What unilateral neglect is
Unilateral (hemispatial) neglect is a difficulty noticing, attending to or responding to things on one side of space. It is most often seen on the left after damage to the right hemisphere, for example after a right middle cerebral artery stroke. A person with left neglect may eat only from the right side of a plate, miss words on the left of a page, or bump into things on their left, often without being aware of it.
Neglect varies a lot between people and over time, which is why simple, repeatable tasks are useful for seeing how much someone is missing and whether that is changing.
How star cancellation works
Star cancellation was developed as part of a behavioural test of visuospatial neglect (Wilson, Cockburn & Halligan, 1987). The page shows small stars scattered among larger stars, letters and short words; the person is asked to cross out all the small stars. Targets are distributed evenly across the page, so a person without neglect finds roughly the same proportion on each side.
Digital versions follow the same idea: targets mixed with distractors, spread across the screen, with the person tapping or clicking each target. A digital task can record which targets were found, in what order and how long it took.
Reading the results
The basic results are the number of targets found on each side. A clear imbalance, many more omissions on one side, suggests a lateralised problem with attention or scanning. Other details are also worth noting:
- Where the person starts. People without neglect usually start on the left (in left-to-right reading cultures). Starting on the right can be a sign of a rightward pull of attention.
- How organised the search is. A systematic row-by-row or column-by-column search looks very different from scattered, revisiting jumps.
- False positives. Crossing out distractors may reflect inattention, impulsivity or visual difficulties rather than neglect.
- Time. A slow but complete search tells a different story from a fast one with omissions.
The centre of cancellation
Rorden and Karnath (2010) proposed the centre of cancellation (CoC) as a simple measure of neglect severity on cancellation tasks. Each target’s horizontal position is expressed from −1 (far left) to +1 (far right). The CoC is the average position of the targets that were found, adjusted for the average position of all targets, so a complete search scores 0.
If the person finds only targets on the right, the found targets sit to the right of centre and the CoC is positive; omissions on the right make it negative. The further from zero, the stronger the bias. The measure uses every target’s position, not just a left/right split, so it is more sensitive to small biases than counting halves.
Published cut-offs compare a person’s CoC with values from healthy controls on the specific task. CogniStride highlights a CoC beyond ±0.081 as a lateral bias worth attention. Treat this as a prompt to look more closely, not as a diagnostic threshold.
Anchoring and cueing
Visual scanning training often uses an anchor: a bright line or mark at the far edge of the neglected side that the person is taught to find before they start scanning (Weinberg et al., 1977). Over time the anchor is made less prominent and then removed, so the person relies on their own scanning strategy.
A simple cueing hierarchy for cancellation practice might be:
- No cue.
- A visual anchor line on the neglected side.
- A more salient cue, such as a flashing border.
- A verbal prompt to start from, and return to, the neglected side.
Recording the cue level with each attempt makes it possible to see whether the person needs less support over time, which is often more informative than the score at a single level.
Using cancellation in practice
- Keep conditions consistent. Use the same screen size, field density and seating position when comparing attempts; screen position relative to the body affects results.
- Watch, as well as score. Head and eye movements, verbal strategies and reactions to feedback all add information that numbers miss.
- Link to function. Cancellation is a means to an end; pair it with functional scanning tasks such as reading, finding items on a shelf or navigating a corridor.
- Mind fatigue. Neglect often worsens when someone is tired; recording effort alongside results helps interpret off days.