Exam 2 Flashcards
Goal of sensory systems
To shape an internal representation of the external world in a way that this internal representation facilitates the processing of/access to crucial information
Name 4 features that sensory systems extract
Modality – labelled line
Location – labelled line
Intensity – firing pattern & rate
Timing– firing pattern & rate
Modality
A property of the sensory nerve fiber.
– Receptors transduce specific type of energy into an electrical signal
– Nerve fibers activate by certain type of stimulus
– They make specific connections to structures in CNS
Location
Receptrive field of a neuron is a region of space in which the presence of a stimulus will alter the response of a neuron
Intensity
Stimulus intensity is encoded by the frequency of action potentials in sensory neurons.
– The intensity of a stimulus is also encoded by the size of the responding receptor population.
– Most of sensory systems have at least two kinds of receptros: low and high threshold receptors that contribute in encoding of the stimulus intensity.
Timing
The duration of a sensation is determined in part by the adaptation rates of receptros.
– There are two types of receptors: rapidly adapting and slowly adapting.
Rules of functional sensory and motor systems
- Each functional system involves several brain regions that carry out different type of information processing.
- Each part of the brain projects in an orderly fashion onto the next, thereby creating topographical maps.
- Identifiable pathways link the components of a functional system.
- Functional systems are hierarchically organized
- Functional systems on one side of the brain control/represent the other side of the body
Which cells form a map in primary visual cortex?
retinal ganglion cells
Where is the map from retinal ganglion cells stored in the brain?
primary visual cortex
To what features is V1 sensitive?
orientation and direction (HUBEL AND WEISEL)
Simple visual cells
Simple:
– Respond best to elongated bars or edges
– are orientation selective
– can be monocular or binocular
– have Separate ON and OFF subregions
– perform length summation
sum LGN inputs
Complex visual cells
Orientation selective
Have spatially homogeneous receptive fields (no separate ON/OFF subregions)
are nearly binocular
How is V1 organized?
In columns!
Functional Segregation of Visual Stream
Dorsal – Leads to MT
– Motion
– Depth
– Form (in V2)
Ventral – leads to V4
– Color
– Form
Depth (in V2)
Perception of Depth depends on what?
Depth vision depends on monocular cues and binocular disparity
Relative Size
objects that are closer appear larger, while objects that are distant appear smaller
Relative motion
apparent slowness indicates an object is distant
interposition
closer objects partially obstruct the view of more distant objects
Relative height
distant objects appear higher in your field of vision than close objects do
Texture gradient
distant objects usually have a much smoother texture than nearby objects
Relative clarity
distant objects are less clear than nearby objects
Linear perspective
parallel lines appear to converge in the distance
Monocular cues for depth perception
familiar size
occlusion
distribution of shadows/illumination
motion
linear perspective
size perspective
binocular depth cues
used by both eyes
Convergence:
– muscle tension in the eyes increases as objects move closer
Retinal Disparity:
– each eye has a slightly different perspective and image than the other
Correspondence problem
the problem of ascertaining which pats of one image correspond to which parts of another image, where differences are due to movement of the camera, the elapse of time, and/or movement of objects in the photos.
Neural mechanism behind depth perception
Input from 2 eyes converges onto same V1 cell w/ binocular receptive fields.
Binocular disparity-tuned neurons:
– Zero disparity-tuned
—- respond best when retinal images are on corresponding points in the 2 retinas.
– non-zero disparity tuned:
—- respond best when similar images occupy slightly different positions on the retinas of the 2 eyes.
Neural mechanism for motion detection
Step 1: two adjacent receptors only a small distance apart
Step 2: intermediary delay neuron
Problems of motion perception
- correspondence:
knowing which feature in frame 2 corresponds to particular feature in frame 1. - aperture problem:
when a moving object is viewed through an aperture (receptive field), the direction of motion of a local feature of part of the object may be ambiguous
Color Perception
– human color perception is based on activities of 3 independent
mechanisms that are differentially sensitive to different wavelengths
- spectral sensitivities of the 3 classes of cones.
- opponent processes
(red + green, yellow+ blue, dark + light)
Receptive Fields and Color
Perception of form
Low level dimension
– determines salient features. Cells in primary visual cortex respond to local features in a scene
Mid-level dimension
– group features into objects
high level vision
– match perceived to encoded representations
V1, V2, V4 cells respond to _____?
V1: cells respond to edges/lines
V2: cells respond to both illusory and actual contours
V4: respond to form
Which cortex responds to form?
