Revision Tutorial Q's Flashcards
There are two patients who have suffered brain damage producing fairly
large damage to their cortex (i.e. macro-lesions, rather than microlesions). Name the
areas or regions (one structure per deficit) that could be damaged by such a macrolesion and produce:
i) Achromatopsia (colour blindness) without any other deficit; (2)
ii) Motion blindness without any other deficit; (2)
iii) Inability to deal with spatial relations and perform visually guided
action; (2)
iv) Inability to recognise objects. (2)
a)
* i) V4
* ii) MT or V5
* iii) Parietal (or dorsal) visual areas
* iv) Temporal (or ventral) visual areas
Name the specific substructure (one per deficit) that if damaged by a micro–lesion, could produce:
i) Localised achromatopsia (colour blindness) without motion blindness; (1)
ii) Localised motion blindness without colour blindness. (1)
b)
* i) Parvocellular LGN, blobs in V1, thin stripes in V2
* ii) Magnocellular LGN, layer 4A in V1, thick stripes in
V1
Explain the cause of the scotoma in blindsight patients. Give the characteristics of patient behaviour that define blindsight. Give a
possible explanation for why these characteristics are observed. (10)
The scotoma in blindsight patients is produced by a lesion in V1
* A patient with blindsight has two characteristics: a) an ability to perform much better than chance on various visual tasks within the scotoma; b) they deny that they are consciously perceiving the stimuli
* Cite also the collicular subcortical system or could cite LGN projection to extrastriate areas
Explain the experimental set-up you would use to map out the on and off regions of a cell’s receptive field, using small spots of light. (6)
An anesthetised animal is set up viewing a screen on which small spots of light can be flashed. An extracellular single-unit recording electrode is inserted into the animal’s optic nerve, which records the currents produced by action potentials of neurons. The electrode is connected to an amplifier, which feeds
the amplified signal into (typically) visual display unit connected to the recording computer to view the response to the stimulus, an audio amplifier for listening to the response to the stimulus, and a computer for recording action potentials. Small spots of light are presented at various locations and their effect on the cell’s spontaneous firing rate is assessed. If the firing rate is
increased when the light is flashed, that location is considered to be part of the “on” part of the receptive field. If the firing rate is decreased when the light is flashed, that location is considered to be part of the “off” part of the receptive field. From recording these locations, one can construct a receptive field.
This question deals with the processing done by the
visual system as reflected in receptive field properties
of the cells listed below. Briefly (in just a few words) list
differences between receptive field properties of
different types of neurons. List no more than the
number of differences indicated for each pair of cells.
i) 2 differences between Cones and Retinal Ganglion
Cells. (4)
ii) 3 differences between Retinal Ganglion Cells and
V1 complex cells. (6)
i) Cones are only “on” (albeit hyperpolarising). RGCs
come in two varieties, on and off centre. Cones have
no centre surround organization RGCs do.
* ii) RGC receptive fields are tightly fixed in space,
complex cells can fire in response to stimuli in a
comparatively wider region. RGCs are approximately linear, complex cells highly non-linear. Complex cells have orientation selectivity RGCs do not
1 difference between Simple Cells and MT cells; (2)
ii) 1 difference between Complex Cells and V4 cells;(2)
iii) 2 differences between MT cells and V4 cells; (4)
iv) 1 difference between Left LGN magno- layer 1 cells and Left
LGN magno- layer 2 cells. (2)
i) MT cells will respond to moving stimuli, not flicker, while simple cells will respond to both
ii) V4 cells can have attentional modulation, complex cells do not.
iii) MT cells have poor colour selectivity; V4 cells have good colour selectivity. MT cells are transient, V4 cells sustained
iv) They get input from different eyes
With reference to retinal neurophysiology discuss why central vision is different to peripheral vision. (10)
Describe the distribution of photoreceptors (cones and rods) across the retina. Consequent convergence of these photoreceptors (130 million) onto the RGC (1 million; do not absorb light but process neural information). How do the RGCs preserve information? Describe convergence/summation refer to the 1:1 relationship in the fovea and approximately 500:1 in the periphery, therefore both spatial and temporal summation may increase sensitivity but will reduce resolution.
* Discuss/list the properties of the different RGC types. X cells mainly central retina: 80%; slow, sustained response, linear summation, small receptive fields, acuity (high contrast) colour opponency and project to the parvocellular layers LGN. Y cells mainly peripheral retina: 10%, fast, transient response, non-linear summation, larger receptive fields, achromatic, detection low contrast, motion, flicker project to the magnocellular layers LGN. W cells that do not fit into X or Y categories, some are on-off, non-concentric receptive fields, more common in lower
animals.
