Exercise XX 



ELECTRICAL ACTIVITY OF A SENSE ORGAN 107 



Compare the responses elicited with three 

 different durations of the stimulating flash, of 

 about ^ sec (as fast as you can move the card- 

 board), 1 sec, and 3 sec. What is the relationship 

 between the ERG and duration of stimulus at 

 constant intensity? 



Now investigate the way in which the ERG 

 varies with light intensity. The intensity is con- 

 trolled with a series of neutral filters. A neutral 

 filter is one that absorbs light more-or-less 

 equally throughout the spectrum, and hence 

 looks colorless (gray). Such filters are graded on 

 a density scale, in which density equals log 

 1/transmission. That is, a filter that transmits 

 1/10 of the light has density log 10 = 1.0; a 

 filter that transmits 1/100 of the light has density 

 log 100 = 2.0; and so on. 



Starting with the dimmest light (neutral filter, 

 density 3.5), and a stimulus of 1 sec, measure the 

 height of the ERG. Now, keeping the duration 

 of the stimulus constant, progressively increase 

 the light intensity by steps of 0.5 log unit; i.e., 

 use progressively lighter filters in which the 

 density falls by steps of 0.5. Make at least two 

 measurements at each intensity that agree with 

 each other. Wait at least one minute between 

 exposures to allow the animal to recover. At the 

 higher intensities you will probably have to re- 

 adjust the amplification setting on the indicator 

 to keep the response on the screen. Plot the 

 magnitude of response (in millivolts) against the 

 light intensity in log units. How big is the range 

 of light intensities over which you find the re- 

 sponse to vary? How big is it in ordinary arith- 

 metic units? Describe the relationship between 

 intensity of stimulus and the ERG in words, and 

 draw what conclusions you can concerning the 

 animal's capacity to respond to and distinguish 

 various brightnesses of light. 



Using a moderate intensity of light (density 

 1.0), and a 1-sec exposure, remeasure the magni- 

 tude of response to a flash. Light-adapt the ani- 

 mal for 5 minutes with the brightest light avail- 

 able, and remeasure the response at density 1.0. 

 Let the animal remain in the dark, and periodi- 

 cally remeasure the response to a flash of this 

 intensity and duration of light. Start by making 



a measurement every minute, and as the change 

 slows down lengthen this interval, eventually to 

 every 3 to 5 minutes. (Don't go on with this 

 longer than 30 minutes.) How long does the 

 horseshoe crab take to dark-adapt? Make a 

 graph of the relation between the height of the 

 ERG and time in the dark. 



Remove the animal from the cage and kill it 

 by turning it on its back and slitting it up and 

 down the middle with a scalpel. Prepare to ex- 

 pose the optic nerve by first cutting a square, 

 about 1 inch on a side, through the carapace 

 around the unused eye of the animal, using a 

 sharp scalpel or one-edged razor blade. Then 

 carefully raise this piece of carapace at its upper 

 edge, and begin to free it from the underlying 

 tissue with the blunt end of a scalpel. Work very 

 slowly, watching carefully for the optic nerve. It 

 is a very fine, glassy structure that runs forward 

 from the eye. If you have trouble, call an in- 

 structor. 



When you find the nerve, free it from the bulk 

 of the surrounding connective tissue and tie a 

 suture around its distal end. Now remove the 

 square of carapace containing the eye with its 

 attached nerve from the animal, and continue to 

 clean away the connective tissue from its back. 

 Go as far with this as you can, but be very care- 

 ful not to damage the eye itself. The cleaner the 

 nerve, the better your experiment will go. Keep 

 the nerve moist with sea water. 



Mount the eye upside down, as shown in the 

 diagram, on a block of plasticene. Position the 

 block of plasticene in the shielded cage, and re- 



