Information Storage in Nerve Cells 



213 



Fig. 17. Characteristics of the dependent mirror focus. In the ink-written tracing 

 the upper two channels record the primary focus and the lower two channels 

 the mirror focus. The ethyl chloride lesion is indicated by cross hatching. Cali- 

 bration: 100 microvolts and one second. In the oscillographic tracing, the upper 

 channel records the primary region and the lower one the secondary region. 

 Calibration: 100 milliseconds (39). 



area of the opposite hemisphere homotopic with that of the primary 

 lesion. The contralateral hemisphere had not been exposed or 

 damaged in any way during the original operative intervention. 

 The paroxysmal discharge in the contralateral hemisphere results 

 directly from massive synaptic bombardment over known ana- 

 tomical pathways from cells of a primary epileptogenic lesion. 

 Consequently the electrical abnormality in the contralateral focus 

 is considered to represent a secondary epileptogenic lesion (38). 

 At first the secondary discharge w^as clearly dependent upon the 

 primary in the sense that spikes only occurred in temporal con- 

 junction with those in the primary lesion, had a measureable 

 latency following the primary spike (Fig. 17) and disappeared 

 altogether after excision or neuronal isolation of the original focus. 

 The pattern of activity in the secondary area looks like a "reflec- 

 tion" of that in the primary and thus has earned the colloquial 

 name of "mirror focus." If the primary lesion was not excised 

 or isolated the mirror focus eventually became independent. 

 Secondary spikes were then unrelated in time to those in the 

 primary focus (Fig. 18) and did not subside if the original lesion 