What do you understand by the following term:
i) retinotopic map (5)
- Retinotopic map
- A visual area of the brain is said to contain a retinotopic map if :- the visual field (or hemifield) is mapped onto this region so that a stimulus in a restricted part of the visual field activates a restricted part of that visual area and that the map of the activated parts of the visual area forms a map whose geometry corresponds, although in a possibly distorted way (fovea overrepresented or, equivalently, the periphery underrepresented), to the geometry of the visual field
What do you understand by the following term:
ii) the aperture problem. (5)
The aperture problem: Each neuron in the visual system is sensitive to visual input in a small part of the visual field, as if each neuron is looking at the visual field through a small window or aperture. The motion of a contour across a small area (e.g. RF LGN or V1) provides ambiguous information about the contour’s movement. This means that a variety of contours of different orientations moving at different speeds can cause identical responses in a motion sensitive neuron in the visual system
What do you understand by the terms dorsal and ventral
pathway? Describe the cortical areas involved in each
pathway?
The dorsal where? how? pathway:
*This route proceeds from striate cortex dorsally into the posterior parietal cortex via areas MT, MST and regions of the posterior parietal cortex.
*Lesions in the posterior parietal cortex in monkeys result in failure to be able to select a response location on the basis of a visual landmark, suggesting that this pathway figures in “perception of spatial relations among objects, and not in their intrinsic qualities”.
*The dorsal pathway is also known as the where? how? pathway since neural responses carry information about spatial location of stimuli and motion direction.
*The ventral what? pathway:
*From striate cortex, one route proceeds ventrally into posterior temporal cortex passing through areas V2, V4 and the sub-regions of the inferior temporal cortex.
*Based on the fact that lesions in the posterior temporal area result in loss of pattern discrimination and that lesions in the inferotemporal cortex in particular result in failure to
recognize previously presented objects, this pathway was assigned the task of analysing “the physical properties of a visual object (such as its size, colour, texture and shape)”.
*This pathway is often known as the what? pathway since the responses of neurons more closely reflect the attributes of the visual stimulus (rather than its location or motion direction).
What do you understand by the terms dorsal and ventral
pathway? Describe their preferred stimulus characteristics in the cortical area?
Dorsal pathway progression — V1/MT/MST
* V1 neurons respond selectively to the direction of motion of an oriented bar. In MT, the middle temporal area, located in the superior temporal sulcus, cells respond to motion of a variety of stimuli including random dots (motion coherence stimuli). At a later stage along the dorsal pathway in area MST (medial superior temporal), cells respond to even more complex motion
patterns (optic flow; radial rotational coherence patterns).
* Ventral pathway progression - V1, V4, InferoTemporal (IT) cortices
* V1 neurons carry information about stimulus orientation and direction. While some V4 neurons are selective for wavelength and also are selective for the orientation of a bar, many cells are selective for more complex stimuli (glass patterns). Neurons encode angles and curves, a higher level image feature than
oriented lines and edges. IT cortex, the last stage in the what pathway, cells are selective for even more complex stimuli. Neurons here may respond to a monkey face or a cartoon face. The neuron will responds only when the outline of the face, eyes and mouth are all included. It does not respond to oriented
bars, or coloured patches.
Define the following conditions including the most likely site of the lesion:-
* i) Visual Neglect;
* ii) Balint’s syndrome;
* iii) Prosopagnosia.
i) Visual neglect (hemispatial inattention) is an attention
disorder that prevents the patient from attending to stimuli
on one side. Neglect is most closely related to damage to
the temporo-parietal junction and posterior parietal cortex.
* ii) Damage in the posterior parietal cortex, clinical signs
include simultanagnosia, optic ataxia and ocular apraxia.
* iii) Inability to recognize faces. The term originally referred
to a condition following acute brain damage, but a
congenital form of the disorder is now recognized. The
specific brain area usually associated with prosopagnosia is
the fusiform gyrus
Receptive Fields of cells in the Visual Pathway:
True or False?
Go from large to small as you move DOWN the visual pathway
If by down is meant from Cortex to retina – this is true. It is false if the opposite is true
Receptive Fields of cells in the Visual Pathway:
True or False?
Become less specific in detail the further you are away from central fixation
True
Receptive Fields of cells in the Visual Pathway:
True or False?
Their response properties are more complex as the stimulus gets more complex
True
