Extrastriate Cortex Flashcards
Is the image processing mediated by the cells in V1 & V2 cortex sufficient to identify this object as an apple?
No! The receptive fields of cells in the hypercolumns of these two ‘early’ cortical areas only analyze each small region of the object (like individual pieces (pixels) of a jigsaw puzzle - refer to the picture).
To Perceive the whole apple requires further processing by cells with much larger & more ‘complex’ receptive cells that are able to ‘see the big picture’……….these are found in higher Extrastriate Cortical Areas
What is the modern view of the human visual cortex as a pose to the traditional view of the human visual cortex?
100 years ago, the Human Visual Cortex was thought to be restricted
to the Occipital Lobe, with the Primary Visual Area (V1, Striate Cortex, area 17) responsible for mediating all Visual Sensations. (So basically that there were only three areas responsible for vision).
Now we now know that V1 is just one – albeit the largest – of several (8-10) other discrete Visual Areas in the Occipital lobe, with area V2 adjoining it & area V3 running parallel to V2, plus others located more dorsally or ventrally, each of which contains its own retinotopic map of the opposite visual hemi-field.
We also know that the entire Posterior Parietal (PP) Cortex and Inferior Occipito-Temporal plus adjoining Inferior Temporal Cortex are also mainly involved in vision, with these regions each containing at least 10 additional Visual Areas.
[That is: ~33% of the Human Cerebral Cortex is specialized for different
aspects of visual function!]
What are the general theories for why there are mutliple Extra-striate areas?
- The theory of Functional Specialization (Zeki)
- The theory of Ventral versus Dorsal Processing ‘Streams’
[Generally tending to accept two more now]
Explain the theory of functional specialisation to explain why there are so many extrastriate areas.
Explain the Ventral Vs Dorsal processing stream theory as an explanation for why there are so many Extra-Striate areas.
It is believed we have two ‘streams’ emerging from V1. One of the streams travels dorsally (sending mainly magno information) to the Parietal-Occipital areas (V5/MT, V3A, V7) and the Posterior Parietal cortex - this stream is involved in determining where objects are in space, their motion, and how to interact with them for visually guided action.
The other stream travels ventrally (sending mainly Parvo information) to areas in Inferior Occipital (V4, V8, LOC) & Inferior Temporal cortex which are Functionally Specialized for Perception (i.e. this stream determines what objects are).
Where are the following areas found?
What are the following areas selectively activated by?
What does damage to the following areas cause?
AREA V4
LOC
FUSEFORM FACE AREA (FFA)
Area V4 (which is found in the Lingual Gyrus, inferior-medial occipital cortex): COLOURS
•Selectively activated by coloured stimuli + localized damage results in a selective deficit in colour perception (Achromatopsia)
Lateral Occipital Cortical (LOC) Area ( found in posterior inferior temporal gyrus): OBJECT FORM
•Selectively activated by shapes & objects + localized damage results in a selective deficit in object perception (Form Agnosia)
Fusiform Face Area (FFA) Area (found in posterior fusiform gyrus): FACES
•Selectively activated only by faces + localized damage results in a selective deficit in face perception (Prosopagnosia)
What is a key piece of evidence that supports the Ventral and Dorsal stream theory to explain why we have so many extra-striate areas?
Patients with ventral/perceptual deficits can still localize objects in space and interact with them without being able to say what they are - this is thought to be because their dorsal stream is still intact!
Equally patients with dorsal deficits ( i.e. unable to dtermine where objects are and interact with them) can still idnetify what the object is.
How do we know V1 isn’t interested in real objects (i.e. that it will still be stimulated even if an object is scarmbled up)?
Why is this?
From looking at fMRI scans of people looking at pictures normally and then the same picture scrambled up. In both cases V1 was activated - cells in the Primary Visual Cortex respond equally well to real objects as to fragmented images containing lots of lines & edges
This is the case because area V1 is only interested in edges and contours and colours rather than trying to fit the whole picture together.
Would LOC be activated if someone was looking at a scrambled up picture?
No - LOC is interested in tryong to determine the ‘big picture’.
The structure of the LOC is as such that there are different ‘object columns’ in it that look for different things- ranging from simple shapes like circles to patterns and features like hands and body parts.
What are the two types of Visual object Agnosia?
What does the patient not interprete in each type of visual object agnosia?
To which area is damage associated with that particular type of Visual object Agnosia?
General Form Agnosia (damage to LOC)
- Conscious visual sensation of the components of objects is preserved (because area V1 is intact)
- But Px cannot recognize whole objects that they see or understand their meaning
- Although memory of what they look like & their purposes are preserved
Category-Specific Agnosia (damage to FG)
- Is restricted to particular classes of object
- Mostly commonly faces (=prosopagnosia), but can be just animals or buildings etc.
Where are the following areas found?
What are the following areas selectively activated by?
What does damage to the following areas cause?
Area V5/ MT ( this area is refered to as either V5 or MT)
The Inferior Parietal Lobe
The Superior Parietal Lobe
Area V5/Middle Temporal (MT) (found in junction of inferior occipital & superior temporal sulci): MOTION
•Selectively activated by moving stimuli + localized damage results in a selective deficit in motion perception (Akinetopsia)
Inferior Parietal Lobule (found in & below Intra-Parietal Sulcus): EYE movements
•Selectively activated during particular eye movements + localized damage disrupts these (e.g., Gaze palsies)
Superior Parietal Lobule (found in & above Intra-Parietal Sulcus): HAND movements
•Different regions selectively activated during reaching & grasping actions + localized damage results in a selective hand movement deficit (Optic Ataxia)
Do the actions of reaching and grasping for an object activate different Posterior Parietal areas?
Yes
How would a patient with optic ataxia behave?
True or false- Area V5/MT is selectively activated by moving compared to stationary patterns.
True
In Extrastriate Cortex- which of the following is true?
a) Thin stripe cells in V2 receive direct inputs from cells in layer 4B of the primary visual area
b) Damage to Area V4 results is prosopagnosia
c) Area V5/MT is considered to be part of the magnocellular processing pathway
d) The superior parietal lobule is associated with the control of eye movements
C
