Visual response properties and afferents of nucleus of the optic tract in the ferret

Basic properties of responses to visual stimulation with large moving random dot patterns were studied in ferret nucleus of the optic tract. Retinal input to NOT was assessed by orthodromic electrical stimulation of the optic chiasm and optic nerves. Presence of an input from visual cortex was tested by orthodromic electrical stimulation of ipsilateral area 17. All 51 NOT neurons studied displayed a non-habituating, clearly direction-specific response: discharge rate strongly increased with the stimulus pattern moving horizontally in ipsiversive direction (motion directed towards the recorded hemisphere) and decreased with contraversive stimulus motion. Most latencies to visual stimulation ranged from 80 to 100 ms. Velocity tuning was studied using stimulus velocities between 4 deg/s and 100 deg/s. Discharge rates were most effectively modulated at a stimulus velocity of 20 deg/s. A large portion of the cells studied (91%) could be binocularly activated, although for almost all neurons the contralateral eye was dominant. Through stimulation of the optic chiasm 46 out of 51 NOT neurons could be electrically activated with a latency of 5.42 +/- 0.66 ms (mean +/- SD). For 15 fibers stimulated from both optic chiasm and contralateral optic nerve, conduction velocities between 2.5 and 8.9 m/s, with a mean of 5.1 m/s, were obtained. A major direct input from the ipsilateral retina was not found. Furthermore, 65% of all neurons could be activated through electrical stimulation of visual cortex with a mean latency of 3.7 +/- 1.5 ms, indicating a strong cortical projection to ferret NOT. The functional relevance of response properties of ferret NOT neurons for horizontal optokinetic nystagmus is discussed. Parameters that could be related to formation of a cortico-pretectal projection in mammals are considered.