Pain perception, aversion and fear in fish.

There is now compelling evidence that teleost fish possess similar nociceptive processing systems to those found in terrestrial vertebrates. Noxious stimulation of these nociceptors--specialised pain receptors -in the skin around the snout of fish generates neural activity that can be electrophysiologically recorded, and induces a number of behavioural and physiological changes. To determine whether changes in behaviour are more than simple responses to the noxious stimulation it is necessary to demonstrate that higher order cognitive processes such as mental state or 'affective state' are involved. However, quantifying the 'motivational affected state' of an animal--a concept encompassing not just pain but also fear, hunger, thirst and pleasure - is difficult owing to its subjectivity. Recent empirical work is beginning to test these concepts in fish, and we review a number of these studies and suggest how these general methodologies could be used to further our understanding of fish cognition and the capacity for fish to experience mental states such as fear or suffering.