GoKawiilSubjects and surgical procedures
Data were acquired from two adult male macaques (Macaca mulatta, average weight of 17 kg (subject 1 (S1)) and 10 kg (S2), average age of 9 years (S1) and 7 years (S2)). This sample size was chosen to match the standard for neural recording studies of behaviour in monkeys22,24. All animal procedures complied with the NIH Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee of the Rockefeller University (protocol 24066-H).
After undergoing initial task training in their home cages, the subjects underwent two surgeries: the first to implant an acrylic head implant with a headpost and the second to implant electrode arrays. Both surgeries followed standard protocols, including for anaesthetic, aseptic and postoperative treatments. In the first surgery, a custom-designed MR-compatible Ultem headpost was implanted, surrounded by a bone cement cranial implant, or a headcap (Metabond, Parkell and Palacos, Heraeus), which was secured to the skull using MR-compatible ceramic screws (Rogue Research). After a 6-month interval, to allow bone to grow around the screws and for the subject to acclimate to performing the task during head fixation via the headpost, we performed a second surgery to implant 16 floating microelectrode arrays (32-channel FMA, Microprobes for Life Science) using standard procedures69. In brief, after performing a craniotomy and durotomy over the target area, arrays were inserted one by one stereotactically while held at the end of a stereotaxic arm with a vacuum suction attachment (Microprobes). Using vacuum suction enabled us to release the arrays, after insertion, with minimal mechanical perturbation by turning off the suction. After all arrays had been implanted, the dura mater was loosely sutured and covered with DuraGen (Integra LifeSciences). The craniotomy was closed with bone cement.
We used standard density arrays (1.8 mm × 4 mm) for all areas, except SMA and preSMA, for which we used four high-density arrays (1.6 mm × 2.95 mm). Four additional electrodes on each array served as the reference (two electrodes) and the ground (two electrodes). Two arrays each were targeted to multiple areas of the frontal cortex, with locations identified stereotactically, and planned using brain surface reconstructions derived from anatomical MRI scans (3D Slicer 5.6.2). Locations were selected on the basis of their published functional and anatomical properties (see below), anatomical sulcal landmarks and a standard macaque brain atlas70. During surgery, locations were further adjusted on the basis of cortical landmarks and to avoid visible blood vessels. Arrays were implanted in the right hemisphere (contralateral to the arm used for behaviour).
Array locations are depicted in Fig. 3 and Extended Data Fig. 6 and confirmed with intraoperative photographs. For M1, we targeted hand and arm representations (F1) directly medial to the bend of the central sulcus (which corresponds roughly to the intersection of the central sulcus and the arcuate spur if the latter was extended caudally), based on retrograde labelling from the spinal cord and microstimulation of M171 and M1 recordings72. For PMd, we placed both arrays lateral to the precentral dimple, with one (more caudal) array directly medial to the arcuate spur (the arm representation71,72,73, F2), and the other more rostral (straddling F2 and F7). For PMv, we targeted areas caudal to the inferior arm of the arcuate sulcus (F5), which are associated with hand movements based on retrograde labelling from the spinal cord71 and M174, microstimulation75 and functional studies51,76,77 and with decision making77,78. These areas contain neurons interconnected with PFC74. For SMA (F3) and preSMA (F6), we targeted the medial wall of the hemisphere, with the boundary between SMA and preSMA defined as the anterior–posterior location of the genu of the arcuate sulcus, consistent with previous studies finding differences across this boundary in anatomical connectivity (for example, direct spinal projections in SMA but not preSMA79) and function42,80. SMA arrays were largely in the arm representation79. For dlPFC, we targeted the region immediately dorsal to the principal sulcus (46d), following previous studies of action sequencing27,28,81 and other cognitive functions82. For vlPFC, we targeted the inferior convexity ventral to the principal sulcus, with one (more rostral) array directly ventral to the principal sulcus (46v) and the other rostral to the inferior arm of the arcuate sulcus (45A/B) based on evidence that encoding of abstract concepts occurs in regions that broadly span these two locations29,83,84, including a possibly heightened role (compared with dlPFC) in encoding abstract concepts in a manner invariant to temporal or spatial parameters84,85,86. For FP, we targeted a rostral location similar to previous recording and imaging studies (one array fully in area 10, the other straddling areas 9 and 10)87,88, including areas associated with executive functions89. In general, array locations targeted the cortical convexity immediately next to sulci, instead of in the banks, to allow shorter insertion depths that minimize the risk of missing the target or damaging blood vessels. The exceptions were SMA and preSMA in the medial wall, for which this was not possible. To avoid damaging the superior sagittal sinus, we positioned the arrays laterally (2 mm from midline) and slanted the electrodes medially (Extended Data Fig. 6).
