Past events

Odor perception - discrimination and recognition of volatile chemicals in the environment – is critical for wide ranging behaviors including kin recognition, mate selection, predator avoidance, homing, and feeding. Most naturally occurring odors are complex mixtures, often containing hundreds of different components. Furthermore, natural odors invariably occur against odorous backgrounds. Thus, olfaction and odor perception involves far more than simple odor ligands binding to receptors in the nose. I will describe recent work

Reelin, a glycoprotein of the extracellular matrix, is secreted by Cajal-Retzius cells in the marginal zone of the cortex and controls the radial migration of cortical neurons. Reelin signaling involves the lipoprotein receptors apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR), the adapter protein Disabled1 (Dab1), and phosphatidylinositol-3-kinase (PI3K). In regulating neuronal migration, Reelin signaling eventually acts on the cytoskeleton; however, its effects on the dynamic reorganization of the cytoskeleton have remained obscure. In reeler mutants deficient in Reelin, the majority of cortical neurons are unable to migrate to their destinations, suggesting Reelin signaling to be essential for the dynamic cytoskeletal reorganization that is required for neurons to migrate. In contrast, we show that Reelin signaling stabilizes the cytoskeleton by serine3 phosphorylation of n-cofilin, an actin-depolymerizing protein. Phosphorylation at serine3 renders n-cofilin unable to depolymerize F-actin. However, depolymerization of F-actin is required for cytoskeletal reorganization. The Reelin receptor ApoER2, Dab1, src family kinases (SFKs), and PI3K were found to be involved in n-cofilin serine3 phosphorylation. Phosphorylation of n-cofilin was observed in the leading processes of migrating neurons when they reached the Reelin-containing marginal zone. Using a stripe choice assay, we found neuronal processes to be stable on Reelin-coated stripes. In contrast, on control stripes they formed lamellipodia as a sign of ongoing growth. These new results indicate that Reelin-induced stabilization of neuronal processes anchors them to the marginal zone which is crucial for directional migration by nuclear translocation. (Supported by the German Research Foundation, DFG: SFB 592)

Abstracting from physiological details, I will present a theory that suggests an explanation behind critical periods in learning as a natural consequence of learning dynamics under a small and realistic set of assumptions. Surprisingly, the same theory offers an explanation for other animal learning phenomena, such as the tendency to reverse to the status quo following a transient learning experience. Additionally, the theory suggests simple experiments that can be used to prove or refute it. If the time permits, as a commercial for future results, I will finish the talk with a brief overview of recent attempts at LANL for petascale simulations of the mammalian visual cortex.

Functional MRI (fMRI) studies have defined a series of visual processing regions in the human cortex, which are believed to enable visual recognition behaviors through a hierarchy of processing stages. At the higher stages in this hierarchy lie regions which preferentially respond to images of intact objects compared to other visual stimuli, a set of regions collectively termed object-selective cortex. Within object-selective cortex exist category-selective regions, which prefer particular categories of images over others (e.g., faces, body parts, houses or scenes). Initially these regions were considered non-retinotopic, but increasing evidence indicates substantial retinal position selectivity, and in some cases retinotopy, in these regions. What is the representation of the visual field in object-selective regions? Are separate object- and category-selective regions part of a single map or embedded within a set of distinct visual field maps? We scanned seven subjects on separate experiments to localize object/category-selective regions, and measure visual field maps (GE 3T scanner). For retinotopic experiments, subjects viewed moving bar stimuli containing different stimuli, including slowly drifting checkerboards and frontal face images. The bars extended out to around 14° eccentricity from the fovea, and had a width of ~2.6°. We employed a recently-developed method for estimating population receptive fields (pRFs) using fMRI (Dumoulin and Wandell, Neuroimage, 2008), which estimates pRF center and size for each cortical location. Face-containing bars produced substantially larger responses than checkerboards along the fusiform gyrus, improving our ability to measure visual field maps in these regions. Eccentricity maps revealed two foveal representations, which may correspond to visual field map clusters previously identified as VO and VT (Wandell et al., Neuro-opth. Jpn., 2006). These foveas are within or adjacent to fusiform face-selective regions, and separated by smoothly-varying extra-foveal maps which are less face-selective. For several subjects, pRF sizes systematically increased with eccentricity in face-selective regions. The distribution of pRF sizes were substantially larger than in earlier visual cortex, but comparable to recent measurements made in lateral occipital cortex. We find two spatially separate face-selective regions along the fusiform gyrus, with comparable visual field coverage, separated by a representation of intermediate eccentricities. This indicates these two regions are likely to fall within different visual field maps. Current work addresses possible effects of low-level visual features (e.g. spatial frequency) and stimulus visibility in driving the observed face-selective retinotopic responses. I will also present some preliminary data from retinotopic mapping with house-containing bars, and an examination of retinotopic organization in house- or scene-selective cortical regions.