L1. Intro to Perception

Course Administration

• Subject: PSYC236 – Cognition & Perception

• Lecturer / Coordinator: Dr Ozgur Karakale (email for subject-level questions)

• Other Lecturers: Steve Palmisano, Mark Schira, Tim Byron

• Tutors: William, Joel, Mark, plus Ozgur in some classes
  • Tutorials start Week 2; no tutorials in Week 1
  • Each tutor will advise preferred communication method next week

• Content for each week (slides, readings, extra videos) is released every Friday for the following week via Moodle

• Two required textbooks; weekly chapter list posted on Moodle; supplementary readings uploaded as needed

• Learning support: university writing & study advisers; slides with contact details will be posted

Course Structure & Weekly Topics

• Week 1 – Introduction; physiology of the visual system (current lecture)

• Week 2 – Size & Depth Perception (Steve) + Quiz 1

• Week 3 – Colour Perception (Mark)

• Week 4 – Motion Perception (Mark) + Quiz 2

• Week 5 – Object & Face Recognition + Quiz 3; data collection for lab report in tutorials

• Week 6 – Research / Study Week (no lecture)

• Week 7 – Face Perception (Ozgur) + Quiz 4

• Weeks 8-11 – Higher-level cognition topics: attention, memory, autobiographical memory, language, cognitive biases
  • Quiz schedule continues; each quiz always covers “latest un-assessed” material (e.g., Quiz 2 = Weeks 3 & 4)

• Week 12 – Revision & wrap-up (no quiz)

Assessments Overview (3 Components)

• 8 Online Quizzes (best 7 counted)
  • 8\times2.5\% = 20\% total
  • Open-book; 5 MCQs × 12 min; window: Fri morning → Sat 23:30
  • Miss 1 quiz → replaced by mean of other 7; miss >1 without consideration → zeros (may incur technical fail)

• Lab Report (30 %)
  • Experiment run in Week 5 tutorials; collated results provided Week 8
  • 4-week write-up period; limited AI use allowed (no AI-generated un-credited text) – see subject outline

• Final Exam (50 %)
  • Closed-book; MCQ + 4 short-answer questions
  • SAQs drawn from a pool of 24 weekly questions (2 per week; prepare throughout semester)

Key Study Tips & Expectations

• Technical content can feel dense; allocate ≈12 hrs/week (lectures, tutorials, readings, revision)

• Minimise distractions; regular review makes quizzes & lab report easier

• Make use of learning advisers, lecturer/tutor emails, and Moodle resources

• "Make time now, save time later" – cumulative nature of quizzes/exam

Perception vs Cognition Fundamentals

• Sensation: raw detection (e.g., photons on retina)

• Transduction: conversion of physical energy → neural signal (e.g., light → electro-chemical)

• Perception: brain’s interpretation of sensory input

• Cognition: umbrella term for mental processes (attention, memory, reasoning, emotion) that provide top-down guidance to perception

• Five traditional senses: sight, hearing, smell, taste, touch – each with dedicated receptors and cortical areas

Top–Down vs Bottom–Up Processing: Demonstrations

• Number/Letter Figure: “12 13 14” vs “A B C” – identical middle glyph read as 13 or B depending on context

• Arcimboldo “vegetable bowl” → upright = bowl; inverted = face (bias toward face detection)

• Hollow-mask illusion: concave interior still perceived as convex face; face templates override depth cues

• "Spot-the-Dog" cloud image: once outlined, impossible to un-see – memory alters subsequent perception

Common Visual Illusions & What They Illustrate

• Hermann Grid: grey smudges at peripheral intersections – receptive-field inhibition

• Hering Illusion: straight red lines appear bowed – depth cues from radial background

• Rotating Snakes / Moving Circles: illusory motion from microsaccades

• Checker-Shadow (Adelson): squares A and B physically identical luminance; perceived differently via context & shadow

• Café Wall & Müller-Lyer: size/angle context distorts length & parallelism judgments

Visual System Anatomy: Eye

• Light path: cornea → aqueous humour → pupil (iris controls diameter) → lens (accommodation) → vitreous humour → retina

• Lens issues (refractive errors) corrected by glasses, contacts, or surgery

• Pupil reflex: bright light → constriction; dim light → dilation (lets more photons in)

Photoreceptors: Cones vs Rods

• Divergence vs Convergence:
  • Cones maintain separate channels → sharp acuity
  • Many rods pool → increased sensitivity, decreased detail

• Blind Spot: optic disc (~15° temporal to fixation); no receptors because ganglion cell axons exit eye

• Octopus/cephalopod eyes have photoreceptors facing the light, so no blind spot (illustrates inverted vertebrate retina)

Visual Angle & Retinal Image Size

• Visual angle \theta depends on object height h and distance d:
  \theta = 2\arctan\left(\frac{h}{2d}\right)

• Halving d doubles \theta → image covers twice the retinal area → stimulates more photoreceptors → greater detail

