Lectures 4 & 5: Vision

Question 1

Light enters the Eye and reaches the Retina

What is light?

Answer 1

• Light can be thought of as discrete particles of energy (photons) or as waves of energy
• Light is defined as waves of electromagnetic energy that are between 380 and 760 nanometres
• Light wavelength has role in perception of colour, intensity has role in perception of brightness

Question 2

Light enters the Eye and reaches the Retina:

The pupil and lens

Answer 2

• Amount of light reaching retinas regulated by contractile tissue – the irises (give eyes their colour)
• Light enters eye though pupil (hole in iris)
• Adjustment of pupil size in response to changes in illumination represents compromise between sensitivity (ability to detect presence of dimly lit objects) and acuity (ability to see the details of objects)
• When illumination is high, visual system constricts pupils, image falling on each retina is sharper creating greater depth of focus
• When illumination is too low to adequately activate the receptors, pupils dilate to let in more light thereby sacrificing acuity and depth of focus
• Lens focuses incoming light on retina

Question 3

Light enters the Eye and reaches the Retina:

Accommodation:

Answer 3

• Process of adjusting configuration of lenses to bring images into focus on retina
• Ciliary muscles adjust tension of ligaments holding lens in place, when gaze is directed lens assumes natural cylindrical shape, increases ability of lens to refract (bend) light and bring close objects into sharp focus
• Lens is flattened when focusing on distant object

Question 4

Light enters the Eye and reaches the Retina:

Eye Position and Binocular Disparity:

Answer 4

• Eyes in vertebrates come in pairs (left and right), see in almost every direction without moving heads except humans have eyes on the front of their heads
• Human arrangement is important, what is in front can be viewed simultaneously allowing visual system to create 3D perceptions to see depth from 2D retinal images
• Movements of eyes are co-ordinated so each point in visual world is projected to corresponding points on both retinas, eyes must converge (turn slightly inward) to achieve this
• Convergence is greatest when you are inspecting things that are close
• Positions of images on retinas can never correspond exactly as eyes do not view world from exactly same position
• -> Binocular disparity is the difference in the position of the same image on the two retinas
• Is greater for close objects than distant therefore visual system can use degree of binocular disparity to construct one 3D perception from 2D retinal images

Question 5

The Retina and Translation of Light into Neural Signals:

Answer 5

Light –> Pupil –> Lens –> Retina

• Retina converts light into neural signals, conducts them towards CNS, participates in processing of signal
• Retina composed of 5 layers of different types of neurons: Receptors, Horizontal Cells, Bipolar Cells, Amacrine Cells and Retinal Ganglion Cells
• Retinal neurons communicate both chemically via synapses and electrically via gap junctions
• Amacrine and horizontal cells are specialised for lateral communication (communication across the major channels of sensory input)
• Light reaches receptor layer only after passing through other 4 layers
• Once receptors activated, neural message transmitted back out through retinal layers to retinal ganglion cells
• Retinal ganglion cell axons project across inside of retina before gathering together in a bundle exiting eyeball

Inside out arrangement creates 2 visual problems:
• Incoming light is distorted by retinal tissue through which it must pass before reaching receptors, minimised by fovea – indentation 0/33cm at centre of retina specialised for high acuity vision (seeing fine details), thinning of retinal ganglion cell layer at fovea reduces distortion of incoming light
• For bundle to leave eye there must be a gap in receptor layer –> gap is Blind Spot, problem solved by completion (or filling in)

Question 6

Light enters the Eye and reaches the Retina:

Completion (Filling in):

Answer 6

• Visual system uses information provided by receptors around blind spot to fill in gaps in retinal images
• When visual system detects straight bar going into one side of blind spot and another leaving the other side, visual system fills in missing bit so what is seen is a continuous bar
• Not only form of completion, when looking at object visual system extracts key info about its edges and location, conducts information to cortex, perception of entire object created from that partial information
• Surface Interpolation: The process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large surfaces

Question 7

Light enters the Eye and reaches the Retina:

Cone and Rod Vision – Receptors

Answer 7

Cones: Cone shaped receptors
Rods: Rod shaped receptors

• Species active only in day tend to have cone-only retinas and species only active at night ted to have rod only retinas

