Ch 11: Localizing stimuliand orienting in space Flashcards
What is the spatial receptive field of a cell?
The spatial receptive field of a cell is the region of physical space in which stimuli elicit robust neuronal responses
Is the 2-point threshold in the fingers, face and toes higher or lower than in the rest of the body?
LOWER.
Lower threshold = more accurate!
Sensory axons innervating the skin of the trunk and limbs ascend through the ___ ___ and terminate in the __ ___ ___, whose axons CROSS TO THE OTHER SIDE and ascend to the ___ ____ ____ ____.
Sensory axons innervating the skin of the trunk and limbs ascend through the SPINAL CORD and terminate in the DORSAL COLUMN NUCLEI, whose axons CROSS TO THE OTHER SIDE and ascend to the THALAMIC VENTRAL POSTERIOR NUCLEUS (VPN)
Axons innervating facial skin project to the ____ ____ ____, which projects to a more medial portion of the _____ ____.
Axons innervating facial skin project to the PRINCIPAL TRIGEMINAL NUCLEUS, which projects to a more medial portion of the CONTRALATERAL VPN.
Information from the trunk, legs, and face converge in the ____
Ventral posterior nucleus (VPN)
Where is converging somatosensory information sent to from the VPN?
From the VPN information is sent to the PRIMARY SOMATOSENSORY CORTEX (S1)
What is somatosensory homunculus and how was it generated?
Somatosensory homunculus is a distorted representation of the human body based on a neurological map of the areas and proportions of the brain dedicated to processing sensory info for different parts of the body.
It was discovered by Penfield, who analyzed SKIN SENSATIONS evoked by ELECTRICAL STIMULATION of the SOMATOSENSORY CORTEX.
What two areas are overrepresented in the somatosensory homunculus?
face and hands
What is the “barrel cortex” of rodents?
- Barrel-shaped clusters of cells in a rat’s somatosensory cortex.
- Each cortical barrel contains neuronal cell bodies and processes sensory information from a specific whisker on a rodent’s snout.
- arranged in columns like on the snout
rat’s whiskers are represented ______ in the barrel cortex
topographically
Define lateral inhibition
The tendency for excited neurons to inhibit their neighbours.
How does lateral inhibition allow for more precise stimulus localization in S1?
Neurons that are strongly excited by stimulation on one spot on the skin supress neighbouring neurons that are weakly excited. This sharpens the edges of the stimulus representation within the somatosensory map.
in the visual system, which region of space is overrepresented?
the region of space at which your fovea is aimed, because it has the highest density of photoreceptors.
Define “visual field” and what is the center of the visual field?
Visual field is the region of space from where a visual stimuli can reach your retina.
The center of the visual field is the location where the foveae are aimed.
What are the retina’s output cells?
retinal ganglion cells
The ____ ____ plays a major cole in eye movements
the SUPERIOR COLLICULUS plays a major role in eye movements
The ___ ___ ___ sends projections to the primary visual cortex (V1)
The LATERAL GENICULATE NUCLEUS (LGN) sends projections to the primary visual cortex (V1)
Damage of V1 on the left side of the brain in human causes blindness where?
in the RIGHT VISUAL HEMIFIELD.
Define right visual hemifield
the visibe region of space that lies to the right of the FIXATION POINT, where the foveae are aimed
The retinal axons cross at the ___ ___
optic chiasm
T/F: lesions of V1 on one side of the brain causes blindness in portions of BOTH EYES
TRUE.
V1 damage on the right side of the brain causes blindness in the left visual hemifield, and damage on the left side causes blindness in the right visual hemifield
Information from the right visual hemifield is sent to the ___ ___ ___ and ___ ___, while info from the left visual field is sent to the ____ side of the brain.
Information from the right visual hemifield is sent to the LEFT LATERAL GENICULATE and VISUAL CORTEX, while info from the left visual field is sent to the RIGHT side of the brain.
Do the axons from the temporal retina cross?
The axons from the temporal retina enter the optic chiasm BUT REMAIN UNCROSSED
Which retinal axons cross to the other side of the brain in humans and primates?
Only the axons from the NASAL PORTION OF EACH RETINA cross to the other side of the brain
how have experimenters tested the hypothesis that the central region of the visual field is over-represented in V1? What were the results of this experiment?
- Experimenters injected animals with 2-DG, which was taken up by active neurons.
- animals were then presented with a patterned dynamic visual stimuli that covered the entire visual field.
- Results showed that the central (fovea) region of the visual field is overrepresented relative to the periphery.
