Sensory Neuro Final

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Leech experiment

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4 somatosensory neurons for avoiding different directionsNormally picks direction from weighted sum of firing, determined by electrical stim (with known vectors) of different neurons

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time varying population codes

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needed for olfaction, same neurons, different timing

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Noise from neuron itself

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Variable number of vesicles are sent out Different firing/excitation thresholds for dif cells Failure to cross synapse Background brain activity

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broad location of categorization in brain

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cortical response

ventral/dorsal parts of visual pathway correspond to ‘what’ and ‘where’, or ‘perception’ and ‘action’ (i.e., ventral à object recognition; dorsal à motion

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inferotemporal cortex (IT)

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contains things such as “hand” (i.e. highly resolved specific object) responsive cells

Such neurons respond only to assembled parts and:Any sizeAny positionAny orientation

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IT KO

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prevents identification of objects by name

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Hierarchical theory of object recognition

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from V1->V4, ventral visual pathway, neurons respond to increasingly complex features, by pooling the information from the neurons in the layer below

evidence: neural responses occur with progressively longer latencies along the ventral pathway

problem: (1): too many objects / not enough cells !! - problem (2): not robust to damage

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plastic theory of object recognition

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neurons that respond to the same object under different conditions become linked together over time, forming an 'ensemble'

supported by morph/troll experiments

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McGurk effect

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ba/fa

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prefrontal neurons track

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the category choice (one or the other), rather than sensory input (graded)

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prefrontal cortex KO

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disrupts ability to flexibly categorize visual objects according multiple criteria

(wisconsin card sort)

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ventral visual pathway

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V1 à V2 à V4 à IT

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Superior colliculus

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subcortical visual area, receives direct input from retina and auditory and somatosensory systems. contains two topographic sensory maps: of visual input and of auditory space. maps are contained in different layers linked to both direction of attention to a specific location, as well as to guidance of eye movements

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multisensory enhancement is strongest when

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(i) both stimuli come from the same region of space, and (ii) receptive fields of the neurons overlap

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how sensory info is weighted

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by accuracy

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Parietal cortex

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neurons have receptive fields that sample the same region in both visual and auditory space (albeit more broadly)

also, firing scales with the amount of reward associated with the target; amount of payoff

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multisensory crosstalk evidence

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Brain regions associated only with vision or audio or smell etc. also show responses to opposite sensory inputs

Ex. The potato chips r so crispy

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frog eye rotation effects

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after eye is rotated: same connections (tectum to colliculus), but retinal visual field is now rotated relative to tectum

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nasal RGC axons -> temporal RGC axons pattern determination

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increased EphA, decreased ephrinA5

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EphB

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does medial/lateral patterning (of visual areas)

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cortical development

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occurs ‘inside out’ - neural stem cell progenitors at the base of the developing X continually divide and produce new neurons that migrate outwards - is build vertically (‘columns')

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FGF8

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alters global position of somatosensory cortex

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chemoaffinity hypothesis evidence

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(Sperry) - stereotyped wiring between retina and optic tectum of frog - connections depend on location in tissue, not in sensory space à neurons in source / target tissue have inherent ‘matching’ cues

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synaptic pruning

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at each, there is a ‘winner’ (the one with more activity) and a ‘loser’

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extra frog eye effects

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competition induces ocular dominance columns (on the cortical side where its inputs go) (not normally present in this system)

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glutamate encourages

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dendritic spine growth

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molecular basis of reliance on correlation for synaptic strengthening

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normally, the channel pore is blocked by Mg2+, so no little/no current even if glutamate binds. Ions pass only when the cell is already depolarized (e.g. during correlated activity from several inputs at once)

many inputs (correlated activity): many AMPA receptors activated; more depolarization; Mg2+ block is removed; NMDA activation à strengthening of synapse

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NMDA receptor KO

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tectum patterning is lost

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effects of vision limited to moving bars

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cortex only ‘learns’ the direction to which it was exposed

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Hubel and Wiesel

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first observation of critical periods was in the visual cortex

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critical period timing across different layers of processing

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areas with more complex functional roles also tend to have timing that is delayed relative to ‘simpler’ / ‘earlier’ areas

ex. inputs to olfactory bulb followed by olfactory cortex

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effects of dark rearing or white noise

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both onset and closure of critical periods is delayed

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What determines the end of critical periods?

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synaptic plasticity & stabilization - inhibitory circuits - structural factors

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Use of NMDA receptors in timing

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(1) channels stay open longer, and allow more Ca2+ influx(2) NR2B is preferentially coupled to downstream signaling pathways for driving changes in synaptic strength

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developmental timing usage of NMDA receptor types

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NMDA-2B to NMDA-2A

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GABA

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connections with inhibitory neurons using X mature with the same time course as critical periods, separates the firing in different populations of neurons, then limits the time window during which neurons are active

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KO or increase of GABA action

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prolongs or hastens the critical period end

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evidence for cortical "flexibility"

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surgically redirecting visual inputs to auditory thalamus results in the formation of normal visual maps and sensory responses in AUDITORY X

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metaplasticity

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thresholds for driving modifications in synaptic strength are altered by previous history. Prolonged periods of lower activity (e.g. darkrearing) lowers threshold for increasing synaptic strength

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Aplysia

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touching siphon leads to withdrawal reflex. Pairing siphon touch with shock increases withdrawal.

