Background: Neurophysiological studies with awake monkeys typically require chronic cranial implants. Headpost and connector-chamber implants are used to allow head stabilization and to house connectors of chronically implanted electrodes, respectively. New Method: We present long-lasting, modular, cement-free headpost implants made of titanium that consist of two pieces: a baseplate and a top-part. The baseplate is implanted first, covered by muscle and skin and allowed to heal and osseointegrate for several weeks to months. The percutaneous part is added in a second, brief surgery. Using a punch tool, a perfectly round skin cut is achieved providing a tight fit around the implant without any sutures. We describe the design, planning and production of manually bent and CNC-milled baseplates. We also developed a remote headposting technique that increases handling safety. Finally, we present a modular, footless connector chamber that is implanted in a similar two-step approach and achieves a minimized footprint on the skull. Results: Twelve monkeys were successfully implanted with a headpost and one with the connector chamber. To date, we report no implant failure, great headpost stability and implant condition, in four cases even more than 9 years post-implantation. Comparison with Existing Methods: The methods presented here build on several related previous methods and provide additional refinements to further increase implant longevity and handling safety. Conclusions: Optimized implants can remain stable and healthy for at least 9 years and thereby exceed the typical experiment durations. This minimizes implant-related complications and corrective surgeries and thereby significantly improves animal welfare.
A hallmark of primate vision is the emphasis on foveal processing, accompanied by frequent saccades that bring the fovea to salient parts of the scene, or to newly appearing stimuli. A saccade to a new stimulus is one of the most fundamental sensory-motor transformations. In macaque monkeys, we show that foveal magnification is not only the reason for saccades, but it also governs the dynamics of saccade initiation. In a task where the monkeys made saccades to peripheral target onsets, saccadic reaction time (SRT) increased with target eccentricity. Notably, we effectively eliminated this increment by scaling the target size according to the foveal magnification factor in the superior colliculus (SC). We repeated the comparison between non-scaled and scaled targets, while changing the task to a delayed saccade task. In this task, the target was presented long before the saccade, and the saccade was triggered by fixation offset rather than target onset, such that target onset was essentially irrelevant for SRT. Crucially, in this task, we found that SRT increased with target eccentricity, for both non-scaled and scaled targets. Furthermore, this increment survived the addition of a salient foveal distracting flash. The results obtained with the delayed saccades task are consistent with an attentional scan from the fovea to the target, a recently hypothesized general mechanism of attention.
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