Disgust discussed

Research in the neuropsychology of human emotion has expanded rapidly in recent years. This is in part because of the widening availability of functional imaging technology but also because of a newfound interest in the idea that certain individual emotions may be served by separate neural substrates. This latter theoretical position is at the heart of the basic emotions concept, the idea that selected emotions with strong evolutionary histories constitute the foundations of human emotion. Although this concept has its origins in the work of Charles Darwin, it really took off only after the research of people such as Tomkins, Ekman, and Izard in 1960s and 1970s. Instead, emotion research up until this point was dominated by the idea that all emotions were coded as values on a limited number of dimensions (eg, valence and arousal). Similarly, early neurological accounts of emotion processing took a similar all-encompassing approach in which all emotions were processed by a circuit of interconnected brain structures such as the limbic system. Two important findings of Ekman and his contemporaries that changed theories of emotion were studies demonstrating that certain emotions were associated with distinct facial signals and additional research showing that these facial signals were recognized by cultures throughout the world, observations that now form two of the defining features of basic emotions. A third posited feature was that each basic emotion should be associated with a distinct (neuro)physiological signature. Although human evidence of this proved difficult to find, support was found in comparative neuroscience. Over the last 8 years, human research has similarly begun to identify that certain emotions may be coded by partially distinct neural substrates. A good deal of this evidence has resulted from research aimed at understanding the neural substrates of recognizing emotion in others (ie, the recognition of human signals of emotion). This work escapes many of the criticisms leveled at earlier emotion research exploring emotional experience, such as a reliance on subjective measures and vague and poorly defined aspects of emotion processing. Indeed, neuropsychological investigations of facial affect recognition have had the added advantage of being able to build on two established areas of human research—sociocognitive studies of facial expression recognition and neuropsychological research on facial identity recognition. To date, functional imaging and patient-based research has shown that the human amygdala plays a significant role in recognizing facial and vocal expressions of fear—an association that concurs with earlier comparative research demonstrating a link between fear conditioning and the amygdala. Calder and colleagues also have recently argued that an additional neural system involving the insula and basal ganglia underlies the recognition of human signals of disgust. Support for this position comes from both functional imaging research and neuropsychological studies of brain-injured patients. A particularly striking example comes from a case (N.K.) with a focal lesion to the left insula and basal ganglia who demonstrates a highly selective impairment in recognition and experience of disgust. However, although these studies demonstrate a link between the insula and disgust, the specific regions of the insula involved are less clear. On balance functional imaging research has pointed to the anterior sections of the insula, although not all studies have confirmed this. In this respect, the study by Krolak-Salmon and colleagues constitutes an important contribution to understanding which insular regions are involved. Krolak-Salmon and colleagues recorded event-related potentials (ERPs) to pictures of facial expressions displaying disgust, fear, happiness, and neutral facial expressions in 11 temporal lobe epilepsy patients with depth electrode implants in the insular cortex. Significantly, all four patients with electrodes in the anterior ventral section of the long insular gyrus showed significantly distinct ERPs to facial expressions of disgust. Moreover, stimulation of the electrodes in three of these patients caused two to report unpleasant sensations in the throat spreading to the mouth, lips, and nose, reactions reminiscent of those reported by Penfield and Faulk in a similar stimulation study reported some 50 years previously. Krolak-Salmon and colleagues’ research is informative not only because four proximal electrodes showed a selective response to disgust, but also because 13 additional electrodes located in more dorsal (mainly posterior) regions of the insula showed no evidence of disgust selectively. The insula constitutes a significant area of cortex; however, functional imaging studies frequently report signals in this region as simply “insula.” In addition, stimuli associated with emotions other than disgust also have been reported to activate the insula in brain imaging research. The study by KrolakSalmon and colleagues emphasizes the need to consider the insula’s role in emotion processing at a more finegrain level than currently has been applied. The need for this approach is further substantiated by nonhuman primate research, showing that the insula is composed of three distinct regions, agranular, granular, and dys-