SUMMARY
Professor: This post is about:
- Q: How do we continuously
- update our ‘body senses’
- while we are exercising (or whatever we are doing)?
- A: We use our brain alpha rhythms,
- like a windshield wiper,
- to disregard ‘unimportant’ neural network activities.
- Q: How are the ‘important’ neural events detected?
- A: We use our
- right anterior insula’s body scanning ‘searchlight’
- to spot unexpected, significant sensations and
allow them into consciousness.
Student: So,
- If we have tonic alertness,
- we can continuously update our ‘body sense’
- using brain alpha oscillations to actively suppress task-irrelevant areas.
- If we are selectively attentive,
- we can rapidly detect critical psychophysiological events, i.e., physiologically evoked potentials,
- by using the Posterior Insula’s Searchlight,
- to sense them trying to get through the Sensory Gates, aka, the thalamic reticular nucleus,
- and allow sensations to enter the pathways to the higher cortical centers.
Professor: Very nice analogy. Yup, and now let’s get into the details. First, our bodies are actively disregarding its ‘normal’ noises.
Tonic Alertness
Professor: In dealing with leg discomfort and other symptoms, it’s important to contrast alertness with arousal and selective attention Sadaghiani and his colleagues (2010). They noted that
- arousal means a
- “sense of wakefulness and responsiveness … and
- is controlled by the brainstem”
- while attention “transiently ensures
- selective local processing of specific features and
- is tightly linked to activity in dorsolateral parietofrontal cortices.”
- In contrast, they state that “In contradistinction to these [other] cognitive functions, tonic alertness refers to
- an intermediate capacity that expresses nonselective readiness for perception and action, and
- is implemented by a corticosubcortical system (consisting of the anterior insula, anterior cingulate cortex, and thalamus].”
- They comment that
- the anatomy and function of the cingulo-insular-thalamic network
- is well suited to underpin this control process.”
Student: Nonselective readiness, eh!? Sorta like keeping the hounds at bay near the chicken coop.
Professor: At bay, eh?! You mean that the thalamus prevents escaping EPs by surrounding them with “barking dogs” (to paraphrase a statement from the internet: Origin_of_the_idiom_keep_at_bay)
Student: Now, please come back to the concept of alpha waves inhibiting excessive neural ‘noise’!
Professor: Then I will again quote Sadaghiani et al. (2010):
“Due to its nonselectivity, alertness involves what amounts to a suppression”
Professor: And this means that enhanced alpha desynchronization can actively subdue EEG activity in brain regions that are not involved with task performance.
Professor: To clarify this concept, let me paraphrase Klimesch et al. (2007):
- Brain waves reveal rhythmic changes in the membrane potential of masses of neurons.
- In fact, oscillations reflect phases of low versus high inhibition of these neurons as shown below on the left axes of their Fig. 1:
Fig. 1 – The basic principles underlying the inhibition–timing hypothesis…
Professor: To further clarify this concept, let me paraphrase their figure 1 legend:
- The basic principles for inhibition of non-necessary brain activity
- can be illustrated by considering the phase of oscillatory activity
- and its amplitude
- together with the level of excitation of single neurons.
- In the figure above, they assume that
- oscillatory activity is induced by inhibitory neurons like pyramidal cells and
- reflects rhythmic changes between phases of maximal and minimal inhibition.
- Depending on the amplitude of the oscillation (and the excitation level of single cells), two different firing patterns can be distinguished:
- (Figure 1A) – If the amplitude of the oscillation is small, neurons like Cell 1 with a high level of excitation fire tonically, not entrained to the phase of the oscillation. Other neurons with low excitability like Cell 2 and 3 fire rhythmically entrained to the oscillation.
- (Figure 1B) – If the amplitude is large, even neurons like Cell 1 with a high level of excitation now will fire rhythmically, entrained to the phase of the oscillation!
The other neurons with low excitability like Cell 2 and 3 still fire rhythmically but with fewer action potentials during their bursts.
Student: Just changing the amplitude of an alpha oscillation could affect many neurons in its vicinity!
