During the past five years Hans Berger (1929-1933) has published a series of papers dealing with a remarkable electric effect which can be detected in the human subject by electrodes applied to the head. It consists of a rhythmic oscillation of potential with a frequency in the neighbourhood of 10 a second, appearing when the subject lies quietly with eyes closed and disappearing if the attention is fully occupied.
Berger has shown that the rhythm can be stopped by non-visual activity if the subject's whole attention is involved, but it is stopped far more effectively by a pattern in the visual field, however uninteresting.
In fact for the rhythm to appear, either the eyes must be shut, or the central part of the visual field must be uniform and the subject must not examine it too closely.
It is the perception of pattern or the attempt to perceive it that interferes with the rhythm. The perception of light does not affect it: if the eyes are closed the rhythm persists, although the subject is aware that the room is light or dark and can say when the illumination is altered. Again, with the eyes open, the visual field can be made more or less uniform by arranging an opal glass bowl in front of the face and lighting it by a ring of lamps. The rhythm will appear after a time, and when it is established the illumination of the bowl can be varied without causing more than a momentary pause in the waves, provided that the light is not unpleasantly bright. On the other hand, a narrow band of shadow thrown across the field will stop the rhythm at once.
Absolute uniformity of the visual field is not essential, however; with the eyes shut the rhythm usually appears a short time before an after image has completely faded, and with the eyes open the peripheral part of the field may contain some pattern. But sharp contrasts nearer the central part of the field are prohibitive. Thus a few black spots on the opal screen, or a few small holes in the shutters of a dark room, will abolish the rhythm if the subject looks directly at them, but if they come into the peripheral field he may be aware that there is something there and yet the rhythm will persist.
Although there is the same contrast in the effect of a uniform and non-uniform field whether the eyes are closed or open, the closure of the eyes seems in itself to favour the development of the rhythm. The regular waves usually appear half a second or less after the eyes are shut, but if they remain open and the field is made uniform the waves may not appear for some minutes. The difference is chiefly noticeable when the subject is first confronted with a uniform field. This may be either the opal screen arrangement or the uniform black field produced by enclosing the head in a box lined with black velvet, or by making the whole room completely dark. To begin with the rhythm will only appear when the eyes are shut, and it will cease when they are opened, although the field is uniform. In a few minutes the rhythm will develop with the eyes open. It may still be disturbed when they are opened after a period of closure, but eventually opening them will cease to have any effect.
The most likely explanation is that the act of closing the eyes is coupled with an automatic withdrawal of the attention from visual phenomena. When our eyes are open we try to see something, even though we are in a room which is too dark for anything to be made out. So with the head before an opal screen or in a dark box the tendency is to look for some trace of detail, and to begin with it is only when the eyes are closed that the subject is completely indifferent to the content of the field. After a time he becomes used to his surroundings and it is easier to open the eyes without expecting and trying to see. It is then that the rhythm persists.
This explanation is based mainly on the introspection of one of us acting as subject. It must be admitted that when the eyes are open in the dark and the rhythm absent the feeling of " trying to see " is often very slight, although at times one may be conscious that one is peering into the blackness. But the explanation is supported by the fact that after the rhythm has become established it can be made to disappear at will by attempting to see some trace of detail in the field. The attempt is often attended with a feeling of muscular effort which probably implies convergence and accommodation, but the rhythm may go before this is felt.
An effort of the same kind can abolish the rhythm although the eyes are shut. In this case the effort causes a method of convergence which can be felt through the closed lids. But it is perhaps a mistake to group this with the other kinds of visual activity which abolish the rhythm, for it involves considerable mental effort and a non-visual activity of the same kind, e.g. straining the ears to hear a nearly inaudible sound (e.g. a watch ticking), is equally effective. On the other hand, when the eyes are first opened in the dark no conscious straining to see is needed to prevent the appearance of the rhythm.
It is concerned with pattern vision, but the rhythm can be abolished by attempting to see a pattern as well as by the sight of one.
It is clear, however, that the area which gives the rhythm is concerned specifically with visual activities and is not some general correlating centre. As was pointed out on p. 371 the visual activities which abolish the rhythm do not claim more than a small part of the subject's attention and if pattern vision is absent the rhythm persists in spite of other activities which demand a fair share of attention for themselves.
We have assumed that the area is so much a part of the visual apparatus that when vision is cut off there will be nothing left to disturb it. But an intense activity in the rest of the brain will do so, and it seems that if vision is permanently cut off the area is not allowed to remain idle but becomes gradually more and more accessible to excitations from other parts. In the end it will be continually disturbed and will have no chance of developing a synchronous beat. Thus the Berger rhythm is absent in the blind.
The essential condition for the appearance of the Berger rhythm is that pattern vision should be absent. It develops when the eyes are closed or if the visual field is uniform, and disappears whenever the central part of the field has any detail. The attempt to see detail, even though the field is uniform, abolishes the waves: for this reason the closure of the eyes, by withdrawing the attention from visual phenomena, aids the development of the rhythm. As Berger has shown, non-visual activities which demand the entire attention (e.g. mental arithmetic) abolish the waves; sensory stimuli which demand attention do so too.
We believe that the potential waves are due to the spontaneous beat of an area in the occipital cortex which is normally occupied by activities connected with pattern vision. When the area is unoccupied the neurones discharge spontaneously at a fixed rate (as in other parts of the central nervous system) and tend to beat in unison. Visual activity and widespread non-visual activity break up the synchronous beat by exposing the area to non-uniform excitation. In man a large area is normally occupied with visual activities; thus when the area has nothing to do and is free to develop a synchronous beat the potential changes are large enough to be detected outside the skull.
