Contrast Enhancement and Distributed Encoding by Bipolar Cells in the Retina Dwight A. Burkhardt and Patrick K. Fahey Presented by Eric Golbek.

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Presentation transcript:

Contrast Enhancement and Distributed Encoding by Bipolar Cells in the Retina Dwight A. Burkhardt and Patrick K. Fahey Presented by Eric Golbek

Contrast These bands each appear darker in some places and lighter in others. The luminosity of each band is uniform across its entire width. The apparent difference in brightness within a band is an illusion due to contrast enhancement.

Bipolar Cells come in two varieties. 2 Classes Hyperpolarizing Bipolar Cells (Bh) Depolarizing Bipolar Cells (Bd) These two types of cells create two complimentary images of opposing signs. This scheme is fundamental for the representation of visual contrast. Other differences

Introduction They analyze the responses of bipolar cell contrast steps by intracellular recording in the light adapted retina of the tiger salamander. All past research has been done with no light, and hence involves the rods instead of the cones.

Hypothesis We will see an increase in amplitude as well as depolarization of photoreceptor input.

Methods They cut off the head of the tiger salamander. The salamander is then rapidly pithed. The cornea, iris, and the lens is removed. Microelectrodes are then inserted into the proper cells that will have their activities recorded. Low intensity background light will be used along with a light that will flash into the center of the retina. The retina which is always adapted to the light will be flashed every ten seconds for 14 contrast levels.

Results of Bipolar Cells Only There was great variation of response of bipolar cells to contrast both within as well as across the Bd and Bh classes. This indicates that the optimal contrast polarity for driving the bipolar cell would probably depend on the specific bipolar cell as much as it would on the generic class to which it belongs.

Results seen in the Cones and Horizontal cells. We see little difference across the different types of cones when concerning polarization. The contrast/responses of horizontal cells were similar to that of the cones. Therefore, we will assume that the difference in contrast must arise later. Namely in the ( Bipolar Cells)

Enhancement Across Cone---Bipolar Synapse We observe a 5 to 10 times amplification across the cone- bipolar synapse. Clear evidence for contrast enhancement of small signals across the cone-bipolar synapse in the light adapted retina has been found by quantitatively comparing the response of cones and bipolar cells. Anything else such as rod input, or convergence have been ruled out. Neither can account for the different shape and values of the contrast/response of bipolars and cones. (results)

This is the first evidence that differences between the latency of Bh and Bd cells also are found in the light adapted retina and that the difference holds for different contrasts of both polarities. Even though there are many issues to be researched, we now know that the bipolar cell population of the light adapted retina is capable of providing contrast to the visual image.

Done