Visual and pain pathways involved in fear conditioning measured with fear-potentiated startle: behavioral and anatomic studies

The visual conditioned stimulus and footshock unconditioned stimulus pathways involved in fear conditioning were investigated. Anatomic tracing studies showed that the posterior triangular nucleus (PoT) of the thalamus receives somatic pain inputs from the spinal cord and, in turn, projects to the amygdala both directly and indirectly via the insular cortex. The lateral posterior nucleus (LP) of the visual thalamus projects to two amygdala-fugal cortical areas: area TE 2 and dorsal perirhinal cortex, and directly to the amygdala as well. Lesions of insular cortex and PoT together disrupt the acquisition, but not expression, of fear-potentiated startle using a footshock US. Lesions of insular cortex or thalamus alone had no effect. In contrast, post training lesions of both the lateral geniculate body (LG) and LP together, or LP and TE2, but not lesions of LG, LP or TE2 alone, completely blocked the expression of fear-potentiated startle to a visual CS. Together, these data suggest parallel cortical and subcortical (thalamo-amygdala) pathways are involved in relaying footshock and visual information to the basolateral amygdala during fear conditioning.