Transgeniculate signal transmission to middle suprasylvian cortex in intact cats and following early removal of areas 17 and 18: a morphological study.
Removal of cat areas 17 and 18 early, but not late, in postnatal development results in the sparing of certain reflexive and nonreflexive visually guided behaviors. These spared behaviors are accompanied by an expansion of geniculocortical projections to middle suprasylvian (MS) cortex. However, little is known about the types of visual signals relayed along these pathways. The purpose of our study was to reveal the morphologies of the neurons participating in the rewired circuits and, by relating them to the morphologies of functionally characterized neurons described by others, infer the types of visual signals transmitted via the lateral geniculate nucleus (LGN) to MS cortex. To do this, we retrogradely labeled LGN neurons from MS cortex with fluorescent microspheres, and subsequently intracellularly filled them with Lucifer Yellow. We then classified well-filled neurons according to a battery of morphological parameters (such as soma size and shape, and dendritic field-form and specializations), and compared them with already defined structure/function relationships. By doing this, we found that the large majority of visual thalamic relay neurons to MS cortex of both normal cats and cats that incurred removal of areas 17 and 18 were types I and IV. These results indicate that visual Y and W signals, respectively, are relayed directly from LGN to MS cortex in both types of cats. Following the early lesions, some of the MS-projecting type I neurons were found in layers A and A1, where they are never found in intact cats. Thus, some layer A and A1 type I neurons redirect axons to MS cortex following early removal of areas 17 and 18. For the type IV MS-projecting neurons in early lesioned cats, the somas were hypertrophied and they had more profuse and broader dendritic arbors than equivalent neurons in intact cats. These results suggest that dynamic interactions take place between inputs and outputs of LGN neurons during development that influence final LGN neuron morphology. Moreover, they suggest that signals transferred to MS cortex by type IV neurons may be modified by early lesions of areas 17 and 18. Overall, these results contribute to our understanding of the types of behaviors that may be spared by early lesions of areas 17 and 18.
MacNeil
M A
MA
Department of Anatomy and Neurobiology, Boston University School of Medicine, MA 02118, USA. macneilm@helix.mgh.harvard.edu
Einstein
G
G
Payne
B R
BR
D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D002415:Cats; D002535:Cerebral Aqueduct / Q000502:physiology*; D005456:Fluorescent Dyes; D005829:Geniculate Bodies / Q000166:cytology / Q000502:physiology*; D007546:Isoquinolines; D008863:Microspheres; D009474:Neurons / Q000145:classification / Q000166:cytology / Q000502:physiology; D009435:Synaptic Transmission; D013788:Thalamus / Q000166:cytology; D014795:Visual Pathways / Q000502:physiology*
D005456:Fluorescent Dyes; D007546:Isoquinolines; C017475:lucifer yellow