CENTRAL MECHANISMS OF VISION 



727 



two avenues ought to affect the pattern of axonal dis- 

 charge from the cell. 



The conditions applying to activation of the neuron 

 via cell-body synapses are as follows. More than one 

 impulse must be delivered to the cell body in order 

 to activate it. When the discharge is once set up, it is 

 of the all-or-none type. The impulses required for cell- 

 body activation may arrive via a single axon branch, 

 or via several of them, provided that they arrive 

 within the required time limits. 



As just implied, when once set into action, the cell 

 body discharges once or more without receiving fur- 

 ther activation. The authors (36, 37) infer that, when 

 once a cell body is .set into action, its action may be 

 sustained by its own dendritic activity. They say that 

 the cell body once sufficiently activated can activate 

 its own dendrites, and they in turn can sustain activity 

 in the cell body. The duration of the dendritic impulse 

 seems to be of the order of 15 msec. This length of 

 time provides for the dendrites acting like a steady- 

 current stimulus to the cell body while it builds and 

 discharges .several times. 



In cases of stimulation of dendrites only, the den- 

 drites do not exhibit all-or-none conduction to the 

 cell body which, therefore, would not be expected to 

 be activated. It is presumed that some effect, never- 

 theless, would be exerted by dendrites upon the cell 

 body by reason of the excited state of one and un- 

 excited state of the other. The various excitations 

 induced in dendrites would sum. The authors sup- 

 pose it proble that, if the dendrites are excited from 

 enough converging sources, they might begin to con- 

 duct and activate cell bodies. The role of the dendrite 

 seems to be to rai.se the level of excitation of the cell 

 body and thereby lower its threshold to influences 

 arriving via axons impinging on it. The chief charac- 

 teristic of dendritic action is its graded character in 

 contrast to the all-or-none manifestations of cell body 

 and axon. This provides for a great deal moie flexi- 

 bility and variety in action than a system limited to 

 all-or-none activity. 



VISUAL PHENOMENA TO BE EXPLAINED 



Gross Response to Gross Intensity Relations 



One of the major considerations in the study of vision 

 and its mechanism is the question of what characteris- 

 tics of vision require the striate cortex and what charac- 

 teristics are demonstrable when the cortex is removed. 

 It has been found that in some animals response to 

 gross intensitv relations in the stimulus field are re- 



acted to in the absence of the occipital cortex. This 

 is not at all surprising in the light of what we know 

 about the bifurcation of the optic pathway, some 

 fibers going to cortex and some going to motor 

 centers that are subcortical, and in the light of our 

 findings on pupillary responses which parallel Fech- 

 ner's paradox (see the subsequent section on bilateral 

 functions). It would seem possible that the structuring 

 of response to gross flux differences could occur in the 

 motor sphere and in the sensory spheres somewhat 

 parallel to each other, according to our interpretation 

 of the parallelism described in connection with Fech- 

 ner's paradox. If one of the two channels were to be 

 destroyed, the other might be able to mediate an end 

 result. With the cortical channels destroyed, motor 

 behavior of some effective kind might still be able to 

 be exhibited. With the motor channel destroyed, one 

 could experiment only on man, for he alone could tell 

 whether experiential reactions to intensive features 

 of stimulation were altered. We should not expect 

 them to be in gross situations. 



The following are some of the characteristics of the 

 ijehavior of monkeys in response to visual stimuli 

 when their occipital cortices are removed, eliminating 

 the geniculostriate systems, as described by Kliiver 

 (50). In such animals, the eyelid reflex to photic 

 stimulation is abolished permanently. The pupillary 

 reflex to photic stimulation is retained, however. The 

 sudden appearance of a stationary or moving photic 

 source does not elicit a turning of the head or eyes 

 towards it, although movements of the head and eyes 

 are elicited by nonphotic stimuli. 



Conjugate movements of the eyes are not destroyed; 

 neither does destruction of the superior coUiculi 

 abolish such movements in response to stimulation of 

 the cortical eye fields. Visual placing reactions are 

 lost. Nevertheless, animals rarely bump into objects 

 when they are left to themselves or are not excited. 



When the bilaterally decorticate monkey is in a 

 limited familiar habitat, its behavior in jumping, 

 swinging and climbing is so readily executed that 

 the unsuspecting observer would suppose the animal 

 to be normal. Variation in the position of some fa- 

 miliar object in its cage elicits considerable fumbling 

 until the animal's hands come into contact with it. 



Such a monkey can respond discriminatively to the 

 more or less intense of two photic sources whether they 

 are indefinitely present orappear suddenly, or whether 

 their presentation is simultaneous or successive. Re- 

 sponses to weak photic stimuli have demonstrated 

 that the ai:)solute threshold in the occipitally operated 