Temporal
Vision as a constructive process
Adaptability
Contextual Effect
Plasticity
Attentional Modulation
Robustness to omission
How are odorants generally thought to be encoded?
as a combinatorial, ‘across-fiber’ code
Receptor activation to glom activation
Patterns of odorant receptor activation are mapped into patterns of glomerular activation in the olfactory bulb
How are gloms tuned at low odor concentrations?
at low odor concentrations, gloms are narrowly tuned and extremely sensitive to ‘best’ odorants
how are gloms spatially clustered?
gloms tuned to basic chemical features are spatially clustered
clustering/tuning properties reflect odorant receptor subfamilies
What’s a unique feature of olfactory circuits that shape glomerular representations of odors?
dendrdodendritic inhibition
what mediates gain control and filters weaker inputs in olfaction?
Intraglomerular feedfoward and presynaptic inhibition.
what circuit motif sharpens mitral cell receptive fields?
lateral (interglomerular inhibition)
additional functions of OB circuits
- decorrelation
– OB outputs are decorelated from sensory inputs - synchronizing OB outputs
–MT cells activated by same odor or mixture fire synchronously
How are odors represented in piriform cortex?
distributed and overlapping across piriform. not an odotopic map
how does piriform cortex represent odors?
neuron representations are combinatorial
Proposed mechanism for odor object recognition in piriform
another possible code for odor identity?
timing-based code for odor identity, where earliest activated glom drives odor perception
how quickly do rats/mice recognize odorants?
< 200 ms
to what is the canonical piriform cortex circuit optimized to do?
Filter earliest inputs in sniff cycle
why would a mouse sniff at higher freq?
sampling at higher sniff freq attenuates input from background odorants. experimental evidence suggests higher snif freqs reformat odor representations
Taste Transduction
- Tastants pass directly through ion channels (Salty, Sour)
- Tastants bind to g-protein-coupled receptors which activate second messenger pathways (umami, sweet, bitter)
How is taste quality encoded in taste cells?
taste quality is encoded via labeled lines. bitter receptors are not expressed in the same cells as sweet/amino acid receptors
to what are gustatory cortex neurons tuned?
gustatory cortex neurons are tuned to multiple tastants AND other aspects of taste-related behavior. they’re also NOT spatially segregated according to tuning
two viewpoints of taste info encoding
viewpoint 1. (mostly) as labeled lines. Taste afferents appear narrowly tuned to one taste
viewpoint 2. taste is encoded (mostly) as an across-fiber pattern
Is taste coding dynamic?
in cortex (at least) taste reponses are dynamic. different aspects of ta taste stimulus are encoded at different times. earlier response may be different than later response.
what do the different epochs of activity encode w/ taste?
1 - somatosensory: taste info never present
2 - chemosensory: distinct patterns of activation for different taste qualities
3 - palatability: Hedonically similar tastes (sour & bitter) evoked similar responses
Principles of taste coding
- In the periphery, different taste modalities have distinct detection and
transduction pathways and largely distinct afferent pathways into the
CNS. - Within the CNS, encoding of taste-related information involves
overlapping neural populations, is dynamic, and reflects distinct aspects
of taste perception and taste-driven behaviors at different times. - Gustatory cortex and amygdala likely both participate in taste-driven
behavioral decisions (i.e., - reject or ingest). - Neural representations of tastes are plastic and change with experience
and internal state
flexors vs. extensors
Flexors reduce joint angle when
contracted
Extensors increase joint angle when
contracted
minimal motor controls
- “Start” signal to initiate movement
- Flexor-extensor pairs on the same side must alternate
- Contralateral limb cycles must be anticorrelated
- Fore- and hindlimb cycles must be anticorrelated
Cell-intrinsic oscillations
arise from
the molecular features of single
neurons
Emergent network oscillations
an
be produced by reciprocal
inhibition of tonically-active cell
pairs. requires synaptic fatigue,
post-inhibitory rebound, or
adaptation to a tonic excitatory
input
what drives pyloric CPG?
intrinsic oscillations
what drives leech cardiac CPG?
reciprocal inhibition
half center model
spinal CPGs in lamprey
- Ipsilateral glutamatergic interneurons provide
rhythmic excitation to primary motoneurons
and to contraterally-projecting inhibitory
neurons - Inhibitory interneurons coordinate L-R
alternation
which mammalian SC neurons cause CPGs?
from Shox2 interneurons
Rubrospinal tract:
Cerebellar output
Tectospinal tract:
Short-latency,
reflexive behaviors
Vestibulospinal tract:
Balance, head
orientation
Reticulospinal tract:
Axial
movements, balance, limb
movements, modulates corticospinal
signals, locomotion
How is zebrafish locomotion encoded?
mix of labelled lines and distributed coding schema
how is speed encoded in zebrafish?
