Single unit response of the Toad's (Bufo americanus) caudal thalamus to visual objects

Summary

In the toad, Bufo americanus, single units of the caudal thalamus were sensitive to either moving or stationary visual objects. Furthermore, some units responded to tactile as well as visual stimulation. On the basis of analysis of the response characteristics of more than 300 single units, the following provisional classification is proposed:

• 1. Spontaneously active units which did not seem to have sensory inputs.

• 2. Units which responded only to tactile stimulation (usually bilateral). The ipsilateral input was typically weaker than the input from the contralateral skin.

• 3. Units with relatively small excitatory receptive visual fields (ERF: 15°–30°) that were best activated by moving black objects of 10° or more in size. To diffuse light changes they showed “off”-responses.

• 4. Large-field units whose excitatory receptive fields (ERF) included the entire contralateral visual field, or the whole visual field, as seen via both eyes. Most of these units were typically responsive to each new kind of object motion, but they quickly adaptated to a repeated movement within a particular region of the field.

• 5. Units that seemed to be variants of types 3 and 4 had ERF's that could change their size, according to circumstances that are not yet known: (a) “Small field” units that could double their size along either the horizontal or vertical axis, (b) Large field units that had a distinct “blind area” centered about the rostral midline; the width of this blind spot could narrow down from the usual 60° distance to only 20–10°. (c) Large field units that were continuously sensitive to the ipsilateral eye, but following tactile stimulation of the contralateral side would respond to contralateral visual movement as well.

• 6. Units that were maximally stimulated by dark objects moved in the “z-axis”. Although the ERF usually included the entire visual field, movements toward the eye from the dorsal direction were most effective.

• 7. Darkness-sensitive units included (a) those with a tonic discharge following the dimming of room lights, and (b) others that gave tonic discharge to light-on and were inhibited during darkness.

• 8. Spontaneously active units with either excitatory (ERF) or inhibitory receptive fields (IRF) that seemed to include the entire visual field. Any moving object within the IRF caused an immediate inhibition, followed by a gradual return of activity to the background level. Some of these units also had a smaller ERF within a large IRF.

• 9. Units activated by moving objects that continued to discharge for 10 seconds or more after the object had disappeared (“memory-units”).

• 10. Units with some of the above characteristics that had the additional property of giving a prolonged response to stationary objects. The ERF-sizes varied from 30° to 90°.