I sometimes wonder if we have all been hoodwinked about the whole Roman Colosseum stories of thousands of supposedly normal everyday Romans, presumably wearing sandals (not that that is important to this), cheered on as their fellow humans were slain, eaten, speared and mutilated. Then again, cage fighting, described by Senator John McCain as human cockfighting, is one of the fastest growing sports in the world, so perhaps the Colosseum Capers did in fact happen.
My seeds of doubt relate to findings such as that reported in Nature this week[1] In a very comprehensive study involving several fairly complicated experiments (budding scientists be inspired), Daejon Jeon and mates have shown that rats acquire fear vicariously if they watch another rat getting electric shocks. That, in itself, is not altogether surprising (although it does suggest a degree of neural sophistication that might surpass that of the 18 000 people who will attend the British Cage Fight Bonanza in Manchester shortly), but the team also show that if the rat who is hurting is related to the rat who is watching, the latter gets a bigger response.
Moreover, even if they stick a screen between them that means the hurting rat is not recognisable visibly, the observer still has a heightened empathetic fear response, presumably on the basis of non-visual cues. Think about that – rats, with brains the size of your thumbnail, pick up on sounds and smells that tell them that their brother or sister is getting hurt and it makes them scared. Actually, they also showed that lovers had a bigger response, but only if they had been lovers for more than 10 weeks (about equivalent to 5 human years).
The research team then looked to determine what parts of the brain might underpin this response. They reported that the anterior cingulate cortex, amygdala and medial thalamic nucleii were important because when they injected these areas with lidocaine, the empathetic fear responses were smaller. These areas are implicated as being important substrates for the affective dimension of pain. Areas implicated in the sensory dimension of pain (S1, lateral thalamic nucleii etc) did not seem important.
Another thing that I like about this paper is that it almost passes the Moseley Pedantic Pain Language Test – although it mentions a ‘pain stimulus’ (which is nonsense), and implies a ‘pain pathway’ (which is nonsense) from the dorsal horn via the thalamus, it does actually state that nociceptive input from the spinal cord is transmitted to the brain. So, a pass perhaps.
My only nitpicking bit is that I don’t think one can conclude, as they have, that empathetic responses and learning in rats uses the medial pain system, because they only identify amygdala and medial thalamus involved in memory of the response. I think they should say that important areas in empathy are also known to be important in affective components of pain. They could just as easily say that empathy uses the social phobia system (people with social phobia have heightened amygdala activation[2] as an aside – you don’t actually need the amygdala to get empathetic fear responses – look at this[3] or the error detection system (the anterior cingulate cortex is important in error detection)[4] but most of us would pick it as a big call if they made it.
Regardless of these issues, I like the paper and I think it deserves its spot in the premier natural sciences journal. Well done!
References
[1] Jeon D, Kim S, Chetana M, Jo D, Ruley HE, Lin SY, Rabah D, Kinet JP, & Shin HS (2010). Observational fear learning involves affective pain system and Ca(v)1.2 Ca(2+) channels in ACC. Nature neuroscience PMID: 20190743
[2] Phan KL, Fitzgerald DA, Nathan PJ, & Tancer ME (2006). Association between amygdala hyperactivity to harsh faces and severity of social anxiety in generalized social phobia. Biological psychiatry, 59 (5), 424-9 PMID: 16256956
[3] Tsuchiya N, Moradi F, Felsen C, Yamazaki M, & Adolphs R (2009). Intact rapid detection of fearful faces in the absence of the amygdala. Nature neuroscience, 12 (10), 1224-5 PMID: 19718036
[4] Nieuwenhuis S, Ridderinkhof KR, Blom J, Band GP, & Kok A (2001). Error-related brain potentials are differentially related to awareness of response errors: evidence from an antisaccade task. Psychophysiology, 38 (5), 752-60 PMID: 11577898