04-10-2012, 11:12 AM
NEURONAL SPECIFICITY & PLASTICITY: FROM SPERRY TO THE BLUE BRAIN
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Abstract
This is a modest review of a couple of papers devoted to an examination of neural specificity and invariance among microcircuits in neocortical columns and minicolums, and the loci of experience dependent plasticity and learning within and between these microcircuits. It focuses on somatosensory cortex, but the appendix covers visual research as well on some of the early work on these issues, from Stratton, to Sperry, Hebb, Mountcastle, Hubel and Wiesel, and T. A. Woolsey and Van der Loos. It starts with a paper from Markram’s group on the invariant properties of the microcircuits, and conjectures on a segue form Hebb to Edleman’s Darwinian brain theories. Next I review Petersen’s paper on neurophysiological studies of experience dependent plasticity in these systems. Some network theory has proved essential to research in these areas, as well as some nifty technical advances in neurophysiological stimulation and recording.
Blue Brain Project
Rosen Industries created the prototype for the Blue Brain. The report is in Do Androids Dream of Electric Sheep, which obviously inspired Markram’s Blue Brain Project in Lausanne to reverse-engineer the mammalian brain, creating a complete virtual brain within IBM’s BlueGene/L supercomputer. The Blue Brain project proposes a fantastic voyage.2 The first phase of this project succeeded in simulating a rat cortical column. It claims to use more realistic models of neurons than most neural net models.3 It has spawned several ancillary projects around the globe, such as the Cajal Blue Brain in Madrid.
Roll out the Barrel
Rats, mice, and other rodents are nocturnal, so they have not developed the complex visual processing capabilities of primates, cats, and many other animals, such as found in the studies by Hubel and Wiesel. But evolution has favored them with a similarly complex ability to analyze 3D properties, including integrating bilateral sensory information, with their whiskers. The neocortical terminus of much of this information is in minicolumns, especially in, but not confined to, L4 pyramidal cells, an area frequently referred to as ‘barrels’.
PLASTIC IN THE BARRELS
Early (window of a few days after birth) destruction of whisker follicles is accompanied by failure of the development of the corresponding barrels which is unaffected by genetic destruction of NMDAR8 (Iwasato et al., 2000). Petersen considers that thus NMDR cannot be important for the developmental sensitivity to deprivation and that the critical period to change the mainly genetically determined large-scale map of the barrel field only lasts until postnatal day four (p. 345).
However plasticity can be supported on a smaller scale. Dendritic filopedia and spines grow in response to LTP from strong synaptic input which implicates them in synaptic plasticity. Injecting the Sindbis virus with its gene for a fluorescent green protein (SIN-EGFP) onto a small area of barrel cortex and using a two photon scanning electron microscope (2PLSM) in vivo to examine the morphodynamics of dendritic filopodia and spines in L2/3 (the limit of the focusing depth of the 2PLSM being approximately 600 μm), Lendai et al. (2002) found a critical period for robust sprouting in the young adult rat.
[attachment=35225]
Abstract
This is a modest review of a couple of papers devoted to an examination of neural specificity and invariance among microcircuits in neocortical columns and minicolums, and the loci of experience dependent plasticity and learning within and between these microcircuits. It focuses on somatosensory cortex, but the appendix covers visual research as well on some of the early work on these issues, from Stratton, to Sperry, Hebb, Mountcastle, Hubel and Wiesel, and T. A. Woolsey and Van der Loos. It starts with a paper from Markram’s group on the invariant properties of the microcircuits, and conjectures on a segue form Hebb to Edleman’s Darwinian brain theories. Next I review Petersen’s paper on neurophysiological studies of experience dependent plasticity in these systems. Some network theory has proved essential to research in these areas, as well as some nifty technical advances in neurophysiological stimulation and recording.
Blue Brain Project
Rosen Industries created the prototype for the Blue Brain. The report is in Do Androids Dream of Electric Sheep, which obviously inspired Markram’s Blue Brain Project in Lausanne to reverse-engineer the mammalian brain, creating a complete virtual brain within IBM’s BlueGene/L supercomputer. The Blue Brain project proposes a fantastic voyage.2 The first phase of this project succeeded in simulating a rat cortical column. It claims to use more realistic models of neurons than most neural net models.3 It has spawned several ancillary projects around the globe, such as the Cajal Blue Brain in Madrid.
Roll out the Barrel
Rats, mice, and other rodents are nocturnal, so they have not developed the complex visual processing capabilities of primates, cats, and many other animals, such as found in the studies by Hubel and Wiesel. But evolution has favored them with a similarly complex ability to analyze 3D properties, including integrating bilateral sensory information, with their whiskers. The neocortical terminus of much of this information is in minicolumns, especially in, but not confined to, L4 pyramidal cells, an area frequently referred to as ‘barrels’.
PLASTIC IN THE BARRELS
Early (window of a few days after birth) destruction of whisker follicles is accompanied by failure of the development of the corresponding barrels which is unaffected by genetic destruction of NMDAR8 (Iwasato et al., 2000). Petersen considers that thus NMDR cannot be important for the developmental sensitivity to deprivation and that the critical period to change the mainly genetically determined large-scale map of the barrel field only lasts until postnatal day four (p. 345).
However plasticity can be supported on a smaller scale. Dendritic filopedia and spines grow in response to LTP from strong synaptic input which implicates them in synaptic plasticity. Injecting the Sindbis virus with its gene for a fluorescent green protein (SIN-EGFP) onto a small area of barrel cortex and using a two photon scanning electron microscope (2PLSM) in vivo to examine the morphodynamics of dendritic filopodia and spines in L2/3 (the limit of the focusing depth of the 2PLSM being approximately 600 μm), Lendai et al. (2002) found a critical period for robust sprouting in the young adult rat.