Response properties of units in the lateral geniculate nucleus of the domestic chick (Gallus domesticus).

One hundred and four visual units of the (ventral) lateral geniculate nucleus (GL) of chicks (Gallus domesticus) were studied with extracellular microelectrodes. Most were extremely responsive to moving targets even at low speeds (0.1 degrees/sec). They were classified as follows. (1) movement-sensitive units with uniform, restricted receptive fields (47%). (2) Movement-sensitive units with small receptive fields possessing regions responding to differing modes of stimulation ('center-periphery units'). (3) Movement-sensitive neurons with uniform, wide receptive fields, larger than 75 degrees x 65 degrees (15%). (4) Movement-insensitive cells, responsive only to illumination changes (7%). (5) Units with indeterminate receptive fields, a poor visual response or a poor response to movement (14%). The firing of neurons in classes I-III was mostly an accurate function of target speed; half of them also provided accurate monitoring of the beginning, course and termination of target movement (tonic units). In view of the above, the high proportion of directionally selective cells (31%) and the large size of many of its receptive fields, the GL is considered as a candidate structure for optomotor integration but the evidence is otherwise scant. There is, however, an apparent contribution to its unit properties from both retinal ganglion cells and neurons from different tectal layers. Whatever its role, it is unlikely to be the functional homologue to the mammalian ventral lateral geniculate as the unit properties of two structures differ substantially.