7i6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



ment is involved, the movement is said to be 'real.' 

 Again it may be said that subhuman species respond 

 in motor ways so as to indicate they are differentially 

 sensitiv-e both in cases of displacement and in cases of 

 stationary localization of targets. 



Color vision is the name given to the fact that both 

 human and some subhuman species give evidence of 

 being differentially sensitive to various portions of the 

 visible spectrum. Color as an experience may also be 

 exoked in the human by nonspectrally selected radia- 

 tion, hence the color end result stems also from condi- 

 tions of nonspectral selection. 



Phenomena of the Optic Palliivnx 



BRAIN waves: spontaneous and EVOKED. One of the 

 major considerations in dealing with the neurophysi- 

 ology of central phenomena, 'brain waves,' for exam- 

 ple, is the assignment of their origin. All the activities 

 which we deal with can be put into two classes : those 

 that occur when no intended peripheral input to the 

 brain is involved (spontaneous activity), and the spe- 

 cific activity that occurs when known inputs are 

 delivered through intended stimulation (specific or 

 evoked responses). 



What do the characteristic waves found in the 

 record of spontaneous activity represent? What ele- 

 ments produce them? There are two quite obvious 

 alternative possibilities. One is that these rhythmic 

 patterns of potential are the summed record of the 

 primary impulses of unit neuronal responses occurring 

 somewhat out of phase and producing wave envelopes 

 of much longer duration than the unit impulses them- 

 selves. The second alternati\e is that the recorded 

 waves are the manifestations of slower longer-lasting 

 processes that are more nearly similar in duration to 

 the recorded waves than would be the case in the first 

 alternative. These waves would apparently be some- 

 thing like after-potentials in elements where activity 

 would fall short of the kind of discharge that produces 

 spikes. Bishop & Clare (21) believe that the first al- 

 ternative is preferable in accounting primarily for 

 spontaneous activity. They believe that this activity 

 may incidentally include slower potentials suggested 

 in the second alternati\ e. As for e\oked responses, the 

 slow surface-negative portions may be an example of 

 a slow potential of the character suggested in the 

 second alternative. 



Whichever alternative may operate, the next ques- 

 tion is whether the spontaneous and the evoked activi- 

 ties as the result of well-controlled peripheral stimula- 



tion occupy the same cortical elements. Of course, 

 there are two alternatives here. At first, it was inferred 

 that they did not, but it has later appeared possi- 

 ble that the two activities share at least some common 

 elements (15, 21). 



The conclusions just given are in line with the find- 

 ings of Adrian & Moruzzi (i). They reported that 

 groups of impulses were discharged via axons in the 

 pyramidal tract in unison with the alpha cycle of the 

 motor cortex. Thus it appears that whatev-er may be 

 said about the cortical waves them.selves, impulse 

 volleys are associated with them. Primary impulses of 

 cortical cells are involved in spontaneous activity. 

 The conclusions are also consistent with the finding 

 of Bartley (4) that, following the cortical response to 

 the afferent input via the optic nerve and radiation, 

 the cortex is refractory to a second stimulus, the de- 

 gree depending upon elapsed time. The moment of 

 full recovery coincides with the point at which the 

 alpha-like portion of the typical evoked response 

 de\elops. This alpha-like portion may be spoken of as 

 a sequel to the specific response (21) or be considered 

 as a less specific but true portion of the response. The 

 conclusions are also in line with Bishop's (18) finding 

 that responses to stimulation of the optic nerve waxed 

 and waned in such a way as to imply that the sponta- 

 neous alpha wave left a depression of the same tem- 

 poral character as the evoked response just mentioned 



(4). 



Interpretations that may be added in this connec- 

 tion are those of Bremer (31), Eccles (40), and Gastaut 

 el al. (42). The first of these attributed spontaneously 

 occurring rhythmic potentials in connection with 

 excitability changes, plus axon discharges to account 

 for the brain waves observed in 'resting' records. The 

 second supposes that the activity in the cortex can be 

 interpreted as being analogous to that in the spinal 

 cord where neuronal activity involves reco\ery from 

 depression. The interpretation is that rhythmicity 

 results from successive re-excitations following periods 

 of depression. Apparently closed neural chains form 

 the sources of the re-excitations. The third, in study- 

 ing what is ordinarih- called photic driving of the 

 cortex, inferred that the spontaneous cycle is an ex- 

 pression of refractoriness following discharge. 



OPTIC pathway. The optic pathway consists of: a) 

 the tract, including the optic nerve; />) the relay nuclei 

 of the lateral geniculate, the pretectal area and the 

 superior colliculus; c) the radiation to cortex, and 

 paths to thalamus and tectal area; and d) the projec- 

 tion areas including the cortical, thalamic and tectal 