• Duplexity Theory:
Theory that cones and rods mediate different kinds of vision
- Phototopic vision (cone-mediated vision) predominates in goof lighting and provides high acuity (finely detailed) coloured perceptions of the world, in dim lighting there is not enough light to reliably excite the cones, more sensitive Scotopic vision (rod mediated vision) predominates, Scotopic vision however lacks both detail and colour of Phototopic vision
–> Differences in types of vision result from a difference in the way the two systems are wired, in the Scotopic system output of several hundred rods converge on a single retinal ganglion cell whereas in the Photopic system only a few cones converge on each retinal ganglion cell to receive input from only a few cones, the effects of dim light simultaneously stimulating many rods can summate (add) to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converge whereas the effects of the same dim light applied to a sheet of cones cannot summate to the same degree and retinal ganglion cells may not respond at all to the light

• Convergent scotopic system pays for high degree of sensitivity with low level acuity, no way of knowing which portion of rods contributed to change
• More intense light is required to change firing of a retinal ganglion cell that receives signals from cones, when such a cell does react there is less ambiguity about the location of the stimulus that triggered the reaction
• There are no rods at all in the fovea, only cones, at boundaries of foveal indentation the proportion of cones decline and the density of rods reaches a maximum (there are more rods in the nasal hemiretina, the half of each retina next to the nose than in the temporal hemiretina, the half next to the temples)

Question 8

Light enters the Eye and reaches the Retina:

Spectral Sensitivity:

Answer 8

• More intense lights appear brighter however wavelength has substantial effect on perception of brightness
• Visual systems are not equally sensitive to all wavelengths, lights of the same intensity but of different wavelengths can differ markedly in brightness
• Graph of relative brightness = spectral sensitivity curve
• Cone = Photopic spectral sensitivity curve, can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the fovea
• Rod = Scotopic spectral sensitivity curve, determined by asking subjects to judge relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity too low to activate the few peripheral cones that are located there
• Purkinje Effect: Tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the colour spectrum at low illumination levels as part of dark adaptation

Question 9

Light enters the Eye and reaches the Retina:

Eye Movement:

Answer 9

• Eyes continually scan visual field, visual perception at any instant is a summation of recent visual information, it is because of this temporal integration that the world does not vanish momentarily each time we blink
• Even when we fix our gaze on an object our eyes constantly move
• Involuntary fixational eye movements are of 3 kinds: Tremor, drifts and saccades (small jerky movements or flicks)
• Normally unaware of these, have critical visual function, must fix our gaze to perceive minute details, if we were to fixate perfectly world would fade/disappear, visual neurons respond to change, if retinal images are artificially stabilised (kept from moving on the retina) the images start to disappear and reappear
• -> Fixational eye movements enable us to see during fixation by keeping the images moving on retina

Question 10

Light enters the Eye and reaches the Retina:

Visual Transduction: The conversion of Light to Neural Signals:

Answer 10

• Transduction is the conversion of one form of energy to another
• Visual transduction is the conversion of light to neural signals by the visual receptors
• Red pigment extracted from rod (rhodopsin), exposed to continuous intense light, became bleached, lost ability to absorb light, when it was returned to dark it regained its redness and light absorbing ability, now clear that rhodopsin’s absorption of light is first step in rod-mediated vision
• Degree to which rhodopsin absorbs lights of different wavelengths is related to the ability of humans and other animal’s rods to detect the presence of different wavelengths of light under scotopic conditions
• Rhodopsin is a G-protein coupled receptor that responds to light rather than to neurotransmitter modules, initiate a cascade of intracellular chemical events when they are activated
• When rods are in darkness their sodium channels are partially open, this keeps the rods slightly depolarised allowing a steady flow of excitatory glutamate neurotransmitter molecules to emerge from them. However, when the rhodopsin receptors are bleached by light the resulting cascade of intracellular chemical events closes the sodium channels, hyperpolarises the rods and reduces the release of glutamate
• The transduction of light by rods exemplifies that signals are often transmitted through neural systems by inhibition

Question 11

From Retina to Primary Visual Cortex

Answer 11

• Largest studied pathways are the retina-geniculate-striate pathways which conduct signals from each retina to the primary visual cortex or striate cortex, via the lateral geniculate nuclei of the thalamus
• 90% of axons of retinal ganglion cells become part of the retina-geniculate-striate pathways
• All signals from left visual field reach the right primary visual cortex, either ipsilaterally from the temporal hemiretina of the right eye or contralaterally via the optic chiasm from the nasal hemiretina of the left eye and that the opposite is true of all signals from the right visual field
• Each lateral geniculate nucleus has 6 layers and each layer of each nucleus receives input from all parts of the contralateral visual field of one eye
• -> Each lateral geniculate nucleus receives visual input from only the contralateral visual field, 3 layers receive input from one eye and 3 from the other
• ->Most of the lateral geniculate neurons that project to the primary visual cortex terminate in the lower part of the cortical layer IV producing a characteristic stripe or striation when viewed in cross section (hence striate cortex name)

