Vision and Perception in Infants and Adults

Vision Introduction

• Humans heavily depend on vision, often prioritizing it over other senses when conflicts arise.
• Visual perception primarily occurs in the brain; the eyes serve to collect, focus, and sense light.

The Stimulus - Light

• Light, a form of electromagnetic radiation traveling in waves, is crucial for sight and eye processing.
• Light has three physical properties: wavelength, amplitude, and purity.

Wavelength

• Wavelength refers to the distance between consecutive peaks of a light wave.
• Variations in wavelength determine the color we perceive.
• Long wavelengths correspond to the red end of the spectrum (around 750 nanometers).
• Short wavelengths are associated with the violet end of the spectrum (around 360 nanometers).
• Humans can only see a small portion of light wavelengths in the visible spectrum.
• Primates have the ability to distinguish both yellow and red from green, which aids in hunting food.

Amplitude

• Amplitude signifies the height of each peak in a light wave.
• Amplitude affects how bright we see the stimulus; greater amplitude indicates higher brightness.

Purity

• Purity influences the saturation of colors.
• Pure light consists of a single wavelength, resulting in saturated colors.
• Natural light comprises multiple wavelengths, leading to desaturated colors.

Physical vs Psychological

• Wavelength corresponds to color
• Amplitude corresponds to brightness
• Purity corresponds to saturation

The Eye

• Light waves are captured onto the retina and converted into neurochemical signals.
• The series of light focusing points are as follows: curved cornea, pupil, lens, and retina
• The curved cornea is responsible for 80% of initial focusing and bends the incoming light.
• The sclera is a tough membrane that protects the eye ball.
• The pupil is the round window that lets light into the eye; its size is controlled by the iris.
• The iris has muscles that dilate the pupil in low light and constrict it in bright light.
• The lens provides the final focusing, projecting images onto the retina upside down and reversed.
• Accommodation is the process where the lens changes shape to focus on objects at varying distances
• The vitreous humor is a jelly-like substance that consist of the main chamber inside the eyeball.

The Retina

• The retina translates light into neural impulses.
• The retina contains light sensitive photoreceptors that translates the visual stimulus into neural signals the brain understands.
• Light passes through two transparent layers to reach the photoreceptors.
• The inside-out arrangement is due to photoreceptors receiving nutrients from the retinal pigment epithelium (RPE) at the back of the eye.
• There are two types of photoreceptors: rods and cones

Rods

• There are 125 million rods in the human eye
• Rods excel in low-light conditions, enabling night vision.
• Rods do not register color, resulting in decreased visual sharpness.
• Rods are concentrated in the periphery and are absent in the fovea.
• Rods are useful for peripheral vision.

Cones

• There are 6 million cones in the human eye
• Cones operate best in high light, providing day vision, color sensation, and sharp detail.
• Cones concentrate towards the fovea, a central retinal spot containing only cones.
• Looking directly at an object focuses its image on the fovea for detailed observation.

Bipolar Cells

• Bipolar cells are photoreceptors that transfer information via a transmitter substance to bipolar cells
• Bipolar cells then send information to ganglion cells

Ganglion Cells

• Information from the retina is collected to ganglion cells
• Ganglion cell axons converge at the optic disc to form the optic nerve.
• The optic disc creates a blind spot due to the absence of photoreceptors in that region.

Horizontal and Amacrine cells

• Horizontal and amacrine cells facilitate communication between areas within the retinal layers.
• Information from 130 million rods and cones converges into 1 million optic nerve axons.
• These cells enable the combination of information from nearby photoreceptors

Receptive Field

• Photoreceptors are organized into groups, and their information is assimilated into a single ganglion cell signal.
• In the fovea, a ganglion cell's photoreceptor group may contain just one cone, maintaining detail.
• Input from numerous rods and cones combined into a single neural signal for one retinal ganglion cell
• Peripheral vision has low visual acuity due to larger grouping
• A receptive field refers to the group of cones and rods that when stimulated, has an affect on the firing of a particular ganglion cell.

Vision 2 Introduction

• Visual perception comes together in the brain
• The visual system functions through assembly lines
• The right and left halves of the visual fields are processed contralaterally
• The right visual field inputs travel along the optic nerve to the left hemisphere
• The left visual field inputs travel along the optic nerve to the right hemisphere

Visual Fields and Hemispheres

• Axons from the inner regions of each retina crossover to the opposite hemisphere.
• The optic chiasm is where optic nerves from the inside of each eye crossover.