Professor: Consequently, Klimesch et al. indicate the functional “state of brain activation” in this vicinity can be either:
- Activation is indicated by Alpha Desynchronization that is found
- when opening your eyes, even in a dark room, and
- in response to undertaking a variety of different types of tasks, or
- Inhibition is shown by Alpha Synchronization that is induced
- By tasks demanding the retention of complex encoding of information or motor performance sequences, and
- Suppression of brain areas not involved during task performance.
Professor: In a very active area of neurons networking as a part of task performance, generally its processes are enhanced by desynchronization, i.e., reduction in synchrony.
Student: I would guess that the neural areas surrounding these networking cells, would become much more synchronized and show an increased amplitude of alpha oscillations.
Professor: Your guess has be substantiated:
- Lower levels of local synchronization, that is, desynchronization, is frequently termed alpha suppression and
- Are found in a variety of tasks,
- Are not a unified waveform,
- since lower alpha activity is wider spread and
- is apparently related to specific attention
- follows a time course set by the task requirements, having on onset at about 200 ms, a desynchronization peak at about 350-650 ms, and a resynchronization afterwards from 900 to 2000 ms.
- If the task is anticipated, the onset may occur on a rising phase of an oscillation and result in a large alpha synchronization peak followed by desynchronization.
- Finally, the duration of the desynchronization corresponds closely to the duration of the task.
- Surrounding areas of increased synchronization initially observed in the eyes closed situation, was thought to be due to brain ‘idling’ or ‘nil-working’, BUT recently been found
- In tasks where a learned response must be withheld, or
- Over brain areas that are not relevant to the task being performed, or
- In an area of previous desynchronization after the task is over and a period of resynchronization results in alpha synchronization preparing for next task.
Student: Ah Ha, do you mean resynchronization is like making and erasing “an exercise symptom list” on a “marker board”?
Window Wipers
Professor: Very nice rhetorical question about exercise symptoms! Sadaghiani and his colleagues (2010) studied our brains’ fluctuating electrical noises during a eyes-closed, resting condition. They observed that:
- “The correlation of activity in the [insulo-cingulo-thalamic] system
- “with upper alpha band oscillations
- “as the most robust electroencephalographic marker of vigilance fluctuations
- “suggests that this system could serve a role in maintaining tonic alertness.”
Professor: You solved your rhetorical question. So did Sadaghiani et al. and from their results, they propose,
- that alertness involves a generalized ‘windshield wiper’ mechanism and
- that alpha oscillations serve this purpose
- by rhythmically and synchronously
- clearing the flood of sensory information
- on a rapid time scale to reduce distraction and
- hence enhance detection of novel and relevant sensory information.
- This proposed mechanism is compatible with evidence of alpha synchronization as an active mechanism for inhibitory top-down control (Klimesch et al., 2007).
Student: Didn’t Klimesch et al. (2007) also describe “traveling waves”
- “… moving in a task-dependent manner, e.g., from anterior to posterior sites
- “… with a “‘travel’ speed … in the range of neural transmission“?
- “In general, [Klimesch et al.] assume that traveling alpha reflects waves of spreading activation moving from one area to another.”
Professor: Actually part of the traveling wave could be following and assisting
- in the resynchronizing of the recently active areas having a focal synchronization and
- in the deactivation of the surrounding areas having an increased desynchronization.
Student: Sure seems like “paying attention” really means “being alert” to focal symptoms and to not being alert to surrounding discomforts.
Take Home: “The [rising] deactivating/inhibitory phase [of an alpha wave] operates as inhibitory filter to achieve a high [signal-to-noise] ratio by allowing only a small number of cells to process information selectively and silencing the majority of other cells.” (Klimesch et al. 2007)
Next: Talking about what allows us to becoming aware of something through our senses and deciding to ameliorate non-important symptoms.
References
Klimesch W, Sauseng P, Hanslmayr S. (2007) EEG alpha oscillations: The inhibition–timing hypothesis. DOI:10.1016/J.BRAINRESREV.2006.06.003.
Sadaghiani S, Scheeringa R, Lehongre K, Morillon B, Giraud A-E, Kleinschmidt A. (2010) Intrinsic Connectivity Networks, Alpha Oscillations, and Tonic Alertness: A Simultaneous Electroencephalography / Functional Magnetic Resonance Imaging Study. DOI:10.1523/JNEUROSCI.1004-10.2010.