Question 12

From Retina to Primary Visual Cortex:

Retinotopic Organisation:

Answer 12

• The retina-geniculate-striate system is Retinopic: each level of the system is organised like a map of the retina
• 2 stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system
• Retinopic layout of the primary visual cortex has a disproportionate representation of the fovea although the fovea is only small part of the retina, large proportion (25%) is dedicated to analysis of input

Question 13

From Retina to Primary Visual Cortex:

The M and P Channels:

Answer 13

• 2 parallel channels of communication flow through each lateral geniculate nucleus
• One channel runs through the top 4 layers (Parvocellular layers) /P layers), are composed of neurons with small cell bodies, are particularly responsive to colour, fine pattern details and stationary or slowly moving objects, cones provide majority of input to P layers
• Other channel runs through bottom 2 layers (Magnocellular layers/M layers), are composed of neurons with large cell bodies, are particularly responsive to movement, rods provide majority of input into M layers
• -> Both neurons project to different sites in the lower part of the layer IV of the striate cortex, in turn the M and P portions of lower layer IV project to different parts of visual cortex

Question 14

SUMMARY TASK:

Answer 14

• Neural signals are carried from the retina to the lateral geniculate nuclei by the axons of retinal ganglion cells
• The axons of retinal ganglion cells leave the eyeball at the blind spot
• The area of the retina that mediates high-acuity vision is the fovea
• Cones are the receptors of the Photopic system, which functions only in good lighting
• The retinal ganglion cells from the nasal hemiretina decussate (cross over to the other side of the brain) via the optic chiasm
• The photo pigment of rods is rhodopsin
• The most important organisational principle of the retina-geniculate-striate system is that it is laid out retonotopically
• Rhodopsin was implicated in scotopic vision by the fit between the absorption spectrum of rhodopsin and the scotopic spectral sensitivity curve
• The high degree of convergence characteristic of scotopic system increases its sensitivity but decreases its acuity

Question 15

Seeing Edges

Answer 15

A visual edge is merely the place where 2 different areas of a visual image meet, the perception of an edge is really the perception of a contrast between 2 adjacent areas of the visual field

Question 16

Seeing Edges:

Lateral Inhibition and Contrast Enhancement:

Answer 16

Adjacent to each edge, the brighter stripe looks brighter than it really is

• The non-existent stripes of brightness and darkness running adjacent to the edges are Mach bands, they enhance the contrast at each edge and make the edge easier to see (contrast enhancement)
• Every edge we look at is highlighted for us by contrast enhancing mechanisms, our perception of edges is better than the real thing (as determined by measurements of the physical properties of the light entering our eyes)
• Axons of ommatidia (very large receptors in horseshoe crab) are interconnected by a lateral neural network, if a single ommatidium is illuminated it fires at a rate that is proportional to the intensity of the light striking it, more intense lights produce more firing, when a receptor fires it inhibits its neighbours via the lateral neural network via lateral inhibition because it spreads laterally across the array of receptors. The amount of lateral inhibition produced by a receptor is greatest when the receptor is intensely illuminated and the inhibition has its greatest effect on the receptors immediate neighbours

Question 17

Receptive Fields of Visual Neurons:

Answer 17

• Hubel and Wiesel, Nobel prize winning technique for studying single neurons in visual systems of lab animals
• Tip of microelectrode positioned near single neuron in part of visual system under investigation
• During testing, eye movements are blocked by paralysing eye muscles and images on screen in front of subject are focused sharply on retina by an adjustable lens
• Next step is to identify receptive field of neuron
• Receptive field of a visual neuron is the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron
• Final step is to record response of neuron to various stimuli within its receptive field in order to characterise the types of stimuli that most influence its activity
• Electrode is advanced slightly, process repeated for another neuron and another etc. gradually working up through higher levels of the system in an effort to understand the increasing complexity of the neural response at each level

