Which of the following stimuli would optimally activate an ON-center ganglion cell? A. A spot of light in the center of the receptive field B. A shadow in the center of the receptive field C. A ring of light covering the surround of the receptive field D. A large spot of light covering both the center and surround portions of the receptive field E. A large shadow covering both the center and surround portions of the receptive field

Explanation:

anglion cells are the first neurons in the retina that respond with action potentials.

Assume we have an electrode either in the ganglion cell layer of the retina or in the optic nerve. The response of the ganglion cell will depend upon the responses of the cells that feed into the ganglion cell, including the photoreceptors, the bipolar cells and the various lateral interconnections via horizontal cells and amacrine cells. However, what we are really interested in is the relationship between this ganglion cell's activity (firing rate) and the visual stimulus image.

H.K. Hartline of Rockefeller University in New York did the first important experiments on this, working in the limulus (horseshoe crab). Later Stephen Kuffler at Harvard University, and Horace Barlow at Cambridge University in England also did some of the pioneering work on this topic in the early 1950s, working on the visual systems of both frogs and cats. They analyzed the ganglion cells' sensitivity to light by using the following simple technique. A very small spot of light was flashed on the surface of the retina for a brief duration. The position of the spot of light was systematically varied across the retinal surface, and while they did this the response of the ganglion cell was continuously monitored.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

hope this .. .