Visual response properties in the tectorecipient zone of the cat's lateral posterior-pulvinar complex: a comparison with the superior colliculus.

The medial portion of the cat's lateral posterior-pulvinar complex (LPm) receives a prominent ascending projection from the superficial layers of the superior colliculus. This region of the thalamus has been suggested to serve as a relay by which visual information from the midbrain could be conveyed to extrastriate cortex. In order to determine how the functional organization within the LPm compares with that of the superior colliculus, visual response properties of LPm and superior collicular neurons were examined under identical experimental conditions. The majority of neurons in the LPm, as in the superior colliculus, respond vigorously to moving stimuli, and a substantial proportion of these cells also exhibit a preference for movements in a particular direction. Furthermore, most cells in the LPm, in common with those of the tectum, respond only in a phasic manner to flashed stimuli, have homogeneous receptive field organization, and show response suppression to stimuli larger than the activating region of the receptive field. As in the colliculus, the ipsilateral visual field is represented in the LPm. In spite of these similarities, there are also some striking differences between the visual responses of LPm and collicular neurons. First, the average size of receptive fields of neurons within the LPm is at least twice that of units in the superficial gray layers of the tectum. Second, more cells in the colliculus are directionally selective than those in the LPm, and the distribution of preferred directions is different in the two regions. Third, an appreciable proportion (27%) of the cells in the LPm are orientation selective, whereas this response property is only rarely encountered in the cat's tectum. Fourth, many LPm neurons can only be activated by binocular stimulation, whereas most collicular units respond equally well to stimulation of either eye. Collectively, these findings indicate that there is a substantial transformation in the lateral posterior-pulvinar complex of the ascending visual influx provided by the superior colliculus.