Question 18

Receptive Fields: Neurons of the Retina-Geniculate-Striate System

Answer 18

• Studying visual system began by recording from 3 levels of retina-geniculate-striate system
• First from ganglion cells, then lateral geniculate neurons and last from the striate neurons of lower layer IV, at the terminus of the system
• Hubel & Wiesel found little change in the receptive fields as they worked through the levels

When comparing the receptive fields recorded from retinal ganglion cells, lateral geniculate nuclei and lower layer IV neurons they found:
• At each level, the receptive fields in the foveal areas of the retina were smaller than those at the periphery, this is consistent with the fact that the fovea mediates fine-grained (high acuity) vision
• All the neurons (retinal ganglion cells, lateral geniculate neurons and lower level IV neurons) had receptive fields that were circulate
• All the neurons were monocular, each neuron had a receptive field in one eye but not the other
• Many neurons at each of the 3 levels of the retina-geniculate striate system had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary

• When Hubel & Wiesel shone a white light onto various parts of the receptive fields of a neuron in the retina-geniculate-striate pathway, they discovered neurons responded with either an ‘on’ firing or ‘off’ firing depending on the location of the spot of light in the receptive field, the neuron either displayed a burst of firing when the light was turned on ‘on firing’ or it displayed an inhibition of firing when the light was turned on and a burst of firing when turned off ‘off firing’
• -> Response depends on whether neurons are on centre or off centre

On centre cells:
- Respond to lights shone in central region of their receptive fields with ‘on’ firing and to lights shone in the periphery of their receptive fields with inhibition, followed by ‘off’ firing when the light is turned off

Off centre cells:

• Display the opposite pattern, respond with inhibition and ‘off’ firing in response to lights in the centre of their receptive fields and with ‘on’ firing to lights in the periphery of their receptive fields
• -> Most effective way to influence the firing rate of an on-centre or off centre cell is to maximise the contrast between the centre and the periphery of its receptive field by illuminating either the entire centre or the entire periphery
• -> One function of the neurons in the retina-geniculate-striate stem is to respond to the degree of brightness contrast between the 2 areas of their receptive fields

Question 19

Receptive Fields: Simple Cortical Cells

Answer 19

• Striate cortex neurons (neurons of lower layer IV) have different receptive fields (are an exception)
• Consist of Simple and Complex cells
• Simple cells like lower IV neurons have receptive fields that can be divided into antagonistic on and off regions, are unresponsive to diffuse light and they are all monocular (respond to stimulation of only one of the eyes), difference is that the borders between the on and off regions of the cortical receptive fields are straight lines rather than circles
• Complex cells are more numerous than simple cells, have rectangular receptive fields (like simple cells), are unresponsive to diffuse light, differ to simple as they have larger receptive fields, it is not possible to divide the receptive fields of complex cells into static on/off regions and are binocular (respond to stimulation of either eye)
• Most binocular cells in primary visual cortex of monkeys display some degree of ocular dominance, respond more robustly to stimulation of one eye than they do the same stimulation of the other, some also fire best when the preferred stimulus is presented to both eyes at the same time but in slightly different positions

Question 20

Columnar Organisation of Primary Visual Cortex:

Answer 20

• Characteristics of receptive fields of visual cortex neurons are attributable to flow of signals from neurons with simpler receptive fields to those with more complex fields, signals flow from on centre and off centre cells in lower layer IV to simple cells and from simple cells to complex cells
• Primary visual cortex neurons are grouped in functional vertical columns (vertical means at right angles to the cortical layers)
• If electrode is advanced vertically with stops to plot receptive fields of many neurons along the way, results show each cell in column has receptive field in same area of visual field
• All cells in a column respond best to straight lines in the same orientation, neurons in a column that are monocular or binocular with ocular dominance are all most sensitive to light in the same eye, left or right
• If electrode advanced horizontally through tissue of primary visual cortex, each successive cell encountered likely to have receptive field in slightly different location and to be maximally responsive to straight lines of a slightly different orientation
• During horizontal electrode pass, tip passes alternately through areas of left eye dominance and right eye dominance (ocular dominance columns)
• -> All functional columns of primary visual cortex that analyse input from one area of retina are clustered together, half a cluster is thought to receive input from left eye and other half from right, each cluster also thought to include neurons with preferences for straight-line stimuli of various orientations

Question 21

Plasticity of Receptive Fields of Neurons in the Visual Cortex:

Answer 21

Plasticity/ability to adapt to change, which has largely been ignored, appears to be a fundamental property of visual cortex function

