Recent Human Neuroimaging Studies Research Articles

Cortico‐basal ganglia pathways are essential for the recall of well–established visuomotor associations

Recent human neuroimaging studies, supported by lesion studies with nonhuman primates, have suggested that learning arbitrary associations between sensory cues and behavioural responses requires interactions between the infero-temporal, prefrontal and premotor cortices. We directly tested the hypothesis suggested from our neuroimaging experiments that functional links between the basal ganglia and premotor cortex are involved in the process via which task performance becomes automatic. We made unilateral excitotoxic lesions, centred on the internal pallidum, in four macaques previously given extensive experience on the associations between nonspatial visual cues and movements of a joystick. The basal ganglia lesion was later combined with a premotor cortical lesion in the opposite hemisphere so as to interrupt the connections between them. Three of the animals were subsequently found to be impaired in relearning pre-operatively acquired associations; they eventually succeeded but made three-times as many errors. A fourth animal was unimpaired but its premotor cortex lesion was later found to be incomplete. Response times were only marginally increased and the learning of novel associations appeared relatively unaffected by these lesions. As a control, the effects of a unilateral premotor cortex lesion were assessed with two additional animals but this lesion did not result in a relearning impairment. We therefore suggest that when visuomotor associations have become well established through over-training, performance depends on connections between the basal ganglia and premotor cortex.

Error Monitoring Using External Feedback: Specific Roles of the Habenular Complex, the Reward System, and the Cingulate Motor Area Revealed by Functional Magnetic Resonance Imaging

The dopaminergic system has been shown to be involved in the processing of rewarding stimuli, specifically of errors in reward prediction, in animal studies as well as in recent neuroimaging studies in humans. Furthermore, a specific role of dopamine in the human homolog of the rostral cingulate motor area (rCMA) was proposed in a recent model of error detection. Negative feedback as well as self-detected errors elicit a negative event-related brain potential probably generated in the rCMA. We performed two experiments using functional magnetic resonance imaging to investigate the brain activity related to negative and positive feedback in a dynamically adaptive motion prediction task. Whereas positive feedback raised hemodynamic activity in the ventral striatum (nucleus accumbens), negative feedback activated the rCMA, the inferior anterior insula, and the epithalamus (habenular complex). These data demonstrate the role of the habenular complex in the control of the human reward system, a function previously hypothesized on the basis of animal research. The rCMA reacted only to errors with negative feedback but not to errors without feedback, which ruled out an influence of response conflict or uncertainty on its role in error detection by external signals.

IS FACE RECOGNITION NOT SO UNIQUE AFTER ALL?

In monkeys, a number of different neocortical as well as limbic structures have cell populations that respond preferentially to face stimuli. Face selectivity is also differentiated within itself: Cells in the inferior temporal and prefrontal cortex tend to respond to facial identity, others in the upper bank of the superior temporal sulcus to gaze directions, and yet another population in the amygdala to facial expression. The great majority of these cells are sensitive to the entire configuration of a face. Changing the spatial arrangement of the facial features greatly diminishes the neurons' response. It would appear, then, that an entire neural network for faces exists which contains units highly selective to complex configurations and that respond to different aspects of the object “face.” Given the vital importance of face recognition in primates, this may not come as a surprise. But are faces the only objects represented in this way? Behavioural work in humans suggests that nonface objects may be processed like faces if subjects are required to discriminate between visually similar exemplars and acquire sufficient expertise in doing so. Recent neuroimaging studies in humans indicate that level of categorisation and expertise interact to produce the specialisation for faces in the middle fusiform gyrus. Here we discuss some new evidence in the monkey suggesting that any arbitrary homogeneous class of artificial objects—which the animal has to individually learn, remember, and recognise again and again from among a large number of distractors sharing a number of common features with the target—can induce configurational selectivity in the response of neurons in the visual system. For all of the animals tested, the neurons from which we recorded were located in the anterior inferotemporal cortex. However, as we have only recorded from the posterior and anterior ventrolateral temporal lobe, other cells with a similar selectivity for the same objects may also exist in areas of the medial temporal lobe or in the limbic structures of the same “expert” monkeys. It seems that the encoding scheme used for faces may also be employed for other classes with similar properties. Thus, regarding their neural encoding, faces are not “special” but rather the “default special” class in the primate recognition system.