Electrophysiology of lateral and dorsal terminal nuclei of the cat accessory optic system

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Abstract
Visual responses were examined quantitatively in 96 units in the lateral (LTN) and dorsal (DTN) terminal nuclei of the cat accessory optic system (AOS). The receptive fields of LTN and DTN cells were quite large, with an average diameter of .apprx. 60.degree.. Individual cell receptive fields, which could be as small as 30.degree. vertically by 15.degree. horizontally or as large as 100 by 100.degree., always included the area centralis. Large moving textured stimuli provoked optimal modulation in these cells. In response to a 100 by 80.degree. randomdot pattern moving at a constant velocity, nearly all cells in both the LTN and DTN displayed a high degree of direction selectivity. Direction response profiles were subjected to a vector analysis that generated 2 quantities proportional to the direction and magnitude of the major excitatory (E vectors) and inhibitory (I vectors) responses of individual cells. Directional vectors of the LTN displayed a strikingly bimodal distribution: E vectors of individual LTN cells pointed either upward (25 of 49) or downward (23 of 49). I vectors also pointed either up or down in a direction opposite to that of the E vector for the same cell. E and I vectors in both LTN and DTN units were separated by .apprx. 180.degree.. With few exceptions, E vectors of DTN cells pointed in a horizontal-medial direction, while DTN I vectors pointed in a horizontal-lateral direction. A relatively broad range of stimulus velocities (0.8-102.4.degree./s) evoked maximal excitation in individual LTN units. The majority of LTN cells, achieved maximal excitation at velocities between 0.8-12.8.degree./s. The deepest inhibition was elicited over a range of velocities from 0.2-102.4.degree./s, with 2 major peaks at 0.8 and 12.8.degree./s. A similar range of velocity sensitivity was observed in DTN cells; maximal excitation was obtained for stimulus velocities from 1.6-102.4.degree./s, with most DTN cells showing the greatest excitatory response between 6.4-12.8.degree./s. A broad range of inhibitory velocity tuning was also observed in DTN units, with most cells exhibiting the deepest inhibitory modulation at 25.6.degree./s. The majority of LTN and DTN units were driven most effectively through the eye contralateral to the recording site. Nonetheless, a large percentage of LTN (78%) and DTN (93%) cells could be driven to some extent through both eyes. Despite this conspicuous ipsilateral eye influence, no units were found in either the LTN or the DTN that were driven solely through the ipsilateral eye. Mean conduction latencies in response to electrical stimulation of the optic chiasm (100-150 .mu.A for 1-50 .mu.s) were 4.91 ms for LTN cells and 5.73 ms for DTN units. The cat AOS may provide information on the direction and velocity of whole-field motion to the CNS.