• Means that research based solely on study of reaction to simple stimuli cannot provide a complete explanation of how the visual system works

Question 22

Component and Opponent Processing:

Answer 22

Component theory:
(trichromatic theory) of colour vision states there are 3 different kinds of colour receptors (cones) each with a different spectral sensitivity and colour of a particular stimulus is presumed to be encoded by the ratio of activity in the 3 kinds of receptors
- Any colour of visible spectrum can be matched by a mixing together of 3 different wavelengths of light in different proportions
- Can be accomplished with any 3 wavelengths provided that the colour of any one of them cannot be matched by the mixing of the other two

Opponent Process theory:

• 2 different classes of cells in visual system for encoding colour and another class of encoding brightness
• Each class of colour coding cells signalled red by changing its activity in one direction (e.g. hyperpolarisation) and signalled reds complementary colour (green) by changing its activity in the other direction
• Hypothesised same for blue and yellow
• Class of brightness-coding cells hypothesised to similarly signal both black and white
• Complementary colours are pairs of colours that produce white or gray when combined in equal measure
• Cannot exist together no such thing as bluish yellow or reddish green –> support
• Further support comes from afterimage produced by starring at yellow see blue after, by starring at red see green after
• ->Technique for measuring absorption of single cone found that there are indeed 3 different kinds of cones in the retinas of those vertebrates with good colour vision, each has a different photo pigment with its own characteristic absorption spectrum
• In retina-geniculate striate system there are cells that respond in one direction e.g. increased firing to one colour and in the opposite direction e.g. decreased firing to its complimentary colour
• Most primates are trichromats (possess 3 colour vision photo pigments) most other are dichromats (possess 2), some birds, fish, reptiles have 4, fourth allows detection of UV light

Question 23

SUMMARY TASK:

Answer 23

Contrast enhancement – Mach bands

Simple cortical cells – static on and off areas

Complex cortical cells – many are binocular

Ocular dominance – striate cortex

Component – three

Opponent – complementary

Retinex – reflectance

Cytochrome oxidase- blobs

Horseshoe crab – ommatidia

Question 24

Colour Constancy and Retinex Theory:

Answer 24

Colour constancy refers to: the fact that the perceived colour of an object is not a simple function of the wavelengths reflected by it

• Light sources differ markedly in the wavelengths they contain, colour does not
• Colour constancy is the tendency for an object to stay the same colour despite major changes in the wavelengths of light that it reflects
• Colour constancy improves our ability to tell objects apart in a memorable way so that we can respond appropriately to them, our ability to recognise objects would be greatly lessened if their colour changed every time there was a change in illumination
• Normally unaware of colour constancy, have no way of appreciating just how much the wavelengths reflected by an object can change without the object changing its colour
• Land’s Study used 3 adjustable projectors, each emitted one wavelength of light (short, medium, long), test displays are Mondrian’s because they resemble the paintings of Dutch artist Piet Mondrian, found that adjusting the amount of light emitted from each projector and thus the amount of light each wavelength being reflected by the Mondrian had no effect at all on the perception of its colours
• Land demonstrated that blue objects stay blue, green stay green etc. regardless of the wavelengths they reflect
• Colour constancy occurs as long as the object is illuminated with light that contains some short, medium and long wavelengths (daylight, firelight and virtually all manufactured lighting) and as long as the object is viewed as part of a scene, not in isolation
• Land’s Retinex Theory: Colour of an object is determined by its reflectance – the proportion of light of different wavelengths that a surface reflects
• Although the wavelength of light reflected by a surface changes in illumination, the efficiency with which a surface absorbs each wavelength and reflects the unabsorbed portion does not change
• Suggests one type of cortical neuron that is likely to be involved in colour vision
• If the perception of colour depends on the analysis of contrast between adjacent areas of the visual field then the critical neurons should be responsive to colour contrast
• Dual-opponent colour cells in monkey visual cortex respond with vigorous ‘on’ firing when the centre of their circular receptive field is illuminated with one wavelength e.g. green and the periphery is simultaneously illuminated with another wavelength e.g. red, same cells display vigorous ‘off’ firing when pattern of illumination is reversed
• Dual-opponent colour cells are not evenly distributed through the primary visual cortex of monkey’s; neurons are concentrated in the primary visual cortex in peglike columns that penetrate the layers of the monkey primary visual cortex with the exception of lower layer IV
• Many of these neurons are rich in mitochondrial enzyme cytochrome oxidase, distribution in primary visual cortex can be visualised if one stains slices of tissue with stains that have an affinity for this enzyme
• When a section of monkey striate tissue is cut parallel to the cortical layers and stained, pegs are seen as ‘blobs’ of stain scattered across cortex (accepted scientific label for peglike, cytochrome oxidase-rich dual opponent colour columns)
• Located in midst of ocular dominance columns
• fMRI studies have provided evidence of dual-opponent colour cells in human visual cortex

