Updating Body Sense: Windshield Wiping of Cortex and Using an Insula Searchlight

December 19th, 2013 by Phil Weiser Leave a reply »

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:

Klimesch etal 2007 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.

Be Sociable, Share!
    Advertisement

    Comments are closed.