Student: Today it was cold outside, and when I started out to exercise I promptly reactivated my cold-induced asthma. I could hardly catch my breath until I put a hand sock, er, ‘glove’, over my mouth and nose. Voila, not so much wheezing, and my breathing became a lot easier.
Professor: Very smart trick, like our study highlighting the simple use of a surgical face mask to block exercise-induced wheezing (Brenner et al., 1980).
Student: Say, in our last blog post you suggested that,
- the thalamic reticular nucleus (TRN) acts like an externally driven searchlight.
Professor: Please, also remember that the TRN acts as a bottleneck allowing only a small part of the thalamus’s activity to travel up into the cortex. Perhaps a better metaphor is a clogged up strainer, and the function of a searchlight is to show holes that ought to be opened.
Student: But what is the mechanism that operates the searchlight?
Professor: Your excellent question can actually be slightly rephrased:
How do processes like attention and focusing operate?
Student: And, here is a more concrete question:
What is involved focusing our attention on tasks involving
- focusing on an internal process like breathing
- in contrast to computer-oriented tasks using externally oriented vision?
Professor: Timely question! Very recently, Farb et al. (2013) reported a study on external versus internal forms of attention. They found that,
- “Visual [External Attention] modulated striate and extrastriate [visual ]cortices and a lateral frontoparietal ‘‘executive’’ network.
- “By contrast, respiratory [Interoceptive Attention] modulated a posterior insula region sensitive to respiratory frequency, consistent with primary interoceptive cortex, [along with] a posterior limbic and medial parietal network, including the hippocampus, precuneus, and midcingulate cortex.”
Professor: These investigators used a “Follow The Breath” task for Interoceptive Attention, and some of the participants included meditators. The exteroceptive tasks were to press a button when a repeated word appeared (maintain task: attend toward external visual stimuli) and to read sequentially presented words (suppress task: “keep the mind blank” to attend away from internal signals).
Student: Did they find any involvement of the thalamus?
Professor: Yes. And again the parts of the thalamus identified, in our last blog post, were active in this study:
- During respiratory Interoceptive Attention, the ventral posterolateral and ventral posteromedial nuclei of the thalamus were highly connected with right posterior insula. The authors referred to this section of the thalamus as the “putative interoceptive thalamus”.
- And during visual External Attention, the ventral anterior nucleus and the pulvinar of the thalamus were connected. Similarly, they identified these hotspots as the “visual thalamus”.
Student: Then is the posterior insula capable of functioning as a searchlight focusing on the TRN?
Professor: Yes, it sure does, and it seems like the posterior insula functions this way.
Student: You mean that when I am running, and I do a “Body Scan“, I am telling my posterior insula to shine the Searchlight over the thalamic reticular nuclei and catalog the activities of various areas of my body?!
Professor: Yes again. In fact, these investigators found that the “posterior insula activity [was] correlated directly with respiratory rate. ” So, it is keeping track of the output of the ventral posterolateral nucleus of the thalamus which is ….
Student: … part of pulmonary homeostatic regulation!
Professor: Let’s emphasize that the posterior, middle, and anterior together perform functions of the primary and secondary interoceptive cortex. Concerning cortical activity overall, we are seeing the reflections of three Central Networks:
- Caudal Reflexive Network, involving the spinal cord, medulla, cerebellum, and pontine pinpointed on the thalamus as the Sensory Gate.
- Salience Network (aka frontoinsular network), involving the insula, cingulate, and frontal operculum with the posterior insula as the Searchlight Operator.
- Executive Control Network, involving many lateral frontoparietal connected structures that are associated with external awareness and focusing.
Student: How come these homeostatic inputs do not go the somatosensory cortex, you know, the one with the funny looking homunculus?
Professor: Only Mother Nature knows the answer to that question, or at least, has not shared the answer with us yet. But we do know that the posterior insula does have a definite somatotopic organization. Farb et al. (2013) made the following comment:
- “Unlike surface bodily sensation that is somatotopically represented in somatosensory cortices,
- interoceptive representation such as pain is first somatotopically organized in dorsal posterior insula regions (e.g., Brooks et al. 2005),
- implicating the POSTERIOR INSULA as a primary viscerosomatic cortex.”
Professor: As far as the functions of the insula is concerned, these scientists further comment,
- “Connectivity, however,
- shifts from somatosensory cortices in the posterior insula
- to the striatum in the middle insula (Menon and Levitin 2005) and
- finally to the ACC and orbitofrontal cortices in anterior insula nuclei (Craig 2003).
Student: Hmm, this indicates that the duties of the insula is not very simplistic.
Professor: Actually, the insula is likely to have many roles in higher orders of behavior. Farb et al. indicate this as they comment,
- “The anterior and middle insula may therefore,
- ntegrate viscerosomatic signals into a motivational space
- that no longer directly represents bodily sensation (Critchley 2005).”
Student: Some interpreting of sensory information may be happening!?
Professor: Farb and coworkers (2013) cite a review of Flynn et al. (1999) whose impression of the insula is that its role “may to integrate [all] these special visceral senses with behavioral and emotional events”.
Student: Just specifically with perception of dyspnea, what modifications do you want to make to your 2004 Model?
(Modified from Weiser et al. (1993))
Professor: In the brainstem,
- add paths could be numbered for efference copies and
- label the symbol of the brainstem comparator, a circle with an X, possible including path from the Nuclei Tractus Solitarius.
In the higher CNS,
- Identify the comparator that has input from Breathing “Comfort” Memory also is labeled thalamic Reticular Nuclei that do the gating of traffic to CNS higher levels.
- Include something like a rectangle for the posterior insula’s searchlight operator.
- Change the “GO?” in the decision diamond to a more universal question: “Keep Going?”
Take Home Message: The Posterior Insula does act like a Sensory Searchlight Operator.
Next: How fast does the insula and the salience network provide attentional control of occluded breathing?
References:
Brenner AM, Weiser PC, Krogh LA, Loren ML. Effectiveness of a portable face mask in attenuating exercise-induced asthma. JAMA l980;244:2l96-2l98
Brooks JCW, Zambreanu L, Godinez A, Craig AD, Tracey I.( 2005) Somatotopic organisation of the human insula to painful heat studied with high resolution functional imaging. Neuroimage. 27: 201–209.
Craig A. (2003) Interoception: the sense of the physiological condition of the body. Curr Opin Neurobiol. 13:500–505.
Critchley HD. (2005) Neural mechanisms of autonomic, affective, and cognitive integration. J Comp Neurol. 493:154–166.
Farb NAS, Segal ZV, Anderson AK. (2013) Attentional modulation of primary interoceptive and exteroceptive cortices. Cereb. Cortex (2013) 23 (1): 114-126. DOI: 10.1093/cercor/bhr385
Flynn FG, Benson DF, Ardila A. (1999) Anatomy of the insula—functional and clinical correlates. Aphasiology. 13:55–78.
Menon V, Uddin LQ. 2010. Saliency, switching, attention and control: a network model of insula function. Brain Struct Funct. 214: 655–667.
Weiser PC, Mahler DA, Ryan KP, Hill KL, Greenspon LW. (1993) Clinical assessment and management of dyspnea. In: Pulmonary rehabilitation: Guidelines to success. 2nd edition. Hodgkin, J, Bell CW, eds. Philadelphia: Lippincott, 478-511.
