VISION

Question 1

what is the difference between sensation and perception?

Answer 1

• sensation: the effects of a stimulus on the sensory organ e.g. eyes, nose, mouth etc
• perception: the elaboration and interpretation of a sensory stimulus
• in vision sensation is light that comes to eyes & perception is our interpretation of this

Question 2

How does the brain construct our perception of the world through visual experiences?

Answer 2

• vision is an inferential constructive process> you don’t see what’s there, you see what’s most likely there based on neural state & assumptions built into visual system

Question 3

what happens in our visual pathway

Answer 3

• light enters to eyes > image formed in retina > retina has photoreceptors that capture light energy> transform to neural activity
• Neural signals generated by photoreceptors > sent to the optic nerve.
• those signals get transmitted down visual pathway> first to thalamus then to occipital lobe = process & interpret info
• the brain determines what is where in the world

Question 4

what is the primary visual cortex?

Answer 4

• primary visual cortex: basic visual features detected & initial processing of visual info occurs > gateway for flow of visual signals through visual processing hierarchy in brain.
• fundamental aspects of visual stimuli e.g > edges, colours, & orientations, are processed > more complex visual processing occurs in higher visual areas.

Question 5

what is the structure of the eye?

Answer 5

• lens: gathers light from world & projects 2d image on retina
• retina: contains receptors that react to light> transmit signals to rest of brain
• optic nerve: where all axons from ganglion cells come & exit eyes > no photoreceptors= blind spot

Question 6

what is the retina?

Answer 6

• contains serval layers of neurones & synapses> has photoreceptors (cones & rods) that react to light= transmit signals to other neurones
• after processing> ganglion cells > send signals to brain through optic nerve
• photoreceptor form synapses w/ bipolar cell neurones > form synapse w/ ganglion cell = signals to brain through optic nerve

Question 7

what are photoreceptors?

Answer 7

• specialised cells found in retina > sensitive to light
• cones: daylight vision, in fovea (centre of vision), good for high resolution vision & colour
• rods: nighttime, not fovea (everywhere else except the blind spot), low res, no colour (BW), low light conditions, more in peripheral regions of retina

Question 8

what is transduction? (what happens in retina)

Answer 8

• transduction in retina = conversion of light energy into electrical signals, which are then transmitted to the brain for further processing and interpretation
• When light strikes photoreceptor cells, > causes a chemical change in the pigments = a change in the cell’s membrane potential > generates electrical signals in the form of nerve impulses
• electrical signals are then transmitted to bipolar cells > & eventually to ganglion cells
• Ganglion cells are the output neurons of the retina = further transmitted through the optic nerve & thalamus > then brain

Question 9

what happens in the optic tract?

Answer 9

• The optic tract = bundle of nerve fibers that continues from the optic nerve > the optic nerve splits into two optic tracts, one from each eye
• These tracts carry visual info to diff parts of the brain > thalamus and other visual processing areas

Question 10

what are receptive fields?

Answer 10

• specific sensory areas activating neurons > Found in all sensory neurons
• Represent regions in sensory space.
• Example: In vision, neurons in the visual cortex respond to patches of the retina.
• Crucial for processing sensory information
• in ganglion cells & LGN: small & respond to presence or absence of light in visual field
• in primary visual cortex: small but more complex> respond to orientation of lines

Question 11

how are receptive fields identified/mapped?

Answer 11

• David Hubel & Torsten Wiesel
• take neuron in brain >stimulate receptor > i.e show light in diff part of visual field = see if there were action potential > high firing rate = that neuron was active

Question 12

what is meant by vision is hierarchical?

Answer 12

• brain starts by processing most simple features & works its way up as processing continues
• progressive increase in receptive field size at higher cortical lvl & receptive feed represents aspects of visual field that are progressively more complex
• visual hierarchy > flow of info from basic visual features to more complex & abstract representations

Question 13

what is meant by vision is modular?

Answer 13

• functional specialisation of areas of cortex> specific areas of brain deal w/ specific information (e.g. colour, movement)
• specialised modules > arranged hierarchically

Question 14

what is associate cortex?

Answer 14

• cortical areas involved in higher-level processing & interpretation of visual info
• processes complex visual info > contributing to tasks e.g. object recognition, contextual understanding, & spatial awareness.
• While primary visual cortex (located in the occipital lobe) processes basic visual features e.g. edges, colors, & orientations > association cortex takes on more complex tasks related to visual perception & cognition.

