Transcript Lecture 16A

What is more important: sensory receptors or
the cortical neurons?
• In the primary gustatory cortex sweet
and bitter are represented by neurons
organized in a spatial map with each
taste quality encoded by distinct
cortical fields.
• EXPERIMENT: activate the brain field
representing bitter taste and give
mouse sweet water - will mouse drink?
• The activity in the primary gustatory
cortex directly controls an animal’s
internal perception of sweet and bitter
taste, and drives behavioral actions.
Essentially, it does not matter what
sensory receptors are reporting. Your
perception is completely controlled
only by the cortical neurons activity.
• In fact, the cortical neurons activity is
your perception
Rubin vase
illusion
• Make a
prediction:
where are the
neurons that
follow the
percept?
My Wife and My
Mother-In-Law by
W. E. Hill
Duck-Rabbit illusion by Joseph Jastrow
(the beak of the duck forms the ears off the rabbit)
Necker cube
• The three-dimensional cube can be perceived in two
different ways, depending on whether you see the
lower-left corner as being on the front face of the cube
or on its rear
Impossible images
• The mind
attempts to
interpret all
available visual
cues at the same
time and …. fails.
• The brain cannot
find a neuronal
ensemble that
could fit the image
falling onto a
retina
The blivet
• This process results in perceptual flickering between
two neuronal ensembles in the case of the blivet
and three in the case of the Penrose triangle.
The Penrose triangle
• Interesting: people with dyslexia are faster at
identification of impossible images
Relativity, 1953
by M. C. Escher
Waterfall, 1961
by M. C. Escher
Salvador Dali: Slave Market with the Disappearing Bust of Voltaire - rivalrous or bistable stimuli
Rivalrous stimuli
Salvador Dali: Slave Market with the Disappearing Bust of Voltaire
• Make a prediction: where are the neurons that follow the percept?
• In V1, most cells’ activity depends on the visual stimulus and does not
depend on what the subject’s perception is. Change in the perception
affected the firing rate in fewer than 10% of V1 and V2 neurons.
• In V4, 40% of the neurons changed their activity when the perception
changed.
• Finally, in the inferior temporal lobe, 90% of the neurons changed their
activity when the perception changed.
David A. Leopold, Nikos K. Logothetis
• Conscious perception flickers between perceiving the nuns and the
bust of Voltaire. When the nuns are perceived, the neuronal
ensemble representing the nuns (light green pyramid) is activated.
When the bust of Voltaire is perceived, the neuronal ensemble
representing Voltaire (dark blue pyramid) is activated.
• Note that most neurons in V1 belong to either one or the other
neuronal ensemble, while only a small population of neurons (e.g.
representing the white apron) belong to both the Voltaire and the
nuns ensembles.
• The proportion of neurons that belong to both neuronal
ensembles decreases as we ascend to more specific areas in V4
and TL.
90%
40%
10%
Necker cube
• The three-dimensional cube can be perceived in two
different ways, depending on whether you see the
lower-left corner as being on the front face of the cube
or on its rear
In the case of the Necker cube, the visual cues for both interpretations are identical.
1. When one first looks at the Necker cube, the lines are projected from the retina
to V1. The population of neurons coding for the lines is indicated in thick red. No
3D cube is yet perceived.
2. The mind attempts to fit 3D objects from memory to the available visual cues.
Thus the neural ensemble representing the 3D cube is activated (shown in light
green) and perceived. Note that additional neurons in V1 and elsewhere were
activated to encode perceived depth.
3. After some staring at the drawing, a second possible interpretation is found. The
first neuronal ensemble disintegrates and the second neuronal ensemble (shown
in dark blue) is perceived.
• A comparison of the neuronal ensembles representing the
two interpretations of the Necker cube.
• The two non-overlapping populations of neurons encode
the depth information.
• Population A is only active when the light green ensemble
is activated and the first 3D cube is perceived;
• Population B is only active when the dark blue ensemble is
activated and the second 3D cube is perceived.
Can you consciously influence the choice of perception?
What part of brain controls the choice?
PFC
• Can you find a camouflaged frog
amongst the dead leaves in this illustration
from the 2015 Johnston Club calendar?
