Visual field topography and binocular responses in frog's nucleus isthmi

Visual responses of 125 units have been extracellularly recorded with glass micropipettes from the left nucleus isthmi in the frog Rana nigromaculata, and 101 electrode tip positions marked with cobalt staining to reconstruct a visual field map in the nucleus. 80% of the units recorded show ON-OFF responses to a stationary spot of light and many are directionally selective in response to black or white targets moving through their receptive fields. All the cobalt-marked spots are within the nucleus, indicating that the nucleus isthmus proper is a restricted part of the frog visual system. There is a visual field map in the nucleus. The entire contralateral hemifield and the nasal 40 degrees of the ipsilateral hemifield project on the nucleus topographically. A cell-free band inside the nucleus is a boundary line separating the contralateral hemifield from the ipsilateral one. Dorsal to it is the contralateral field representation. The upper visual field projects on the rostral half of the nucleus and the central and medio-ventral portion of its caudal half. The lower field is represented on the dorsal and lateral part of the caudal half. Fifteen binocular units have been found from the nucleus, 13 of which are dominantly activated by the contralateral eye, the other two are almost equally excited by either eye. These binocular units are mainly situated in the medulla of the rostral half of the nucleus isthmus.