The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2667-2675
Copyright ©1999 by the American Physiological Society

Effects of Feedback Projections From Area 18 Layers 2/3 to Area 17 Layers 2/3 in the Cat Visual Cortex

 ¹Laboratorios de Neurociencia y Computacion Neuronal (asociados al Instituto Cajal-CSIC), Facultad de Medicina y Servicio Neurofisiologia Clinica-Complejo Hospitalario Universidad de Santiago de Compostela, Santiago de Compostela E-15705, Spain;  ²Departamento de Ciencias de la Salud II, Universidad de A Coruña, A Coruña E-15006 Spain; and  ³Department of Optometry and Vision Sciences, University of Manchester Institute of Science and Technology, Manchester M60 1QD, United Kingdom

Martinez-Conde, Susana, Javier Cudeiro, Kenneth L. Grieve, Rosa Rodriguez, Casto Rivadulla, and Carlos Acuña. Effects of Feedback Projections From Area 18 Layers 2/3 to Area 17 Layers 2/3 in the Cat Visual Cortex. J. Neurophysiol. 82: 2667-2675, 1999. In the absence of a direct geniculate input, area 17 cells in the cat are nevertheless able to respond to visual stimuli because of feedback connections from area 18. Anatomic studies have shown that, in the cat visual cortex, layer 5 of area 18 projects to layer 5 of area 17, and layers 2/3 of area 18 project to layers 2/3 of area 17. What is the specific role of these connections? Previous studies have examined the effect of area 18 layer 5 blockade on cells in area 17 layer 5. Here we examine whether the feedback connections from layers 2/3 of area 18 influence the orientation tuning and velocity tuning of cells in layers 2/3 of area 17. Experiments were carried out in anesthetized and paralyzed cats. We blocked reversibly a small region (300 µm radius) in layers 2/3 of area 18 by iontophoretic application of GABA and recorded simultaneously from cells in layers 2/3 of area 17 while stimulating with oriented sweeping bars. Area 17 cells showed either enhanced or suppressed visual responses to sweeping bars of various orientations and velocities during area 18 blockade. For most area 17 cells, orientation bandwidths remained unaltered, and we never observed visual responses during blockade that were absent completely in the preblockade condition. This suggests that area 18 layers 2/3 modulate visual responses in area 17 layers 2/3 without fundamentally altering their specificity.