Crowding: a cortical constraint on object recognition

The external world is mapped retinotopically onto the primary visual cortex (V1). We show here that objects in the world, unless they are very dissimilar, can be recognized only if they are sufficiently separated in visual cortex: specifically, in V1, at least 6 mm apart in the radial direction (increasing eccentricity) or 1 mm apart in the circumferential direction (equal eccentricity). Objects closer together than this critical spacing are perceived as an unidentifiable jumble. This is called ‘crowding’. It severely limits visual processing, including speed of reading and searching. The conclusion about visual cortex rests on three findings. First, psychophysically, the necessary ‘critical’ spacing, in the visual field, is proportional to (roughly half) the eccentricity of the objects. Second, the critical spacing is independent of the size and kind of object. Third, anatomically, the representation of the visual field on the cortical surface is such that the position in V1 (and several other areas) is the logarithm of eccentricity in the visual field. Furthermore, we show that much of this can be accounted for by supposing that each ‘combining field’, defined by the critical spacing measurements, is implemented by a fixed number of cortical neurons.Symbolsaarea (mm2) of cortex occupied by the set of neurons that implement one combining fieldAarea (in mm2) of cortex availablebBouma’s proportionality constant for Eqn (1). “Roughly 0.5” [1••]dposition (in mm) on the cortical surfaceΔdcritical spacing (in mm) on the cortical surfaceNnumber of combining fields implemented by an area A of cortexooverlap factor. The number of combining fields that include any given point in the visual fieldα, βcorrespond to the Larsson and Heeger parameters a and b in Figure 5 of [ 2•]ηanisotropy of crowding, the ratio of radial to circumferential critical spacing. About 2 [3•]φeccentricity (in °) in the visual field [1••]Δφcritical spacing (in °) at the visual field