Lecture 7 Flashcards
what information goes to each part of the brain from the eye
Contralateral– information from the nasal part of the retina crosses to the other side of the brain, such that the left visual field is seen by the right hemisphere and vice-versa
what does LGN stand for
Lateral Geniculate Nucleus
what is the LGN
where the information passes through to get to the brain (from vision)
there are 2, the left and the right
what are the Types of layes in the LGN
Magnocellular (1 and 2)
Parvocellular (3-6):
what is the Magnocellular (1 and 2) layer
Large cells, bottom two layers. Receive input from M ganglion cells. Respond best to large, fast-moving objects
what is the Parvocellular (3-6): layer
Smaller cells, top four layers. Receive input from P ganglion cells. Respond best to fine spatial details of stationary objects.
what are the Properties of the LGN:
Controlateral representation: left LGN recieves info from the right visual field and vice-versa.
Each LGN layer receives signal from one eye only.
Within each layer of the LGN, the neurons are arranged in a retinotopic map of the visual field. In other words, RGCs (receptive ganglion cells) with adjacent receptive fields connect to adjacent neurons in the LGN (AKA each spot in retina has a corresponding place in the LGN)
what is another name for the Primary visual cortex
striate cortex
what are Two important features of striate cortex
Retinotopic mapping
Cortical magnification
what is Cortical magnification
Dramatic scaling of information from different parts of visual field
Proportionally much more cortex devoted to processing the fovea than to processing the periphery (aka lots more space in the cortical magnification dedicated to the fovea than periphery)
what is Retinotopic mapping
each part in your visual field corresponds to a place in the straits cortex
Why sine gratings?
Any black-and-white image can be described in terms of a weighted combination of different : (there are 4 properties)
aka every image can be decomposed into what?
frequencies,
contrast,
phases, and
orientation
Low frequencies = broad outline
High frequencies = details
Retinal ganglion cells are sensitive to what
1) frequencies, 2) contrast, 3) phase, but not 4) orientation…
is it possible to detect the orientation fo an image with just one ganglion cell
no,
By combining information from SEVERAL retinal ganglion cells, it is possible to detect the orientation of lines.
what is the Tilt after-effect
We have different populations of neurons that specialise in certain frequencies and orientations.
Adaptation: Looking at a pattern of stripes for a certain time will “tire” the neurons and shift the balance in the opposite direction.
-> Note: this also implies that orientation is encoded by a population of neurons
(this is the example with the red dot embedded in the black and while lines)
define Sensation
Sensory receptors activated after receiving a specific input
define Perception
Interpretation of the sensory input
define Visual Sensation:
light activating photoreceptors (rods and cones)
in the retina
define Visual Perception
how the environment is consciously perceived after organization and interpretation of the retinal input
Visual Perception occurs at what level (where)
This occurs at the level of the cortex and involves both bottom-up and top-down processes including attention, context and preconceived ideas about the world.
how does sensation get translated into perception
via the visual pathway
how does the visual pathway/system work
From the retina, the visual information travels to the lateral geniculate nucleus (LGN) in the thalamus.
From the LGN, neurons project either to subcortical structures or to the visual cortex.
what is The subcortical structures for vision responsible for
are responsible for eye movements.
what is The visual cortex responsible for
INTERPRETING the retinal input and giving us a visual experience
The visual cortex is composed of what
the striate and the extrastriate areas
From the LGN, visual information travels directly to where
the striate primary visual cortex (V1).
From the LGN, visual information travels directly to the striate primary visual cortex (V1).
The visual information then travels into where
the extrastriate cortex that splits into two different streams.
the extrastriate cortex is split into two different streams, what are they
the dorsal stream and the ventral stream
what is the dorsal stream
motion processing
The ‘where’ or ‘how’ pathway
what is The ventral stream =
object recognition
- The ‘what’ pathway
V1 extracts what ind of information
V1 extracts very basic information from the visual input including the orientation of lines and direction of simple motion.
The extrastriate areas extract what kind of information
more complex information in a hierarchical fashion
what happens if you have Lesion in area MT (middle temporal)
Akinetopsia
what is Akinetopsia
motion blindness
what happens if there is a Lesion in Fusiform Face Area within the Infero-Temporal Cortex
Prosopagnosia
what is Prosopagnosia
face blindness
Lesions at any point in the two streams (dorsal or ventral) causes very specific deficits which are what
deficit in PERCEPTION not sensation– you can still see okay just cant interpret it
what happens if there is a lesion before the V1 cortex (before the area where things are processed as being seen)
Cortical blindness
what is Hemianopia
Cortical Blindness just another name
what is ‘Cortical Blindness’ (CB)
severe loss of vision that follows damage to the primary visual cortex (V1) – affects the contralateral visual field of both eyes
Cortical Blindness is most often caused by what
an ischemic stroke to the posterior cerebral artery
how many stroke patients have permanent homonymous hemianopia (CB)
8-10%
how does CB impact the patients life
affects the patients’ quality of life significantly, with many patients having difficulties reading, driving and navigating the world
what are the treatments for CB
Currently, no treatments exist to recover vision loss after stroke
Most strategies used in rehabilitative centers used ‘compensatory mechanisms’ rather than recovery.
If the eyes and the rest of the visual cortex are spared (in CB), is there a way vision could be rehabilitated ? Why do people think there might be?
Numerous studies have shown that CB subjects possess residual abilities in their blindfield, a phenomenon known as blindsight
what is blindsight
cortically blind patients are able to discriminate or respond to stimuli without being aware of them.
Blindsight differs from normal vision: in what 3 ways
1) It is largely unconscious
2) It is elicited most reliably by large stimuli
3) It seems to be restricted to coarse, flickering/moving stimuli
does blindsight have functional utility
Although discrimination improves (in the experiment), there is no increase in awareness.
Blindsight has a limited functional utility
Can cortically blind subjects be trained with other stimuli (that do not elicit blindsight)?
according to experiments: Yes
Visual recovery was attained with a wide variety of stimuli
Detection of shape, shape comparison, orientation discrimination, letter identification, motion direction discrimination
All stimuli were specially designed not to elicit blindsight
Stimuli are successful at reducing the size of the blindfield
Eye movements were controlled with eye trackers
Stimuli were presented fully in the blind field and light scattered was minimized
How significant is the improvement? (with regard to the cortically blind subjects trained with other stimuli (that do not elicit blindsight))
The recovery is spatially limited and remains suboptimal
How can we increase the clinical significance of recovered vision?
(for CB)
targeted visual plasticity
what is Targeted Visual Plasticity
inducing plasticity in a targeted visual area through training
the dorsal stream is primarily good for what
motion perception
the ventral stream is primarily good for what
object recognition
for the experiment regarding CB explain how improvements were made to allow for the patient (monkey) to use other parts of the dorsal stream to compensate
After training with a simple stimulus, deactivating area MT does not affect motion perception of the simple stimulus significantly
After training with a complex stimulus, deactivating area MT does affect motion perception of the complex stimulus significantly
After training with a complex stimulus, deactivating area MT ALSO affects motion perception of the simple stimulus significantly
