Insula and Plasticity: A Hypothetical Outline

Author: Chun Lok (Leo) Wong ︱Editor: Amelia Kobylarz

The opinions expressed here are solely those of the author and do not represent the views of U-Lingua, its editors, or any affiliated organisation. Any copyright issues should be taken up with the author themselves.

Insula— “Island of Reid”, hidden within bilateral lateral sulci, is a reciprocally connected area that serves a variety of functions from visceral-somatic processing to managing attention [1]. Although its modification is evident in the literature on auditory deprivation and sign language, its significance is rarely explored. Neuroplastic changes associated with sensory deprivation are observed across modality [2]. Specifically, neural reorganization, attributed to auditory deprivation and sign language acquisition, in terms of brain morphology [3,4], functional connectivity (FC) [5,6], and task-based activations [7], has been explored.

The auditory area, in the absence of auditory input, is adapted cross-modally to process visual information [8, 13]. However, modulations in the insula, with its multiplicity of input, are not as obvious to explain. Morphological changes associated with auditory deprivation and loss, specifically decreased gray matter volume of the insula [3,9] and reduced cortical thickness in the left insula [4,10], have been observed. Studies have shown no significant changes in the insula attributable to sign language alone [7, 28-30]¹. Functional connectivity changes has been explored in a recent fMRI study [5] which found decreased resting state-FC (RSFC) of the left anterior insula (AI) with left angular gyrus, left middle frontal gyrus, and left frontal pole, by seed-based analysis, as well as potential “temporally dynamic linkage” (p. 1140) between bilateral AI, left fusiform gyrus, left superior temporal gyrus, and left cerebellum by independent component analysis. These results suggest insular involvement in motor planning and speech articulation, and insula as a site of inter-network interactions, respectively.

These modifications might be due to one of two forms of neuroplasticity outlined in [8]: intra-modal plasticity and cross-modal plasticity². By examining evidence regarding modification in superior temporal cortex (STC), cross-modal plasticity is hypothesised to adhere to certain organization principles, namely maintaining functional specialization³ and mediated by “heterometal [...] structural connections and minor functional reorganization of cortico-cortical connectivity” (p. 133 [8]). Whether this theory can be generalised to plasticity outside of STC remains a future research direction. Stemming from this assumption, neuroplasticity in the insula is likely dependent on, and indicative of, the functions of the insula in wider processing in hearing individuals [1]. In fact, insular microstructural organization mirrors that of the anterior cingulate cortex [12], serving as important components of the salience network, suggesting functional-structural specificity.

Several studies have examined task-based activation in comprehension. Significant task-based bilateral insular activation in the reading comprehension task is observed, associated with task difficulty [7]. A Systematic review and ALE meta-analysis [14] showed convergence in bilateral insula cortex during sign language comprehension tasks as well as nonlinguistic sign-like actions observation, suggesting instead its involvement with lower-level non-linguistic processing [15] instead of comprehension. The latter is consistent with current literature of the formal language network, which is structurally and functionally dissociated from cognitive processes outside of linguistic processing [16,17]. The insula does not contribute to the formal language network [17]. A recent functional Magnetic Resonance Imaging (fMRI) study [13] provides temporal-spatial evidence for cross-modal plasticity within the auditory cortex but did not find significant insula involvement when viewing natural and distorted silent movies, suggesting, however, that the insula is not involved in visual processing per se in deaf individuals.

The insula is also thought to be an important site for language production in hearing individuals. fMRI studies suggested that AI contributes to both language and speech perception and production [18, 19]. In particular, the superior precentral gyrus of the insula has been hypothesized to be crucial for articulation in lesion studies [20]. However, more recent studies did not support this claim [21, 17]. Recently, utilising intracranial recordings implanted in patients with severe intractable epilepsy, researchers were able to map on-task cortical activity with high temporal and spatial resolution [22]. Contrary to existing literature, they found pre-articulatory activity in the proximal Frontal Operculum (FO) but no pre-articulatory activity in the AI, inferring no clear insular involvement in speech planning. Bilateral posterior insula (PI), however, displayed post-articulatory activity and was most active during self-generated speech, suggesting a monitoring role during speech production. Increased RSFC between the auditory right superior parietal gyrus and the right insula is observed in prelingually deaf adolescents, the former involved in visual-spatial processing [6]. However, further temporally specific research is needed to distinguish whether plasticity can be attributed to speech monitoring.

Among the variety of functions that the AI subserves in hearing individuals [1, 23], its role as a “gatekeeper” in cognitive control [24] is a plausible attribute of plasticity. The high density of von Economo neurons in the AI supports rapid switching of signal relays [12], mediating and driving the activity of large-scale networks such as default mode networks and central executive networks [24]. As part of the salience network (SAN) [25], which is responsible for salience processing, the AI is thought to manage both interoceptive and external sensory input. RSFC of left AI/FO and dorsal Anterior Cingulate Cortex has been shown to correlate with second-language lexical retrieval in hearing individuals [26], suppressing first-language activation. A recent resting state fMRI study examining network-based dynamics in deaf and inattentive adolescents found an increase in FC between deprived auditory cortices and the SAN compared to the control [27]. This could be related to selecting or suppressing visual input relayed by the STC [6] against visual background noise. Whether the connection with SAN produces compensatory behavioural changes, and to what extent neuroplasticity progresses in adolescent growth, remains open.

The study of insular function and plasticity is filled with speculation and lacks substantial certainty. Searching for overarching computational goals fulfilled by the insula is a research direction. The insula is a compact area, consistent parcellation in neuroimaging studies [1] with structural specificity [31] into at least three central parts (the dorsal AI, ventral AI, and PI) might provide insight into its topographical specialization. This, in turn, could help determine whether observed modifications are due to intra-modal or cross-modal neuroplasticity, a central question instrumental in integrating function, structure, and microstructure of the insula.

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1. Footnote 1: It is, however, possible that the neuroplastic effect of deafness and sign language is mutually dependent, as deprivation of input signal seems to drive plasticity (see [8]).

2. Footnote 2: Intramodal plasticity and cross-modal plasticity differ in functional-topographically specificity. The former involved regions “dedicated to the processing [...] the remaining senses”, while the latter occurs in “regions that normally would process the lost sensory modality” ([8] p.128).

3. Footnote 3: Benetti and Collignon quoted Collignon et al. [11] in order to explain this concept which I will include here: selective cross-modal recruitment of a sensory-deprived region would “find ‘neuronal niches’ in a set of circuits that perform functions sufficiently close to the ones required by the remaining senses” ([8] p.133)