Introducing The Eye

Inattentional Blindness

Inattentional blindness is when a person fails to recognise stimuli that is in plain sight, and obvious (seemingly unmissable) once pointed out.

Typically this happens because of the stimulus/input overload, and the inbuilt need to prioritise for ecologically relevant information. Being unaware that things are being missed leads people to believe they do not experience inattentional blindness, but there have been many experiments performed to demonstrate the phenomenon.

E.g.

Introduction to Light

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

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Luminance

Luminance is a measure of the per-area intensity of light travelling in a particular direction. The SI unit for luminance is candela per square metre (cd/m2).

Illuminance is a measure of the per-area incident of luminous flux (a measure of the perceived (adjusted based on human sensitivity to different wavelengths) power of light).

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There are several sources of illumination, and our eyes can cope with a vast range:

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Brightness

Our brain takes in illuminance, and from there computes brightness (stimuli and sensation). Our brain can be tricked into perceiving two equiluminant things as having different brightness, e.g.:

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Basic Anatomy of the Eye

The Eye is an organ that collects light and converts it to electro-chemical impulses in neurons.

The human eye allows for vision. It is composed of rod and cone cells in the retina which allow for conscious light perception including colour differentiation an depth perception.

The properties of the light source (wavelength, intensity, etc), as well as the absorbance properties of the object, affect the property of the light going into the eye.

A basic anatomy of the eye is below:

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Light's Travel

Light travels

  1. Through the cornea
  2. Through the aqueous humour
  3. Through the lens
  4. Through the vitreous humour
  5. Possibly through blood vessels
  6. Through the optic nerve (retinal ganglion cells)
  7. Through horizontal cells
  8. Through bipolar cells
  9. Through amacrine cells
  10. Through the cells bodies of photoreceptors

…before it reaches the transducing sections of the photoreceptors

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Visual Angle

The angle at which an object is seen, and focussed onto the back of the eye, can be used to specify/determine the size of the stimulus, and allow us to determine if an object is actually small, or simply far away.

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The Retina

The retina is a light-sensitive layer of tissue that lines the inside of the eye, and is where light proceeds to after the upside down image has been projected onto it - it converts the light into a variety of electro-chemical impulses and sends them to different visual centres of the brain.

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Rod cells and Cone cells are the photoreceptor cells in the retina, they're responsible for the transduction of the light information.

Rod cells perform better in dimmer light and are located around the periphery of the retina - they're responsible for peripheral vision and night vision. Cone cells have a lower absolute sensitivity than rod cells (better at brighter light), and are located in the centre of the retina (the fovea) - they're hence responsible for colour vision.

There are about 6 million cone cells, compared to 120 million rods.

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Cone Cells

There are three types of cones; Short, Medium and Long. These cones each pick up a different range of wavelengths, as indicated below.

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At moderate to bright light the eye is most sensitive to yellowish/green light (red x green), whereas at low light (where the rod cells operate), its most sensitive to blueish/green light.

Distribution

Whilst there are typically more M and L cones than S ones, the distribution varies between people, and between sections of the retina; the cones are not evenly distributed.

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(Clearly different cone distribution implies different wavelength-pickingup-abilities).

Rod Cells

Because of the lack of colour receptors (only one type of rod cell, measures luminosity), we have monochromatic night vision. The lack of rods in the fovea means we can't see dim objects by looking at them directly.

Horizontal Cells

Horizontal cells are laterally spread connector cells in the back of the receptor - they integrate and moderate input from several cones/rods.

Bipolar Cells

Bipolar cells convey responses from the photoreceptors to the retinal ganglion cells. Because both they and horizontal cells take input from the cones/rods, our spatial resolution and sensitivity to differences in intensity is increased.

Amacrine Cells

Amacrine cells regulate bipolar cells, and provide the other 70% of the input to the retinal ganglion cells.

Retinal Ganglion Cells

Retinal Ganglion Cells receive information from amacrine and bipolar cells, and transmit visual information out of the retina. Their fibres form the optic never.

