pontis (NRTP), a structure lying adjacent to the medial parts of the PN, but still much less dependent on input from cerebral cortex than the PN.

The Pontine Nuclei

The dorsolateral PN (DLPN) and neighboring parts of the PN receive ample input from a number of cerebrocortical and subcortical structures known to be involved in saccadic eye movements such as the FEF, parietal areas LIP and MP, or the SC. Hence, the anatomy strongly suggests that the PN might be involved in information processing for saccades as well, rather than being confined to the slow visually guided eye movements emphasized by the early electrophysiological and lesion work on the PN [74, 75]. Actually, as it turns out, saccade-related single units can be encountered almost as frequently as single units activated by smooth pursuit eye movements if the dorsal parts of the PN are explored without any bias for the one or the other type of oculomotor behavior. In two rhesus monkeys trained to perform smooth pursuit eye movements as well as visually and memory-guided saccades, out of 281 neurons isolated from the dorsal PN (DPN), 138 were responsive in oculomotor tasks. Forty-five were exclusively activated in saccade paradigms, 68 exclusively by smooth pursuit, and 25 neurons showed responses in both [76]. The various types of oculomotor neurons could be encountered in the lateral as well as medial parts of the DPN without any distinctive differences in their relative frequencies, further putting into perspective the notion of the DLPN as the only oculomotor part of the PN. Saccade-related neurons in the DPN were found intermingled with those discharging in conjunction with smooth pursuit eye movements. Most saccaderelated neurons had a preferred saccade direction. However, with respect to other features, they were quite heterogeneous, exhibiting a wide variety of response patterns when tested in a memory-guided saccade task. Whereas some discharged only at the time of the eye movement, others displayed additional visual responses or activity in the ‘memory’ period. Even the features of saccade-related bursts differed substantially between neurons, as among others reflected by the wide distribution of burst onset latencies, varying between substantial lead and lag relative to eye movement onset. The sources of afferents impinging on the DPN involve probably all cerebrocortical representations of saccadic eye movements, areas which house neurons with very different types of saccade-related responses. The heterogeneity of saccade-related responses in the DPN is therefore most probably a reflection of the diversity of the cerebrocortical input. While about 90% of the afferents impinging on the DPN are of cerebrocortical origin [77], there is additional input from a number of subcortical sources, including the SC [78]. Hence, in principal saccade-related signals in the DPN might also reflect saccade-related input from the SC, rather than information originating from the saccade-related areas of the cerebral cortex.

While some of the saccade-related neurons encountered in the DPN may indeed have been driven by input from the SC, it seems unlikely to be true for the majority of these neurons. This is suggested by the fact that the projection from the SC is not only small, compared with the one originating from cerebral cortex, but, moreover, largely restricted to the rostral DLPN proper [78]. However, saccade-related neurons were found in extended parts of the DPN, most probably also in locations far away from the putative target zones of the SC projection, and, moreover, without any clear differences in the properties of saccade-related responses in different parts of the DPN.

Neurons showing combined sensitivities to saccades and to smooth pursuit, surprisingly frequent in the DPN, do not seem to have a cerebrocortical counterpart. This might suggest that they are constructed by convergence of more specialized oculomotor streams originating from different parts of the cerebral cortex. The functional role of these ‘combination’ neurons is unclear. One might speculate that they play a specific role in the generation of catch-up saccades, executed in an attempt to bring the eye back on target in case of insufficient smooth pursuit eye movements. However, such a role would probably require coinciding preferred directions for saccades and smooth pursuit, a coincidence these ‘combination’ neurons typically lack.

Unlike the effects on smooth pursuit eye movements, small experimental lesions of the monkey DLPN do not affect saccades made to stationary visual targets. However, saccades made to targets moving away from the starting position of the eyes become hypometric for target movement toward the side of the lesion [74]. Larger lesions of the human basilar pons, sparing the brainstem tegmentum, may cause hypometria also of saccades made toward stationary targets without changing saccade velocity and its dependence on saccade amplitude [Bunjes and Thier, unpubl. obs.].

The Nucleus Reticularis Tegmenti Pontis

The dominating type of saccade-related neurons in the NRTP produces bursts of spikes before and during a saccadic eye movement directed toward circumscribed movement fields. Unlike neurons in the nearby PPRF, the discharge intensity or duration does not reflect the saccade metrics. Some of these neurons exhibit additional visual sensitivity to spots of light turned on within the movement field. These neurons are functionally intermediate between the saccade-only neurons mentioned before and neurons with purely visual responses found in the same area. The features of these three types of neurons are reminiscent of the neurons in the SC, from which some of the input of the NRTP is derived. However, unlike movement fields of saccade neurons in the SC, those in the NRTP have a 3-D organization, reflecting eye torsion as well as the vertical and the horizontal excursions of the eye [79]. Moreover, unlike