The serial position effect is one of the most replicated findings in memory research (Murdock 1962; Atkinson & Shiffrin 1968): free recall of a list traces a U-shape — early items (primacy) and the last few items (recency) are recalled far better than the middle.
This 2D view exposes the mechanism that a 3D bar scene can only hint at: a scrolling buffer-occupancy timeline. Each column is one study tick; each colored tile is an item token sitting in one of the K rehearsal-buffer slots at that tick. Watch tokens get bumped out (a slot goes empty, then a new hue appears) once the buffer fills up.
buffer.length >= K → evict one random occupant, insert new item
every tick: rehearsal[i] += gain for every i currently in the buffer
LTM(i) = rehearsal[i] / max(rehearsal)
Early items enter before the buffer is full, so they rack up rehearsal for longer before ever risking eviction — that surplus is the mechanistic source of primacy, and it is directly visible in the timeline as longer unbroken colored streaks near the left edge. Faster presentation rates shrink the rehearsal gain per tick, flattening the primacy limb.
Items still sitting in the buffer at the end of the list carry a separate, fast-decaying short-term (STM) activation:
STM(i) = inBuffer(i) · e^(−λ · delay) λ ≈ 0.15 s⁻¹
A recall delay lets STM decay before test, exactly as Glanzer & Cunitz (1966) showed: the recency bump on the curve below the timeline selectively vanishes with delay while primacy survives. Final recall probability blends both stores:
P(recall | i) = clamp( 0.15 + 0.55·LTM(i) + 0.75·STM(i), 0, 1 )
- List length — longer lists overflow the buffer sooner, sharpening the primacy/middle contrast.
- Presentation rate — slower study time per item increases rehearsal gain and strengthens primacy.
- Recall delay — drains STM activation; push it high enough and the recency bump disappears while primacy remains.
- Buffer capacity — how many items can be actively rehearsed at once; a smaller buffer means fewer items enjoy the primacy advantage.