Module 6: Vision and the Brain
Properties of Light
Visible Light is a narrow band of the spectrum of electromagnetic radiation that receptors in our eyes detect
Three Dimensions determine the perceived color:
Hue: dominant wavelength
Saturation: purity
Brightness: intensity
Anatomy of the Eye
Sensory information from the environment (e.g., light) is detected by specialised neurons called sensory receptors. The information detected is transformed into changes in the electrical potential of neurons (i.e., the way neurons talk to one another) via a process of sensory transduction. The changes in neuron charge generated by sensory information are called receptor potentials.
The sensory receptors for light information are called photoreceptors which are located in the retina of the eye.
Retina
The retina is located at the back of they eye and is part of the brain
Light must be focused on the retina for vision because it contains billions of sensory receptors called photoreceptors, which collect information and send it to the CNS via the optic nerve
Focusing Light on the Retina
Extraocular muscles hold our eyes in place and move them
The muscles are attached to the sclera - the white outer coating of most of the eye
The sclera is opaque and does not let the light enter
The cornea is the transparent part of the eye at the front
The amount of light hitting the retina is determined by the pupil size
The lens is behind the iris, it changes shape to help focus light on the retina via the process of accomodation
Focusing on a far point makes the lens narrower, wider when it a near point.
The Retina and Sensory Transduction
The retina has three cellular levels: photoreceptor cells, bipolar cells, and ganglion cells
Photoreceptors
Two types:
Rods (92 mil)
Most prevalent in the peripheral retina, not found in the fovea
Sensitive to low levels of light
Provide only monochromatic information
Poor acuity
Cones (4.6 mil)
Most prevalent in the central retina, found in fovea
Sensitive to moderate to high levels of light
Provide information about hue
Provide excellent acuity
The Fovea is the central region of the retina very important for visual acuity (fine spatial detail)
Cones are found in the fovea, and predominate the central retina
Rods are NOT found in the fovea and predominate the peripheral retina
Blind Spot
The blind spot occurs is because of the optic disk - the place where the axons of neurons sending information onwards to the cortex (and other places!) gather together and leave the eye
There are no photoreceptor cells at the optic disk so there is no way for light to be detected
The Layers of Cells
Photoreceptor cells form synapses with Bipolar Cells, which then form synapses with Ganglion Cells
The axons of ganglion cells converge to form the Optic Nerve
Visual Transduction and Receptive Fields in the Retina
Transduction is the process by which environmental energy is converted into a change in a neuron's membrane potential.
In the visual system, transduction occurs in photoreceptors in the retina.
Photopigments are molecules in photoreceptors made of:
A protein (opsin)
A lipid (retinal)
When exposed to light, opsin and retinal separate, triggering intracellular events that change the photoreceptor’s membrane potential.
The arrangement of rods and cones affects the receptive field of ganglion cells.
The number of photoreceptors per ganglion cell differs between central and peripheral vision:
In the peripheral retina, many photoreceptors connect to one ganglion cell → large receptive fields → low visual acuity
In the central fovea, one cone connects to one ganglion cell → small receptive fields → high visual acuity
High acuity in the fovea allows for fine visual detail, such as reading or recognizing faces.
To see detail, humans must move their eyes to bring objects into the fovea.
Example: When reading, our eyes focus on words to clearly process letters for comprehension.
Central and Peripheral Vision: Eye Movements
Because peripheral vision is blurry we want the light to project onto the fovea for detailed processing of relevant information, so we move our eyes
Vergence movements
Eyes rotating inwards and outwards depending on the distance from the object of focus
Saccade movements
Rapid jerky movements of the eyes used when we scan a scene
Pursuit movements
Steady and smooth eye movements following a moving object
The Visual Pathway
The axons of ganglion cells in the retina converge to form the optic nerve
The optic nerves from each eye meet at the base of the brain to form the optic chiasm
Information is then sent on to the lateral geniculate nucleus (LGN) of the thalamus
Then information travels to the primary visual cortex (V1, striate cortex)
Then information goes to the visual association cortex (V2, extrastriate cortex)
The ganglion cell axons from the inner halves of the retina cross the chiasm and go to the LGN on the opposite side of the brain; but axons from the outer halves of the retina go to the LGN on the respective side
Thus, each hemisphere of the brain receives information from the opposite side of the visual field (contralateral)
Lateral Geniculate Nucleus (LGN)
LGN is layered, some layers receive input from ganglion cells of the contralateral eye, some from the ipsilateral eye
Magnocellular layers: two inner layers
Relay information to the visual cortex that is important for the perception of form, movement, depth, and small differences in contrast (larger receptive fields)
Parvocellular layers: four outer layers
Relay information to the visual cortex important for colour perception and the processing of fine spatial detail (similar receptive fields)
Koniocellular sublayers: beneath other two types
Relay information to the visual cortex important for processing blue light (colour)
Striate Cortex
After processing in LGN information is passed on to the primary (striate, V1) visual cortex
V1 is the first cortical region that combines information from several sources to detect visual features bigger than the receptive fields of ganglion and LNG cells
Highly structured + has layers like LGN
The striate cortex of one hemisphere of the brain contains a map of the contralateral half of the visual field = topographic mapping.
Has 12 segments of the visual fields assigned a personal part of the cortex
Processing from the fovea utilises four of the segments (1-4)
Extrastriate Cortex
The Striate Cortex cannot perceive entire objects or scenes and only processes basic visual features. For more complex information it sends information to the Extrastriate Cortex
Extrastriate Cortex (V2–V5)
Contains specialised regions, each with a topographic map of the visual field
Neurons are tuned to specific types of visual information (e.g., V4 = colour, V5/MT = motion)
Organised hierarchically: information flows from areas closer to V1 to higher-level regions for advanced processing
Dorsal Stream ("Where" Pathway)
Processes object location, movement, speed, and direction
Supports spatial awareness and action guidance
Ventral Stream ("What" Pathway)
Processes object identity and colour
Supports object recognition and meaning
Colour Perception
Different cells in the visual system are specialised to process different types of information. This specialisation is certainly important when it comes to colour perception.
