The Accessory Optic System Analyzer of Self‐Motion a

Accessory optic system (AOS) neurons in the medial, lateral, and dorsal terminal nuclei (MTN, LTN, and DTN) of the rabbit have contralateral receptive fields and the common property of direction and speed selectivity in response to movement of large, textured Neurons in each nucleus prefer slow speeds, on the order of 0So/sec, but differ in their direction preferences. DTN neurons prefer horizontal movement, whereas MTN and LTN neurons prefer near-vertical movement. An unusual feature of the direction selectivity of MTN and LTN neurons is that their preferred excitatory and inhibitory directions are noncollinear. This geometrical arrangement has been further studied in conjunction with an investigation of the spatial organization of direction selectivity within the receptive field of neurons in the MTN and the neighboring visual tegmental relay zone (VTRZ) of the ventral midbrain. In the rabbit, many DTN neurons project directly to the dorsal cap of the inferior olive, but only a few MTN and LTN neurons do so.&* The caudal half of the dorsal cap receives an input directly from the ipsilateral DTN, while the rostral half of the dorsal cap and its lateral extension, the ventrolateral outgrowth (VLO), receive an input from neurons located ipsilaterally in a crescent-shaped region immediately dorsal to the MTN. These neurons are within the visual tegmental relay zone (VTRZ), which is the part of the ventral tegmentum to which contralateral MTN neurons project, largely via the posterioi c o ~ i ~ m i s s u r e . ~ ~ ~ The anatomical data indicating that VTRZ neurons are interposed between the MTN and the rostral dorsal cap raises questions about the nature of the intermediate processing of the transmitted visual signals. With respect to direction selectivity, the structure of the receptive field of rostral dorsal cap neurons is substantially more complicated than that of the usual MTN Neurons in the rostral dorsal cap