`saccade-time-sync`

Measures how long it takes for something on this participant's screen to reach their camera, and subtracts that from every later gaze timestamp. The screen steps between two brightness levels at random moments about once a second while the camera watches. There is at most one change per second, so it is not a flickering display.


Package	`@saccadejs/plugin-time-sync`
Browser global	`jsPsychSaccadeTimeSync`
Trial type	`saccade-time-sync`
Requires	the extension and a running camera, so it comes after `saccade-preview`
Wraps	`runLoopback`

timeline.push({ type: jsPsychSaccadeTimeSync });

Parameters

Parameter	Type	Default	Description
`duration`	`number`	`15000`	Length of the measurement, in ms. Longer runs give more edges and a tighter estimate.
`gap_min`	`number`	`500`	Minimum interval between brightness changes, in ms.
`gap_max`	`number`	`1000`	Maximum interval, in ms.
`contrast`	`"full" \| "reduced"`	`"full"`	`"full"` is black and white; `"reduced"` is dark grey and light grey, gentler but noisier.
`instructions`	`HTML string`	an explanation of the brightness test	Shown before the run.
`button_text`	`string`	`"Start"`	Text of the button that starts the measurement.
`require_ok`	`boolean`	`false`	Rerun once on an `UNRELIABLE` verdict. The trial continues either way.
`apply_offset`	`boolean`	`true`	Hand the measured lag to the extension, so later trials have it subtracted.

Data

Field	Type	Description
`lag_ms`	`number`	The measured display plus camera lag, in ms.
`plateau_width_ms`	`number`	Width of the range of lags consistent with every edge: the uncertainty. One camera frame (about 33 ms) or less is expected.
`peak_d`	`number`	Height of the edge-difference peak, 0–1. Below 0.5 the camera did not clearly see the screen change.
`halves_ms`	`[number, number]`	The estimate from each half of the run.
`camera_period_ms`	`number`	Mean camera frame interval.
`camera_jitter_ms`	`number`	Its standard deviation.
`dropped_frames`	`number`	Camera frames the browser reported dropping.
`raf_period_ms`	`number`	Mean animation-frame interval, effectively the display refresh.
`clock_source`	`"captureTime" \| "receiveTime" \| "callback"`	Anything but `"captureTime"` makes `lag_ms` advisory.
`verdict`	`"OK" \| "INCONCLUSIVE" \| "UNRELIABLE"`
`reason`	`string \| null`	Why, when the verdict is not `"OK"`.
`applied`	`boolean`	Whether the offset was set on the extension.
`rt`	`number`	Milliseconds from trial start to the end of the measurement.

The verdict is "OK" when peak_d is at least 0.5, the plateau is at most 34 ms, and the two halves agree: within 8 ms when there are at least 15 edges per half, within 20 ms otherwise.

Example

timeline.push({
  type: jsPsychSaccadeTimeSync,
  contrast: "reduced",
  duration: 20000,
  instructions: `
    <h3>One quick screen test</h3>
    <p>For the next twenty seconds the background will change between dark and light every
    second or so, while your camera watches the screen. It does not flash or flicker.</p>
    <p>This measures how long your screen and camera take to respond, so that we can line up
    what you looked at with when it appeared.</p>`,
});

See Timing and synchrony for how to read the verdict and what the correction leaves behind.

Parameters​

Data​

Example​

Parameters

Data

Example