In the lateral geniculate nucleus (LGN) of 18 kittens whose ages ranged from 6 to 40 days, 445 cells were studied. 2. LGN cells of kittens younger than 21 days are characterized by very low maintained rates, long latencies to full-field flash, response fatigue, large receptive field, absence of surround responses and surround inhibition, poor responses to fast-moving stimuli, and low-amplitude responses to flashing spots. 3. Cells were characterized as sustained or transient, and on, off, or on-off by their responses to flashing spots of light, and as X-like of Y-like by their responses to contrast reversal. Prior to 21 days, cells are hard to classify as X or Y. 4. A large proportion of cells in kitten LGN have both on- and off-responses to small and center-sized spots of light. This proportion decreases with development. 5. A small number of cells develop mature receptive-field properties very early (14--20 days). These are cells with X-type responses (linear summation) to contrast reversal and tend to have sustained responses to flashing spots. 6. Y-like cells, with nonlinear summation, develop mature receptive-field properties later than 34 days of age and later than all X-cells. 7. We conclude that there are different developmental patterns for cells of the kitten LGN. These different patterns may be important in determining the visual responses of cortical cells and their degree of susceptibility to environmental modification.