• If you interpret fragments
17, 18, 19, 20, 21 separately,
they look like dead leaves
PFC
• Once you interpret these
fragments together (as a single
entity), you perceive a frog
• Notice that these fragments did NOT
jump into the coherent percept of the
frog on their own.
• Your mind had to VOLUNTARILY
integrate the fragments.
PFC
• Similar to this frog recognition, the PFC of
hominins had to integrate multiple visual clues into
a coherent object.
• The PFC had to decide whether a motionless patch
of grass was a boulder, or a wild boar, or a leopard.
per min
• Make a prediction of ability to consciously
control the choice of perception in patients
with lesions of the prefrontal cortex due to
brain tumors?
Bottom-up activation of a neuronal
ensemble by imprecise sensory
stimulation
TL
V2, V4
a
u
t
o … y
V1
automatically
automatically
TL
V2, V4
4
V1
u
t
o … y
Psychology blot test
Self-organization of a
neuronal ensemble
Bottom-up activation
Perception of a bat
PFC
Self-organization of a
neuronal ensemble
Bottom-up activation
Perception of a butterfly
PFC
Self-organization of a
neuronal ensemble
Bottom-up activation
Perception of a black
mask
• For most people the first self-organizing neuronal
ensemble in this photograph is incorrect.
• PFC thinks: “it cannot be true” …and exerts topdown influence: it tries to actively reorganize
fragments of visual information into correct
neuronal ensembles
• For most people the first
self-organizing neuronal
ensemble in this
photograph is incorrect.
• PFC thinks: “it cannot be
true” …and exerts topdown influence: it tries
to actively reorganize
fragments of visual
information into correct
neuronal ensembles
Without the influence of PFC, we have all kinds
of weird dreams…
• Dreams are vivid, sensorimotor hallucinations with a narrative structure
• During dreaming we are for the most part unable to introspect about our
uncanny ability to fly or to meet somebody long dead; things happen, and
we go along for the ride (recall: lateral PFC (the puppeteer) is inhibited and
puppets (neuronal ensembles in posterior cortex) activate spontaneously).
• Although we often have trouble remembering dreams, our dreaming selves
have full access to our pasts. In dreams we recall earlier episodes from our
lives, and we often experience intense feelings of sadness, fear, anxiety or
joy (increased activity in the medial PFC).
• REM sleep behavior disorder = lack the muscle paralysis (known as atonia).
Patients act out their dreams: their actions match their dream reports. For
example, when they say they have dreamed about walking, they moved
their legs during REM sleep. They could dive from the bed or imagine they
are playing American football and tackle their bed partner,
Post-sleep wakefulness when
compared with REM sleep as baseline
red indicates greatest increase of rCBF
• Note dramatic increase in lateral PFC activation. The
PFC puppeteer is back to work controlling the puppets
(neuronal ensembles in the posterior cortex).
• In wakefulness your mental world is much more
controlled by your PFC (top-down recall, mental
synthesis, analysis) than during sleep.
PFC
We covered:
• Activation of a neuronal ensemble by an external
electrical current
• Activation of a neuronal ensemble by the PFC
• Activation of a neuronal ensemble by precise sensory
stimulation
• Activation of a neuronal ensemble by imprecise sensory
stimulation
• Activation of a neuronal ensemble by the hippocampus
 memory consolidation during sleep
• Spontaneous activation of a neuronal ensembles during
vivid dreaming
Innate neuronal ensembles
• Driven by sex
Driven by common experience: convex or concave?
• E.g., the sun is normally shining on top, so we
perceive the top row as convex
• Bottom row: concave
• The brain interprets the
line nearer the
apparent horizon as
farther away and
therefore longer than
the other identical line.
• Make a prediction:
is the Ponzo illusion
based on the innate
wiring of our brains; or
it is a product of
learning?
•
•
If the Ponzo illusion were the result of visual learning, the Prakash kids wouldn’t fall for it. But to the team’s
surprise, the children were just as susceptible to the Ponzo illusion as were control subjects with normal vision:
They consistently found the line closer to the horizon longer, the team reported in Current Biology in May.
