Visual Streak

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The Visual Streak is the area of highest acuity in the retina of many species. In humans and some other mammals, the location of highest acuity in the retina is a circular area termed the fovea or area centralis. In contrast, some animals (such as rabbits) have a visual streak. In such animals, the area of greatest acuity in their retina is not a single point, but rather an elongated "streak" running across the retina. This allows for better detection of movement in the periphery. In all species studied, the area of highest acuity (whether fovea or visual streak) boasts the highest concentration of cones, the lowest concentration of rods, and much smaller receptive field sizes for all cells.

Species Differences

One interesting aspect of the visual streak is that its presence is not phylogenetically determined. That is to say, whether a given type of animal (you can't even say species) has a visual streak, an area centralis, or both can not be predicted by looking at a phylogenetic tree. Rather, environmental pressures can affect which type of acuity area an animal has. The perhaps best-studied example of this is dogs. It has been discovered that some dog "species" have an area centralis while others have a visual streak. Since the visual streak facilitates better resolution vision in the periphery, dogs that are good hunters or that chase everything that moves generally have a visual streak. On the other hand, those dogs that don't chase things — or who seem to enjoy watching the TV — are likely to have an area centralis. It also turns out that the length of the nose of a dog is a good predictor of whether they have a visual streak (longer nose = visual streak). It has been hypothesized that dogs that can focus better on faces and thus better read human emotions were selected for in breeding certain pet dog "species" and this is why they have "lost" the visual streak.

Cell Type Differences

Many cell types display differences in connections, shape, and size as their distance from the visual streak changes. Bipolar cells near the visual streak are smaller, as are ganglion cells. Bipolar cells in this region receive inputs from fewer photoreceptors (as few as one) and ganglion cells receive inputs from fewer bipolar cells (as few as one in midget ganglion cells).

Starburst Amacrine Cells

One specific example of a cell type whose shape is highly correlated to their location with respect to the visual streak is starburst amacrine cells (SACs). SACs also display structural changes as the perpendicular distance from the visual streak increases. Nearest the visual streak, both type a and b SACs have a relatively small cell body size and dendritic field diameter as well as a relatively high frequency of branching and frequency of synaptic boutons. As we move perpendicularly away from the visual streak, cell body size and dendritic field diameter increases, while the frequency of branching and synaptic boutons decreases.