**2. Color processing in the human brain**

All mammals possess dichromatic or monochromatic color vision, but only primates have trichromatic color vision. What is the ecological advantage of having trichromatic color

The Contribution of Color to Object Recognition 75

showed that, when he viewed and recognized colors, significant increases in activity were restricted to V1 and V2, and no significant activation of V4 was observed. Finally, a third and final stage in color processing involves object colors. This is supported by the inferior temporal and probably also by the prefrontal cortex (Zeki & Marini, 1998). Little is known, however, about the neural mechanisms underlying higher-level aspects of color processing (cortical brain regions believed to be important for color perception are shown in **Figure 2**). Given that the brain has developed specialized mechanisms to handle color perception information in the visual environment, it is a fair question to ask what functional role color

Fig. 2. Schematic view of the human brain. The regions that are important for various aspects of color perception are shown. These regions include the lingual gyrus and the

Traditionally, theories of object recognition suggest that objects are recognized based only on shape information, largely ignoring the potential role of color information (Biederman, 1987; Marr & Nishihara, 1978). For instance, in the recognition-by-components (RBC) model, proposed by Biederman (1987), objects are described as spatial arrangements of a restricted set of roughly 30 basic component shapes, such has wedges and cylinders, called geons. This idea suggests an analogy with words, which are constructed from a restricted set of phonemes. Biederman (1987) suggested that the first stage of object recognition involves the segmentation of the contour in regions of sharp concavity. This segmentation divides the contour into a number of parts that then are matched against the set of geons. Biederman (1987) used view-invariant representations. According with the RBC model, geons are defined by properties that are invariant over different views. Object representations are simply assemblies of geons constructed by inferring the qualitative spatial relations between them. Because geons and the relationships between them are viewpoint-invariant, the recognition process is likewise viewpoint-invariant. One strong point of this theory is the fact that geons are not only view-invariant, but also to other surface properties, such as size,

posterior portion of fusiform gyrus, located below the calcarine fissure.

**3. Does color information improve object recognition?** 

color or texture.

might play in everyday vision, in particular in object recognition.

vision? Primates evolved trichromatic vision from their dichromatic ancestors approximately 40 million years ago following the duplication of a gene coding for the Lcone (Jacobs, 1993; Jacobs & Rowe, 2004; Yokoyama, 2000). It is likely that color serve as a cue for object recognition; for example, animals may use color to assess the health of other members of their species; and color can aid image segmentation (Allen, 1879). But the dominant view is that trichromatic color vision emerged as a specific adaptation for finding fruits and young leaves against a background of mature leaves (e.g., Osorio & Vorobyev, 1996; Regan et al., 2001). This notion is particularly attractive, as many fruits gradually turn yellow, red or orange, and finally brown during ripening. These colors are strikingly visible to trichromats, but dichromats have difficulty distinguishing them from a dappled background of green leaves (**Figure 1**).

Fig. 1. The left image (A) shows in full color a picture of ripe fruit against a leafy background. To remove any advantage in seeing fruit conferred by trichromacy, (B) on the right side have had all the hue and saturation information removed, but are otherwise identical to image (A). The fruit in (B) is much less salient than in (A).

As the human brain evolved, it preserved the mechanism to handle color vision. Several physiological and anatomical studies have established the human color center in the V4 area located in the posterior part of the fusiform gyrus. However, this color center is a part of a more broadly distributed cortical network responsible for color processing, which includes V1, V2, V4, and regions beyond the inferior temporal cortex (e.g., Bartels & Zeki, 2000; Lueck et al., 1989; McKeefry & Zeki, 1997; Zeki & Bartels, 1999; Zeki et al., 1991). Nevertheless, it is unclear what role these brain regions play within the color processing system. Evidence suggests that the first stage of color processing, located in the V1 and V2, primarily registers the presence and intensity of different wavelengths. A second stage, located in the V4, is involved in automatic color constancy operations (Zeki & Marini, 1998). Color constancy is a property of the visual system that ensures that the perceived surface color remains relatively constant under varying illumination conditions. A very interesting case study reported by Zeki and colleagues (Zeki, Aglioti, McKeefry, & Berlucchi, 1999) shows the specific roles of V1, V2 and V4 within the color processing system. After an electric shock that led to vascular insufficiency, the patient PB became virtually blind, although he retained the capacity to perceive colors consciously. The psychophysical results suggested that color constancy were severely deficient in the patient and that his color vision was merely based on wavelength discrimination. Functional neuroimaging studies 74 Advances in Object Recognition Systems

vision? Primates evolved trichromatic vision from their dichromatic ancestors approximately 40 million years ago following the duplication of a gene coding for the Lcone (Jacobs, 1993; Jacobs & Rowe, 2004; Yokoyama, 2000). It is likely that color serve as a cue for object recognition; for example, animals may use color to assess the health of other members of their species; and color can aid image segmentation (Allen, 1879). But the dominant view is that trichromatic color vision emerged as a specific adaptation for finding fruits and young leaves against a background of mature leaves (e.g., Osorio & Vorobyev, 1996; Regan et al., 2001). This notion is particularly attractive, as many fruits gradually turn yellow, red or orange, and finally brown during ripening. These colors are strikingly visible to trichromats, but dichromats have difficulty distinguishing them from a dappled

Fig. 1. The left image (A) shows in full color a picture of ripe fruit against a leafy

identical to image (A). The fruit in (B) is much less salient than in (A).

background. To remove any advantage in seeing fruit conferred by trichromacy, (B) on the right side have had all the hue and saturation information removed, but are otherwise

As the human brain evolved, it preserved the mechanism to handle color vision. Several physiological and anatomical studies have established the human color center in the V4 area located in the posterior part of the fusiform gyrus. However, this color center is a part of a more broadly distributed cortical network responsible for color processing, which includes V1, V2, V4, and regions beyond the inferior temporal cortex (e.g., Bartels & Zeki, 2000; Lueck et al., 1989; McKeefry & Zeki, 1997; Zeki & Bartels, 1999; Zeki et al., 1991). Nevertheless, it is unclear what role these brain regions play within the color processing system. Evidence suggests that the first stage of color processing, located in the V1 and V2, primarily registers the presence and intensity of different wavelengths. A second stage, located in the V4, is involved in automatic color constancy operations (Zeki & Marini, 1998). Color constancy is a property of the visual system that ensures that the perceived surface color remains relatively constant under varying illumination conditions. A very interesting case study reported by Zeki and colleagues (Zeki, Aglioti, McKeefry, & Berlucchi, 1999) shows the specific roles of V1, V2 and V4 within the color processing system. After an electric shock that led to vascular insufficiency, the patient PB became virtually blind, although he retained the capacity to perceive colors consciously. The psychophysical results suggested that color constancy were severely deficient in the patient and that his color vision was merely based on wavelength discrimination. Functional neuroimaging studies

background of green leaves (**Figure 1**).

showed that, when he viewed and recognized colors, significant increases in activity were restricted to V1 and V2, and no significant activation of V4 was observed. Finally, a third and final stage in color processing involves object colors. This is supported by the inferior temporal and probably also by the prefrontal cortex (Zeki & Marini, 1998). Little is known, however, about the neural mechanisms underlying higher-level aspects of color processing (cortical brain regions believed to be important for color perception are shown in **Figure 2**).

Given that the brain has developed specialized mechanisms to handle color perception information in the visual environment, it is a fair question to ask what functional role color might play in everyday vision, in particular in object recognition.

Fig. 2. Schematic view of the human brain. The regions that are important for various aspects of color perception are shown. These regions include the lingual gyrus and the posterior portion of fusiform gyrus, located below the calcarine fissure.
