Effects of ectopic hilar granule cells on normal signal processing in the trisynaptic circuit. It is proposed that the development of ectopic hilar granule cells could contribute to cognitive deficits in epileptics by interfering with the normal trisynaptic circuit. A) The trisynaptic circuit is schematized as perforant path axons innervating distal granule cell dendrites (1), granule cell mossy fibers innervating proximal CA3 pyramidal cell dendrites (2), and CA3 pyramidal cell projections to apical dendrites of CA1 pyramidal cells (3). The accumulation of hilar ectopic granule cells with axons that innervate pyramidal cells, hilar neurons, and the inner molecular layer, suggests a multitude of complex pathways that would potentially interfere with normal transmission along the trisynaptic pathway, as shown diagrammatically in B.