The effect of certain mental operations upon the alpha rhythm

Citation preview

THE EFFECT OF CERTAIN MENTAL OPERATIONS UPON THE ALPHA RHYTHM

A Dissertation Presented to the Faculty of the Graduate School The University of Southern California

In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy

by George Donald Gray February 1950

UMI Number: DP30390

All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion.

Dissertation Ptibiisfung

UMI DP30390 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code

ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48 10 6 - 1346

p

k

i

0 .

D*. /^ro 15) carried out the first systematic study of the EEG- of human subjects. He made several important dis­ coveries and proposed some theories.

He noted that the spon­

taneous rhythm is recordable from, the surface of the intact skull of the conscious human subject, and that the rhythm gives the greatest effect over bone defects in the skull.

He

further noted that the same frequency rhythm is obtainable over most any part of the haad and with particular prominence over the occiput, and that, for the entire head between elec­ trode pairs, there were always secondary fast rhythms super­ imposed on the slower dominant ones.

To these slow dominant

rhythms, most prominent over the occiput and having a fre­ quency of about 10 vibrations per second, he gave the name aloha waves; and to the superimposed low amplitude, higher frequency activity he applied the name beta waves.

He con­

cluded that the alpha rhythm originates in the cortex, that it represents a fundamental activity of the brain, and that it has a wide and less specific origin than the occipital lobes —

that every part of the cortex may contribute to the

recorded potential changes.

He ruled out the possibility

that the waves come from the blood vessels or connective tis­ sue; and concluded that the alpha rhythm results from the activity of a large mass of neurons in the cortex as a whole, and that it is a reflection of biologic activities associated with psychophysical processes.

3 Berger found that the alpha waves appear most favor­ ably when the subject is lying quietly with his eyes closed; and that the rhythm can be modified by the application of sensory stimuli, or by having the subject engage in some­ thing that occupies his whole attention, e.g., a problem in mental arithmetic.

He noted in these situations that beta

waves are apparently unaffected, and concluded that they arise from nutritive and metabolic functions.

It was posited

that attention to specific sensory stimuli is associated with facilitative processes in one specific locus of the cortex, and inhibition of activities in all other parts. Depression of the alpha rhythm reflects this process of in­ hibition in the greater part of the cortex. Adrian and Matthews (2) confirmed the work of Berger, but disagreed as to the locus of the alpha rhythm, contend­ ing that the rhythm is occipital in origin.

They even sus­

pected that muscle potentials picked up by the scalp might account for the record.

Accordingly, they conducted an ex­

tensive study, in which they were able to rule out muscle sources, but in which they became further convinced that the rhythm was entirely occipital in origin.

In fact, they

applied the expression f,Berger rhythm” to the occipital alpha waves in order to distinguish them from Berger's alleged over-the-whole-cortex alpha rhythm.

4 I.

PSYCHOLOGY AND THE EEG

It is well established that the EEG reflects physio­ logic activity in the brain.

If factors in the brain on

which the EEG depends are also factors in psychological phe­ nomena, them the EEG might be an indicator of physiologic mechanisms underlying psychological phenomena.

If this be

true, then it lies within the province of the psychologist to look into the nature of the brain activities which regu­ late psychological performance.

The suggested steps to be

followed are (a) a determination of an empirical correlation between any psychologic variables and some property of the EEG; if the result is positive, then (b) a determination of the factors in the brain which are responsible for the partic­ ular type of EEG obtained; and then, (c) the formulation of a working hypothesis of the role played by these factors in the psychologic phenomena under consideration (104). II

TYPES OF WAVES CONSTITUTING THE EEG

The alpha waves.

With respect to frequency, the

upper and lower limits and the range assigned to these waves are somewhat arbitrary.

The lowest and highest limit and

the broadest spread are allowed by Narrow (36), who gives 7-14 waves per second; the other extreme is found with Adrian and Matthews (2 ), who give 9.5-10.5 waves per second.

