Horseshoe Crab Sight:
     EACH TIME A PHOTON OF LIGHT PASSES THROUGH YOUR LENS AND LANDS ON
YOUR RETINA IT SENDS OFF AN ELECTRICAL SIGNAL TO THE BRAIN.  

     Barlow: " the retina converts visual images into nerve signals, electrical signals that
are transmitted form the eye to the brain as if it were being transmitted over a telephone
line."

     BOB BARLOW IS AN OPTHAMOLOGIST AT SUNY SYRACUSE.  HE'D LIKE TO FIGURE
OUT JUST HOW THAT CIRCUITRY WORKS.  BUT WITH 100S OF MILLIONS OF RECEPTORS
CONNECTED TO AS MANY NEURONS--OUR EYE IS AN ELECTRICAL SYSTEM SO COMPLEX
THAT NO TELEPHONE SYSTEM COULD MIMIC IT.  TO TRY TOMAKE SENSE OF THE COMPLEX
CIRCUITS, BARLOW HAS BEEN STUDYING A MUCH SIMPLER SYSTEM--THE HORSEHOE CRAB
EYE.

     Barlow: "We try determine how such encoding methods work in much simpler
eyes and once you understand how these mechanisms work in simpler eyes you can
often exrapolate that information or extend it to understanding how more complex
systems such as our own eye, works."

     BARLOW CONNECTS ELECTRODES TO THE HORSESHOE CRAB'S EYE AND SITS A
MOVIE CAMERA ON THE CRAB'S HEAD.  HE THEN RELEASES THE CRAB INTO THE WATER SO
THAT HE CAN MONITOR WHAT THE CRAB IS SEEING WHILE MEASURING THE EYE'S
ELECTRICAL SIGNALS.  WITH SUCH TECHNIQUES, BARLOW HAS CRACKED THE CODE FOR
HOW THE CRAB'S EYE TURNS A PHOTON OF LIGHT INTO A MESSAGE THAT YOUR BRAIN CAN
UNDERSTAND.  SOME RELATIONSHIPS ARE SIMPLE. . . A PHOTORECEPTOR GETS MORE
LIGHT, IT SENDS MORE PULSES; LESS LIGHT AND IT SENDS FEWER.  BUT OTHERS ARE MORE
COMPLICATED, LIKE THOSE THAT ENHANCE THE ABILITY TO TRACK DOWN A MATE OR SPOT
A PREDATOR. 
     
     Barlow: "we feel we have a fairly good understanding of the neural code the eye
sends to the brain, the next question is how does the brain decypher this code."