- Results also showed that the visual field is topographically mapped onto the primary visual cortex (although distorted).
Are neurons in the superior colliculus of primates monocular or binocular? What about those in the LGN?
- Neurons in the superior colliculus are BINOCULAR (ie. visually responsive neurons receive CONVERGING INPUT from BOTH EYES).
- Neurons in the LGN are MONOCULAR (they respond to input from one eye or the other, but NOT both).
Neurons in the LGN project mainly to deep layer __ of V1.
Neurons in the LGN project mainly to deep layer 4 of V1.
How can our visual system estimate the distance between an object and the retina?
- V1 neurons are highly sensitive to BINOCULAR DISPARITIES.
- By measuring binocular disparities, our visual system can estimate the distance between an object and the retina.
explain canonical cortical circuit
- sensory input from the DORSAL THALAMUS terminates mainly on the STELLATE neurons of layer 4.
- The stellate neurons project mainly to the PYRAMIDAL neurons radially above them, in layers 2 and 3.
- Most of these pyramidal neurons terminate on the dendrites of the LARGE PYRAMIDAL neurons that have cell bodies in layers 5 and 6
- These large pyramidal neurons then project to various subcortical areas.
All these interconnected neurons lie within a relatively narrow radial column (cortical minicolumn)
in cortical minicolumns, layer 4 contains ____ neurons and layers 2, 3, 4 and 5 contain _____ neurons
in cortical minicolumns, layer 4 contains STELLATE neurons and layers 2, 3, 4 and 5 contain PYRAMIDAL neurons
to determine a sound’s location, neurons must compare the sound’s ___ and ____ between the two ears
to determine a sound’s location, neurons must compare the sound’s TIMING and INTENSITY between the two ears
Will there be a difference in perceived sound between the two ears if the source of the sound is directly in front of you?
A sound coming from directly in front of you will be EQUALLY LOUD in your two ears.
What do you call the level of difference of sound between the ears?
interaural level difference (ILD)
Interaural level differences are ___ for high-frequency sounds and ____ for low-frequency sounds because…
Interaural level differences are LARGER for high-frequency sounds and LOWER for low-frequency sounds because LOW-FREQUENCY SOUNDS TRAVEL MORE EASILY PAST OBSTACLES, SUCH AS YOUR HEAD.
What do you call the time difference at which sound reaches both ears?
interaural time difference (ITD)
What is “binaural input?”
auditory information received by both ears
Where is binaural convergence accomplished?
binaural convergence is accomplished in the MEDULLA, specifically the LATERAL and MEDIAL SUPERIOR OLIVES
The ascending auditory pathway
- auditory nerve axons project to the COCHLEAR NUCLEI in the MEDULLA, which project ipsolaterally to the LATERAL SUPERIOR OLIVARY (LSO) NUCLEUS, bilaterally to the MEDIAL SUPERIOS OLIVE (MSO), and contralaterally to the MEDIAL NUCLEUS of the TRAPEZOID BODY (MNTB).
- The MNTB has inhibitory projections to the MSO and LSO, whch project to the inferior colliculi.
The lateral superior olive (LSO) receives excitatory inputs from the _____ ____ ____ and indirect inhibitory inputs from the ____ ___ ____.
The lateral superior olive (LSO) receives excitatory inputs from the IPSILATERAL COCHLEAR NUCLEUS and indirect inhibitory inputs from the CONTRALATERAL COCHLEAR NUCLEUS.
What occurs whenever the ipsilateral excitation is stronger than the contralateral inhibition of the lateral superior olive?
This occurs whenever sound is louder in the ipsilateral ear.
NEURONS IN THE LATERAL SUPERIOR OLIVE INCREASE THEIR FIRING RATE
The more the neurons in the lateral superior olive fire, the further ____ the sound must be. Therefore, …
The more the neurons in the lateral superior olive fire, the further IPSILATERAL the sound must be. Therefore, a sound sourcce’s horizaontal location is encoded in the firing rate of lateral superior olive neurons.
ILDs are processed by the ____ superior olive and ITDs are analyzed by the ____ superior olive
ILDs are processed by the LATERAL superior olive and ITDs are analyzed by the MEDIAL superior olive
Axons from the lateral and medial superior olives ascend to the midbrain’s ____ ____
inferior colliculus
Individual neurons in he inferior colliculus of barn owls have relatively ____ spatial receptive fields
SMALL.
ie. they respond only to sounds from very limited regions of space.