It is

generally agreed that the waves have a frequency of about 10

5 per second*

Witn respect to amplitude, the waves lie be­

tween 10 and 65 microvolts (90,113). The beta waves*

The lowest limit assigned to these

waves is 12 vibrations per second (133), and the highest is 60 per second (113); the minimum spread is 13-22 waves per second (133) and the maximum spread is 18-50 waves per second (45)*

The lowest assigned amplitude is 5 microvolts,

and the highest is 20 microvolts (87). The gamma waves*

These are a 35-45 per second band

which was set aside by Jasper and Andrews (87).

The ampli­

tude of' the waves is around 5 microvolts. The delta waves*

Walter (151) suggested delta waves

as a generic term for slow, large-amplitude waves.

Davis

(48) used the term restrictedly to include irregular wave lengths greater than 2.5 centimeters or frequencies up to and including 4 waves per second. The theta waves.

This is an expression used by

Walter and Dovey (152) to describe a 4-7 per second rhythm arising in the parietotemporal region, and characteristic of the EEG- in light ,sleep. The kappa waves* by Kennedy, ot al. (94).

This is a rhythm recently reported It lies within the alpha range of

frequency, and has an amplitude of 20-30 microvolts.

It is

6 alleged to be unrelated to alpha activity, and is tentative­ ly assigned to the temporal lobe. Lindsley (113) has worked out a somewhat elaborate scheme of wave classification, which it seems worth while to outline here. Type 1+

This is the alpha rhythm of Berger;

frequency

8-13 per second, average 10.2 per second; amplitude 10-75 microvolts;

most prominent in the occiput.

Type II.

This is the beta rhythm of Berger —

also

reported by Jasper and Andrews (86), who limit beta to this band; frequency 20-30 per second; amplitude 10-20 micro­ volts; most prominent in precentral. Type III.

This is also the beta rhythm or Berger;

frequency 40-60 per second; amplitude 2-10 microvolts; more often in the occiput than type II.

G-amma waves fall into

this category. Type IV.

A1 20-24 per second rhythm with an amplitude

one-fourth to one-half that of alpha. Type V.

This is a slow rhythm of frequency 0.6-1.5

per second, and an amplitude one-half to three-quarters that of alpha. wave s.

This type is extended to include the delta

7 Other waves.

Here is mentioned a 4-7 per second

frequency, approximately equal to alpha in amplitude, and falling into delta frequency during sleep.• It is found during stimulation in the absence of alpha activity. III.

VARIABLES OF THE EEG- AND THEIR MEASUREMENT

Frequency.

This has reference to the number of

waves of a specific rhythm occuring uninterrupted in a specified unit of time. Amplitude.

This is a measure of the height of a

wave in terms of microvolts.

Contrary to the practice

followed in ordinary wave analysis, where amplitude is taken as the vertical height or a wave crest or trough from the base line, electroencephalography measures from trough to crest (69)* Waves per unit time.

This measurement calls for the

number of waves of a particular rhythm during a specified time in the record (45,48,93). Alpha index. time alpha.

This is also referred to as the percent

It measures the amount of time alpha waves are

present during a specified time in the record.

That is, it

indicates the percentage of time alpha activity is present, regardless of the presence or absence of other rhythms (45,48,93).

Delta index.

This is a measure of slow-wave

(less than alpha) activity.

The procedure consists in

measuring, for 100 centimeters of record, the wave trace of the slow waves with a map measurer calibrated in centimeters -- the measurer being allowed to pass through the base line of all the other waves.

The excess length, in centimeters,

of the wave trace above 100 centimeters is the delta index (48,82). Duration-di stribution curve. A plot of the number and duration of all countable waves in a given length of record (146). Weighted average frequency.

This is obtained by

counting all waves in a meter of record, .dividing the number of waves of each frequency by the frequency, and dividing by the total number of waves (108). Rating scales. Methods for evaluating differences in pattern, particularly as the pattern is affected by alpha waves, are in use.

One of these makes use of a scale from

1.(very rhythmic and regular) to 4 -.(.mainly arhythmie and irregular) (113).

Another assumes four categories based

upon percent time alpha in the record: "Rare11: below 25 percent alpha, f,Mixedf*: 25-50 percent alpha, 11SubdominantH : 50-75 percent alpha, and 11DominantM : 75 percent and above for alpha (45).

9 Filtering and Fourier wave analysis are used to get at dicrotic-like

and complex wave forms (31*118).

