VISN1101: PERSPECTIVES IN VISION SCIENCE

SENSORY PERCEPTION

(1) THE PROBLEM OF PERCEPTION

SENSATION & PERCEPTION

• Sensation = how senses transform physical properties of environment à electrical signals relayed in the brain • Perception = turning neural signals à meaningful experience (e.g. faces) o Ability for brain – select, organize & interpret sensory information it receives o Perception à response

THE 6 SENSES

Sense Energy Receptor Vision Light Eyes Hearing Sound + vibrations Ears Touch (somatosensation) Pressure + temperature Skin Taste Chemicals Tongue Smell (olfaction) Chemicals Odour receptors in nose Vestibular Gravity + inertial forces applied to the head Inner ear

LUMINANCE

• Luminance = R x I o R = reflectiveness o I = absorbance • Dark objects à absorb more + reflect less • Light objects à absorb less + reflect more • R & I = fractions

THE INVERSE PROBLEM

• Cameras only sense – convert light into electrical potentials (bits + bytes) o They don’t perceive - pixels seen as independent o Humans can integrate across pixels – cameras can’t

• Inverse problem = turning 2D à 3D shape, colour, texture o Brain can perceive by solving the inverse problem • We recognise things in different sizes + orientations o Can compare by mentally rotating objects in our mind • Different views + perceptions o Accidental view = only one face visible (improbable when shapes are irregular) § E.g. perceiving a blue square (one face of a cube) o Generic view = adopted when an object is partially obstructed from view

ILLUSIONS

Importance: • Illusions indicate we don’t know exactly how things are structured in the world around us • Provide insight into how perceptual systems break down • Reveal the kinds of assumptions/general rules we use to make guesses/inferences about the physical world

1. Colours in shadows • Brain gives us best estimate of what we are looking at • Deduces colour of the tile based off illumination o Tile ‘B’ = most efficient at reflecting light in the shadow area à therefore perceived as the same as the lighter tile in the light area, but is actually the darker tile in the light area (A)

2. Café Wall Illusion • Staggered black and white squares/rectangles – looks like it warps/tapers along rows o Apparent warping – happening in your head o Lines are actually straight + parallel

3. “Margaret Thatcher” Illusion (Thompson, 1980) • Harder to detect inverted facial features relative to the face, if the whole head is upside- down o Not much difference perceptually in the inverted faces

o BIG difference perceptually when faces are upright

o Also applies to other facial features e.g. facial hair

• Explanation à Face Inversion Effect (Yin. 1969)– variations in recognition performance depending on: o Inverted faces – ­ errors/ ­ time taken / ¯ confident o Upright faces – ¯ errors/ ¯ time taken / ­ confident o Inverted/upright non-faces – no difference in recognition performance • Facial recognition works well if facial features = upright, even if head is upside-down

FACE DETECTION

• Hard problem to solve • Cameras can compute simple location of face-like objects – but do not ‘see’ faces • Top-down processing – cognitive process of how we make sense of information brought into the brain by the senses. (Thoughts àsenses) o Bottom-up processing – senses à brain • We use top-down knowledge from prior learning to turn inverted faces inside out à nose = convex

SEGMENTATION & GROUPING

Brain can select key features to recover complex information about an object (e.g. faces)

• Necker Cube o We group lines to perceive 3D o Necker cube = bi-stable b/c we get reversals § i.e. over time, we alternate b/w 2 different orientations • Can perceive 3D cube by grouping forks alone and having missing lines drawn in using subjective contours generated inside your head

GESTALT PSYCHOLOGY

“Whole is other than the sum of its parts”

• Proposes you cannot predict appearance of complex form by simply adding up individual components • Object perception is an achievement

Grouping & Figure-Ground • Construct experience of an image from patterns (image features) o Depends on what you attribute the features in the image to be – we experience the entire image differently o E.g. ear/eye à young/old woman

• Perceptual attribution of image features determines perceived figure + ground o E.g. Rubin’s Vase § Common edge owned by either 2 faces/1 chalice à cannot see both simultaneously § When one is figure (allocated as foreground)à adj. region becomes ground (background)

Gestalt Grouping Laws - Used to group features into larger units Law of Proximity Grouped by closeness • Lines 1-2 = proximal • Lines 2-3 = distal

Law of Similarity Grouped by similarities • E.g. colour • Similar vertically + alternating along rows causes grouping into 6 vertical bars

Law of Common Grouped by movement in same direction + Fate velocity

Law of Pragnanz We group the max no. of visual elements à (simplicity) min no. of perceptual constructs • “We see the forest before the trees” • E.g. 2 objects perceived, rather than 100s of triangles + squares

Law of Good Certain constraints help determine figure- Continuation ground relationships: • Foregrounded image: o Owns common edge perceived to be owned by foregrounded image o Good continuity of the border (+ at the common edge) • Backgrounded image: o T-junctions producing poor continuity in background

Law of Familiarity We use prior knowledge to unscramble sensory information OR/ knowledge acquired in the immediate context can affect perception of sensory information. • E.g. (Bugelski & Alampay, 1961) Perceived as man’s face when preceded by images of faces • Perceived as a rat when preceded by images of non-human animals

(2) OVERVIEW OF VISUAL PATHWAYS & PROCESSES

THE EYE

• Cornea o Performs most of the initial focusing of the incoming image § 2/3 of refraction done by the eye • Accommodation (lens) o Allows us to focus on objects at different distances by changing the shape of the lens Lens Location of focused distant object Location of focused near object Far accommodation Flatter On retina Behind retina Near accommodation Rounder In front of retina On retina

• Retina o Light reflected by surfaces = captured by retina (sensory tissue at back of the eye) o Image = flipped upside down + back to front § Corrected in the brain o Contains: § Photoreceptors • Rods: cones = 20:1 Rods Cones Number 120 million 6 million Type of Night (scotopic) Daylight (photopic) vision Unable to discriminate colour Visual So many rods sending information at once Each cone gets its own ganglion cell – very Acuity – brain cannot make out much detail detailed colour vision

Relative Peripheral Fovea abundance (therefore, ­ peripheral vision at night)