Cortical Alpha Rhythms Research Articles

Alpha waves--artifacts?

Kennedy has argued that alpha rhythm recordings are artifacts caused by the mechanical pulsation (choroid plexus pump) of a gel (the living brain) with different electrical potentials. This paper has been republished in a recent collection by Evans and Robertson. Kennedy's hypothesis seemed insufficiently supported, and contradicted the consensus concerning the origin of alpha waves which existed at the time. Significant recent research is examined which provides strong, if not conclusive, evidence that alpha waves represent a basic cellular process probably related to excitability. It is suggested that Kennedy's phenomenon be explored further to clarify any effect it may have on standard EEC recordings. A few years ago, Kennedy (1959) presented a rather surprising article suggesting that the externally recorded cortical alpha rhythm could possibly be—in fact probably was—a recording artifact. This was in direct contradiction to the trend of theorizing about the nature of alpha waves. Kennedy's paper did not cause any great uproar nor did it change the direction of research or writing on the EEC. Recently, however, this same paper has made a reappearance in a book of collected readings (Evans & Robertson, 1966). This renewed presentation compels comment. In essence, Kennedy suggested that the alpha rhythm does not represent a basic process at the cellular level, but is merely an artifact due to the gross physical properties of materials from which the recordings are made. He suggested that the alpha rhythm may be due to three sets of conditions normally found in the brain: (a) an electrically charged state, presumed to arise through chemical oxidation reduction reactions; (6) a gel mass with a co-efficient of elasticity approximated by commercial gelatin; and (c) a periodic mechanical pulse, to set the gel into oscillation at its resonant frequency. It is proposed that the interaction of these conditions in the skull may produce the oscillating potentials recorded from normal humans by standard electroencephalographic equipment [Kennedy, 1959, p. 352], In order to demonstrate this mechanism, Kennedy used a mass of commercial gelatin under conditions roughly similar to the human brain. He mechanically pulsed this mass to simulate the arterial pulsing of the brain so that it resonated at its particular frequency. He demonstrated that this resonance is capable of producing rhythmic potential changes which roughly resemble the alpha wave of the cortical EEC. As further support, he established an alpha rhythm in a subject in whom it was not previously evident by restoring his intracranial pressure to normal through use of a head plate to cover a small cranial wound. These demonstrations are ingenious and they should certainly dictate caution in any interpretations of such recordings. They do not, however, seem to disprove the more orthodox hypotheses.

Simultaneous Changes in Visual Separation Threshold and Voltage of Cortical Alpha Rhythm

An automated psychophysical method allows a human subject to track his own separation threshold for two flashes delivered in sequence without interfering with the simultaneous recording of the alpha rhythm of his electroencephalogram. A systematic rise in this threshold is observed when the alpha voltage declines from its normal alert level. This suggests that an increase in the minimum resolvable temporal sequence may contribute to the development of the negative time errors associated with drowsiness and sleep.

Electroencephalographic abnormalities in hyperthyroidism.

IT WAS recognized early in the course of our electroencephalographic studies on endocrine disorders that some hyperthyroid patients had abnormal electroencephalograms (1). Gibbs and Gibbs (2) in their Atlas of Electroencephalography reported that the electrical activity of the cortex tends to be abnormally fast in hyperthyroid patients. Lindsley and Rubenstein (3) found a correlation in normal subjects between the alpha frequency and the basal metabolic rate. Odoriz et al. (4), in their study of parathyroid insufficiency, observed fast activity, simple spike discharges, and slow waves (2 to 3 per second). Ross and Schwab (5) reported that when the cortical alpha rhythm is determined during a “basal state” (patient fasting and resting one half hour) the correlation is not as good as when the patient is well fed, satisfied and relaxed. Fabrykant (6) noted that, in a group of 15 labile diabetics, electroencephalographic abnormalities were recorded in 80 per cent and severely disordered patterns consistent wi...

The effects of polarizing currents on cell potentials and their significance in the interpretation of central nervous system activity

Since no potential could be recorded from the medium surrounding a cell which was uniformly and synchronously active over its whole surface, potentials recorded across cell layers represent the difference in activity at apical and basal regions. Since potentials led from simple cell layers are effectively monophasic, the conduction time along the soma is a minor factor in determining the form of the record. Diphasic records are inferred to be summations of cell discharges some of which, represented predominantly in one phase, have a difference of potential of one polarity, others represented chiefly in the other phase having a potential difference of opposite polarity. If this is valid, potential differences of opposite polarity must be exhibited by cortical pyramids with a uniform anatomical orientation. A difference in the distribution or intensity of response of apical and basal dendrites is suggested as a possible factor. Polarization from an external source across a simple cell layer may either increase the amplitude of the monophasic difference of potential led from across it, or decrease and finally reverse its sign; presumably by altering the relative amplitudes of electrical activity at the two poles of the cells, but without blocking conduction from pre- to post-synaptic axons. The interpretation of this fact involves that polarizing currents act on cells essentially as they do on peripheral nerve axons. The change in polarity of record is in a direction opposite to the sign of the polarizing current. Polarization of optic tract axons may result in prolonged after-effects, negative under anodal polarization and positive under cathodal. This is interpreted as representing a repolarization, after discharge during the spike of the externally applied component of membrane charge, along with its resting metabolically induced component. This effect also occurs at axon end-arborizations, and changes in synaptic conduction may be assigned to such effects of sufficient intensity as well as to polarization of post-synaptic dendrites in the same region. Under polarizing currents the changes in amplitude and reversals of sign of the cortical alpha rhythm, of evoked potentials from shocks to the optic nerve including the evoked alpha waves, evoked potentials from the superior colliculus, strychnine spikes, and paroxysmal spikes of cortex set up by repetitive stimulation or otherwise all may be accounted for by the interpretation offered for the effects of polarization at the geniculate.

THE INFLUENCE OF THYROIDECTOMY ON THE VARIABILITY OF NEUROMUSCULAR ACTIVITY IN THE RAT

IT is GENERALLY AGREED that thyroidectomy leads to a reduction in the level of neuromuscular activity (1). The converse, however, does not hold. Feeding thy roid hasbeen shown to result in a decrease rather than an increase in activity (2). The decreasedactivity following thyroidectomy may be attributed in part to lowered levels of excitation and irritability of the nervous system. This hypothesis tends to be confirmed by experiments indicating lowered resistance to narcotics (3), increased thresholds to light and sound stimulations (4), decreased speed of simple reaction to an electric shock (5), and lowered rate of cortical alpha-rhythms (6) in the hypothyroid state. Since the variabilityof behavior is in large part a function of sensitivity to environmental stimuli, andassuming that the thyroid secretion maintains the normal level of irritability of the nervoussystem, it is reasonable to predict that the daytO-day variability of spontaneous activity would be less in hypothyroid than in normal animals.