Criterion of occipital alpha waves. Alpha rhythm is looked upon as only those frequencies near 10 per second which are characteristically suppressed upon opening and closing the eyes (45). Criterion what

of presence of alpha rhythm.

arbitrarily set up.

This is some­

In one case a series of waves is

considered rhythmic only when composed of at least five suc­ cessive waves separated by approximately equal intervals (87); on the other hand, 3 consecutive alpha waves with an amplitude of 7 microvolts may be accepted as the criterion

(132). A few remarks upon some of the above measures seem pertinent.

The waves per unit time is a very rough index

which neglects completely variations in amplitude, and which depends somewhat upon the criterion accepted for the presence of alpha waves.

It is, however, very quickly calculated, and

is a rather good index of the amount of activity over a peri­ od of time.

This.measure is not to be confused with fre­

quency. since blank periods in which no detectable rhythm is present are included (93)♦

With respect to alpha index

versus frequency, it may be noted that, in general, when the eyes are open alpha frequency is the same as, or perhaps one-

10 half cycle faster than, when the eyes are closed; but the percent time alpha Is greatly decreased or eliminated: with alpha around 10 per second for a group, percent time alpha may vary from 10 to 90 percent.

In this connection it may

be noted that alpha frequency and percent time alpha appear to be independent variables, and must therefore be controlled by

quite different mechanisms.

From which it follows that

analysis must not measure them indiscriminately (93). IV.

CHARACTERISTICS OF THE ALPHA WAVES

Because of its prominence, regularity, and its easy response to environmental variation (68,98), the early workers regarded the alpha rhythm as practically the whole of the EEC-.

The low voltage, apparently irregular waves,

since they did not seem to respond to stimulation, were, with a few exceptions (87)> set aside as irrelevant to a study of the phenomena under consideration.

A dichotomy was

thus erected between the familiar slow waves and the low volt­ age fast waves.

However, as early as 1936 Travis (138)

pointed out that frequency of the EEG- is a continuous, -unimodal distribution.

Most of the data down to the present

time treat of alpha phenomena only; and since it was found that the alpha rhythm is most conspicuous over the occipital lobes, most of the electrode placements have been such as to favor recordings from these areas.

From which it follows

11 that not all characteristics of the situations studied have been placed under analysis*

However, since most of the

studies have been confined to the rhythm as recorded from the_occiput, the greater part of what follows will of neces­ sity be limited to this rhythm. In studying the alpha waves, W o kinds of records are recognized: those present in the absence of any known envi­ ronmental stimuli, including any known proprioceptive or interoceptive factors; and those in the record as a result of some form of stimulation, including the ill-understood stimu­ lation involved in the bringing into play, or in the main­ taining of, the so-called mental processes.

Before proceed­

ing into the material touching upon the observed phenomena, it seems appropriate to give at this point some definitions pertinent to an understanding of them. Blocking time (also called the latent time of block­ ing (97)» or the latency (149)).

The time required for the

stimulus to diminish or abolish the-alpha rhythm (90), Recovery time (Travis and Knott (147) use -persevera­ tion time). The duration between cessation of the stimulus and the reappearance of the alpha waves (90). Adaptation.

A condition wherein the rhythm fails to

respond further to stimulation (68)*^

12 Complexity.

The introduction of a new stimulus into

a stimulus-response stiuation (98). Facilitation.

In these connections, the word is used

to indicate an increase in alpha activity (154). Standard conditions. A state of quiescence on the part of the subject: the subject is relaxed in a comfortable position, is thinking about nothing in particular, and has the eyes closed (6b).

The eyes may be left open if the cen­

tral part of the visual field is uniform, and if the subject does not attempt to examine it too closely (2).

In some ex­

periments "mind blankness" is an added condition: the subject is instructed to refrain from thinking at all, in so far as that is possible (149). Proceeding now into the body of the material, various phenomena observed with respect to the alpha rhythm will be treated under several topic headings. Inter- and intra-individual differences. A normal subject under standard conditions gives an EEG- —

not just an

alpha rhythm, though this rhythm is the most outstanding aspect —

which is characteristic for him; but the EEG- may

differ considerably from that of another subject (45,68,132, 143,144).