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locus ceruleus

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releases noradrenaline widely throughout the brain and LC activity linked to arousal state and strongly engaging contexts

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basal forebrain (nucleus basalis) and brainstem

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ACh systems: widespread ACh release; facilitates plasticity, activated most strongly when a sensory stimulus is salient / surprising / engages attention

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CF-FM call

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constant, narrow band frequency: ideal for Doppler type detection - any movement of the target will shift frequency of the returned echo

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FM-call

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frequency modulated: rapid upward/downward shifts in frequency spectrum over a few milliseconds

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FM-FM area

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different sets of neurons that will be activated at different distances away from the target (echo return delay), organised in increasing order in cortex

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DSCF (doppler shift constant frequency) area

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area expanded representation of frequencies close to the emitted pulse

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bat active sampling strategies

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systematic modulation of sonar emission frequency depending on approach phase & distance to prey (avoids overlap for successive pulses) - directional emission of sonar calls; sequential targeting of different objects / obstacles

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passive electroreception method

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Tight junctions in the skin make it a HIGH-resistance pathway. - Ohm’s law: current flow driven by local voltage fields thus preferentially passes through the canal of the ampullae instead; where it flows through the tissue surrounding the electroreceptors, creating a local voltage gradient

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active electric organ

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modified muscle cells - polarized cells: one side has a high concentration of ATP-dependent Na+/K+ pumps, producing resting membrane potential - other side has a high concentration of ACh receptors and is innervated by motor inputs from a pacemaker nucleus. Synchronized inputs open receptors causing current flow into cell

objects in the field created by the fish will either increase or decrease local current densit

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corollory discharge

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a copy of a motor command (electrosensing) that is sent to the muscles to produce a movement. This copy or corollary does not produce any movement itself but instead is directed to other regions of the brain to inform them of the impending movement

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MUPs and female hormones

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sexy during estrus, downregged by prog

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sexually dimorphic processing of same cue example

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CVA causes aggression or receptiveness by alt splicing of “fruitless”. gay flies produced by changing fru splicing

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Sketchy evidence of pheromone detection in humans

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exposure to axillary-derived odors affects the timing of the menstrual cycle in human women. Also vno receptors in main olfac bulb

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AOB neurons

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encode chemosignals with information about individual identity: show strong selectivity for sex, gender, and strain

used in Bruce effect

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Bruce effect

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Mating suppresses mitral cell output. Suppression is specific to stud-activated MCs

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limbic system

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(hypothalamus); -‘emotional/salience’ areas (amygdala) - reward structures (dopamine system) -memory structures (hippocampus); - higher cortical areas (prefrontal cortex)

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Rodent TAARs

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respond to amine containing compounds found in the urine of carnivores/predators

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conditioned fear extinction

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first encoded in amygdala

then not really lost; but overwritten by another memory - extinction is controlled by top-down inputs to the amygdala from prefrontal cortex

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Hippocampus

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provides data to PFC about overall sensory context to decide whether fear memory should be expressed or suppressed

also, Navigation is robust to lost placemarks pattern completion

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MPC (medial premotor cortex)

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increases in firing are nearly perfectly correlated with the monkey’s subjective report of perception.

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Attractor network definition/formation

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neurons in brain area ‘X’ that receive sensory input; neurons are heavily interconnected with each other.

Sensory input causes hebbian plasticity

subsequently , partial sensory input will activate the whole circuit

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LIP of parietal cortex

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neurons act as if they are accumulating sensory information: - more coherence between dots -> more bias in evidence for one direction-> faster increase in firing rate of neurons

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Bottom up attention

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arises from properties of raw sensory input; ‘pop-out effect’ arises from disruption of background regularity in some dimension

saliency depends on level of difference from background features

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ACh systems roles in attention

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a stimulus entering activates X cells in nucleus basalis leads to changes in the cortical processing of sensory stimuli

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Top down attention

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directing attention, improves speed/sensitivity
in general, attentional changes become more prominent in higher processing areas (ex. V4, MT)

heavily involves parietal regions and pulvinar nucleus of the thalamus

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PREMOTOR theory of covert attention

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the same systems that produce overt attention via eye movements may also be responsible for the amplifying effects of X, eye movements are driven by activation of neurons in the frontal eye field (FEF) , stimulating the FEF also increases the firing of visual neurons

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posterior->anterior brain

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Emx2 down, Pax6 up