In the case of broadband stimuli (e.g., letter optotypes), impaired performance has also been found when the subjects were required to identify a test letter in the presence of nearby objects. This phenomenon is referred to as a ‘crowding effect’ and the objects causing the deterioration are called distractors. Dr. H. Strasburger (personal communication at 18th ECVP in Tubingen, 1995) traced the usage of the term ‘crowding’ to H. Ehlers who in 1936 (Ehlers,
1936) distinguished the separations needed for reading when the letters are isolated and when they are ‘in close type (provided the reading distance and the size of types are the same)’; and in 1953 Ehlers wrote that ‘if the visual field is crowded with letters, the area of the visual field in which the letters can be recognized narrows' (Ehlers,
1953). Later Stuart and Burian (
1962) adopted the term ‘crowding phenomena’ instead of ‘separation difficulty’. At present, many other words are used to describe the impaired perception of letters surrounded by different types of distractors; and conversely, the word ‘crowding’ is applied to various visual phenomena not involving letter discrimination (for example, see Parkes, Lund, Angelucci, Solomon, & Morgan,
2001). Alternative terms include
lateral masking or
lateral interference (Huckauf, Heller, & Nazir,
1999; Taylor & Brown,
1972; Townsend, Taylor, & Brown,
1971; Wolford & Chambers,
1983),
mutual or
cognitive inhibition(Woodrow,
1938),
lateral inhibition (Townsend et al.,
1971), and
contour interaction (Flom, Weymouth, & Kahneman,
1963; Jacobs,
1979; Kooi, Toet, Levi, & Tripathy,
1992). When researchers talk about letter recognition, all of the above terms are used as synonyms, although Flom (
1991) made a distinction between
contour interactions, which he considered as a loss of information within the visual system, and
crowding effects, which are more complex phenomena and include both contour interactions and attentional factors. For simplicity, we follow this distinction in our paper and refer to
crowding effects when talking about complex surrounds (letters, etc.) and
contour interaction when the distractors are simple bars. However, under ‘crowding phenomena’, we shall include all types of distractors.