The brain mechanisms of punishment

Abstract

Punishment involves the reduction of responses that lead to negative outcomes. This thesis investigated the behavioural and neural underpinnings of this phenomenon. A review of the literature suggests punishment results in a reduction of responding via instrumental suppression, and not other forms of learning. Experiment 1 developed a protocol for punishment whereby rats responded on two levers for food and responses on one lever (punished), but not another (unpunished) were additionally punished with brief, aversive footshock. Rats selectively reduced responses on the punished lever and also showed a preference for the unpunished lever in a choice test.

The brain mechanisms of this behaviour were investigated in Experiments 2 – 9 using bilateral cannulations of BLA, AcbSh, PFC, VTA, LHb and dmStr, permitting microinfusions of selected drugs. Inactivations of the BLA using baclofen/muscimol (BM) impaired the acquisition and expression of punishment suppression, but not unpunished choice. This effect was attributable to manipulations of caudal but not rostral BLA. GABAA antagonist into the VTA during punishment acquisition resulted in long-lasting impulsivity/punishment insensitivity and hyperactivity, but had more selected effects in other punishment tasks. Microinfusions of relevant drugs into the AcbSh, PFC, LHb and dmStr did not cause any change in punished leverpressing during punishment acquisition, expression or choice.

These findings were interpreted to mean that: 1) caudal BLA serves a role in encoding the aversive value of the footshock punisher and this encoding is critical for acquiring and expressing punishment suppression; 2) VTA disinhibition during initial punishment perturbs learning-related plasticity, manifesting as an impulsivity/hyperactivity behavioural phenotype; and 3) AcbSh, PFC, LHb and dmStr are not crucial for punishment learning or behaviour. The pathways that mediate the caudal BLA role in punishment, and the precise neural and behavioural loci of the effects observed in the VTA experiment, remain unclear and are avenues for future research. These findings have important implications for considering the effects of punishment within populations with altered BLA (e.g. depression, psychopathy) or VTA activity (e.g. ADHD, substance abuse disorders).