Indexing the archive…
Your Universe of Digital Possibilities
A ring circles a cross and a small disc flashes once beside it. You will see the flash trail the ring — by tens of milliseconds. Answer ahead or behind, trial after trial, and the instrument measures your lag in ms. Then it runs the two conditions that tell whether your brain predicts where the ring will be, or waits a moment and decides what it saw.
The probability of one answer as the stimulus x changes: F a sigmoid (logistic or cumulative normal) centred on α, the point of subjective equality, with spread β; γ the guess rate, λ the lapse rate. Every Iris instrument fits it to the visitor’s own trials by maximum likelihood, with the PSE’s error from the profile likelihood.
Step down after n positive answers in a row, up after one negative: the track can only stop moving where a run of n positives is as likely as not. Two interleaved tracks — one rising, one its mirror — bracket the PSE from both sides and spend the trials where the curve is steep.
The flash must be placed ahead of the moving ring by θPSE to be seen aligned with it; divided by the ring’s angular speed ω, that offset is a time — how far behind the present the flash is placed by the moving thing’s percept.
Each model predicts the three conditions’ PSEs from one lag: extrapolation (L, 0, L), postdiction (L, L, 0) for continuous, flash-initiated, flash-terminated. Each is fitted to every trial; the ratio of their best likelihoods says which the data favour — and how strongly.
The third percept is about time. Light from the ring and light from the flash leave the screen together, and what you report as “now” puts them apart — your sense of the present is itself an estimate, assembled over a short window. This is where the edition’s procedure is written down once for every instrument after it: a psychometric function fitted by maximum likelihood, and interleaved Levitt staircases to feed it. What a strobe does to motion on the screen’s side is The Strobe; what “at the same time” means once light has a speed is The Cone.