rate code of nMLF spinal projection neurons. topographical recruitment of motor neurons at different swim freqs. Dorsal neurons fire during fast swims. ventral neurons fire during slow swims.
motor cortex involvement w/ learning and reproducing a task
cortex is required for non-dexterous skill acquisition but not necessary for reproduction
Grassmans Laws (vision)
1) any light can be matched w/ a mixture of 3 primaries
2) rescaling light results in rescaled mixture
3) adding 2 lights together results in a sum of their mixtures
Color matching can be described as an N x 3 linear system
on what does retinal projection depend?
size and distance
Wavelength encoding (trichromacy)
Three cone types with different spectral sensitivities. Each cone
outputs only a single number that depends on how many photons
were absorbed. If two physically different lights evoke the same
responses in the 3 cones then the two lights will look the same
(metamers). Explains when two lights will look the same, not what
they will look like.
Color appearance
Color opponency: appearance depends on the differences between cone
responses (R-G and B-Y).
Chromatic adaptation: color appearance also depends on context
because the each cone adapts (like light and dark adaptation) to
the ambient illumination.
Color constancy: visual system infers surface color, despite changes in
illumination.
where are rods and cones primarily found?
cones primarily line the fovea whereas rods are found in the periphery
Mechanisms of light/dark adaptation
- Pupil size
- Switchover between rods and cones
- Bleaching/regeneration of photopigment
- Feedback from horizontal cells to control the
responsiveness of photoreceptors
Parasol ganglion cell:
- Inputs from many
photoreceptors - Fast/transient
responses - Poor spatial resolution
- Combine all cones
(“color blind”)
Midget ganglion cell:
- Inputs from few (or
one) photoreceptors - Slow/sustained
responses - High spatial resolution
example retinal functions:
ØLight Detection
ØMotion Detection and Discrimination
ØTexture Motion
ØObject Motion
ØApproaching Motion
ØAnticipation:
ØMotion Extrapolation
ØOmitted Stimulus Response
ØSaccadic Vision:
ØSaccadic Suppression
ØLatency Coding
ØAdaptive Computation
Øgain control mechanisms
All LGN neurons
- are monocular - respond to stimulation of one eye only
- have concentric (ON/OFF or OFF/ON) receptive fields
Type mLGN neurons in LGN magnocellular layers
- synapse with Type M retinal ganglion axons
- have large concentric receptive fields
- are insensitive to colorare insensitive to color
- sensitive to small changes in brightness levels (scotopic vision)
- are rapidly-adapting ( motion sensitive)
Type pLGN neurons in LGN parvocellular layers
- synapse with Type P retinal ganglion axons
- ha e small concentric recepti e fields (high acuity)* have small concentric receptive fields (high acuity)
- are sensitive to color ( color sensitivity)
- are not sensitive to small changes in brightness levels
- are slowly-adapting (indicate the duration stimulus is “on”)
bipolar cell receptive field
The field’s CENTER is formed by the receptor cell(s)
with which the bipolar make(s) direct synaptic contact.
RFs and synapeses of Rods/Cones
Rod Bipolars have large receptive fields whereas Cone Bipolars
have small receptive fields
Cone Bipolars in the fovea synapse with few cones, whereas
Rod Bipolars in peripheral retina synapse with many rods. p p p y p y
LGN physiology
*The LGN brings retinotopic maps from both eyes into register to make it
easy for cortex to combine inputs from the two eyes.
*The LGN is a convenient bottleneck for the modulatory inputs from the
brainstem and cortex.
V1
*V1 is located in the Calcarine sulcus in the medial occipital lobe of the brain.
*V1 is the first visual processing area in the cortex.
*All 6 layers of LGN project to area V1 in cortex.
*The magno and parvo layers project separately in the input layers of V1.
Normalization
computes a ratio between the response
of an individual neuron and the summed activity of a
pool of neurons.
Where/When do we see Normalization?
primary visual cortex (non-linear properties)
light adaptation in retina
size invariance in fly visual system
associative memory in hippocampus
representation of odours
the modulatory effects of visual attention
the encoding of value and the integration of multisensory information.
Rationales for Normalization
Maximizing sensitivity.
Invariance with respect to some stimulus dimensions.
Decoding a distributed neural representation.
Discriminating among stimuli.
Max-pooling (winner-take-all).
Redundancy reduction.
Whats common about normalization across all these different modalities
not the biophysical implementation but the computation that is done
effect of conductance on firing rates
It is now agreed that the effect of conductance increases on firing rates is divisive, but only if the source of increased conductance varies in time.
Possible normalization motifs
Feed forward inhibition
feedback inhibition
change in conductances
synaptic depression
fluctuations in membrane potential
may rely on amplification instead of suppression
noise
Draw the pyloric circuit