Moment to moment changes in pattern detail, but

the same general appearance, are characteristic of the indi-

13 vidual subject*

An individual shows alpha rhythm (a) with

clearness and regularity, or (b) only occasionally, or (c) scarcely at all.

Subjects vary under stimulation, particu­

larly with respect to adaptation and modification, and any one subject may vary from time to time.

However, under

standard conditions a given subject produces on successive test his own type of record (45) characterized by (a) the presence or absence of a regular alpha rhythm; (b) the per­ cent of time that the rhythm is present; (c) the degree and duration of the alpha rhythm under a standard stimulus — opening the eyes in darkness, in moderate illumination, or in bright light —

(d) the prominence of other waves, whether

or not of regular frequencies, such as beta; (e) the activity level of alpha, usually 20-75 microvolts; (f) differences in amplitude between occipital alpha and alpha at the vertex. It may be noted here that alpha frequency for an individual rarely varies more than 2-3 percent; but, during the second or third hour of long experiments, it usually shows a 5-10 percent variation (118). Some special characteristics of the rhythm*

It blocks

from time to time, sometimes showing periodic fluctuations of amplitude occuring at 1-2 second intervals, even though no known stimuli are introduced into the environment (45,90, >98,99)*

Momentary speed up of alpha by as much as 1-2 cycles

14 per second following closure of the eyes is a characteristic modification (48,84). Drowsiness and sleep.

Drowsiness may cause the alpha

waves to become slo\?er and less regular in shape and rhythm (45).

With sleep (a) at first the 10 per second waves dis­

appear, and are usually replaced by large, slow, irregular waves; (b) as sleep continues the whole activity may be con­ siderably depressed, or at another time it may approach the waking pattern; (c) sometimes a new frequency of about 14 waves per second appears for brief intervals; and (d) the sleep record may be quite as variable and complex as the wak­ ing one, and may be modified by stimuli, which do not waken the subject (116). Personality.

Borne aspects of personality, with re­

spect to their bearing on bradn waves, have been studied. Lindsley (113), in attempting to correlate results on tests of ascendance-submission and emotional stability with alpha frequency and pattern, found no significant relationship. Shagass (134) found no significant correlation when alpha was compared with results on the R.C.A.F. classification test — a group test comparable in form and validity to the usual type of self-administering mental ability test.

Imbeciles

may show a normal EEG-, except for those under four years mental age, indicating that the rhythm does not correlate

15 with amentia (69, 103)*

In fact, in cases of cortical

atrophy the EEG- may be reported normal (150).

On the other

hand, cases have been reported where normal intellection is present in the absence of any electrical activity recordable at the usual amplifications (36)* Aloha response to stimulation.

In general, the waves

are modified by visual, auditory, and tactual stimulation (98,113,140), and perhaps by affective states and conditions of attention (12,117,147); and there is evidence that the rhythm can be conditioned (32,91,141). The nature of the response of alpha to stimulation. Upon stimulation there is first a latent period, and than a fairly rapid depression of the rhythm.

This effect may reach

zero amplitude, or there may be a displacement by other fre­ quencies.

This state ensues, outlasting brief stimuli, and

declining with prolonged stimulation (98).

The magnitude of

the response varies with the sense mode, and the potency of the stimuli in order: visual, auditory, and tactual; and, within a given mode, varies as a function of intensity, -du­ ration, frequency (i.e., adapting with increased numbers of stimulus presentations), and complexity (32,147,148). Visual activity and the alpha rhythm.

Lindsley (113)

found the blocking time for light (on for 0.2-0.4 second) to be 0.30 second; and the recovery time to be 0.96 second.

16 These results accord closely x^ith the findings of Jasper and Cruikshank (*90) and Travis and Knott (147)*

Jasper and

Cruikshank found the blocking time to light (on for 15 sec­ onds) to be 0.16-0.52 second, with an average of 0.28 second; and attribute the wide variability in blocking time to be due in part to differences in mental set of different indi­ viduals to the same instructions.

Knott (98) reports that

blocking time is a function of the intensity of the light stimulus.

