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Of all sensory cortical areas, barrel cortex is among the best understood in terms of circuitry, yet least understood in terms of sensory function. Because sensory cortical areas have stereotyped anatomies, understanding computations in one sensory area may inform us of computations being performed by other sensory areas or sensory microcircuits all over the brain. Functional studies of barrel cortex are therefore important for marrying our immense and increasing knowledge of the cortical circuitry with the computations being performed in a cortical microcircuit. This thesis is an investigation of barrel cortex function as it pertains to 1) site specific sensory evoked plasticity in cortical microcircuit and 2) sensory receptive fields of the different cortical lamina in S1. The brain's capacity to rewire is thought to diminish with age. It is widely believed that development stabilizes the synapses from thalamus to cortex and that adult experience alters only synaptic connections between cortical neurons. We combined whole-cell recording from individual thalamocortical neurons in adult rats with a newly developed automatic tracing technique to reconstruct individual axonal trees. Whisker trimming substantially reduced thalamocortical axon length in barrel cortex but not the density of TC synapses along a fiber. Thus, sensory experience alters the total number of TC synapses. After trimming, sensory stimulation evoked more tightly time-locked responses among thalamorecipient layer 4 cortical neurons. Axonal plasticity was topographically specific, with robust changes in L4 and modest changes in the septal and infragranular layers. These results indicate that plasticity is mediated by interactions with the local cortical subcircuit and may be suggestive of laminar specific roles in sensory learning/coding. Next we sought to examine spatiotemporal coding properties of neurons in the different layers of the cortical microcircuit in S1. We combined intracellular recording and a novel multi-directional multi-whisker stimulator system to estimate receptive fields by reverse correlation of stimuli to synaptic inputs. Spatiotemporal receptive fields were identified orders of magnitude faster than by conventional spike-based approaches, even for neurons with little or no spiking activity. Given a suitable stimulus representation, a simple linear model captured the stimulus-response relationship for all neurons with unprecedented accuracy. In contrast to conventional single-whisker stimuli, complex stimuli revealed dramatically sharpened receptive fields, largely due to the effects of adaptation. Surprisingly, this phenomenon allows the surround to facilitate rather than suppress responses to the principal whisker. Optimized stimuli enhanced firing in layers 4-6, but not 2/3, which remained sparsely active. Surround facilitation through adaptation may be required for discriminating complex shapes and textures during natural sensing.

Thesis (Ph.D.). / Written for the Dept. of Neurology & Neurosurgery. Title from title page of PDF (viewed ). Errata sheets included. Includes bibliographical references.

No description available.

Three groups of rats were tested both before and after the bilateral ablation of the taste sensory cortex. The first group, exposed to quinine hydrochloride (QHCL) in a two-bottle preference situation, showed a large deficit postoperatively, but these were considerably reduced by the fourth postoperative week. A second group, tested for sodium chloride (NaCl) discrimination in a modified signal detection situation, also showed significant postoperative impairment. A third group, QHCL discrimination, was discarded for failure to learn the detection task. The results which were very unclear compared with NaCl discrimination and QHCL preference. It is concluded that preference tests are unsatisfactory measures of taste sensitivity unless the stimuli possess extreme aversive or preferred qualities. / Thesis / Master of Arts (MA)

Diss. (sammanfattning) Stockholm : Karol. inst., 2004. / Härtill 4 uppsatser.

Human brain evolution granted us cognitive and behavioral capabilities that are unique amongst animals. SRGAP2 is a gene that was specifically duplicated in the human lineage and plays roles in the regulation of cortical development and synapse dynamics. As paralogs of one of the few known genes that regulates excitatory and inhibitory synapses concurrently, the duplications of SRGAP2 were well-positioned during human evolution to gain novel functions leading to the cognitive and behavioral phenotypes exhibited in humans. SRGAP2C, a human-specific paralog of the ancestral SRGAP2 gene, inhibits every known function of SRGAP2 and induces a phenotype similar to SRGAP2 knockdown. This induces neoteny in the maturation of synapses in mice, allowing us to study a putatively “human-like” phenotype in the mouse brain. While studies have been conducted on the effects of SRGAP2 manipulation in juvenile and young adult mice, its effects on older mice has yet to be determined. In this dissertation, we perform longitudinal imaging experiments to determine the effects of SRGAP2 manipulation in the cortex of adult mice. In Chapter 3, we first examine the effects of SRGAP2 knockdown on the spine dynamics on apical dendrites of layer 5 pyramidal cells in the barrel cortex of adult mice, determining how it regulates spine density, turnover, and survival at baseline and in response to sensory deprivation. In Chapter 4, we study how SRGAP2 knockdown affects the clustered formations of new dendritic spines on the apical dendrites of layer 5 pyramidal cells in the barrel cortex of adult mice. Together, these results represent the first demonstration of SRGAP2 regulating on synapse dynamics in vivo and show that SRGAP2 knockdown can be used to model human brain evolution in adult mice.

Thesis (Ph.D.)--Drexel University, 2008. / Includes abstract and vita. Includes bibliographical references (leaves 95-101).

The sensorimotor network receives a rich variety of somesthetic afferents and outputs considerable motor efferents, both of which drive experience-dependent plasticity in the system. It remains unclear to what extent subtle changes in somaesthesis and motor function extrinsic to the brain drive plasticity in the functional organisation and anatomy of the sensorimotor network. This thesis contains a series of multimodal MRI experiments to investigate how altered-use and disuse can induce plastic changes in the sensorimotor network of the human brain. In Chapter 3, a method of mapping digit somatotopy in primary somatosensory cortex at the single-subject level using 7.0 tesla fMRI was developed and applied for a study of healthy participants. Using a phase-encoding paradigm, digit representations were accurately mapped in under 10 minutes. These maps were reproducible over time and comparable to a standard block design. In Chapter 4, a further fMRI study assessed the potential for short-term reorganisation of digit representations in primary somatosensory cortex following a manipulation whereby the right index and right middle fingers were glued together for 24 hours. There was a marked shift in the cortical overlap of adjacent digits after the glued manipulation, not seen across an equivalent control period, providing strong evidence for short-term remapping of primary somatosensory cortex. In Chapter 5, a patient study investigated plasticity associated with chronic unilateral disuse of the upper limb. A cross-sectional comparison with control participants showed reduced grey matter density in the posterior right temporoparietal junction, and increased radial diffusivity in the white matter of the right superior longitudinal fasciculus, consistent with change in the right ventral attention network. A complementary longitudinal study in Chapter 6 investigated structural plasticity associated with rehabilitation of the disused limb. There were localised increases in grey matter density, notably in the right temporoparietal junction, further implicating a potential role for regions responsible for egocentric attention in regaining upper limb use. In Chapter 7, a further patient study investigated candidate predictive biomarkers at the sub-acute stage of stroke recovery, identifying CST-lesion cross-section and sensorimotor network strength as correlates of motor function, which warrant further study. The results of the studies presented in this thesis provide a novel insight into the nature and time frame of functional and structural plasticity associated with altered use and disuse. Further study of how subtle changes in our sensory and motor use shape the sensorimotor network is warranted, particularly in the context of disuse in non-neurological clinical populations.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2005. / Härtill 4 uppsatser.