• Despite changing retinal size, perceived size remains constant (size constancy) using depth cues and top-down knowledge

Visual Pathways: Retina → LGN → V1

1. Retinal Ganglion Cell axons bundle as optic nerve

2. Partial decussation at optic chiasm
   • Nasal hemiretina fibres cross; temporal fibres stay ipsilateral

3. Post-chiasm fibres = optic tract → Lateral Geniculate Nucleus (LGN) of thalamus

4. LGN → optic radiations → Primary Visual Cortex (V1/Striate cortex) in occipital lobe

5. From V1, two major streams:
   • Ventral “What” (V4 etc.) – colour & object identity
   • Dorsal “Where/How” (V5/MT etc.) – motion & spatial location

Lateral Geniculate Nucleus (LGN)

• 6 layered structure per hemisphere
  • Layers 1-2: Magnocellular (M) – motion, flicker
  • Layers 3-6: Parvocellular (P) – colour (red/green), fine detail
  • Thin inter-laminar Koniocellular (K) slabs – blue/yellow & alerting signals

• Layer eye-segregation: each layer receives monocular input (e.g., Layer 1 right-eye, Layer 2 left-eye) enabling later binocular integration

Receptive Fields & Centre–Surround Antagonism

• RGC & LGN cells exhibit on-centre/off-surround or off-centre/on-surround organisation

• Function: enhance contrast & highlight edges / boundaries / luminance changes

• Example response logic:
  • Light only in excitatory centre → many spikes
  • Uniform illumination centre+surround → excitation ≈ inhibition → baseline firing
  • Light only in inhibitory surround → suppressed firing

Lesion Sites & Resulting Visual Field Deficits

(Central/foveal input projects bilaterally, explaining macular sparing.)

Key Terminology Reference

• Photopic vs Scotopic vision;

• Transduction, Divergence, Convergence;

• Visual Angle, Size Constancy;

• Magnocellular / Parvocellular / Koniocellular;

• On-centre / Off-centre receptive fields;

• V1 (Primary Visual Cortex), V4 (ventral/"what"), V5 (= MT, dorsal/"where");

• Optic nerve → chiasm → tract → LGN → radiations → V1

Ethical & Practical Notes

• AI tools permitted for lab report only in limited, transparent ways (no ghost-writing) – see outline

• Collaboration on weekly SAQs encouraged, but answers must be written individually to avoid academic misconduct

Quick-Fire Self-Check Questions

1. Why is reading difficult under scotopic (rod-mediated) conditions?
   • Cones (needed for acuity & colour) are inactive; rods converge heavily → low resolution.

2. Which LGN layers carry blue–yellow opponency?
   • Koniocellular layers (inter-laminar zones).

3. Formula for visual angle? \theta = 2\arctan\left(\frac{h}{2d}\right)

4. Damage to left optic tract produces loss in which field?
   • Right visual hemifield of both eyes.

End of Week 1 study notes – ensure you can sketch the visual pathway and explain at least two illusions.

Perception vs Cognition Fundamentals

• Sensation: The initial process of raw detection of physical energy from the environment by specialized sensory receptors (e.g., photons hitting specialized cells in the retina of the eye, sound waves vibrating hair cells in the cochlea). It is generally considered a passive, bottom-up process where information flows directly from the sensory organs to the brain.

• Transduction: The critical step where physical energy from the environment is converted into neural signals (electro-chemical impulses) that the brain can understand and process (e.g., light energy is converted into electrical signals by photoreceptors in the eye; sound waves into nerve impulses in the inner ear). Each sensory system has specialized cells or mechanisms for this conversion.

• Perception: The brain’s active process of interpreting, organizing, and selecting sensory input to create a meaningful and coherent representation of the world. This involves integrating sensory information with prior knowledge, expectations, and context. It is how we consciously experience and make sense of the sensory raw data.

• Cognition: An umbrella term for a broad range of higher-level mental processes including attention, memory, executive functions (planning, problem-solving), reasoning, language, and emotion. Cognition provides top-down guidance to perception, meaning our thoughts, expectations, and experiences can profoundly influence what and how we perceive, often overriding purely sensory input.

• Five traditional senses: sight (vision), hearing (audition), smell (olfaction), taste (gustation), touch (somatosensation) – each with dedicated receptor organs and specialized cortical areas for processing. Beyond these, senses like proprioception (body position in space) and equilibrioception (balance) also provide crucial sensory input.

Top–Down vs Bottom–Up Processing: Demonstrations

• Number/Letter Figure: An identical middle glyph can be perceived as "13" within the sequence "12 13 14" or as "B" within "A B C." This demonstrates top-down processing, where the surrounding context (a sequence of numbers or letters) creates an expectation that influences how an ambiguous sensory input is interpreted by the brain, rather than purely relying on the visual features of the glyph itself.