Question 25

Cortical Mechanisms of Vision and Conscious Awareness:

Answer 25

Visual cortex is considered to be of 3 different types…

• Primary visual cortex – area of cortex that receives most of its input from the visual nuclei of the thalamus i.e. from the lateral geniculate nuclei
- Located in the posterior region of the occipital lobes, hidden from view in longitudinal fissure

• Secondary visual cortex – Areas that receive most of their input from the primary visual cortex
- Most areas are located in 2 general regions: The prestriate cortex – band of tissue in occipital lobe that surrounds primary visual cortex and The Inferotemporal cortex of the inferior temporal lobe

• Visual association cortex – Areas that receive input from areas of secondary visual cortex as well as form the secondary areas of other sensory systems

• Areas of association cortex that receive visual input are located in several parts of the cerebral cortex but the largest single area is the posterior parietal cortex
• -> Major flow of visual information in the cortex is from the primary visual cortex to the various areas of secondary visual cortex to the areas of association cortex
• ->Neurons have larger receptive fields to the specific and more complex stimuli they respond to the further up the visual hierarchy you go

Question 26

Damage to Primary Visual Cortex – Scotomas and Completion:

Answer 26

• Damage to an area of the primary visual cortex produces a Scotoma
• Scotoma is an area of blindness in the corresponding area of the contralateral visual field of both eyes
• Those with suspected damage to primary visual cortex are given Perimetry Test, patients head held motionless on chin rest, stares with one eye at fixation point on screen, small dot of light flashed on various parts of screen, patient presses button to record when dot is seen, result is map of visual field of each eye which indicates any areas of blindness
• -> Many patients are not consciously aware of own extensive scotoma deficits due to completion, completion may depend on residual visual capacities in scotoma, patients who are hemianopsic (have scotoma covering half of visual field) may see an entire face when they focus on a person’s nose even when the side of the face in the scotoma has been covered by a blank card

Question 27

Damage to Primary Visual Cortex – Scotomas, Blindsight and Conscious Awareness:

Answer 27

• In human’s conscious awareness is inferred from the ability to verbally describe the object of awareness
• Blindsight: Displayed by patients with scotomas resulting from damage to primary visual cortex, ability to respond to visual stimuli in their scotomas even though they have no conscious awareness of the stimuli
• Of all visual abilities, perception of motion is most likely to survive damage to primary visual cortex, subject might reach out and grab moving object from scotoma whilst claiming not to see it
• Interpretations of Blindsight: Striate cortex is not completely destroyed, remaining islands of functional cells are capable of mediating some visual abilities in the absence of conscious awareness, other is that visual pathways ascending directly to secondary visual cortex from subcortical visual structures without passing through primary visual cortex are capable of maintaining some visual abilities in the absence of cognitive awareness

Question 28

Functional Areas of Secondary and Association Visual Cortex:

Answer 28

• Secondary visual cortex and portions of association cortex that are involved in visual analysis are both composed of different areas each specialised for a particular type of visual analysis
• Neurons in each functional area respond most to different aspects of visual stimuli e.g. to their colour, movement or shape, selective lesions to the different areas produce different visual losses, there are anatomical differences among the areas and each are laid out retinotypically
• Major flow of interconnecting signals re from simple to more complex areas
• PET, fMRI and evoked potentials have been used to identify areas of visual cortex in humans, activity monitored whilst they inspect various types of visual stimuli, 12 different functional areas of human visual cortex identified

Question 29

Dorsal and Ventral Streams:

Dorsal - where

Ventral - what

Answer 29

Many pathways that conduct information from the primary visual cortex through carious specialised areas of secondary and association cortex are parts of 2 major streams

• Dorsal Stream:

• Flows form primary visual cortex to dorsal prestriate cortex to the posterior parietal cortex
• Most visual cortex neurons in dorsal stream respond best to spatial stimuli e.g. location of objects or direction of movement
• Involved in perception of where objects are

• Ventral Stream:

