2 results
Distribution and structure of efferent synapses in the chicken retina
- S. H. LINDSTROM, N. NACSA, T. BLANKENSHIP, P. G. FITZGERALD, C. WELLER, D. I. VANEY, MARTIN WILSON
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- Journal:
- Visual Neuroscience / Volume 26 / Issue 2 / March 2009
- Published online by Cambridge University Press:
- 01 March 2009, pp. 215-226
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- Article
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The visual system of birds includes an efferent projection from a visual area, the isthmo-optic nucleus in the midbrain, back to the retina. Using a combination of anterograde labeling of efferent fibers, reconstruction of dye-filled neurons, NADPH-diaphorase staining, and transmission electron microscopy, we have examined the distribution of efferent fibers and their synaptic structures in the chicken retina. We show that efferent fibers terminate strictly within the ventral retina. In two completely mapped retinas, only 2 fibers from a total of 15,359 terminated in the dorsal retina. The major synapse made by each efferent fiber is with a single efferent target amacrine cell (TC). This synapse consists of 5–25 boutons of 2 μm diameter, each with multiple active zones, pressed into the TC soma or synapsing with a basketwork of rudimentary TC dendrites in the inner nuclear layer (INL). This basketwork, which is sheathed by Muller cell processes, defines a private neuropil in the INL within which TCs were also seen to receive input from retinal neurons. In addition to the major synapse, efferent fibers typically produce several very thin processes that terminate nearby in single small boutons and for which the soma of a local amacrine cell is one of the likely postsynaptic partners. A minority of efferent fibers also give rise to a thicker process, terminating in a strongly diaphorase-positive ball about 5 μm in diameter.
7 - Is there more than meets the eye?
- Colin Blakemore, University of Oxford, K. Adler, M. Pointon
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- Book:
- Vision
- Published online:
- 05 May 2010
- Print publication:
- 24 January 1991, pp 74-83
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Summary
In a recent review entitled ‘Why can't the eye see better?’, Horace Barlow (1986) discussed how optical factors, photoreceptor characteristics and the dynamic range of retinal ganglion cells limit visual performance. Barlow's question, like all good riddles, demands a shift in perspective before it can be tackled. In this essay we pose a complementary puzzle, ‘is there more than meets the eye?’ Although there is no doubt that much remains to be discovered about the retina, we question whether the apparent simplicity of retinal function critically underestimates the sophistication of early stages of visual processing.
The complexity of visual coding by retinal ganglion cells is often presented in terms of those receptive field characteristics that may provide the neural substrate for psychophysical phenomena. Thus the centre–surround organization of concentric cells is related to the coding of chromatic or luminance contrast; the On and Off pathways underpin the efficient signalling of increased light and darkness; the spatial and temporal properties of X and Y ganglion cells seem matched to the requirements of pattern vision and motion detection. Although this approach has inherent appeal, the sophistication that psychophysicists and central physiologists demand of retinal function is, in fact, rather limited.
Hierarchical concepts of visual processing deny to retinal function those characteristics that are presently perceived to be intrinsic to cortical function. For example, the long-range horizontal interactions between cortical modules are thought to underlie such diverse processes as vernier acuity and figure–ground discrimination. In the retina, however, the many amacrine connections beyond the classic receptive field are credited with little more than producing the periphery effect.