How does the visual system work?

Visual Development How does the visual system work? Look at how it develops over time - what do neonate babies see? - at what age do various abilities appear?

Visual systems: Basis for conscious vision -relatively late to develop (2-3 years) -relatively late to evolve

Development of area V1 - synaptic connections - age 0-8 months: increasing connectivity

Development of area V1 - synaptic connections - age 8 months - 12 years: neuronal pruning

Development of area V1 - synaptic connections - age 12-30 years: stable connectivity

Development of area V1 - synaptic connections - age 30+ years: gradual loss (aging)

Measuring performance of babies Need a way to measure responses to stimuli - how can babies communicate what they see?

A second visual system: Basis for eye movements -relatively early to develop (2-3 months) -relatively early to evolve

Measuring performance of babies Can use eye movements - developed by 2-3 months - tell us what baby is interested in Various techniques based on this…

1. Habituation/Dishabituation - babies get bored quickly (habituate) - stop looking when bored - start looking again (dishabituate) when something new is encountered E.g., colour categories are adult-like at 2-4 months How do we know? a. Show baby something blue b. Wait until they stop looking c. Show baby something in a different colour d. See if they start looking again

2. Preferential Looking - babies look most at what they find interesting E.g., colour categories are adult-like at 2-4 months How do we know? • Show baby something blue, and at the same time a similar item, but inyellow b. See if baby looks at one item more c. If so, then baby can distinguish the colours

3. Surprise (startle) - babies get startled (sometimes frightened) when something unexpected happens E.g., colour categories are adult-like at 2-4 months How do we know? Show baby something blue (in a picture) b. Suddenly change its colour to yellow c. If baby startles, then must have seen a difference in the colours

Stages of increasing neural connectivity At birth: • brightness perception • red-green color vision • motion — objects moving around • looming — objects getting nearer At 1.5 months • depth via accommodation • collision avoidance

At 2 months: • perception of blue (tritanopia ends) • perception of familiar objects (face recognition) - prefer coherent faces over scrambled ones (same parts, different arrangements)

At 3 months: • depth via binocular disparity • depth via vergence At 4 months: • depth via motion (kinetic depth) • biological motion perception At 5 months: • depth via pictorial cues (T-junctions) • relative size

At 6 months: • Gestalt grouping • size constancy At 7 months: • familiar size • shading cues -> Most processes in operation at 8 months of age -> Subsequent development (neuronal pruning) leads to refinement of visual abilities - better control of visual attention?

Nature vs. Nuture What governs the development of vision? Possibility 1: Innate mechanisms - people are born with a fixed program - this program unfolds regardless of environment - cf. Rationalist (Nativist) philosophers Possibility 2: Environmental influences - people born only with general ability to learn - vision results via interaction with environment - cf. Empiricist philosophers

Test 1: Raise observer in deprived visual environment Blakemore — raise kittens in environment with verticalstripes only What happens when kittens are in normal environment? -> Good at seeing vertical stripes -> Bad at seeing horizontal stripes • edge detectors in striate cortex developed only for vertical lines, not horizontalones Need to interact with environment to develop visual abilities

Note 1: For normal development, must have exposure to environment during critical period Cannot recover ability later. Note 2: This can happen to humans, too -> e.g., astigmatism - focusing of astigmatic lens is imperfect -> some orientations not perceived well Correction must be done in infancy - child will not recover if done later

What about higher-level activities? (e.g., object perception) Gregory — S.B. - vision in early childhood; blind afterwards - given corneal transplant in adult life Could S.B. recognize objects by sight? -> No -> Needed extensive interaction with objects before he could recognize them visually (could always recognize them by touch)

Test 2: change thevisual environment Stratton — goggles invert incoming image (i.e., flip it upside down) Can observer learn to adapt to this switch? -> Yes! -> Although it can take several days… Need to interact with environment to get correct adaptation

Note 1: Animals do not adapt so easily - amphibia, chickens never adapt - only humans and monkeys? Note 2: Limits to human adaptation - good (but not perfect) adaptation to shifts, inversions, rotations - unable to adapt to time delays • unable to adapt to remappings of colour • (e.g., photographic negative) Development of visual system involves both innate (genetic) and environmental factors

How does the visual system work?