted by means of perceptual hypotheses .
• Gestalt states that sentory input is grouped, segmented (perceptually organised) following principles of the good Gestalt.
• There are more than 200 forms of visual illuson.
• Visual illusions are based on colour, luminance, contrast, motion, geometry etc,