T/F: the inferior colliculus of barn owls contains a topographic map of auditory space that is analogous to the map of the visua field in V1 and to the map of the body surface in S1
true
What allows barn owls to better collect sound?
- stiff feathers on their face collect sound
- their heart-shaped face acts as a radar dsh
- owls are able to hear in 3D
- their right ear aims up and their left ear aims downward, creating interaural level differences for high frequency sounds that differ in elevation.
- Thus they are able to localize sounds in both azimuth and elevation
Do mammalian brains employ a population code or a single-neuron code to represent sound location? Explain.
Population code.
As auditory stimuli move contralaterally, different neurons increase their firing at different rates; but stimulus at any given location ACTIVATES MANY DIFFERENT NEURONS.
Owls, however, employ the single-neuron code.
What does turning your head towards an interesting sound accomplish?
This brings the sound source into the ACOUSTIC FOVEA, where sounds are localized more precisely
What do we do when trying o localize a sound source in the vertical dimension?
tilt our head sideways, ensuring that sounds above the horizon will be louder and arrive earlier in one ear than the other.
you have the highest spatial acuity in the ___ ___
visual fovea
The retina encodes stimulus in ____ _____ coordinates
eye-centered coordinates
in animals with immobile ears, information about the location of sound is initially computed in ____ ___ coordinates
head-centered coordinates
How did experimenters find that auditory spatial receptive fields move as eyes move?
- They implanted electrodes into a monkey’s brain and trained it to fixate on light presented at 3 different locations
- They then played sounds on a moveable speaker and recorded how individua neurons in the monkeys superior colliculus responded.
- As the speaker moved closer to the point of fixation, the neurons in the superior colliculus fired more
what is blindsight?
Being aware of movement, even when you can’t see. It is implicit (unconscious).
If the spatial receptive field moves with the eyes, then the neuron codes space in:
eye-entered coordinates
If the spatial receptive field is stable as the eye moves, but shifts with head movements, then the neuron codes space in:
head-centered coordinates
If the spatial receptive field is stable as the eyes or head move, but shifts with body movements, then the neuron codes space in:
body-centered coordinates
If the spatial receptive field is stable as the eyes, head or body move, then the neuron codes space in:
world-centered coordinates
When looking at a static landscape and then suddenly there is a movement, your eyes will be drawn towards that movement. What is this eye movement called? (not smooth, but choppy. Follow a straight trajectory)
saccade
The motor neurons that innervate the extraocular muscles (that allow for side to side and up and down movement) receive major inputs from two ___ ___ ___ in the brainstem
The motor neurons that innervate the extraocular muscles (that allow for side to side and up and down movement) receive major inputs from two SACCADE GENERATOR REGIONS in the brainstem
How are diagonal saccades possible?
for diagonal saccades, the horizontal and vertical saccade generators are activated simultaneously to varying degrees (vector addition)
2 visual streams of info proceeding away from the visual cortex:
- Dorsal stream: stretches from V1 in the occipetal lobe into the parietal lobe.
- Ventral stream: from V1 to the temporal lobe.
Which visual stream is responsible for discriminating fine features and which is responsible for vision for action?
Ventral stream: discrimination of fine features
Dorsal: vision for action
What effect does sustained damage to the parietal lobe have?
- exhibits problems with hand-eye coordination because it affects the DORSAL STREAM (vision for action).
- patients do NOT have agnosia and are still able to identify objects
What occurs when a patient sustains damage to parts of the ventral stream or visual association cortex?
They experience VISUAL AGNOSIA: the inability to recognize visually presented objects.
Patients with agnosia do not follow the same saccades that healthy individuals do.
Command center in the ___ ___ controls eye movement
superior colliculus
Deficit in the ___ stream affects the ability of a subject to grasp an egg
DORSAL
Patient D.F. posting and matching
- due to hypoxia from CO poisoning, patient D.F. sustained injuries in her VENTRAL STREAM.
- patient D.F. had VISUAL AGNOSIA (unable to recognize objects visually presented)
- When asked to match a card in her hand with the orientation on a picture, she was unable to MATCH the orientation
- However, patient D.F. could correctly POST a letter through a slot
- SHE DEMONSTRATES INTACT VISUOMOTOR SKILLS, WHILE LACKING PERCEPTUAL MATCHING ABILITIES
Looking in which direction requires the least amount of muscle energy?
looking straight ahead
The ___ ___ causes the eyeball to look inwards, the ___ ___ downwards, and the ____ ____ upwards
The MEDIAL RECTUS causes the eyeball to look inwards, the INFERIOR RECTUS downwards, and the SUPERIOR RECTUS upwards
Neurons in the midbrain RF command ___ eye movements, whereas neurons in the pontine and medullary RF generate commands for ____ saccades.
