Visual responses of single neurons in the caudal lateral pulvinar of the macaque monkey

Single unit recordings were made in the portion of the lateral pulvinar which forms the lateral aspect of the caudal pole of the thalamus, i.e., PL gamma (Rezak, M., and L. A. Benevento (1977) Soc. Neurosci. Abstr. 3:573; Rezak, M. (1978) Soc. Neurosci. Abstr. 4: 642), of macaque monkeys. PL gamma receives convergent inputs from the occipital cortex and has strong reciprocal interconnections with the visual association cortex, including the inferotemporal cortex (areas 20 and 21). It was found the that PL gamma has a poor or nonexistent retinotopic organization. Many of the neurons had large, unflanked, overlapping receptive fields which often included the fovea. A few neurons could be influenced by a visual stimulus placed anywhere in the visual field described by a tangent screen. The receptive fields could be bilateral or located entirely within the contralateral or ipsilateral hemifields. The majority of units were binocular and exhibited various types of binocular interaction which could be quite complex. The binocular response was not predictable from the algebraic sum of the monocular responses and could be of the opposite sign (e.g., excitatory when the monocular response was inhibitory). Neurons which were also sensitive to the direction of movement of stimuli projected upon the tangent screen formed a major group. Of the units sensitive to tangentially moving stimuli, two special subgroups were found. One group of neurons gave sustained responses to static levels of luminance, while the other group was sensitive to simuli which moved toward or away from the eyes. The nonlinear rate of change of the apparent size of approaching or receding stimuli was described by a mathematical function which also describes the response of the neurons to the same stimuli. For many of these units which were sensitive to tangentially moving stimuli and one other class of stimuli, such as luminance levels of movement in depth, the responses to one class were seemingly unrelated to the responses to the other class. The same statement may be made for monocular and binocular responses. It may be, then, that different wiring diagrams describe these different types of inputs. These physiological results are discussed in terms of the inputs to PL gamma as well as its cortical targets.