Two Visual Pathways

• Visual field information arrives in the opposite hemisphere after the optic chiasm.
• Optic nerve fibers split and travel along two pathways.
• Most ganglion cell axons synapse in the lateral geniculate nuclues (LGN)
• LGN is part of the thalamus (relay station for sensory information)
• After processing, visual signals travel to the primary visual cortex in the occipital lobe.
• A smaller portion of axon takes a detour to the superior colliculi in the midbrain.
• Secondary Pathway info from the visual input with other senses, and localizes objects in space through head and eye movement

Primary visual pathway

• Striate cortex (Area V1) is located in the occipital lobe, which is the primary visual cortex.
• Extrastriate Cortex is the visual processing area outside the striate cortex
• Extrastriate areas functions include colour, motion and object recognition

Dorsal and Ventral Streams of Extrastriate Cortex

• The dorsal steam, also called the "where pathway", processed objects locations, depth and motion.
• The ventral stream, also called the "what pathway", processes the object its self, like colour and form.

Neural Convergence

• Neurons converge their imput as they travel down the visual system
• Combinations of receptive fields combine to form single target visual information
• V1 has Topographic organization in the primary Visual Cortex

Topographic organization

• Neighbouring regions/inputs in the retina correspond to neighbouring regions in the visual cortex
• Visual information is compressed (photoreceptors -> ganglia -> LGM-> primary visual cortical cells)
• Neurons respond strongly to a specific stimului/attribute

Evolution of the Eye

• Darwin thought complexity of eye challenged ideas of natural selection, but linked it to natural selection
• eyes evolved from simple light-sensitive patches.

Light Sensitive Patch

• Eyes could've started off as something simple that detects light.
• First light sensors were used by Algae to harness energy and create action potentials

Curved Cup Eye

• A sensitive patch formed into a slight depression allowed for the sensing of direction of light.
• Improved survival advantage.

Crude Lens

• A crude lends allowed processing of visuals at different distances
• A lens can progressively improve focus and accommodation with better curvature and transparency.
• Crude Lens does not allow for visual acuity animals like fish

Adjustable Lens

• Is Flexible and allows for accommodation (adjust focus)
• Ability to adjust focus help processing at varying distances (humans)
• The complex vertebrate eye evolved gradually with new adaptations layered on top of old.
• Cumulative Selection drives eye evolution

Cumulative Selection

• Each small change was layered on top of each other
• Gradually changing eye sophistication

Environmental factors

• Environmental circumstances greatly affect a spices visual architecture
• Factors include
  • Light
  • Food
  • Movement
  • Shape and Colour of Prey
• Simple eyes (Vertebrates) VS Compound eyes (anthropods)
• Each point on the Ommatidia faces a different direction
• Compound eyes enable really good movement detections but only at close distances
• Resolution and sensitivity depends on the size of the object
  • Resolution (acuity) is sharpness/clarity
  • Sensitivity is ability to get enough light

Eye Position

• Laterally facing eyes: WIDE field of view, and poor depth of view
• Front facing eyes: NARROW field of view, increased binocular overlap, and good depth perception

Development of Visual Architecture

• Vision is the least developed at birth
• Eyes develop since conception (prenatally) and continues to develop throughout life
• Relies on visual input

Factors for Development

• Lens muscles are weak at birth
• Newborn pupils do not respond to light changes blur clarity
• Retina has lower cell desnity and continues to develop
• Visual cortex still requires a lot to mature
• Full "maturity" is complete at age 11

Measuring Infant Visual Acuity

• There are characteristic patterns of development
• Two key methods: the Preferential Looking Technique and Visual Evoked Potentials.

Preferential Looking Technique

• Babies are naturally inclined to observe sharp lights and dark patterns (such as black/white stripes) verus uniform colours like grey.
• Stripes closer together -> becomes harder to see (visual acuity isn't sharp enough).

Visual Evoked Potentials

• Measure electrical activity in a baby by differentiating two slightly different images
• The measurements help gauge when a baby is able to differentiate between various visuals

Baby Visual Acuity Stats

• Measurements have proved similair results
• Newborns at 20 ft distance compare to an adult that is 600 ft away
• By 6 months, detail at 20 ft for a baby, looks like 100 ft for an adult
• By 1 year, babies are nearing adult level-acuity, but it is still developing till the age of 4-6 years
• Brain and eye need to mature (11 yrs)

Explore visual acuity development, physiological factors affecting newborn vision, and brain processing of visual stimuli. Learn about the impact of wavelength on color perception and the dominance of visual information. Understand the development of the visual system.