Extant euprimates (=crown primates) have a characteristically expanded neocortical region of the brain relative to that of other mammals, but the timing of that expansion in their evolutionary history is poorly resolved. Examination of anatomical landmarks on fossil endocasts of Eocene euprimates suggests that significant neocortical expansion relative to contemporaneous mammals was already underway. Here, we provide quantitative estimates of neocorticalization in stem primates (plesiadapiforms) relevant to the question of whether relative neocortical expansion was uniquely characteristic of the crown primate radiation. Ratios of neocortex to endocast surface areas were calculated for plesiadapiforms using measurements from virtual endocasts of the paromomyid Ignacius graybullianus (early Eocene, Wyoming) and the microsyopid Microsyops annectens (middle Eocene, Wyoming). These data are similar to a published estimate for the plesiadapid, Plesiadapis tricuspidens, but contrast with those calculated for early Tertiary euprimates in being within the 95% confidence intervals for archaic mammals generally. Interpretation of these values is complicated by the paucity of sampled endocasts for older stem primates and euarchontogliran outgroups, as well as by a combination of effects related to temporal trends, allometry, and taxon-unique specializations. Regardless, these results are consistent with the hypothesis that a shift in brain organization occurred in the first euprimates, likely in association with elaborations to the visual system.

SUMMARY1. The responses of units in the superficial layers of the superior colliculus to stretch of extrinsic ocular muscles (e.o.m.) and to visual stimuli, delivered singly and paired at various inter-stimulus intervals, were studied in chloralose-anaesthetized cats.2. Most units responded to visual stimuli and about half also gave phasic excitatory responses to stretch of e.o.m.3. Signals from the e.o.m. of each eye reach both superior colliculi; only those in the colliculus ipsilateral to the e.o.m. stretched were studied in detail.4. A variety of control experiments provided evidence that the signal leading to the responses to e.o.m. stretch was extraretinal. The strong probability is that the receptors responsible were in the extrinsic ocular muscles or their tendons.5. Of fifty-six units, twenty-four (43 %) showed definite interactions between the effects of visual stimuli delivered to the left eye and those due to stretch of e.o.m. of the right eye whose retina had been destroyed.6. Interactions were found with both stationary and moving visual stimuli. They involved either enhancement or reduction (sometimes abolition) of the response to either e.o.m. or visual stimulation, particularly the latter.7. Units with interactions showed one of three types of behaviour.(1) Excitatory responses to visual and e.o.m. stimuli given singly, and interactions when the two types of stimulus were paired at some time intervals. Suppression and abolition of visual responses by preceding e.o.m. stretch was common. (2) Units with little or no excitatory response to e.o.m. stretch applied alone, but which showed reduction of their visual responses by preceding e.o.m. stretch. (3) Units with minimal responses to either type of stimulus presented alone but which gave markedly enhanced responses when visual and e.o.m. stimuli were paired.8. These interactions between proprioceptive and retinal signals are thought to allow retinal image movements which result from saccades to be distinguished from those due to movement of objects in the external world.

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