Visual Illusions Flashcards
(8 cards)
Intro
Introduce the topic: Define visual illusions and their role in understanding perception.
Introduce visually guided actions: Actions such as reaching, grasping, or walking that rely on visual input.
Introduce the central debate: Do such actions remain immune to illusions, or are they influenced similarly to perceptual judgments?
Thesis statement: While early evidence suggested that visually guided actions resist visual illusions, more recent research demonstrates that this resistance is partial, task-dependent, and influenced by methodological factors.
para 1 - background
Define visual illusions as discrepancies between physical reality and perceptual experience.
Outline their value in studying visual processing (e.g., Titchener Circles, Ponzo Illusion).
Introduce the two-visual-streams hypothesis (Goodale & Milner, 1992):
Ventral stream for perception (allocentric coding).
Dorsal stream for action (egocentric coding).
Key claim: Illusions distort perception (ventral stream) but not action (dorsal stream).
para 2 - supporting resistance
Aglioti et al. (1995)
Explain Titchener Circles experiment:
Participants judged disks surrounded by small/large circles.
Illusion affected perception (misjudged size), but grip aperture during grasping reflected true size.
Conclusion: Grasping behaviour seemed immune to the illusion → evidence for dorsal stream insulation.
Quote key conclusion: “What we think we see may not always be what guides our actions.”
Interpretation: Strong support for the two-stream model.
para 3 - challenging resistance
Brenner & Smeets (1996):
Found grip aperture unaffected, but lifting force was influenced by illusory size (Ponzo illusion).
Implies some aspects of action (e.g., force) may still rely on distorted perceptual cues.
Franz & Gegenfurtner (2008):
Meta-analysis showed illusion effects > 0 mm on grasping across many studies.
Suggests actions are not entirely immune; effects are smaller and more variable than perceptual ones.
Methodological insight: Measurement technique affects results (e.g., direct comparison vs. implicit action).
Schenk & McIntosh (2010): Evidence of illusion effects on action, depending on task design and time constraints.
Overall: Resistance is contextual, not absolute.
para 4 - real world applications
Driving:
Use of visual illusions (fake speed bumps, patterns) to reduce speed (Denton, 1980).
Optic flow illusions influence perceived speed, prompting action adjustment.
Walking:
Floor patterns influence walking speed and heading direction (Ludwig et al., 2018; Leonards et al., 2015).
Effects small, but statistically significant.
These examples show actions can be influenced by illusions in complex, real-world tasks, especially where timing, complexity, or unfamiliarity is involved
para 5 - limitations and method considerations
llusion strength varies based on:
Task constraints (e.g., speed, familiarity).
Measurement type (grip aperture vs. lift force vs. endpoint error).
Whether perception and action tasks are truly equivalent.
Awareness of illusion doesn’t always reduce effect → actions not necessarily immune.
Ecological validity: Lab tasks (e.g., reaching for disks) may not reflect real-world actions.
Visual-motor tasks in daily life often blend ventral and dorsal processing.
conclusion
Recap key points:
Strong initial support for resistance of actions to illusions.
Subsequent studies show partial resistance, depending on what aspect of action is measured, and how.
Evidence from real-world nudging shows illusions can influence motor behaviour.
Final argument: Visually guided actions do not wholly resist visual illusions. Rather, they show graded susceptibility depending on context and task demands.
Implication: Visual processing is more integrated than strictly dual-stream theories may suggest.
Aglioti et al 1995 crit ev
STRENGTHS
Controlled laboratory conditions and objective measurement of grip aperture (motion tracking) support strong internal validity.
The study operationalized perceptual judgment and grip aperture well as separate measures of perception and action. (Construct)
WEAKNESSES
External validity is limited, and findings may not fully apply to complex real-world actions. - Simplified lab task
