The Relationship Between Spectral Stimuli and Sensations of

Colour: Taking the Right Shirt.

Everyday, when we wake up, the light reach our retinas given us one part of material what

we need to see what we see. Even though what se see is not always what is outside of our

minds. Even if our vision cannot grasp the properties of physical world, we can get the shirt

what we want. If we are focus on our target we will not make a mistake. Then, if we agree

with this idea that what wee see is not what it is because the visual system cannot access the

physical world by means of retinal light patterns per se (Purves, 2000, 2011, 2014), how is

possible getting the thing what we want or a least what we need? How Vision works? Well,

It was a big question. I cannot solve this question and I don not want to do it. The purpose

of this short assay is just to take a little problem about the relationship between spectral

stimuli and sensations of colour. Specifically, why we see colours that are not there and

keep being successful into an unknowable world?

What we know about colours what we see? Colour experience is composed by three

perceptual qualities. One of these is hue, which is the perception of color per se (i.e. Pure

colors as Red, Blue, Green and Yellow). Another one is saturation, which is the degree to

which hue differs from the neutral gray. The last one, color brightness, perceived intensity

of colours (Purves, Lotto & Polger, 2000; Purves, 2009).

Now, we need to state that any color percepts “should accord with the way

reflectances and illuminants interact in generating the distributions of wavelengths that fall

on the eye.” (Lotto & Purves, 2002, p. 614). Despite the colours we perceive are not a direct

outcome of the spectral composition of light returned by an object, spectral stimuli

necessarily conflate the contribution of the consequence of the reflectance of a surface, the

spectrum of the illuminant and the transmittance of the intervening medium (Ibid).

Thus, up to the present moment, we have 1) three perceptual qualities and 2) a

definition about spectral stimuli. Going ahead with these, we can deal with one of the most

interesting problems about color vision, Color Contrast. How two spectrally identical

colours can be perceived as different colours? Understanding how a perceived color can

look different from another one, which has the same spectrum colour, we might understand

that different contexts (different chromatic surrounds) can induce changes in the perception

of the hue, saturation and brightness of a spectral stimuli (Fig.1). In the same way, these

changes are in each instance “complementary to the way spectral returns from surfaces

change under different chromatic illuminants”(Ibid, p. 618).

Fig. 1. “Two identical target stimuli are physically different surfaces,

then they will appear different” -color contrast- (Lotto & Purves, 2002)

Hence, we can perceive different colours coming from the same spectra as well as same

spectra under different illumination can be perceived like two opposite colors. Alike we see

in the figure 2. where the blue tiles on the in the left cube are physically identical to the

yellow tiles on the right cube, both kind of tiles coming from a gray patches.

Fig. 2 . “The effects on color perception when the same or differently reflective surfaces are

presented in scenes that are consistent with illumination by spectrally different light sources.” (Lotto &

Purves, 2002)

Despite these sorts of tricks, it’s the way what see. But how have we been

successful into this world if we have never ever taken a real picture of the world and we

will not do it? It’s seem to be a outcome from phylogenetic (evolution) and ontogenetic

(development and learning processes) history interaction, which allow us to get a set of

patterns of neural activity triggered by past experiences of what generated the same or

similar stimuli previously (Purves, 2001, Lotto, 2010). Thereby, we always get the

right shirt not because we have solved the inverse problem (or our vision could access

physical reality), but “because the perceptual values assigned by the frequency of

occurrence of visual stimuli accord with the reproductive success of the species and

individual” (Purves, 2014, p .4753).

In conclusion, if we attend 1) the following rules:

“1. When a target is presented on a saturated background whose perceived hue is similar to the hue the target would elicit if on a neutral background, the apparent color of the target decreases in saturation and brightness, with little change in hue2. When a target is presented on a background whose perceived hue is opposite that of the target on a neutral background, the apparent color of the target increases in saturation and brightness, with little change in hue.3. When a target is presented on a background whose perceived hue is neither the same as nor complementary to that of the target on a neutral background, the apparent hue of the target moves away from the apparent hue of the surround , with little change in saturation and brightness.4. These contextual effects on hue, saturation and brightness are all diminished by increasing the apparent saturation of the chromatic surround.” (Lotto & purves, 2002)

And 2) the role of the physical world is just to provide empirical feedback regarding which perceptions and behaviors have been related reproductive success, we can understand why we see colours that are not there and we will keep being successful into an unknowable world.

References

Lotto R.B. & Purves D. (2002). The empirical basis of color perception. Consciousness and

Cognition 11: 609-629.

Lotto, R.B., Clarke, R., Corney, D. and Purves, D. (2011). Seeing in colour. Optics and

Laser Technology, 43(2), 261-269.

Purves D, Lotto RB and Polger T (2000). Color vision and the four-color-map problem. J

Cog Neurosci 12: 233-237.

Purves D, Lotto RB, Williams SM, Nundy S, and Yang Z (2001). Why we see things the

way we do: Evidence for a wholly empirical stategy of vision. Philos Trans R Soc Lond B,

356: 285-297.

Purves D., Wojtach W.T. and Lotto RB (2011) Understanding vision in wholly empirical

terms. Proc Natl Acad Sci, 108 (3): 15588-15595.

Purves, D., Monsona, B.B., Sundararajana, J. and Wojtachc W.T. (2014). How biological

vision succeeds in the physical world Proc. Natl. Acad. Sci. USA April 1, 111 (13) 4750-

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