Colour Vision and the Retina
In terms of photoreceptors, the responsibility of processing colour falls onto cones, rather than rods.
The retina detects different colours because it contains three types of cones, each sensitive to different hues, which is referred to as Trichromatic Coding (blue, green, and red)
How does it work?
The different types of cones are sensitive to different wavelengths due to them having different absorption characteristics
Absorption is determined by the opsin (protein) in photopigments - specifically different opsins absorb different wavelengths
Therefore, trichromatic coding can explain many differences in colour vision
Colour Vision Differences
Protanopia: confuse red and green and see the world in shades of blue and yellow; is thought to be due to red cones, in individuals with such condition, being filled with green cone opsin
Deuteranopia: also confuse red and green; thought to occur because green cones are filled with red cone opsin
Tritanopia: see the world in greens and reds; retina lacks blue cones
Monochromatic: do not perceive hue differences; retina lacks all three cones
Colour Processing in Ganglion Cells
Ganglion cells use an opponent colour system
Ganglion cells respond to pairs of primary colours (red vs green, yellow vs blue)
Therefore, the brain has red-green ganglion cells, and yellow-blue ganglion cells
Each ganglion cell is excited by one colour in the pair and inhibited by the other. For example:
A red-green opponent cell increases firing for red (excitatory) and decreases firing for green (inhibitory).
A yellow-blue opponent cell decreases firing for blue (inhibitory)
Yellow light causes both excitatory and inhibitory signals to be sent to the red-green cell, because it activates both types of cones, which cancels out the signal.
At the same time, yellow light causes red and green cones to send excitatory messages to the yellow-blue ganglion cells, which causes it to increase firing signalising yellow.
This system helps the brain detect colour contrast and prevents us from seeing “impossible” colours like reddish-green.
Colour Processing in LGN
After retinal processing, colour information is sent to the visual cortex via the lateral geniculate nucleus (LGN) in the thalamus. Different layers of the LGN process different aspects of visual input:
The parvocellular layers receive input from red and green cones and process wavelength (colour) and fine detail.
The koniocellular layers carry information from blue cones.
The magnocellular layers are mainly involved in processing motion and brightness, not colour.
This separation allows the brain to process different features of a visual scene in parallel.
Colour Processing the Extrastriate Cortex
The extrastriate cortex plays a crucial role in colour perception. Colour information from the parvocellular and koniocellular systems is sent along the ventral pathway to the inferior temporal lobe, which is responsible for processing object identity, including colour. The ventral pathway also receives input from the magnocellular system(mainly related to motion and brightness).
In contrast, the dorsal pathway primarily processes magnocellular information and is more involved in spatial awareness and motion than in colour processing.
Interestingly, it is possible to experience a loss of colour vision following a lesion to a specific region of the extrastriate cortex, without impacting visual acuity (as the photoreceptors are unaffected). This condition is called cerebral achromatopsia, and can even cause individuals to have difficulties imagining and remembering the colours of objects they once knew (before the brain lesion).
Form Perception
Studying individuals with visual agnosia has provided a lot of insight into form perception.
Visual agnosia is characterised by inability to identify common items by sight, although visual acuity remains; it is thought to be caused by injury to parts of the extrastriate cortex contributing to the ventral stream
A region of the extrastriate cortex called the Lateral Occipital Complex (LOC) appears to respond to a wide variety of objects and shapes
There also appears to be a few regions that primarily process specific categories - specialisation (faces, bodies, scenes - image on right)
Fusiform Face Area
Within the ventral stream, there are special face recognising circuits in the fusiform face area (FFA) which is a region of the visual association cortex located in the fusiform gyrus on the base of the temporal lobe.
People can experience prosopagnosia following injury to the FFA (acquired prosopagnosia), or they can experience prosopagnosia from birth (congenital prosopagnosia).
Finally, it is worth knowing that the fusiform face area has also been called the flexible face area. This name has arisen because the FFA has been implicated in expert object recognition more generally. For example, when bird and car experts view images of birds and cars respectively, activation in the FFA changes - this is not the case for non-experts (Gauthier et al., 2000; Tarr & Gauthier, 200; Xu, 2005).
Perception of Depth and Location
Depth Perception
Monocular Depth Perception
Depth can be perceived using monocular cues (one eye).
For instance, perspective, and the relative size of objects on the retina can be used to determine where objects are located.
Binocular Depth Perception
Depth perception is also informed by binocular (two eyes) information and binocular disparity (the small difference between the image on the retinas of both eyes).
Disparity sensitive neurons are found throughout the striate and extrastriate cortex, and in both the ventral and dorsal pathways.
Those found in the dorsal stream are involved in spatial perception and respond to large, extended visual surfaces
Those found in the ventral stream are involved in object perception and respond to the contours of 3D objects
The parietal lobe is also involved in the perception of spatial location and somatosensory perception.
It receives auditory, visual, somatosensory and vestibular information, and is important for object location memory and perception, as well as for controlling eye and limb movements
The dorsal visual stream terminates in the posterior parietal cortex and is involved in processing movement and location information.
Motion Perception
Extrastriate Cortex and Motion Perception
Area V5/MT of the extrastriate contains neurons that respond to movement
Receives input directly from the striate cortex and other areas of the extrastriate,
Also receives input from superior colliculus (involved in reflexes and eye movements)