5 Interference: A loss of independence

The pixels in the “camera of the human mind” do not work independently. A strong case in point are illusory brightness or colour shifts. Patches of the exact same brightness may be perceived as entirely different, depending on their surroundings, and depending on the global configuration of brightness and contrast. A striking example is the cylinder with checkerboard illusion shown in the right half of figure 2. Similar illusions exist in the domain of colour (figure 2, left). Relatedly, everyone who has ever tried to paint a picture has experienced that it takes an astonishingly rich palette of reds, purples, browns, yellows, and even greens or blues to construe a veridical depiction of a simple red apple. The unitary experience of seeing a red apple is in fact composed of the detection of a multitude of wavelengths, all interacting to compose that one colour. Only with extreme focused scrutiny (or by covering surrounding elements) are we able to isolate the elements that make up our unitary conscious experiences.

Figure 2: Two strong shifts in the perception of colour and brightness. Although the indicated patches are identical, they are perceived as having quite different colour and brightness. Visit Michael Bach’s website (http://michaelbach.de/ot/) for these and many other examples.

Another illustration is the phenomenon of colour constancy. When we look at a bowl of fruit in the blue morning light the spectral composition of wavelengths reflected from the fruits is very different from the wavelengths coming from the fruits at sunset (figure 3). Nevertheless, we see the banana or the apple as having the same colour whether it is dusk or dawn. Our visual system is not interested in the wavelength coming from fruits; it is interested in their potential taste or edibility. Therefore, it discounts the illumination, and computes “colour”, which is a property of the object, rather than of the light coming from it.[15] Colour is not wavelength; colour is a meaningful property of objects that is based on wavelengths, yet transcends it.

To what extent do these phenomena depend on consciousness? Harris et al. (2011) studied a brightness illusion much like that in figure 2. Two circles were shown, of either the same or different brightness. By placing these circles in a dark and bright surround respectively, their brightness suffered from an illusory shift. In the critical condition, the surrounds were made invisible by presenting them to one eye, and filling the other eye with a continuously flashing Mondrian stimulus. This resulted in CFS of the surrounds. Cleverly, the two circles were shown in both eyes, so remained visible throughout. Regardless of the CFS-induced invisibility of the surrounds, the circles still showed illusory brightness shifts.[16]

Figure 3: These images show a bowl of fruit photographed in three lighting conditions—artificial light (left), hazy daylight (middle), and clear blue sky (right). Notice the marked variation in colour balance caused by the spectral properties of the illuminant. We are not normally aware of this variation because colour constancy mechanisms discount illumination effects (image and legend from http://www.psypress.co.uk/mather/resources/topic.asp?topic=ch12-tp-04).

The neural mechanisms of illusory brightness perception were studied extensively in the macaque monkey and cat visual cortex. It was found that perceived brightness (modulated by flanking regions) influenced neural responses in area V1 of the cat, but not at earlier stages such as the LGN or the optic tract, thereby showing a gradual progression from physical brightness to perceptual brightness in the visual pathways (Rossi & Paradiso 1999; Rossi et al. 1996). Using the Cornsweet brightness illusion,[17] it was found that in the monkey’s visual cortex, V2 cells represents surface brightness whereas V1 cells do not, pushing the level at which perceived brightness is calculated somewhat higher (Roe et al. 2005). Either way, these results were recorded in anaesthetized animals, showing their independence from consciousness.

How the visual system goes from the detection of wavelength towards the representation of colour is still a topic of controversy. Initially, there was thought to be a modular progression from V1 cells encoding wavelength towards V4 cells encoding colour. That view was challenged by various findings showing that the responses of V1 cells are influenced by surrounding hues. The view that V4 is the “colour module” has also been challenged, in part by strong disagreement on the homology between monkey V4 and alleged human counterparts.[18] Moreover, the coding of colour is intricately linked to the coding of object shape, and hence can no longer be viewed as a simple “add-on” to our visual percept.[19] It is now thought that the perception of colour depends on the interaction between neuronal groups, or is best understood as a population code (Shapley & Hawken 2011).

Given this controversy, it is difficult to know to what extent colour perception depends on consciousness. Many of the recordings in monkey visual cortex were performed in awake animals, some in anaesthetized animals (Shapley & Hawken 2011). A clear-cut difference in results between the two conditions is hard to establish. A remarkable finding is that blindsight patients report no conscious sensation of colour, yet may have spectral sensitivity curves that have a similar shape in the lesioned and intact hemi-fields (Stoerig & Cowey 1989). Spectral sensitivity is, however, mostly carried by wavelength. Similarly, patients with cortical colour blindness (achromatopsia) do not consciously perceive colour, yet can detect objects or patterns based on wavelength contrasts (Cowey & Heywood 1997). Colour constancy mechanisms, on the other hand, are absent in the lesioned hemi-field of blindsight patients (Barbur et al. 2004; Barbur & Spang 2008), and hence seem more closely linked to conscious perception.[20]

The difference between perceived colour and wavelength, and its relation to conscious vision, has been directly addressed in a masked priming experiment. In this experiment, subjects were shown desaturated blue, green, or white coloured disks. Perceptually, the white was closer to the blue disk. From the point of view of the phosphor activations on the monitor screen, on which the disks were shown (i.e., their “wavelength composition”), the white disk was, however, closer to the green disk. What was studied was the effects of these disks when they acted as primes for a subsequent colour discrimination. It was found that masked, and hence invisible white disks, acted more like green primes than like blue ones. Visible white disks, on the other hand, acted more like blue primes than like green ones (Breitmeyer et al. 2004; Breitmeyer et al. 2007). Apparently, unconscious priming acts on wavelength similarity, whereas conscious priming acts on perceived colour similarity.

All in all, it remains difficult to assess the relation between consciousness and phenomena like brightness or perceived colour illusions, or mechanisms related to colour constancy. Perceived brightness seems to depend on largely unconscious mechanisms, and on fairly low level and short range mechanisms. The transition from wavelength analysis to the perception of colour is more likely to accompany the transition from unconscious processing to conscious vision. A firm conclusion, however, relies upon settling the debate about mechanisms of colour perception and their neural substrates in humans and animals, and more direct experimentation on how these mechanisms are affected by manipulations of consciousness.[21]