Jasper and Cruikshank (90) noted a mean frequency,

during light stimulation, of 2.1, which is 21 percent greater than the pre-stimulus frequency.

With respect to Recovery

time, it appears to be shorter for brief stimuli; and for long periods of light (3-15 seconds) there is a tendency for recovery of alpha before the cessation of the stimulus; if this occurs, cessation of the light may cause the dropping out of the rhythm.

Jasper (84) notes that, for brief stimuli,

recovery time is a function of the intensity of the stimulus— greater intensity giving greater depression of the rhythm. Lindsley (113) reports that the reappearance of alpha usually is accompanied by a frequency increase up to 15 percent, and an amplitude increase up to 100 percent, with respect to the pre-stimulus values; and that the duration of the increase is from 0.5 to 1.0 second. Comolex visual stimulation.

Travis and Knott (148)

found that the visual presentation of meaningful stimuli

XT (words) gives longer perseveration time than does light alone —

intensity of light held constant.

It has "been noted

that the latent time of blocking to light is shorter when light is used as a signal for manual response than when light is used alone (148,149). Modification of the rhythm. Loomis, Harvey, and Hobart (117) found that even the faintest light is sufficient to abolish the waves.

It is to be expected, they note, that

very faint lights would be fully as effective as bright ones, since faint lights would require a greater effort to see than bright ones.

Change in amplitude of the waves gives, perhaps,

the best measure of modification, but it presents too tedious a task to reckon (113).

Using frequency, and conditions of

eyes closed; eyes open and reading silently; eyes open and reading orally; and a second condition of eyes closed; and using the mean frequency for each condition, Knott (96) found a significant increase in reading periods over initial con­ trol periods —

but no significant relation between final

and initial control periods. The aloha rhythm and after-images.

In an experiment

to test the hypothesis that absence of alpha subsequent to light stimulation corresponds in detail with the phenomenon of visual after-image, Jasper and Cruikshank (90) found that, with the light stimulus on for 15 seconds, the first after-

lb image, in terms of wave count per unit time, is about as effective, as light itself in blocking the rhythm; and that successive after-images have somewhat less effect.

They

noted that the amount of a,lpha activity between successive after-images is greater than that during the presence of images, but it is much less than that during pre- and post­ stimulation periods.

Short bursts (small amplitudes) were

found toward the end of the 15-second period of light stimu­ lation; and, during the 15 seconds following the disappear­ ance of all after-images, median frequency was higher than before stimulation.

Travis and Hall (145) found that, for a

high degree of attention to after-sensations, the total du­ ration of alpha and the mean duration of bursts during the after-sensation periods were less, and the length of time for the first burst to appear after the light went off was greater than for a low degree of attention. Other factors influencing the rhythm.

In addition to

the factors mentioned above, factors such as the amount of previous -stimulation, reflecting surfaces, change in illumi­ nation, and pattern of the visual field may influence not only recovery, but also return of the rhythm to its pre­ stimulation constancy (90).

If a continuous or repeated

stimulus is presented, adaptation to it gradually takes place, alpha returning to 50 percent of its pre-stimulation value within 2-8 seconds (4).

According to Adrian and

19 Matthews (2), the perception of light itself does not affect the rhythm: it is the perception of pattern, no matter how uninteresting, or the attempt to perceive it that interferes with the waves. Flicker -phenomenon.

It has been observed that the

alpha rhythm can be caused to fall into step with a flicker­ ing light stimulus.

Adrian and Matthews t2) have observed

the pacing up to 20-25 waves per second.

Jasper (85) reports

synchrony up to as high as 55-60 waves per second.

Frequent­

ly the wave response is double the flicker frequency, with the alternate waves sometimes of much lower amplitude (117); and, in some cases, when certain multiples of the spontaneous alpha frequencies are reached, the rhythm modifies in pattern, or shifts back to its original frequency.

Loomis, Harvey,

and Hobart (117) note that in the case of one subject —

the

one showing the highest regularity of alpha of these tested — there was no indication of flicker response, but a slight slowing of alpha with slowing of flicker, and a slight speed­ up with a speed up of flicker. Aloha response to auditory stimulation.

