Visual Perception Flashcards
(38 cards)
What happens after the retina - Parvocellular (P) Pathway
- Sensitive to colour and fine detail
- Most input comes from cones
What happens after the retina - Magnocellular (M) Pathway
- Most sensitive to motion
- Most input comes from rods
Pathway from eye to the brain
Retina –> Optic nerve –> Optic chiasm –>Lateral Geniculate Nucleus (LGN) –>Cortical area
- Left visual cortex comes from the
left sides of the two retinas, and signals reaching the right
visual cortex comes from the right sides of the two retinas
Property of Visual Neurons - Receptive fields
The region of the sensory space (i.e. retina) within which light will cause the neuron to fire.
Property of Visual Neurons - Retinotopy
Things that are near to each
other in space are processed
in cells that are physically
near to one another
Property of Visual Neurons - Lateral inhibition
A reduction of activity
(inhibition) in one
neuron that is caused by
a neighbouring neuron
* Useful for enhancing
contrast at edges of
objects
1st Stop - Lateral Geniculate Nucleus
Part of the thalamus – a subcortical relay for most of the brain’s sensory input and motor output
- Cells have a centre-surround receptive field
- Responds to differences in light across their receptive field (e.g. light in centre, dark in surround)
- Maintains a retinotopic map
- Correlates signals from the retina in space and time
- Provides the early 3D representation of space for action
Primary Visual Cortex (V1)
Extracts basic information from the visual scene (e.g. edges, orientations, wavelength of light)
- Sends this information for later stages of processing of shape, colour, movement, etc.
- Maintains retinotopy
Single-cell recordings by Hubel and Wiesel (1979)
- Indicate that some cells respond to simple features (e.g. points of light)
- Others combine those features into more complex ones (e.g. adjacent points of light may
combine into a line)
What is the result of damage to V1?
- Leads to cortical blindness - patient cannot consciously report
objects presented in region of
space - Patient can still make visual discriminations in ‘blind’ area - Blindsight (Weiskrantz, 1974)
- This is due to other routes between eye and brain
- Geniculostraite route may be specialised for conscious vision + others act unconsciously
Blindsight
Filling-in of ‘blind’ regions similar to filling-in of normal blind spot
Functional Specialisation Theory (Zeki, 1992-93)
Different parts of the visual cortex are specialised for different visual
functions
- V1 + V2: Early stage of visual perception like shapes
- V3 + V3a: Responsive to form (especially of moving objects)
- V4: Responsive to colour
- V5/MT: Responsive to visual motion
Central Assumption of FST
Colour, form, and motion are
processed in anatomically
separate parts of the visual
cortex
Brain Imaging (PET) of Human V4 and V5 - Zeki et al, 1991
V4 more active for coloured than
greyscale images –>specialised for
colour
* V5 more active for moving dots compared with static dots –>
specialised for motion
V4: The colour centre of the brain + Cortical achromatopsia
Patients with cortical
achromatopsia can’t see
colours because of damage to V4,
but often also due to damage to
V2 and V3 (despite a fully functioning retina)
* Case studies indicate intact implicit colour processing in patients with achromatopsia.
* Conclusion: V4 is involved in colour processing but the link between colour processing and V4 is not perfect
V5/MT: The motion centre of the brain + case study of LM
- Damage to V5/MT leads to akinetopsia
Patient LM:
- Bilateral damage to V5/MT
- Was good at locating stationary
objects
- Had good colour vision
- Motion perception was grossly
deficient
A Challenge for Functional Specialization– The Binding Problem
- Sighted people don’t perceive colour of things separately so how are different features bound to create coherent object processing?
Possible solution: Coherent perception depends on synchronised neural activity between brain regions, which my depend on attention
Beyond visual cortex: Parietal processing pathway
Also known as dorsal
Concerned with movement processing - vision for action
Beyond Visual Cortex - Temporal processing pathway
Also known as ventral
- Concerned with colour + form processing
Case study of DF - Vision for perception vs action
-Had a lesion on lateral occipital cortex - trouble locating and identifying objects
- But, conscious perception was different from information available to her motor system - had where but not what
Model of Object Recognition - Step by Step
- Early visual processing (colour,
motion, edges etc.) - Perceptual segregation: grouping of visual elements (Gestalt principles, figure–ground segmentation)
- Matching grouped visual description onto a representation of the object stored in the brain (called structural descriptions)
- Attaching meaning to object based on prior semantic knowledge
Perceptual Segregation
- Separating visual input into individual objects
- Thought to occur before object
recognition
Gestalt Psychology
Fundamental principle: the “Law of
Prägnanz”:
- “Of several geometrically possible
organisations, that one will actually
occur which possesses the best,
simplest, and most stable shape”
(Koffka, 1935, p. 138)
– Assumes a set of rules that operate early in visual processing
Gestalt Psychology: Problems
- Segmentation processes aren’t always bottom-up and following the laws of perceptual organisation
- Most evidence is only descriptive not explanatory
- Relies on introspection + evidence from 2D drawings
- Some segmentation clearly occurs via top-down prior knowledge
