The problem of isolating the activity of single hippocampal neurons can be solved by simultaneously recording the potentials with multiple electrodes (McNaughton et al. 1983a). McNaughton and co-workers developed a method in which an action
p o te n tia l is re c o rd e d w ith d if f e r e n t a m p litu d e s o n tw o a d ja c e n t e le c tr o d e s , c a lle d a
stereotrode. If th e s o u rc e o f th e s ig n a l is n e a re r to o n e o f th e e le c tr o d e s , th e n it is la r g e r o n th a t e le c tr o d e . A n e x a m p le o f d a ta re c o r d e d in th is m a n n e r is s h o w n in F ig u r e 3 .4 .
T h is fig u re is a s c a tte r p lo t o f th e p e a k - to - p e a k a m p litu d e f o r e a c h d ig itis e d s p ik e o n th e tw o e le c tr o d e s o f th e s te re o tro d e . T h e y -a x is is th e a m p litu d e o n o n e e le c tr o d e a n d th e x -a x is is th e a m p litu d e o n th e s e c o n d e le c tr o d e . E a c h c lu s te r r e p r e s e n ts th e a c tiv ity o f a p u ta tiv e s in g le n e u ro n . In th e a b s e n c e o f n o is e , all o f th e a c tio n p o te n tia ls fro m o n e n e u ro n w o u ld b e p lo tte d o n to p o f e a c h o th e r. A d d itiv e n o is e p r o d u c e s a s p re a d u n ifo rm ly in th e x a n d y d ir e c tio n s , a n d m u ltip lic a tiv e n o is e ( p r o p o r tio n a l to a m p litu d e ) p ro d u c e s a la rg e r s p re a d w ith d is ta n c e fro m th e o rig in . T h e f ig u r e c o n ta in s se v e ra l c le a rly d is tin g u is h a b le c lu s te r s a n d b a c k g r o u n d n o is e n e a r th e o rig in .
C hannel 1 Vp-p
Channel 2 Vp-p
F ig u re 3 .4 Stereotrode plot show ing the activity o f several hippocam pal neurons. In the plot the peak to peak voltage on one channel is plotted against the peak to peak voltage on a second channel.
It is c le a r fro m th e d a ta th a t th e c lu s te rs a re o b lo n g r a th e r th a n c ir c u la r a n d s y m m e tric a l, a n d th e ir lo n g a x is is d ia g o n a l w ith a p o s itiv e slo p e . A s M c N a u g h to n e t al. (1 9 8 3 a ) p o in te d o u t, th is s tru c tu re is d u e to th e c o m p le x s p ik e f ir in g p a tte rn o f th e c e lls . S in c e th e tru e a m p litu d e o f th e s p ik e s m o n o to n ic a lly d e c r e a s e s , th e re c o r d e d sig n a l d e c r e a s e s p r o p o r tio n a lly on b o th e le c tro d e s .
If only one electrode had been used to collect these data, none of the separation between these clusters would be evident. Using a single threshold on one of the electrodes would be equivalent to drawing a dividing line perpendicular to the chosen axis. It is clear that the best dividing line perpendicular to the axis contains many false positives and false negatives (dotted line in figure).