The kids also fell for the Müller-Lyer illusion, a pair of lines with arrowheads on both ends; one set of arrowheads
points outward, the other inward toward the line. The line with the inward arrowheads seems longer. “All we can
say based on these results is that it’s not experience,” Sinha says. “It’s something else. It’s probably being driven
by very simple factors in the image that the brain is probably innately programmed to respond to.”
Conclusions
• Bayesian probability: the brain evaluates the probability
of a hypothesis, using some prior probability,
• i.e. the brain is not calculating but finding the closest
matching neuronal ensemble (forms a hypothesis).
• The hypothesis is then updated in the light of new
evidence,
• i.e. the brain adjusts the neuronal ensemble to fit the
new object.
No sensory perception is
absolute – everything is based
on our previous experience even colors are subjective
• In this animation, Mach bands exaggerate the
contrast between edges of the slightly
differing shades of gray, as soon as they come
in contact with one another.
Colors are
subjective
• A dress worn recently
to a Scottish wedding
– blue and black or
– white and gold?
• The dress is really
royal blue and black
• There really are no
colors outside of the
brain
• We always adjust the
perceived color (the
word label) based on
the background
Sometimes we can find
a neuronal correlate
• Ponzo perceives the upper bar as larger than the lower one
• The magnitude of this effect differs substantially across individuals.
(The size of the illusion is established by asking how much larger
the lower bar has to be to make it look the same size as the upper
one.)
• Surprisingly, these differences are reflected in the surface area of
the primary visual cortex (V1).
• For unknown reasons, the area of V1 can differ by a factor of
three among people (unfolded, the size and width of a typical V1
compares with that of a credit card).
• the smaller a person’s V1, the more powerfully he experiences the
illusion. Those individuals with a large V1 judged the size of the
bars to be more similar than those with a smaller V1.
PFC
Attention
• Attention (or more precisely top-down attention) can
be defined as consistent top-down effort to activate a
neuronal ensemble.
• This is consistent with increased blood flow to the
frontal lobe in all tasks that require maintenance of
attention
• In “top-down” attention the subject deliberately directs
the focus to an object.
• Count passes:
• https://www.youtube.com/watch?v=vJG698U
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Attention
• Conclusions: conscious perception
can often be different from physical
reality.
• Our senses can be fooled—we can
see something that is not there.
• Conclusion: “Vision is controlled
hallucination”
• We are just matching existing
neuronal ensembles to a pattern of
incoming sensory stimulation …
• Stop here
Persistence of vision
• Recall that beta rhythm is about 15Hz.
• Silent movies were shot at a rate that was between 16 frames per second and 23
frames per second. (When sound-film was introduced in the late 1920s, 24 frames
per second was chosen because it was the slowest (and thus cheapest) speed
which allowed for sufficient sound quality. TV currently shows 24 frames per
second.)
• The frame-rate of 16 to 23 frames per second was chosen because the viewer could
not see the flickering between frames due to an effect known as persistence of
vision, whereby the eye cannot react faster than about 60 milliseconds.
• At a frequency below the 16 frames per second threshold, the eye registers a
distracting flicker caused by the shutter of the film projector. Note that a 16 frame
per second threshold is just 1Hz greater than the 15 Hz frequency of the beta
rhythm.
• One can argue that persistence of vision is a property of the eye and has nothing to
do with the beta rhythm. Not so. The eye sensory properties developed during
evolution to serve the full potential of the mind. We do not need an eye that can
change its activity faster than the mind can register the change. The eye has a 16
Hz threshold because this was the simplest construction that served a mind with a
15 Hz rhythm. If our beta rhythm were faster, the eye structure would have evolved
to register faster changes. In other words, the eye is using the least complicated
(and therefore the slowest) mechanism that serves the conscious mind with its 15
frames per second beta rhythm.
• The visual system is optimized for the conscious experience.
More illusion examples
• https://www.youtube.com/watch?v=HhvOP_q
33Hw
• https://youtu.be/VxTFGVp2R-8
• https://youtu.be/Sk3OOA9LdaM
• Hybrid Images developed to test healthy
Eyesight:
http://cvcl.mit.edu/hybrid_gallery/gallery.htm
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Our perceptions are always influenced by the existing
neuronal ensemble that are ready to jump into synchrony