Semantic Memory

How is it that we know what a dog and a tree are, or, for that matter, what knowledge is? Our semantic memory consists of knowledge about the world, including concepts, facts and beliefs. This knowledge is essential for recognizing entities and objects, and for making inferences and predictions about the world. In essence, our semantic knowledge determines how we understand and interact with the world around us. In this chapter, we examine semantic memory from cognitive, sensorimotor, cognitive neuroscientific, and computational perspectives. We consider the cognitive and neural processes (and biases) that allow people to learn and represent concepts, and discuss how and where in the brain sensory and motor information may be integrated to allow for the perception of a coherent “concept”. We suggest that our understanding of semantic memory can be enriched by considering how semantic knowledge develops across the lifespan within individuals

Dimorphic Expression of Various Transcription Factor and Steroidogenic Enzyme Genes during Gonadal Ontogeny in the Air-Breathing Catfish, Clarias gariepinus

In the present study the expression of 13 genes known to be involved in sex differentiation and steroidogenesis in catfish was analyzed during gonadal ontogeny by quantitative real-time RT-PCR. Dmrt1 and sox9a showed exclusive expression in male gonads while ovarian aromatase (cyp19a1) and foxl2 were abundant in differentiating female gonads. Most of the genes related to steroidogenesis were expressed only after gonadal differentiation. However, genes coding for 3β-hydroxysteroid dehydrogenase (3β-hsd), 17α-hydroxylase/C17–20 lyase type 1 (cyp17) and steroidogenic acute regulatory protein (star) were barely detectable during gonadal differentiation. Ovarian aromatase, cyp19a1, which is responsible for estradiol-17β biosynthesis in females, was expressed very early in the undifferentiated gonads of catfish, around 30–40 days post hatch (dph). The steroidogenic enzyme, 11β-hydroxylase (cyp11b1) required for the production of 11-ketotestosterone (11-KT) was expressed only after differentiation of testis. These results suggest that estradiol-17β has a critical role in ovarian differentiation, while the role of 11-KT in testicular differentiation is doubtful. In conclusion, dimorphic expression of dmrt1 and sox9a in gonads during early development is required for testicular differentiation, and sex-specific expression of cyp19a1 and foxl2 in females plays a critical role in ovarian development. Our study reveals that the critical period of gonadal differentiation in catfish starts around 30–40 dph when sex-specific genes showed differential expression