Ralph Nelson And Helga Kolb

ON and OFF pathways are feature detectors A key concept in visual system physiology is the notion of feature detectors. The idea of feature detection is that the visual system separates the visual image into components, much the way a publisher creates a color print by separating the picture into separate masks for different colored inks. The idea of feature detection was most forcefully presented by Lettvin et al. (1959) in the classic paper, What the Frog's Eye Tells the Frog's Brain. In this...

Instructive factors and modulatory factors

We will distinguish between factors that affect the instructive pathway for plasticity, factors that may modulate this instructive pathway, and factors that are involved with both. Given that plasticity is driven by the pattern of activity reaching the visual cortex from the retina, glutamate receptors are clearly on the instructive pathway, and so are calcium and second messengers activated by glutamate receptors. Modulatory factors include state of attention, state of sleep, nonspecific...

On the central course of the optic nerve fibers and the pattern of decussation in the chiasma

Early anatomists saw a prominent nerve exiting the back of each eye directed toward the brain. It was usually assumed that the nerves originated in the brain and extended out to the eye. The fibers arising from each eye appeared first to unite and then to cross the midline in the X-shaped optic chiasm. With earlier techniques of crude dissection, the pattern of crossing was not clear, so the true picture was not accepted until the late nineteenth century. The rearrangement of fibers in the...

Principles of optic chiasm organization

The Binocular Plan The patterns of RGC axon projection at the optic chiasm in different species range from complete crossing and segregation of fibers from each eye to partial decussation and formation of an ipsilateral projection with complex intermingling of the fibers from the two eyes. The presence and relative size of the ipsilateral projection depend on the degree of binocular overlap in the visual field. In lower vertebrates, such as fish, that lack binocular vision, the fibers from each...

Maturation of retinal cells

The complex set of interactions that trigger the appearance of characteristic molecules is only the first step in the formation of a fully differentiated retinal cell type. Rod photoreceptors undergo a major developmental step of outer segment formation about a week after opsin expression can first be detected. Rod outer segments represent a complex morphological specialization and contain the light-sensing Figure 3.4. Comparison of the sequences upstream of the opsin gene of various...

References

Martin, and K. S. Rockland, 1998. The connection from cortical area V1 to V5 a light and electron microscopic study, J. Neurosci., 18 10525-10540. Antonini, A., and C. J. Shatz, 1990. Relation between putative transmitter phenotypes and connectivity of subplate neurons during cerebral cortical development, Eur. J. Neurosci., 2 744-761. Antonini, A., and P. M. Stryker, 1993. Rapid remodeling of axonal arbors in the visual cortex, Science, 260 1819-1821....

Extrastriate cortex

We have until now considered the development of and effects of abnormal visual experience on the structure and function of the visual pathway, up to and including the primary visual cortex, V1. But in primates, there is a very extensive collection of cortical areas outside V1, which in aggregate involve about three times as much cortical tissue as V1 (Felleman and Van Essen, 1991). The functional properties of neurons in these areas and their relationship to behavior are a very active focus of...

Lynne Kiorpes And J Anthony Movshon

Their visual capacities improve over time, with a course that varies somewhat depending on the measure used to define visual function and the species studied. Many common measures of vision reach adult levels by the age of about 1 year in macaque monkeys and about 5 years in humans during the period of maturation, performance typically improves roughly 10- to 30-fold. Figure 12.1 caricatures the effect on vision of two of these measures, spatial resolution and...

Functional perspective on cell population size

Need for Large Numbers An enormous number of cells and synapses are needed to generate and interpret multiple neural representations of the visual world. The advantage of having large numbers of cells is most obvious in the vertebrate fovea (e.g., Collin and Collin, 1999 Wikler et al., 1990). Cones are often packed together in a tight triangular mosaic with peak densities that reach up to 300,000 mm2 in humans (Curcio et al., 1987, 1990). Trios of photorecep-tors, midget bipolar cells, and...

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Light-evoked responses from the eye of a wild-type mouse (+ +) and a mutant mouse ( ) deficient in the gene encoding mGluR6, the metabotropic glutamate receptor used in ON bipolar cells. The b wave, which originates in the light responses of ON bipolar cells, is absent in the mutant mouse. (From Masu et al., 1995.) teleost retinas (Saito et al., 1981), and is likely to be a fast mechanism as compared to metabotropic pathways which are often relatively slow (Nelson, 1973). OFF...

