Alpha waves stock: characteristics and role in the visual system

The editorial staff of Emianopsia is pleased to host Dr Cadri, a Doctor in Neuroscience and Neuropsychological Rehabilitation, who will give us an overview of what alpha waves are.

All cortical activity presents electrical oscillations, referred to as rhythms or brain waves. They differ according to oscillatory parameters such as amplitude (or power), measured in Mv, and frequency, measured in Hz.

The waves and their oscillation parameters are recorded with the Electroencephalogram (EEG) and play an important role in all functional mechanisms promoted by the cognitive system.

They are conventionally divided into five rhythms named after the Greek letters:

• Alpha
• Beta
• Delta
• Theta
• Gamma.

Of these, the oscillatory activity in the Alpha band (8-12Hz) seems to be the one that, more than others, has attracted the most interest and scientific attention over the years. 

What are Alpha waves?

The definition of the Alpha rhythm most widely used to date has been proposed by the International Federation of Societies for Electro-encephalography and Clinical Neurophysiology (IFSECN), which describes it as a rhythm with a frequency ranging from 8 to 12 Hz that occurs during wakefulness in the posterior regions of the head, generally with a higher voltage in the occipital areas.

It tends to be best observed with the eyes closed and in a state of physical relaxation with relative mental inactivity. The rhythm is blocked or attenuated by attention, especially visual effort. 

Thus, Alpha is the dominant EEG rhythm in the occipito-parietal regions in the resting state with eyes closed.

In this condition, Alpha presents itself in the EEG tracing with a slower frequency and greater amplitude, whereas the moment the eyes are opened, or an activity requiring cognitive effort is initiated, the oscillatory rhythm tends to become faster and the amplitude to decrease.

The link with the visual system

The changes that occur about Alpha rhythm activity depend on external stimulation or internal states and reflect different cognitive processes. In particular, it is thought that the complex of these oscillatory dynamics of Alpha may be partly related to the reactivity and functioning of the visual system.

For this reason, Alpha has been investigated during the performance of perceptual tasks and in a resting condition with eyes closed, in which it is prevalent.

More precisely, the most recent EEG research shows how the electrophysiological parameters of alpha waves can mediate different aspects of visual perception.

Studies conducted with different perceptual paradigms maintain that there is a relationship between the modulation of alpha amplitude when the eyes are opened and the perception of a visual stimulus, defining this rhythm as a possible index of the excitability of the visual cortex, the area involved in perception.

What is observed is a correlation between the amplitude of Alpha and the excitability of the visual cortex, in which, when the eyes are closed, there is an increase in the amplitude of Alpha indicating a lower excitability of the visual cortex, while when the eyes are opened, the amplitude of Alpha decreases and cortical excitability increases.

In other words, this correlation, which involves modulation of Alpha amplitude, results in whether or not the visual system is ready to perceive a stimulus and then process it.

In addition, a trend of studies argues that Alpha, with its oscillatory activity in certain situations, plays a role in inhibiting activities that are not relevant to the execution of a task, thereby facilitating it.

The oscillatory frequency of Alpha

Regarding the oscillatory frequency of the Alpha rhythm, it is thought that it may affect reaction times and the speed of information processing, working as a sampling or sampling system of the visual stimulus.

A higher Alpha frequency corresponds to faster information processing and more accurate visual discrimination, compared to a slower frequency associated with poorer perceptual processing. Alpha frequency plays a role in the temporal resolution of information and causes the rhythm to have cyclic perceptual gates and time windows for information filtering.

Alpha wave activity in subjects with posterior cortex lesions

Although the behavior of Alpha and its relationship to visual system function are well known in healthy individuals, little is known about rhythm activity in subjects with lesions of the posterior cortices, especially in the occipital and parietal areas where Alpha activity is predominant.

Indeed, damage to circuits of the visual system, which induces a deficit in the visual field as in the case of hemianopsia, may result in alterations in Alpha oscillatory activity. When recording EEG activity in resting condition in patients with right or left hemianopsia, pathological patterns of Alpha rhythm oscillation were found.

In patients with hemianopsia following a posterior lesion, the typical reduction in alpha wave amplitude at eye-opening is altered, although partially preserved, resulting in reduced cortical excitability and decreased responsiveness of the visual system. A slowing of Alpha’s oscillation frequency was also found, which can be translated as a lower efficiency in the information sampling mechanism.

Thus, Alpha may represent a reliable electrophysiological fingerprint of the visual system in humans and a possible biomarker of the functioning of the posterior cortices, reflecting their integrity in both healthy and clinical reference populations.