Neurons in the midbrain RF command VERTICAL eye movements, whereas neurons in the pontine and medullary RF generate commands for HORIZONTAL saccades.
Why does an alignment of the visual and motor maps occur?
Because individual neurons in the superficial layers of the superior colliculus tend to project to deep collicular neurons directly below them
When a light flashes somewhere in your visual field, what does the spot of activity generated in the superficial layers of the superior colliculus trigger?
The spot of visual activity generated in the superficial layers of the superior colliculus triggers activity in the underlying deep collicular neurons, which makes your eyes move rapidly towards the flashing light.
What occured to the saccade of monkeys whose rectus muscles were surgically weakened in one eye?
the eye with the weakened muscled would make abnormally small saccades (undershoot the target)
When an eye patch was placed over the weakened eye of a monkey, what was observed when they made horizontal saccades to visual targets?
the weakened eye undershot the target consistently, even as the intact eye made a correct saccade
What happened when experimenters switched the eye patch onto the healthy eye of the monkeys? (saccade experiment)
The weakened eye still made saccades that were too small, but became accurate after 5 trials.
When the eye patch was removed, the healthy eye now OVERSHOT the target.
This demonstrates that the plasticity was not due to the weakened eye’s muscles growing stronger, but the MOTOR COMMANDS to BOTH EYES must have GROWN IN STRENGTH
Given enough trials, errors in saccade magnitude can be corrected by adjusting:
the strength of the responsible motor commands
define “smooth pursuit eye movements”
- not saccades
- automatic eye movement you make when tracking a moving stimulus
most direct pathway from the retina to the extraocular motor neurons
retina –> superior colliculus –> reticular formation –> motor neurons
What happens to artificially stimulates sacades of the primary visual cortex when the superior colliculus is lesioned?
they dissappear.
This suggests that the V1 can trigger saccades by means of its direct projections to the superior colliculus
Rapid and automatic head turns to a target are called ___ ____
head saccades
What are the primary functions of head saccades?
- to move the target location into our acoustdic fovea
2. to recenter the eyes within their sockets after a saccadic eye movement
Why is it better to keep the eye centered in its socket?
because keeping it off-center requires the steady contraction of at least one extraocular muscle. This takes energy
Why does the eye not turn with the head as the head rotates to look at a target?
due to the operation of the VESTIBULO-OCULAR REFLEX.
This counterrotates the eyes as the head turns, to maintain line of sight locked on the target
What happens when the superior colliculus is silenced with lidocaine?
the frontal eye field no longer elicits saccades
define optic ataxia
inability to make accurate hand movements to visual targets
World-centered representations of space are also called ____
allocentric
Tolman maze experiment
- rats placed in cross-shaped maze
- experimenters wanted to see whether they would use allocentric or egocentric strategies to find food.
- initially, rats used allocentric cues to find food, but once they because really good at the task, they started using egocentric strategies.
Morris water maze
-rat placed in circular tub filled with opaque water
-rat swims around until they find submerged platform
-as training progresses, swim path shortens
-rats with hippocampal lesions took longer to find the submerged platform, but had no problem locating it when it was not submerged.
HIPPOCAMPAL LESIONS IMPAIR ALLOCENTRIC NAVIGATION
What do rats with lesioned neocortex display in the Morris water maze?
lesion in the neocortex does NOT impair learning the water maze (hippocampal lesion does)
What occurred when rats were lesioned in the hippocampus in the Tolman cross maze?
They did not display allocentric navigation, just performed by chance
what are place ells?
neurons in the hippocampus which respond selectively to particular places in the world
what evidence exists to support that place cells tend to be ALLOCENTRIC?
When patients with epilepsy were implanted with recording electrodes in the hippocampus and asked to navigate a virtual town, their place cells tended to fire in their preferred location regardless of direction of travel. This indicates that they probably encode spatial location in allocentric, rather than egocentric, coordinates.
Where are grid cells located?
in the entorhinal cortex.
What image does connecting the centers of firing of grid cells generate?
a triangular grid
London taxi drivers experiment methods, results and conclusion
- the hippocampal volume in individuals undergoing training to be London cab drivers were measured before and after training. Compared with hippocampal volumes of bus drivers as a control.
- RIGHT POSTERIOR HIPPOCAMPUS volume was GREATER in London taxi drivers than the control subjects
