Figure 1.

High-density recording methods. (A) Location of the 64-channel silicon multi-electrode probe. An 8 × 8 electrode grid with 200 μm inter-site spacing extends over a 1,400 × 1,400 μm plane in the hyperpallium. Probes were inserted horizontally (red plane) or sagitally (yellow plane). The horizontal plane is orthogonal to the sagittal plane and is shown as a thin line in the sagittal brain section. The orientation of horizontal electrode grids is always depicted so that the medial side is on the left and the anterior side (that is, brain surface) is on top; sagittal grids are depicted so that the caudal side is on the left and the anterior side (that is, brain surface) is on top. Together, horizontal and sagittal recordings provide insight into the three-dimensional spatial extent of activity propagation patterns. Thalamic input to the hyperpallium (H), which is where most recordings were performed (n = 11 birds), terminates primarily in the interstitial part of hyperpallium apicale (IHA), a thin region underlying the rest of hyperpallium apicale (HA) and overlying the hyperpallium densocellulare (HD). Output projections from the HA course medially over the surface of the brain before descending down the septomesencephalic tract. The hyperpallium overlies and is interconnected with the mesopallium (M) and nidopallium (N), large nuclear structures composed of similar stellate neurons, involved in high-order cognitive processes comparable to those performed by the neocortex [26]. Two additional recordings were performed in caudal nidopallium (NC; n = 2 birds). (B) Raw intracerebral signals were recorded and off-line filtered to a signal containing local field potentials (LFP, 0.1 to 350 Hz), and to a signal containing multiunit action potentials (MUA, 0.5 to 5 kHz). To obtain a signal that corresponds to the level of multiunit action potential firing, the MUA signal was rectified and decimated (analogue multiunit activity, AMUA).