Question 15

how do we know about vision beyond visual cortex?

Answer 15

• single cell recordings are much rarer > recordings in animals also rarer cos can’t report what they see
• we know from patient studies, fMRI, neuropsychological cases etc

Question 16

what is v4?

Answer 16

• V4 (Striate Cortex): Specialised for colour processing & form perception > Posterior part of the occipital lobe along ventral pathway.
• we now from patients studies w/ damage to v4 = Achromotopsia > inability to see colour after cortical damage
• colour is also processed in other parts of visual pathway > (inc retina, v2, LGN..)

Question 17

what is v5?

Answer 17

• V5 (Middle Temporal Area or MT): Specialised for motion processing & visual motion perception > Middle temporal gyrus, extending into superior temporal sulcus.
• Akinetopsia: motion blindness> inability to see objects in motion after corticall called damage= damage in v5

Question 18

what ventral pathways?

Answer 18

• further specialised processing in dorsal & ventral pathways
• ventral = (What Pathway)
• Function: Processes info for object recognition & identification
• Characteristics: Analyses shape, colour, & texture > perception of “what” aspects of visual info
• recognition of objects, faces, and scenes
• Location: Extends from primary visual cortex (v1) to inferotemporal cortex.

Question 19

what are Dorsal pathways?

Answer 19

• Function: Processes spatial information & guides actions.
• Characteristics: Analyses motion, depth, & spatial relationships.
• Location: Extends from primary visual cortex to posterior parietal cortex.
• uses info abt size, shape, location of an object to perform an action e.g. grasp
• where/how pathway

Question 20

what happens when there is damage to ventral visual pathway?

Answer 20

• visual agnosia = disorder of visual object recognition
• person can ‘see’ - perceive colour, movement + understand there’s object infront of them but don’t know WHAT the object is
• no problems with words > can say words or w/ memory can identify object from verbal description

Question 21

what is visual agnosia>

Answer 21

• disorder of visual object recognition
• Lissauer (1890) divided visual agnosia into 2 types:
  
  • Apperceptive (form) agnosia: inability to perceive shapes & objects> shape discrimination > i.e. recognising, matching, copying (can’t copy shapes) visual stimuli
  • Associative agnosia: perception is ok but can’t associate w/ meaning of object < can copy shapes etc but don’t know what it is

Question 22

what happened to patient DF?

Answer 22

• suffered brain damage > ventral pathway> Apperceptive (form) agnosia: inability to perceive shapes & objects
• Severe perceptual & object recognition problems: e.g. called a cup an ashtray; a fork a knife; couldn’t identify any drawings of common objects
• walked normal despite visual agnosia > was able to do appropriate grasp for tools e.g. ask for knife or cup was able to grasp
• done posting experiment: letter in hand & slot to. ‘ post letter’ > 2 conditions perception & action =. perception condition> couldn’t tell orientation of slot but in action condition was posting letter correctly = dorsal system working fine even though ventral damaged

Question 23

what happens when there is damage to dorsal pathway?

Answer 23

• optic ataxia: difficulty reaching & pointing > unable to scale grip size to match object size
• unable to grasp object at appropriate places
• no deficit in object recognition or in action pantomime (mimic actions)

Question 24

how do we process faces?

Answer 24

• Face Detection: brain is sensitive to facial features from an early age. e.g. Newborns preferentially look at faces & show a preference for face-like stimuli.
• specific brain areas, inc > fusiform face area (FFA) & occipital face area (OFA) > involved in early face detection.
• Face Perception: brain processes facial components, such as eyes, nose, and mouth, holistically rather than as individual parts.
• easier to recognise people we know in lots of diff distortions e.g. blurred pics etc
• fusiform face area found to have more activity in recognition of faces than objects

Question 25

what happens when we lose inability to recognise faces?

Answer 25

• prosopagnosia: inability to recognise faces
• can describe faces but not recognise ppl the knew before
• nothing wrong with their intellect or vision
• acquired prosopagnosia: after brain damage > lose ability to recognise faces

Question 26

what is the optic chiasm?

Answer 26

• Acts as a crossing point for some nerve fibers, ensuring information from both eyes is processed together in the brain
• fibers from each eye cross to the opposite side > image from left visual field is directed to right visual cortex