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The Functional Organization of the Barrel Cortex Alfred Li Cassie Qiu Outline ● How is Barrel Cortex important for mouse (and us!) ● From whisker to cortex: functional anatomy. ● How barrels are mapped. ● Connection within barrel cortex. Why Barrel Cortex? • Barrel Cortex: Primary Somatosensory Cortex (S1). • Somatotopic map • sensory information actively acquired • Important Because: • Mouse and Rat nocturnal: lack of vision • Solution: Tactile information by whiskers: spatial information, object position and texture • Understanding of active perception, experience-dependent plasticity and learning Active Exploration • https://www.youtube.com/watch?v=5gLKtw8gDZU • Whisking is an attractive model for studying sensory processing and sensorimotor integration. Barrel Cortex Somatosensory very important to rodents, larger portion of the brain. From Whisker to Cortex Cortex (barrels) Inhibit (by motor cortex) Thalamus (barreloids) Zona Incerta inhibit Brain Stem (barrelettes) Sensory Neuron From Whisker to Cortex Somatotopic map throughout Brainstem and thalamus reliable coding, cortex has large variability, Single whisker receptive field in ganglion contrast with broad receptive field in Cortex. Suggesting interaction with other cortical activity, and suggest that neo cortex generate association, context dependent. Role of different pathways • Lemniscal pathway: Major signal pathway Strongly modulated by touch and phase of whisking • Paralemniscal pathway: weakly modulated by touch and phase of whisking problem: POM inhibited by ZI, disinhibit when whisking, possibly encode touch, still unclear. Barrel • Barrel = Cortical column responding to stimulus to one whisker. • Primarily due to projection to layer 4. • Identical layout as Whiskers: Barrel Map • Determined genetically • Can be visualized with histology. How Barrels are mapped • Electrophysiology • Single electrode, Electrode array Intrinsic Optical Imaging • A type of BOLD (blood oxygenation dependent signal) • Differential Absorbance of Oxy and Deoxyhemoglobin at higher than 600nm (highest contrast there) • Measure back scattering of red light from tissue. • Fast initial rise of deoxyhemoglobin, follows by slow not very localized drop of deoxy, rise of oxyhemoglobin. IOI measure latter. Averaged intrinsic optical response to whisker stimuli Mapping of Barrel Across intact skull. Left to Right: Green light imaging of blood vessel, Intrinsic imaging of response to one whisker stimulus, insert Dil crystal (dye) to response site. DAPI (stain for nucleus) and Dil fluorescence picture, after sectioning. Match between optical mapping and anatomical barrel map. Voltage Sensitive Dye Mapping • High temporal resolution. Graphics: Response to a single deflection of C2 whisker. • Subthreshold Potential. • Only reflect activity at surface. Difference in Spatial extent: Subthreshold vs. Suprathreshold measurement. Low frequency vs. High frequency stimulus (known to evoke more localized response) Mapping Techniques • Provide functional evidence for somatotopic sensory processing precisely aligned to the anatomical barrel map • dynamic pattern for object localization • Texture • different whiskers exhibit different resonant frequencies Structure Within Barrel: Orientation Map (In rat) • Layer 2,3 measured with tetrode (clustered four electrodes) recordings. • Deflect toward a nearby whisker = stronger response near the barrel for that whisker. The future: Calcium Imaging Influx of Ca2+ during action potential. Fluorescent Protein response to [Ca2+]. • Single Cell Activity • Sectioning (Certain Layer) • Mediocre Temporal resolution (slow removal of Ca2+) Connections within Cortical Column Histology and in-vitro studies shows that L4 excitatory neuron have axon laterally confined to single barrel. B. Red square is simulation point Right most: GABA inhibition blocked, signal spread. C. Uncaging = release. Connection in Barrel Cortex • L2/3 neuron extend well beyond boundaries of barrel column. (spreading of VSD signal) • L2/3 neuron also have axon to L5, which receive input from L4 and thalamus etc. Integration and Computation. • Signal spread more when GABA inhibited: real cortex can be more excitable. • Other Connection: Cortex-Cortex (from S2, M1) Paralemniscal, extralemniscal pathway. The Functional Organization of the Barrel Cortex (PART II) Alfred Li Cassie Qiu Development and Plasticity of the Barrel Cortex Patterning • somatotopic patterning of the barrel cortex appears to be determined by genetic programs • position and dimension of the barrel field - FGF8 • ectopic posterior expression of FGF8 induces formation of a secondary barrel field Development and Plasticity • Refinement of the map might be guided by activity-dependent mechanisms • barrels less defined or absent in NMDA receptor KO mice • development is within a few days of birth (early critical period) • lesion of whisker follicles - no formation of corresponding barrels Development and Plasticity • Next critical period relates to NMDA receptor-dependent plasticity • LTP cannot be induced after P8 • increase in axon and dendrite complexity Development and Plasticity • whisker deflection localized to individual cortical columns • reduced synaptic connectivity • barrel cortex neurons receive information relating to their principle whiskers early in development Experience-Dependent Map Plasticity in Mature Rodents • depression of evoked responses to deflection of the trimmed whiskers • presynaptic reduction in neurotransmitter release probability • consistent with a Hebbian spike-timing-dependent plasticity • increasing response to remaining whiskers • using intrinsic optical imaging (caged vs. novel environment) Experience-Dependent Map Plasticity (A) Single-whisker experience induced a profound and reversible expansion of the spared whisker representation. (B) reversible contraction of the spared whisker representation State-Dependent Whisker Perception • Spontaneous cortical response • Whole-cell recording of layer 2/ 3 (membrane potential changes) • quiet wakefulness - slow largeamplitude • active whisking - variance smaller, depolarize • VSD shows spontaneous slow oscillation, spread like wave. State-Dependent Whisker Perception • processing of sensory stimuli • Deflection of C2 whisker • quiet wakefulness - strong cortical sensory response, wild spread • active whisking - weak response, more localized. • Difference not due to sensory organ (follicle): trigeminal ganglion stimulation. Actively Acquired Sensory Information • Recordings from the first-order sensory neurons in the trigeminal ganglion of awake rodents • in the absence of whisker movement - no spontaneous action potential firing in the trigeminal ganglion. • ‘‘whisking in air,’’- a low level of spiking activity in the sensory neurons. • phase-locked signals could form the basis of a map of positional information • many action potentials in the sensory neurons were evoked when the whiskers contacted objects Whisker-related trigeminal ganglion neurons are therefore sensitive object detectors Actively Acquired Sensory Information Actively Acquired Sensory Information • single-whisker active touch responses can also propagate across the barrel map • Contrast with artificial whisker stimulation, which is weak. Actively Acquired Sensory Information • Real whisker-object contacts, but not remotely applied passive stimuli, might be specifically amplified by a rapid low-level sensorimotor loop Sensory Information Processing during Learned Behaviors ● Two behavior task using whisker ○ detection of edge locations: reach other side for reward ○ discrimination of textures (finger level performance) ● Studies: ○ edge location detection only need one whisker, require S1. ○ Single whisker can also detect object position ○ two separate psychophysical channels when responding to single whisker stimuli. ■ small A high f, and small f high A. ■ rapidly adapting & slowly adapting trigeminal Sensory Information Processing during Learned Behaviors • different action-potential activities in different cortical layers. • top-down input.