On the termination of the optic tract fibers in the lateral geniculate nucleus

Vision, like all sensory inputs, except for olfaction, is relayed to the cerebral cortex by way of the thalamus. The thalamic relay for vision is the LGN. The LGN in humans and Old World monkeys has an obvious striped appearance, with six layers of neurons separated by interleaved fiber layers. Although, by the end of the nineteenth century, it was clear that the eye projects to the LGN, the pattern of termination of optic tract fibers was not well understood. The true picture was revealed by...

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Development of contrast sensitivity for 6-Hz reversing gratings. Between 4 weeks (open squares) and 9 weeks (open circles), the contrast sensitivity function shifts upward by a factor of approximately 5 to 6. Between 9 and 33 weeks (filled circles), the function shifts rightward, with little increase in maximum sensitivity at low spatial frequencies. Between 33 weeks and adulthood (x's), further increases in resolution occur (rightward shift), along with a small increase in overall...

Henry Kennedy And Andreas Burkhalter

An understanding of the neuronal mechanisms underlying visual perception has played a major role in shaping our understanding of how the brain extracts information concerning the world. In turn, insight into the development of this process is enormously conditioned by existing theories of brain function. Over the last century, anatomical and physiological studies have provided a description of the point-to-point connectivity of the visual system where neighboring relations in the retina are...

Failure of optic chiasm development in the absence of other brain abnormalities

In a breed of Belgian sheepdogs with an autosomal recessive mutation in an unkown gene, the optic chiasm fails to develop (Williams et al., 1994). Instead, all RGCs, irrespective of their site of origin within the retina, extend into the ipsilateral optic tract. Despite the fact that approximately 80 of RGCs in these animals project to targets on the wrong side of the brain, the RGCs terminate appropriately in their targets. Consequently, in the LGN, noncongruent mirror-image maps of visual...

Development of corticocortical pathways

During phylogeny, there is a progressive shifting of visual processing to more central structures of the brain. One consequence is that, in the mammal, more than 90 of the synapses in the brain occur in the cerebral cortex. Quantitative investigation of the cortex shows that 95 or more of synapses in the cortex are of cortical origin (Kennedy and Falchier, unpublished). One consequence of these figures is that developmental processes of the corticocortical pathways could have a major impact on...

Synthesis of results

It is tempting to suggest that there are two parallel series of reactions involved in ocular dominance plasticity (Fig. 10.1). One leads from afferent activity to NMDA and metabotropic glutamate receptors to entry of calcium into the cell, to activation of adenylate cyclase, production of cAMP, and activation of PKA and CaMKII. The other leads from afferent activity to activation of BDNF and the GABA system. They may converge at the level of MAP kinase or CREB. They must diverge to produce...

Chiayu Chiu And Michael Weliky

In the adult visual cortex, neurons are selectively responsive to particular stimulus features in the visual world. One of the most well-known functional properties of cortical neurons is orientation selectivity, in which cortical cells fire most intensely to lines and edges of specific orientations. Since the discovery of orientation-tuned cells in the primary visual cortex (Hubel and Wiesel, 1962), much attention has been devoted to elucidating the mechanisms underlying its development....

Ocular dominance plasticity

Ocular dominance plasticity is a shift in the ocular dominance of cells in the visual cortex, that is, a shift in whether the cells are dominated by the left eye, the right eye, or both. Changes in ocular dominance are found with monocular deprivation, strabismus, anisometropia, and other conditions. Most experiments on mechanisms have been done with monocular deprivation, which leads to total dominance by the nondeprived eye after a few days in young animals (see Chapters 8 and 14). There is a...

BDNF overexpression accelerates the functional development of the visual cortex

The experiments on the role of neurotrophins in modulating synaptic release show that BDNF has a strong effect in potentiating GABA release which is not shared by NGF. These results show that neurotrophins (in particular BDNF and NT4) also act on the inhibitory circuitry. trkB is present on cortical interneurons, and BDNF regulates the development of at least one class of inhibitory interneurons. The relationship between neurotrophins and the development of inhibitory processes has been...

Abnormal visual experience and amblyopia

It seems paradoxical that V1 receptive field development passively follows peripheral organization, which is not influenced by visual experience in primates (Blakemore and Vital-Durand, 1986b Hendrickson et al., 1987 Levitt et al., 2001 Movshon et al., 1987), whereas decades of evidence has accumulated for experience-dependent plasticity in V1. Most studies of the effect of visual experience on development in primates have used monocular or binocular deprivation to manipulate visual experience...