Memory or a Clock? Auditing a Memory Advantage in XLand-MiniGrid with Seeds, Maps and Evaluation Noise Apart

Overview & Synopsis

On a partially observable XLand-MiniGrid task, an LSTM agent beats a memory-free MLP by 25 percentage points. The gap looks like a clear case of memory. We ask two questions about it. First, how much of a measured gap is noise? Separating training seed, map and evaluation randomness in 60 runs, each re-evaluated three times, we find that about ten paired seeds detect a 10-point gap in final success. The binary "task acquired" verdict, a common summary of learning curves, is 76% evaluation noise and would need about a hundred seeds for the same gap. Second, is the gap memory? On a sealed panel of new maps, opened once, a memory-free MLP of the LSTM's size comes within 3.3 points of the LSTM when it is given the time step, and gains no measurable benefit from its width without it. With the time step, an MLP a quarter of the LSTM's size, and the LSTM with its state erased at every step, also come within about 5 points. A linear probe reads the time step from the trained LSTM's state. On this task, most of the apparent memory advantage is an advantage of knowing the time, which matters because episodes are cut at a fixed limit and the reward shrinks with time. We recommend giving memory-free baselines the time step, and we release the protocol, the event-driven tooling and every checkpoint.