Faculty Bibliography

The effects of hippocampal lesions on the conditioning of fear responses (freezing responses) to contextual stimuli (static, continuously present stimuli) were examined in three conditioning paradigms: forward pairing of a phasic tone conditioned stimulus (CS) with a footshock unconditioned stimulus (US), unpaired presentations of the CS and US, or presentations of the US alone. All three procedures resulted in the acquisition of conditioned freezing to contextual stimuli. Lesions of the dorsal hippocampus prevented the acquisition of contextual conditioning in the Paired procedure, as reported previously, but not in the Unpaired or US Alone procedures. In the Paired procedure, static contextual cues occur in the background, with the phasic tone CS being the primary stimulus that enters into the association with the US. However, in the other two procedures, where there is no phasic CS, the primary associations with the US involve static contextual stimuli, which are therefore in the foreground. We refer to these types of contextual conditioning as background and foreground contextual conditioning, respectively, and argue that the hippocampus is only involved in background contextual conditioning. These results have implications for understanding both fear conditioning and hippocampal function

Recent studies have identified major components of the neural system mediating the classical conditioning of defense or fear responses to sensory stimuli. The pathways involve transmission to the amygdala from sensory processing areas in the thalamus and cortex. Within the amygdala, the lateral nucleus receives the sensory inputs and the central nucleus provides the interface with motor systems controlling specific defense responses. Internal connections between the lateral and central nuclei allow the structures involved in receiving inputs and generating outputs to communicate. This circuitry contributes to stress reactions in two important ways. First, by way of these pathways environmental events that are interpreted as threatening activate the hypothalamic-pituitary-adrenal axis and thereby initiate so-called stress reactions. Second, nuclear regions of the amygdala contain receptors for adrenal steroids. Steroids released from the adrenal gland as a result of amygdala activty can therefore influence the processing of the environment by the amygdala. Although the amygdala is likely to play a major role in stress responses, relatively little work has been done to elucidate the nature of its role. This is an important topic for future research aimed at understanding how the biological cascade that constitutes the stress response fits into a broader network involved in emotional and cognitive information processing functions. In contrast to other models of stress, fear conditioning allows us to approach this complex problem armed with a clear understanding of major aspects of the circuitry involved in processing stress-inducing stimuli. © 1993 Academic Press Inc.

The purpose of this study was to further our understanding of the contribution of auditory thalamoamygdala projections to conditioned emotional memories formed when auditory and noxious somatosensory stimuli are associated. Single unit activity was recorded in the acoustic thalamus of chloral hydrate-anesthetized rats in response to auditory (white noise, clicks, tones) and somatosensory (foot-shock) stimulation. The thalamic areas focused on were the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from both the inferior colliculus and the spinal cord and that project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv), which receives projections from the inferior colliculus but not from the spinal cord. Auditory but not somatosensory responses were recorded from MGv, while both auditory and somatosensory responses were frequently found in MGm, PIN, and SG. In these areas, convergent auditory and somatosensory responses were more frequently found rostrally than caudally. Within a thalamic subregion, the acoustic response properties of the convergence cells were not different from the response properties of unimodal auditory cells. Some cells that responded to somatosensory but not auditory stimuli showed a potentiated response when tested with simultaneous presentation of auditory and somatosensory stimuli. In some studies, thalamic cells that project to the amygdala were antidromically activated by stimulation of the AL. Consistent with anatomical tracing results, antidromically activated cells were found in MGm, PIN, and SG, but not in MGv. Antidromically activated cells were more likely to respond to auditory stimuli than to somatosensory stimuli, but unimodal somatosensory and convergence cells were also found. These findings, which provide the first characterization of acoustic response properties of multimodal cells in the auditory thalamus and of cells in the auditory thalamus that project to amygdala, suggest insights into the emotional functions of the thalamoamygdala pathway

Projections from the auditory thalamus to the amygdala have been implicated in the processing of the emotional significance of auditory stimuli. In order to further our understanding of the contribution of thalamoamygdala projections to auditory emotional processing, acoustic response properties of single neurons were examined in the auditory thalamus of chloral hydrate-anesthetized rats. The emphasis was on the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from the inferior colliculus and project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv). Responses latencies were not statistically different in MGv, MGm, PIN, and SG, but were longer in the posterior thalamic region (PO). Overall, frequency tuning functions were narrower in MGv than in the other areas but many cells in MGm were as narrowly tuned as cells in MGv. There was some organization of MGv, with low frequencies represented dorsally and high frequencies ventrally. A similar but considerably weaker organization was observed in MGm. While the full range of frequencies tested (1-30 kHz) was represented in MGv, cells in MGm, PIN, and SG tended to respond best to higher frequencies (16-30 kHz). Thresholds were higher in PIN than in MGv (other areas did not differ from MGv). Nevertheless, across the various areas, the breadth of tuning was inversely related to threshold, such that more narrowly tuned cells tended to have lower thresholds. Many of the response properties observed in MGm, PIN, and SG correspond with properties found in AL neurons and thus add support to the notion that auditory responses in AL reflect thalamoamygdala transmission