The blocking

time of the alpha rhythm to auditory stimulation is about the same as that for visual, but the recovery time is frequently much less (113).

Martinson, according to Travis (140), gives

the latent time as approximately equal to 0.3 second, and the

20 perseveration time as about 1.4 seconds.

While the blocking

time to. light may vary logarithmically with intensity and duration of a single light flash (89)., this variable has not been found to operate in this way. ^

*

Sound stimulation is ineffectual much more frequently than light in depressing the rhythm (149).

There are cases

where a sound stimulus alone produces no effect (90); and the same auditory stimulus does not produce a depression of alpha in all subjects (4).

For most subjects there is no increase

of frequency or amplitude after blocking, which suggests a different mechanism at work from that operating in visual situations (113).

However, Bagchi (4) found in 36 percent of

his records a facilitation of alpha, following a period of adaptation to an auditory stimulus.

The average alpha ampli­

tude during adaptation was 149 percent of the pre-stimulus value, with a maximum of 348 percent stretching over quite some time.

This phenomenon was found in 38 percent of the

periods following complete cessation of all stimulation. Complexity. Martinson, according to Knott (98), found that where a tone was ineffective in itself — adaptation —

owing to rapid

it was made effective by presenting it at two

intensities, and in a random order, with two intensities of light.

In an experiment where sound was used as a prepara­

tory signal to light stimulation, after 5 or 6 presentations

21 of the stimuli, alpha would frequently drop out following the end of the preparatory sound signal, and before the appearance of the light.

This tended to show that the reaction

of the

brain rhythm was not necessarily specific to light (90). Travis and Egan (114) found in their conditioning experiment that the electrical response of the cortex preceded the light (unconditioned stimulus) in 35 percent of the records, but that the increased effectiveness of tone when presented with light did not persist when the tone was presented alone, following the paired stimulation series.

In another experi­

ment (142), using complex auditory stimulation (speech) against a control (standard conditions), they found a signif­ icant increase in alpha frequency for the stimulation period, as compared to the control period. Tactual stimulation and the aloha rhythm.

This modal­

ity has received the least attention, and is, of the three studied, the least effective in altering the rhythm.

From

the study of Travis and Barber (140) a few facts will be noted. The areas stimulated were the forehead, eyelid, and shin (3 inches above the ankle), and the stimuli were both blunt and sharp instruments.

In the majority of the legible

records, stimuli \fere ineffective: the rhythm was either un­ affected, or the waves became even longer, at the time of

22 application of the stimulus or immediately thereafter.

For

all series in which the stimuli were effective, the mean latency was found to be 0.48 second, and the mean persever­ ation time 1.08 seconds. lid and shin —

For widely separated areas —

eye­

in one pair only was there found a signif­

icant mean difference in the alpha activity. Emotional stimulation.

The question has been raised

as to whether the blocking is a result of sensory stimulation as such, or emotion aroused by the stimulus, or by states of attention (113)*

Berger (9,10) stressed the importance of

heightened attention, as well as the effect of strong stimuli (e.g., an explosion); and pointed out that intense worry, emotional excitement, or apprehension may often result in a partial suppression of alpha.

Davis and Davis (45) noticed

much the same things: that nervousness or apprehension may prevent the appearance or the rhythm, and any stimulus that startles the subject or particularly attracts his attention may cause a temporary depression of the waves.

However,

Lindsley (113), using a startle stimulus (buzzer), and obtain­ ing adaptation of the galvanic skin response, continued the presentation of the stimulus.

It was observed that the stimu­

lus often continued to suppress alpha just as effectively. From this it was concluded that (a) the emotional or star­ tling value of the stimulus is not essential to the abolish­

23 ment of alpha, but that (b) It does not preclude the possi­ bility that in some instances the emotional nature of the stimulus may prolong or otherwise modify the record. It has been noted that many subjects when first tested show few waves; but later, when they have become ac­ customed to the procedure, alpha waves appear.

This blocking

is inferentially attributed to embarrassment or apprehension (117)*

Loomis, Harvey, and Hobart (117) describe a subject

who could imagine himself in a terrifying situation. phantasy caused the rhythm to disappear.

