Lecture 21- Cortical Vision Flashcards
Is there a single representation of the visual world in the visual cortex?
-Multiple representations of the visual world exist in visual cortex.
-Beyond the striate cortex lie the extrastriate areas, which are higher-order visual areas that also contain
representations of the retina.
What is a receptive field? Why do we need to identify this for a neuron?
-With the use of single cell recording, neurophysiologists can identify the representational characteristics of neurons in different visual areas.
-Remember that visually sensitive cells respond to stimuli within a limited region of space (i.e., their
receptive field).
-Thus, before you can test which type of stimuli cause a neuron to fire, you need to map the neuron’s
receptive field so that you know where to present the stimuli.
What is area MT?
Equivalent of V5 in humans, just what it tends to be called in monkeys
What does a single cell recording in area MT show?
Participants: Macaque monkeys
* Method: The activity of a single neuron in
area MT was recorded while a white bar
passed through the neuron’s receptive field
in varying directions.
* The figure on the right shows the activity
of a single neuron in area MT when the bar
passed through its receptive field.
* Results: The neuron fired vigorously when
the bar moved downward toward the left,
and opposite movement elicited the least
activity (see figure, which illustrates the
firing rate for each direction of movement).
* Conclusion: Neurons in area MT are
selective for the direction of motion.
What was shown with regards to speed tuning for a single neuron in area MT?
Method: The motion of the white bar was
always in the optimal direction, but the
speed varied.
• Results: This neuron fired most vigorously
to movement at 64 degrees/second.
• Conclusion: Neurons in area MT are
selective for the speed of motion.
What type of imaging technique was analyzed during visual stimulation? How was this done?
-PET
-Participants: Nine neurologically-healthy adults
• Method: Positron emission tomography was used to measure regional cerebral blood flow in order to
identify which parts of the brain are involved in the perception of different types of visual stimuli.
-Stimuli: Participants viewed either coloured stimuli or moving stimuli
-Results: Note the relative location of the region activated by coloured stimuli (left figure: lower temporal lobe), and the
region activated by moving stimuli (right figure: further back, more the occipital lobe).
• Conclusion: This data shows that, as in macaque monkeys, different areas of human extrastriate visual
cortex are involved in processing different attributes of vision.
What is PET?
– PET scanners detect radioactive material.
– Participants either inhale or are injected with a radioactive material.
– When the radioactive material gets into the bloodstream, it goes to areas of the brain that are
metabolically active.
– PET provides a functional view of the brain by measuring regional cerebral blood flow.
– As with event-related potentials (ERPs), the background noise must be removed from the
data in order to observe the event-related signal.
– PET data provides a functional view of the brain, but not a structural view of the brain.
Thus, the PET data is overlaid on a structural image.
In the monkey experiment where PET was used how did they control for other factors so that just the activity associated with the desired function was being measured?
-In order to remove activity that is associated with vision but is not specific to colour perception, activity
elicited by the abstract scene when shown in greyscale was subtracted from the activity elicited by the
abstract scene when shown in colour.
-In order to remove activity that is associated with vision but is not specific to motion perception,
activity elicited by the display when it was stationary was subtracted from the activity elicited by the
display when it was moving.
What are the two main projection routes from primary visual cortex (V1) to extrastriate visual cortex?
-The dorsal stream codes motion and location. (‘where’)
– The ventral stream processes detailed stimulus features and object identity.
(‘what’)
They have functional differences!
What are the connections between the different areas of the extrastriate cortex like?
-Interconnectivity of the different areas of visual
cortex is complex but not random.
-Not every area is connected with every other.
-The numbering scheme should not be taken to mean
that the synapses proceed sequentially from one area
to the next.
What is the topographical representation like in each visual field of extrastriate cortex? What happens as information projects from one area to another?
-Each visual area has a topographical representation
of the contralateral hemifield, and the boundaries
between anatomically adjacent visual areas are
marked by topographic discontinuities.
-As one area projects to another, topography is
preserved.
-Although each visual area contains a map of your
visual experience, the maps differ with regard to the
type of information they represent.
-The type of information represented in each areas map is determined by the response properties of the
neurons in that area (i.e., neuronal activity provides the “map”).
• Each visual area provides its own limited analysis.
• After different extrastriate areas process the initial information provided by the retinogeniculostriate
pathway, the information is integrated in order to provide coherent visual experience.
What is V4?
V4 is located along the “ventral stream”.
• Initial observations of neurons in V4 revealed that they were selective for colour, and it was thought that
they distinguished colour only.
• We now know that some cells in V4 respond to combinations of colour and form.
What is V5?
• V5 is located along the “dorsal stream”.
• Neurons in V5 are selective for the direction of motion and speed of motion.
• Damage to V5 can disrupt motion perception (e.g., the ability to discriminate the direction in which a
stimulus moved).
• Note that V5 is not located as far dorsal or rostral as the above cartoon drawing indicates (for more
accurate positioning, see PET data on previous page, right panel).