• Flows from the primary visual cortex to the ventral prestriate cortex to the inferotemporal cortex
• Most ventral neurons respond to characteristics of objects e.g. colour and shape
• Evidence suggests there are clusters of neurons on ventral stream each of which responds specifically to a particular class of objects e.g. faces, bodies, letters, animals, tools
• Involved in perception of what objects are
• Conscious awareness mediated by ventral stream is one thing that distinguishes humans and their close relatives from evolutionary ancestors

Question 30

Dorsal and Ventral Streams:

Dorsal - where

Ventral - what

continued…

Answer 30

Implication of where vs what theory:

Damage to some areas of cortex may abolish certain aspects of vision while leaving others unaffected, patients with damage to inferotemporal cortex often have no difficulty reaching accurately for objects they have difficulty describing

• -> Key difference between streams is not the kinds of information they carry but the use to which that information is put, function of dorsal stream is to direct behavioural interactions with objects whereas function of ventral stream is to mediate conscious perception of objects (control of behaviour vs conscious perception theory)  Theory suggests patients with dorsal stream damage may do poorly on tests of location and movement because such tests involve performance measures and patients with ventral stream damage may do poorly on tests of visual recognition as such tests involve verbal and thus conscious awareness
• -> Support: Some patients with bilateral lesions to ventral stream have no conscious experience of seeing yet are able to interact with objects under visual guidance AND some patients with bilateral lesions to dorsal stream can continuously see objects but cannot interact with them under visual guidance

Question 31

Prosopagnosia:

Answer 31

• Visual agnosia for faces associated with damage to an area of the ventral stream
• Agnosia is a failure of recognition (gnosis means to know) that is not attributable to a sensory deficit or to verbal or intellectual impairment
• Visual agnosia is a specific agnosia for visual stimuli, patients can see visual stimuli but don’t know what they are
• Visual agnosia’s are often specific to particular aspect of visual input e.g. movement agnosia, object agnosia and colour agnosia
• Presumed each specific visual agnosia results from damage to an area of secondary visual cortex that mediates the recognition of that particular attribute
• Prosopagnosics can usually recognise a face as a face but have problems reporting whose face it is, report seeing jumble of facial parts that are never fused or bound into an easy to recognise whole
• In extreme cases patients cannot recognise themselves
• -> Diagnosis typically applied to patients who have difficulty recognising particular faces but can readily identify other test objects e.g. chair, prosopagnosia may not be specific to faces however as although patients can recognise a chair, it is whether they can identify specifically which chair in the same way it is whether or not they can identify whose face, seems unlikely it is a unitary disorder
• -> Diagnosis associated with bilateral damage to ventral stream in the area of the boundary between the occipital and temporal lobes (fusiform face area), parts of it are selectively activated by human faces as well as other visual stimuli, extent to which the development of the fusiform face area depends on a person’s early experience with faces is still unclear
• Dorsal stream function still intact; patients can unconsciously recognise faces that they cannot recognise consciously, indicated by large skin conductance response to being presented with familiar faces (undamaged areas unconsciously perform function)

Question 32

Akinetopsia:

Answer 32

• Deficiency in the ability to see movement progress in a normal smooth fashion, associated with damage to an area of the dorsal stream
• Can be triggered by high doses of certain antidepressants, see moving object followed by a trail of multiple freeze-frame images
• Associated with damage to middle temporal (MT) area of the cortex, location of MT is near junction of the temporal, parietal and occipital lobes
• MT = V5, MT/M5 due to different systems of neuroanatomical classification, all refer to comparable areas
• Function of MT appears to be the perception of motion
• Neurons at lower levels of visual hierarchy (e.g. primary visual cortex) respond to movement as well as colour and shape however provide little information about the direction of movement because their receptive fields are so small
• In contrast 95% of neurons of MT respond to specific directions of movement and little else, each neuron has a large binocular receptive field allowing it to track movement over a wide range

–> Research implicating MT in visual perception of motion and damage to MT as cause for Akinetopsia
• Patients tend to have unilateral or bilateral damage to MT
• Activity in MT increases when humans view movement as measured by fMRI
• Blocking activity in MT with transcranial magnetic stimulation produces motion blindness
• Electrical stimulation of MT in human patients induces the visual perception of motion

Question 33

Conclusion:

Answer 33

Visual system does not transmit complete and intact visual images of world to cortex, carries information about few critical features of visual field and creates a perception, can also create perception without conscious awareness