The lengths of each electrode were custom designed to target half-way through the grey matter and to substantially vary across the array to maximize sampling of the cortical depth. The following electrode lengths were used (in mm): 1.5−3.5 (M1), 1.5−3.1 (PMd and PMv), 2.8−5.8 (SMA and preSMA), 1.5−2.5 (dlPFC and vlPFC) and 1.5−2.6 (FP) for S1; and 1.7−3.75 (M1), 1.5−3.3 (PMv), 1.5−3.1 (PMd), 2.65−5.95 (SMA and preSMA), 1.75−3.15 (dlPFC), 1.35−3.2 (vlPFC) and 1.6−2.9 (FP) for S2. Reference electrodes were longer (6 mm) to anchor the arrays. All electrodeswere Pt/Ir (0.5 MΩ), except 4, which were Ir (10 kΩ). Array connectors (Omnetics, A79022) were housed in custom-made Ultem pedestals (Crist), which were secured with bone cement onto the cranial implant. Four pedestals were used per subject, holding 5, 5, 4 and 2 connectors each.
Behavioural task
Task overview
The subjects were seated comfortably in the dark with their head restrained by headpost fixation. They faced a touchscreen (Elo 1590L 15-inch E334335, PCAP, 768 × 1,024 pixels, refresh rate of 60 Hz, with a matte screen protector to reduce finger friction) that presented images and was drawn on. The touchscreen location was optimized to allow each subject to easily draw at all relevant locations on the screen (23–26 cm away; diagram in Extended Data Fig. 1). Both subjects decided on their own over the course of learning to perform the task with the left hand. The chairs were designed to minimize movements of the torso and legs (by using a loosely restricting ‘belly plate’) and the non-drawing arm (by resting on the belly plate and having movement restricted to within the chair). Gravity-delivered reward (water–juice mixture) was controlled by the opening and closing of a solenoid pinch valve (Cole-Parmer, 1/8-inch inner diameter). The subjects were water-regulated, with careful monitoring to ensure that consumption met the minimum requirement per day (typically exceeding it), and body weight was closely monitored to ensure good health. The task was controlled with custom-written software using the MonkeyLogic (v.2.2.45) behavioural control and data acquisition MATLAB package90 (PC: Windows 10 Pro, Intel Core i7-4790K, 32GB RAM; DAQ: National Instruments PCIe-6343). All stimuli (images of line figures defined as point sets, with points rendered large enough to appear as continuous curves) were also generated with custom-written MATLAB (R2021a) code. Images were presented in a workspace area on the screen (16.6 cm × 16.9 cm, corresponding to approximately 37° by 38° visual angle). Shape components in images were on average 4.0 cm (9°) (maximum of width and height).
Each recording session consisted of 2−3.5 h of recording. We collected 5–20 trials per condition (that is, each unique image for Figs. 4 and 5 and the single-shape task in Fig. 6, and each primitive stroke for the character task in Fig. 6). All trials were shuffled across all conditions in the session and presented in a randomly interleaved fashion, except for one case, the experiment in Fig. 6, in which character and single-shape tasks were switched in blocks.
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