• Arcimboldo “vegetable bowl” ightarrowightarrow upright = bowl; inverted = face: This illusion showcases a strong top-down perceptual bias or template for face detection built into our visual system. Our prior knowledge and strong drive to identify faces cause us to readily perceive a face when the image is inverted, even though the constituent parts are everyday objects.

• Hollow-mask illusion: A concave interior of a mask is still perceived as a convex face. This illustrates the powerful influence of top-down knowledge and strong prior expectations that faces are typically convex. Our brain overrides conflicting bottom-up depth cues (like shading and shadows that indicate concavity) due to ingrained templates for what a face 'should' look like.

• "Spot-the-Dog" cloud image: Once the outline of the dog is perceived within the ambiguous cloud formations (e.g., by having someone point it out or seeing an initial hint), it often becomes impossible to un-see it. This highlights how memory and prior recognition (top-down influence) alter subsequent perception, making it difficult to revert to the initial ambiguous state. Our cognitive understanding permanently changes our visual experience of the image.

Common Visual Illusions & What They Illustrate

• Hermann Grid: Grey smudges appear at the peripheral intersections of a white grid on a black background, which disappear when directly fixated. This is explained by lateral inhibition, a bottom-up retinal process where the firing of neurons stimulated by white lines inhibits the firing of neighboring neurons at intersections more strongly than along continuous lines, causing the illusion of darker spots. When directly fixated, the fovea's smaller receptive fields reduce this effect.

• Hering Illusion: Two straight, parallel red lines appear bowed outwards when presented over a radial background that seems to recede. This occurs because the radial lines provide false depth cues; the brain interprets the background as extending into the distance, thereby distorting the perception of the overlaying straight lines due to how our brain processes perspective and depth.

• Rotating Snakes / Moving Circles: These static images create an illusion of motion without any actual movement. This is believed to be due to subtle involuntary eye movements called microsaccades or a complex interaction of brightness, color, and pattern processing that creates differential neural responses, particularly in motion-sensitive areas, leading to perceived motion. This is largely a bottom-up sensory effect.

• Checker-Shadow (Adelson): Squares A and B, which are physically identical in luminance (brightness), are perceived differently. Square B, which appears to be in a perceived shadow, is interpreted as being intrinsically lighter than square A, which is in direct light. This illustrates lightness constancy and the brain's ability to factor in inferred lighting conditions and contextual information (shadows, surrounding luminance) to determine an object's intrinsic brightness, a strong top-down influence.

• Café Wall & Müller-Lyer: These illusions demonstrate how contextual cues and the arrangement of surrounding elements distort judgments of length, angle, or parallelism. For example, in the Müller-Lyer illusion, the inward or outward pointing arrowheads (or 'fins') at the ends of a line segment influence our perception of its length due to the angles and perceived depth cues they create, impacting feature detectors in the visual cortex.

Visual System Anatomy: Eye

• Light path: Light enters the eye through the cornea (a transparent, protective outer layer that also provides most of the eye's focusing power) ightarrowightarrow passes through the aqueous humour (a clear fluid filling the front of the eye) ightarrowightarrow enters the pupil (the adjustable opening in the center of the iris, which controls its diameter to regulate light entry) ightarrowightarrow passes through the lens (a flexible structure that changes shape via accommodation to fine-tune focus for different distances) ightarrowightarrow crosses the vitreous humour (a clear, gel-like substance filling the main cavity of the eye) ightarrowightarrow finally reaches the retina (the light-sensitive layer at the back of the eye containing photoreceptors).

• Lens issues (refractive errors) corrected by glasses, contacts, or surgery: Common conditions include myopia (nearsightedness, where the focal point falls in front of the retina), hyperopia (farsightedness, where the focal point falls behind the retina), and astigmatism (irregular curvature of the cornea or lens, causing distorted vision). Correction aims to refocus light accurately onto the retina.

• Pupil reflex: In bright light ightarrowightarrow the pupil undergoes constriction (narrows via the iris sphincter muscle) to reduce the amount of light entering the eye, which also increases the depth of field and sharpens focus. In dim light ightarrowightarrow the pupil undergoes dilation (widens via the iris dilator muscle) to let more photons in, thereby increasing sensitivity for vision under low-light conditions.

Photoreceptors: Cones vs Rods

• Divergence vs Convergence:
  • Cones maintain separate channels onto ganglion cells (low convergence ratio, sometimes 1:11:1 in the fovea) ightarrowightarrow results in sharp acuity because each cone's signal is preserved and resolved individually, allowing for high detail resolution.
  • Many rods converge their signals onto a single ganglion cell (high convergence ratio, e.g., 100:1100:1 or more) ightarrowightarrow results in increased sensitivity (summation of weak signals boosts detection in dim light) but significantly decreased detail (spatial information is lost due to pooling).

• Blind Spot: A specific area on the retina known as the optic disc (located approximately 15exto15exto temporal to fixation). This region contains no photoreceptors because it is where the retinal ganglion cell axons bundle together to form the optic nerve and exit the eye, along with blood vessels entering and exiting. The brain