Fig. 4: The structured memory buffers model.

a, A hypothetical task-structured memory buffer (SMB) in the ABCD task. This ring-shaped SMB is a buffer for a specific behavioural step (goal in location 1): neurons along the SMB represent task position relative to this behavioural step. This behavioural step is therefore the anchor for this SMB. Aligning neurons by when a rewarded goal is encountered in location 1 reveals the invariant relationships between neurons across any two tasks where location 1 is rewarded. The dark blue anchor neuron (neuron 1) responds directly to goal location 1. Conversely, 3 other neurons fire at lags of 90° (one state away), 180° and 270° from the anchor. Other neurons (white circles) encode lags that are not necessarily multiples of 90°. The SMB is shaped by the task, which in the ABCD loop task means that activity circles back to the anchor point after four rewarded goals. b, Two example ABCD tasks that share one goal location (location 1, marked by X). Shaded regions show the spatial firing fields of each of the four neurons shown in a. Whereas the anchor neuron (neuron 1; dark blue) fires consistently at goal location 1 across tasks, other neurons (neurons 2–4; lighter shades of blue) fire in different locations in the two tasks. This spatial remapping is not random, but rather preserves encoding of elapsed task progress from goal in location 1. c, The same ring as in a, when aligned by the abstract task states (for example, state A), appears to rotate by 180° across tasks. This is because location 1 is rewarded in different states across tasks (state A in task 1 and state C in task 2). All neurons on the SMB remap by the same amount, not just the spatially tuned anchor neuron. d, A time series showing the flow of activity along 4 SMBs, each anchored to one of the 4 rewarded locations in task 1. A bump of activity is initiated in each SMB when its anchor is visited (top) and moves along the SMB paced by the progress of the mouse in task space. When it circles back close to the start, it biases the mouse to return to the behavioural step encoded by the anchor of the SMB. Multiple SMBs have active bumps at any one time, thereby simultaneously tracking a sequence of behavioural steps for an entire trial. In principle, the same computational logic can also be used even when individual neurons respond to more than one anchor and/or lag. The readout in such a scenario would involve combinatorial activity across anchor neurons from multiple SMBs. Reproduced/adapted with permission from Gil Costa.