Stimulus in Ganglion Cells

Retinal Ganglion Cells have receptive fields - the region of space in which the presence of a stimulus will alter the firing of that neuron.

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Effect of This

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Principles of Colour Vision

Reason for Colour Vision

Perhaps we evolved to have colour vision because of bright fruits (salient information), or perhaps the correlation goes the other way, or they're unrelated. We just don't know.

Trichromatic Theory (Young – Helmholtz, 1866)

Trichromatic Theory is the theory that our perception of colour is determined by the relative responses of each of the three cone types. It defines the way the retina allows the visual system to detect colour.

Opponent Process Theory (Herring, 1920)

Opponent Process Theory is the theory that our perception of colour is determined by the outputs of each of the three cone types, and composed of pairs of opposites - red/green, yellow/blue, black/white (light variance). It accounts for mechanisms that receive and process the information given to the visual system.

Basic Eye Movements

From Saccade; quick simultaneous movements of both eyes in a new direction, to Microsaccade, the human eye is in constant motion.

Microsaccade are a kind of fixational eye movement that occurs during long fixations. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades.

We move our eyes with “saccades” and then fixate (stare at) regions of an image.

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Blind Spots

Because of the way the light leaves the eye through the optic nerve, there is necessarily a lack of photoreceptor cells in that particular place. This leaves us with a Blindspot

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Optic Chiasma

The optic chiasma is the part of the brain where the optic nerves partially cross.

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The nasal side of the eye is the inside one near the nose, and the temporal side is the outside one, near the temporal lobes. The left side of each eye attends to the right side of the visual field, and vice versa, meaning that when the nasal side images cross over to the opposite side of the brain it is the image from the other side of the visual field that is being analysed, leaving it true that the right visual system processes the left visual field.

From the Optic Chiasma the nerves go into the Lateral Geniculate Nucleus (LGN), and then into the Visual Cortex for processing.

Visual Cortex Processing

The visual cortex is the part of the cerebral cortex responsible for processing visual information, and is located in the occipital lobe.

The term visual cortex refers to the primary visual cortex (also known as striate cortex or V1) and extrastriate visual cortical areas such as V2, V3, V4, and V5.

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Primary Visual Cortex (V1)

V1 is the sensory area located in and around the calcarine fissure in the occipital lobe.

V1 transmits information to two primary pathways - the dorsal stream and the ventral stream.
The dorsal stream is known as the where/action pathway, and goes upwards, whereas the ventral stream is the what/perception pathway, and goes downwards.

Dorsal Stream

The dorsal stream is associated with motion, the position of objects in the world, engagement with the environment and control of the eyes/arms (especially when visual information is used to guide saccades).

Ventral Steam

The ventral stream is responsible for form recognition, object representation, conscious perception of environment and is associated with LTM storage.

The two streams exchange lots of information - they're not cut off.

V1 Neurons

Neurons in V1 respond to specific features

  • colour
  • direction of movement
  • contour (form)
  • depth

They're orientation selective (it matters where the light comes from), and can be split into simple and complex cells.

Simple Cells

  • Simple cells have distinct excitatory/inhibitory regions within the receptor fields, meaning that there is a very distinct section for light sensitivity
  • Stimulus covering larger area inside the excitatory region evokes larger response than a stimulus covering a smaller area in the region
  • There is antagonism between the excitatory and inhibitory regions (stimuli covering both regions are ineffective in evoking a response as the excitatory and inhibitory responses are cancelled out)

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Complex Cells

  • Complex cells have no clear division of excitatory and inhibitory regions inside their receptor fields
  • A bar with width about 1/3 - 1/2 of the width of the receptor field in the correct orientation will evoke maximal response, independent of where it is placed inside
  • Stimulus with uniform intensity covering the entire RF will evoke no response

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Other Visual Areas

We don't know much about exactly what the other visual cortexes do, but after V1 the dorsal and ventral streams carry the information to the other areas for specialisation.