The

A second trial

proved successful, but a third trial produced only momentary blocking —

the subject reporting that the phantasy produc­

tion was difficult to repeat. Attention. sensory stimuli —

In observing the effects on the rhythm by particularly visual —

Berger (10) noticed

that on several repetitions a stimulus tended to lose its effectiveness.

He concluded that effectiveness depends upon

the subject*s attending to the stimulus: that the attention value of the stimulus, rather than the stimulus as such, caused the depression.

Anything which engages the subject,

or tends to focus his attention upon a stimulus or task blocks the rhythm (11).

However, Adrian and Matthews (2)

pointed out that alpha is most closely connected with vision, and noticed that much can go on in a subject*s mind without

24 upsetting the rhythm, e.g., he „can count numbers aloud, re- . peat familiar poetry, and join in an unimportant conversation with, at most, an initial reduction inthe size of the waves. They concluded that the adequate stimulus for alpha depres­ sion is pattern vision. Other investigators agree with Berger that depression of alpha is related to attention: stimuli with the greatest attention or arousal value are most effective in depressing the rhythm (4,88,117,145).

Williams (154) decided to con­

sider attention as a factor separate from afferent stimu­ lation, because he was able to show that an increase in at­ tention alone was sufficient to decrease the amount of alpha activity as indicated by the alpha index.

He held that the

kind of attention which depresses the rhythm is attention to a specific stimulus well above threshold.

He noted that

a mere state of attention, i.e., attentiveness, where no stimulus is present to be attended to, will cause no depres­ sion in alpha, and may even cause facilitation; and emphasized that both stimulation and attention are effective in depress­ ing the rhythm.

He further noted that the factors, stimula­

tion and attention, are related, but not identical; that they usually occur together, but to some extent are Independent in their effect upon the rhythm.

25 Central factors and their Influence on the rhythm.

In

addition to the psychological events present in the cases of stimulation and attention, there appears to be a second group of events which is predominantly central.

The latter

has to do with the so-called mental processes evoked in vis­ ual imagery, mental effort, and abstract thinking*

In atten­

tion, and perhaps in response to an afferent stimulation, one is attempting to carry on some mental process, one aspect of which is the act of attending under an external environmental urging; in thinking and imagery, one is engaged in some men­ tal processes which have been set into motion by environmen­ tal factors immediately past or remote.

To what extent the

events comprehended under stimulation and attention represent different underlying neurologic factors from those subsumed under mental processes is unknown.

The question to be an­

swered is whether the latter are able to affect the EEG-, as is known to be the case with the former. In most of the literature dealing with psychological events as they affect the rhythm, no distinction is drawn between these two categories, or, if one is noted, it is merely to state that mental processes per se have no effect upon the waves.

Adrian and Matthews (2), summing up their

findings with Berger's, note that a problem claiming the whole attention of the subject will abolish alpha; that

26 difficult problems in mental arithmetic will do so, so long as the subject tries hard to solve them; but that the rhythm returns as soon as the subjectfs attention wanders,

Gibbs,

Davis, and Lennox (68) note that mental arithmetic flattens the record,

Williams (15^) reports that evidence from ex­

periments involving cognition in the form of problem solving suggests that cognition as -such is not effective in depress­ ing the rhythm. Unless certain variables have been overlooked, it appears that mental processes do affect the record (73#74, 139)*

Id clarify this point on possibly-overlooked variables,

it may be noted that some investigators posit the presence of automatic factors in all mental events — well as in the mental processes,

in attention as

Hoagland, Cameron, and

Rubin (81) note that there is no measure available for dis­ tinguishing between emotion and mental effort.

They go on

to observe that fixation of attention involves some emotional mechanism; but, on the contrary, qualitative comparable degrees of attention appear subjectively to vary widely in emotional content.

With respect to the modification of the

record in problem solving, they are of the opinion that the change in the waves occurs when the subject is expected to get results in the presence of others —

the change is at­

tendant upon emotional rather than mental factors.

Davis and

27 Davis (45) note that concentrated mental effort, such as solving difficult problems in mental arithmetic, particularly if an emotional element of hurry or embarrassment is present, may suppress the rhythm* On the aspect of visual imagery, Adrian and Matthews (2 ) reported that the