8o4 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



Penficld, VVoolsey and others, there is a significant 

 distortion in the relative size of body parts when rep- 

 resented in the form of a "homunculus." However, 

 such a method of depicting the cortical representation 

 must be treated with reserve by reason of the great 

 overlap in the responsive zones found at any one point. 

 Using sine wave monopolar stimulation of the 

 precentral gyrus of adult chimpanzees at frequencies 

 from 5 to 1440 cps, Hines (201 ) found that the optimal 

 frequency for eliciting movements was around 90 

 cps. She observed jerky uncompleted movements at 

 frequencies from 1260 to 1440 cps. Delay in the onset 

 of the response was related to both stimulus intensity 

 and frequency. The delay at liminal or supraliminal 

 intensities was longer for low frequencies than for 

 high frequencies. Boynton & Hines (53), in similar 

 studies with sine wave stimulation in cat and monkey, 

 had previously observed two optimal frequencies 

 (80 to 120 and 500 to 600 cps). Exploration of the 

 cortex in depth indicated a drop in threshold from 

 3.0 v. at the surface to i.o v. in layer \'. Brown & 

 Blackett (66), in studying motor responses to sub- 

 cortical stimulation, have confirmed that the delay 

 in the onset of motor responses is related to the in- 

 tensity of the stimulus. Adrian (g) has pointed out 

 that stimulation of the cortex at one-half to one-third 

 the movement threshold is sufficient to evoke a nega- 

 tive potential for a distance of 2 to 4 mm around a 

 unipolar electrode. Cortical unit studies in relation 

 to motor responses will be described below. 



McCulloch (294) has indicated that increasing 

 duration of the stimulating pulse leads to excitation 

 of an increasing proportion of the motor neuron pool. 

 There is, of course, a limit to the duration of rectangu- 

 lar pulses which may be applied without risk of cellu- 

 lar damage. For this reason, Lilly et al. (274, 275) have 

 investigated the use of a dififerentiated square wave 

 with an invariant period between the positive and 

 negative phases. They claim that this form of stimu- 

 lus does not detectably injure cellular function when 

 passed through the cortex at strengths near threshold 

 4 to 5 hours per day for 5 to 15 weeks, and suggest 

 that other similarly balanced brief wave forms might 

 be expected to produce equivalent results. 



Cure & Rasmussen (106) have investigated the 

 separate roles of frequency, wave form and pulse 

 duration, as well as intensity, on motor responses in 

 the macaque. They used bipolar silver electrodes 

 with a tip separation of 2 to 3 mm, and pulse durations 

 of o. I to 3.0 msec, at frequencies of 2 to 200 per sec. 

 A burst of stimuli was delivered for 1.2 sec, once each 

 minute. Under these conditions, they observed from 



many points reproducible changes in motor responses 

 directly attributable to alterations in frequency. 

 These effects were more often seen from points near 

 the junction of primary areas of representation, or 

 from those areas usually assumed to represent more 

 proximal muscle groups of the extremities. Thus, 

 stimulation of a point in the "arm' area near its junc- 

 tion with the "face' area produced only thumb re- 

 sponses at voltages of threshold intensity when the 

 frequency was less than 30 per sec, and only lip re- 

 sponses at frequencies above 30 per sec, with voltages 

 at or near threshold. Farther medially in the thumb 

 area, high frequencies often produced movement of 

 more proximal muscle groups of the extremity, either 

 alone or with thumb mo\'ement. Tliumi) mos'ements 

 occurred at stimulus rates of 2 to 30 per sec, while 

 with frequencies from 6 to 150 per sec, wrist extension 

 occurred, either alone or with thumb movement. 

 These investigators prepared three separate response 

 maps at 2 to 10, 60 and 200 stimuli per sec. The high 

 frequencv map was characterized by a paucity of 

 toe, thumb and finger movements. The order of 

 motor sequence was identical in each, and the bounda- 

 ries between face, arm and leg subdivisions, as de- 

 termined b\' these threshold stimulations, were almost 

 identical. Similarh', stimulation of the frontal eye 

 fields in area 8 has produced conjugate deviation ot 

 the eyes with brief pulses at 50 per sec, but at 10 to 20 

 per sec. ipsilateral deviation occurred (278). From the 

 upper part of area 8, a "waking" reaction in eye move- 

 ments occurred at high frequencies and a "sleeping' 

 response at low frequencies. 



The consequences of altered pulse duration have 

 also been studied by Cure & Rasmussen (106). Usually 

 motor responses at a given frequency were the same 

 over the range of durations tested (o. i to 3.0 msec). 

 Occasional reproducible alterations were seen which 

 seemed to be correlated with alteration in pulse 

 length. Thus, at one cortical point pronation of the 

 proximal forelimb with 3.0 msec, stimuli changed to 

 finger movements with shorter stimuli (o. i to i .0 

 msec.) at 30 stimuli per sec. Longer pulse durations 

 would fire cells more peripheralh' placed. A fortuitous 

 placement in relation to the neuronal foci for the 

 pertinent muscle groups, as postulated by Chang et 

 al. (88), might permit such an alteration of response 

 as varying groups of cells were activated. 



The effects of increasing intensity of stimulation 

 have been variously reported. Clark & Ward (92) 

 found that occasionally strong stimuli might produce 

 a different pattern of response in which the movement 

 elicited b\- weak stimuli did not occur or was com- 



