EyeWire is an online video-game that is fun to play and useful to neuroscience!
The retina is perhaps the most accessible part of the brain, and in some ways the simplest (though it's still amazingly complicated) - see the comparison below of the retina and the neocortex:
The retina has at least 4 jobs.
1. It converts the current pattern of light intensity and color falling on the retina (in particular, on the outer segments of the photoreceptors) into an electrical pattern (in particular, the potentials inside the photoreceptors, which act as 100 million "image pixels").
2. This electrical pattern is then compressed 100-fold into a pattern of spikes on the axons of the ganglion cells and sent to the rest of the brain (especially the visual thalamus and superior colliculus) for interpretation.
3. Some specialized ganglion cells can already "interpret" aspects of the visual image. For example, some ganglion cells respond to directed motion at particular locations.
1 is accomplished by the phototransduction machinery. Light causes a structural change in the (one of four) photopigments expressed by the photoreceptor, which activates an enzyme that breaks down cGMP. This in turn closes some of the CNG-channels that normally allow a background influx of sodium ions into the photoreceptor, thus hyperpolarizing the photoreceptor, reducing ongoing glutamate release.
2. Compression is achieved by the center-surround receptive field organization. In a nutshell, since neighboring points in retinal images tend to show similar light intensities, it's usually more informative (more "surprising") to send to the brain information about differences in local intensities. Thus an "on" ganglion cell could be caused to fire by light hitting a "central" photoreceptor that provides (via a bipolar) depolarizing input, especially when the illumination of immediately neighboring photoreceptors decreases. These neighbors provide input to GABAergic "horizontal cells", which reduce release from the central photoreceptor, as well as inhibiting the corresponding bipolar. Vice-versa for "off" ganglion cells. In statistics terms this corresponds to a "local" version of PCA called ZCA, as discussed in class. The comparison with neighbors implies sensitivity to pairwise statistics, also a feature of PCA. However, straight PCA is not practical in the retina, because it involves global (and long-distance) connections, which would enormously thicken the retina. Because the pairwise image correlations (mostly caused by optical imperfections of the eye itself) are usually highly local, the local wiring needed for ZCA is much more practical, and equally efficient. Because of inevitable off-axis chromatic aberrations in the lens, local green/blue or red/green center-surround comparisons may also do good compression.
Note that the goal of PCA is to find directions in multidimensional pixel space along which image projections vary maximally.
In the foveola, which aligns with the optical axis of the eye, image blur is minimal, and here each ganglion cell has input from just 1 cone, so there is no image compression: the brain receives the full, detailed, RAW image. Of course it has to interpret it - which is why the visual cortex is so complex (see top image). Note that though the fovea is only a small part of the retina, a large part of the visual cortex is devoted to its analysis - this "magnification factor" is at least 100 fold.
3. The mechanism of the direction selectivity of ganglion cells has been recently worked out (e.g http://www.nature.com.proxy.library.stonybrook.edu/nature/journal/v471/n7337/full/nature09818.html) These ganglion cells come in on and off types, reflecting the sign of the bipolar to which they are connected, and the inner plexiform sublayer where their axons/dendrites meet . But in addition they respond selectively when the light or dark spot to which they are tuned moves in a particular direction. This directionality arises from additional inhibitory input from GABAergic "starburst amacrine cells", which inhibits firing when the spot moves in one particular direction, but not the other. Amacrine cells do not have axons. This type is aptly named, because their dendrites spread out in all directions in ether the on or off sublayer, making synapses on ganglion cell dendrites in that layer. The starburst dendrites also get input from bipolars. Each spreading SBA dendrite branches out in a sector, and within that sector it gets excitatory input from overlying bipolars. However these BP-SBA epsps arrive at different times because of cable properties. In particular, if a spot of light moves away from the SBA cell body, it first depolarises the proximal SB dendrite, then the distal. The distally and proximally generated epsps will thus peak at the same time in the distal dendrite; dendrodendritic SBA-GC synapses in the distal dendrites will thus be strongly activated. Notice that this arrangement makes each separate dendrite respond to centrifugal motion in a particular direction (eg north, south east or west). Now it turns out that a northward GC receives inhibitory synapses from a northwards-tuned SBA dendrite, and so forth, and therefore inherits its directional tuning. Note that individual SBA cells are NOT tuned to individual directions, though its dendrites are. Here we have an example of local dendritic computation, a principle which some neuroscientists are vainly trying to extend to excitatory neurons with axons (e.g. pyramidal cells - see Hebbery Notes.).
The SBA is in black, and the synapses it makes on 4 (N,S,E,W; different colors) directional GCs is shown as colored balls. One of these synapses is shown in detail. All the other neuron processes are shown in gray. Seung and Denk Nature
514,394(16 October 2014) doi:10.1038/nature13877
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