1064 COMMENTARY ( see article on p . 1074 )
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Advances in dermatology and venereology Acta Dermato-Venereologica
The Brain Network of Itch Distraction
CLEMENS FORSTER Department of Physiology and Pathophysiology , University of Erlangen / Nuremberg , DE-91054 Erlangen , Germany . E-mail : clemens . forster @ fau . de
Itch is a daily experience that is mostly induced by a more or less obvious occurrence on the skin . However , the sensation of itch is something that originates from a pattern of activity changes in various regions of the brain . Therefore , the same skin irritation can induce very different itch perceptions depending on the state of consciousness , on concurrent sensations like pain , touch or scratch , or on distraction . Any of these will change the activation pattern within the affected brain network and the itch perception will be modified . The study of Stumpf et al . ( 1 ) focuses on brain areas that may play a role in the distraction from itch . Since it is obvious that the related mechanisms have some common features with the central pain processing , the authors also included regions of the brainstem in their analysis which have been previously described to be involved in pain modulation .
A number of brain imaging studies on the distraction from pain have been performed . Several groups could show that distraction is a process which seems to originate from activation changes in frontal cortical regions , in particular the dorsolateral prefrontal cortex ( DLPFC ) ( 2 , 3 ). On the other hand focusing to a stimulus can increase the perceived intensity leading to increased activation of affected brain areas ( 4 ). It is assumed that the DLPFC plays an important role in the top-down control of pain and together with the anterior cingulate cortex ( ACC ) it modulates brainstem regions like the periaqueductal gray ( PAG ), the rostral ventromedial medulla ( RVM ), and the locus coeruleus ( LC ).
Among the brain regions involved in attention modulated perception of pain the PAG has a pivotal role since it drives the descending pain modulation system ( 5 ). Do brainstem structures also modulate pruritic input ? It seems likely as similar pathways starting at the spinal dorsal horn transmit itch input and pain and these starting points are the targets of the descending system . There are some observations in animal ( 6 ) and human studies ( 7 ) which indicate that even scratching an itch involves the descending modulation of the PAG .
From this it is a quite interesting approach to study the role of the PAG and other brainstem areas during itch using brain imaging techniques as done by Stumpf et al . ( 1 ). For the induction of itch they used a protocol that has been introduced by our group and allows the generation of the typical block design required for fMRI BOLD analysis . With this the sensation of interest ( itch ) can be easily switched on and off . Further , a modified colour Stroop task was used to distract from the itch . Stroop tasks of different variations have repeatedly been used to distract from pain and its impact on pain related brain areas have been demonstrated ( 3 ). The results of the study of Stumpf et al . ( 1 ) are surprising : The Stroop task does not reduce itch sensations as shown by the ratings of the subject . Of course , the ratings were given as a summary after a stimulus which could make it difficult for the subject to remember the real experienced sensation when he / she was performing the Stroop task or the Stroop task could be too simple to effectively distract . Interestingly , they describe effects of the distraction on brainstem structures and related cortical regions , although the psychophysical results do not support the effect of the Stroop task . There was an increased activation in the DLPFC , in the PAG and the nucleus cuneiformis ( NCF ), structures which have been described to be involved in the ( pain ) modulation system . Could it be that the Stroop task alone can activate the attentional frontal system including the descending system ? An indication for this is that an elevated activation occurred within these areas during the Stroop task during itch and during no itch ( saline stimulation ).
An explanation of these results could be that the “ salience network ” including the DLPFC is a key structure . This network is involved in the detection of stimuli that could be relevant or that need attention . There is no doubt that a Stroop task requires attention but will this attention be strong enough to suppress itch by activating the descending inhibitory system ? Kucyi & Davis ( 8 , 9 ) report that during distraction from pain by “ mind wandering ” away from pain the “ default mode network ” ( DMN ) is additionally more active and the antinociceptive system increases the functional connectivity between the prefrontal cortex and the PAG . Can these findings from the pain connectome be transferred to the central mechanisms of itch and its centrally mediated suppression ? Preliminary results from our group ( 10 ) and reports from atopic dermatitis patients ( 11 ) suggest that there is an itch related brain network including insular cortex , cingulate cortex , basal ganglia and frontal areas as central nodes and which is increased during itch . The areas mentioned overlap with those which have been described to form the emotional – aversive network of pain processing ( 12 ). Due to these commonalities it is very likely that the central modulations of pain and itch are likewise organized .
Stumpf et al . ( 1 ) found brainstem activation during the Stroop task in the PAG and the NCF . Indeed , brainstem structures play an important role in the modulation of sensory input and in emotion but considering size and doi : 10.2340 / 00015555-2766 Acta Derm Venereol 2017 ; 97 : 1064 – 1065
This is an open access article under the CC BY-NC license . www . medicaljournals . se / acta Journal Compilation © 2017 Acta Dermato-Venereologica .