# Where Does the Line Exit? — align the continuation across the barrier

A diagonal line enters an opaque column. Drag the handle on the right until both segments form a single straight line, then pull the thread to measure the shift in pixels and millimetres.

[Open the interactive instrument](https://mariuscomper.uk/unde-iese-linia/en/)

## How it works

1. A line enters from the left and stops at the boundary of an opaque central column.
2. Drag the handle on the column's right edge or use the nudge buttons (▲/▼) to place the continuation on the same straight path.
3. Tap "Pull the thread" to reveal the true geometric line and view your error.
4. Complete five trials with varied angles and barrier widths.
5. Receive your final diagnostic: mean error, best trial, and your visual cortex's acute expansion rate.

## Why the line breaks in the mind

In 1860, Johann Christian Poggendorff discovered that acute angles are systematically misperceived when a line crosses an occluder. In early visual cortex (area V1), orientation-tuned neurons engage in mutual lateral inhibition. This inhibitory interaction expands perceived acute angles by 3 to 5 degrees, projecting the exit segment noticeably higher than its true geometric course.

Results reflect performance across five trials on your current display. They do not constitute medical or ophthalmological diagnostics.

Text and implementation: Marius Comper, September 2026.