The Judd illusion

evidence for two visual streams or two experimental conditions?

Mark Mon-Williams*, Rebecca Bull

*Corresponding author for this work

Research output: Contribution to journalArticle

40 Citations (Scopus)


In the Judd illusion, observers inaccurately bisect the shaft located between two arrowheads pointing in the same direction. The magnitude of error is greater when verbal judgements are compared to action based responses (reaching out and grasping the centre of the bar). This difference has been attributed to the presence of two visual streams within cortical processing. In contrast, we provide evidence that the improved accuracy in the reaching condition may be due to occlusion of the illusory background during the transport phase of the movement. We suggest that caution is required when interpreting performance differences between two conditions that are not strictly equivalent.

Original languageEnglish
Pages (from-to)273-276
Number of pages4
JournalExperimental Brain Research
Issue number2
Publication statusPublished - Jan 2000
Externally publishedYes


  • prehension
  • visual pathways
  • illusions
  • human
  • motor control

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