As suggested above, claim (3’) may be unnecessarily ambitious. Maybe one can carve out a smaller region of the HSB space by narrowing down the values represented along only a subset of HSB, and maybe that is sufficient for a color experience to have narrower range contents. Thus, one might replace (3’) with (3’’):

(3’’) Attentive color experiences represent narrower intervals of values, along some of H, S or B, than inattentive color experiences.

The lack of attentional effects on represented hue values is not a problem for (3’’). This claim can still turn out true if either (or both) of the following are true:

One could characterize color range contents in this more modest way, regardless of their views on the relation between hue, saturation and brightness. The modest strategy is compatible with the proposal that hue is the primary dimension of color content. For example, one could argue that the phenomenology of hue is determined by represented values of saturation and brightness, maybe in this sense: narrower saturation and brightness ranges make hues more easily discriminable in our experience, even if the represented hue ranges remain unchanged.Footnote 24 On the face of it, this proposal seems supported by the observed effects of attention on perceived saturation (Fuller and Carrasco 2006; see also the discussion below). Increasing the saturation of a color patch plausibly facilitates discriminating its hue, while desaturating it makes discrimination harder (imagine desaturating a hot pink patch to make it grayish pinkFootnote 25).

However, this strategy does not entail that the only roles for saturation and brightness in modulating the determinacy of color content are tied to hue discrimination. Think of our experiences of different shades of gray, which can be more or less determinate. Increasing the saturation of a gray color patch (for example, with attention) might enable us to see it as smoke gray rather than as the more indeterminate “light gray”. But it is not obvious that this phenomenal switch from light gray to smoke gray also improves our ability to pick the exact hue this shade belongs to (that is, its angle in the 360° hue space). So at least in gray perception, increases in determinacy need not be accompanied by better hue discriminability.

Thus, the modest strategy is compatible with different positions about the relations between hue, saturation and brightness and the determinacy of color content. But is the strategy sound? Are claims (3’ii) or (3’iii) supported by our current evidence about the effects of attention on visual perception? To assess this, we will now take a closer look at the studies we briefly introduced in §2 (Fuller and Carrasco 2006; Kim et al. 2014; Tsal et al. 1994; Tse 2005; see also Carrasco et al. 2004).

Let us start with (3’ii), the claim that attentive experiences represent narrower ranges of saturation values. Fuller and Carrasco (2006) presented participants with red, green, and blue tilted ovals, which varied in saturation while having constant hue and luminance. Participants fixated their gaze on a cross in the middle of a computer screen and were given either a neutral (central, on the fixation) or a peripheral cue. After another short fixation, two ovals appeared on either side of the screen: a standard one, with saturation in the middle of the saturation range, and a test one, with changing saturation. The two ovals were tilted in different directions. Participants must report the orientation of the more colorful oval (the one that looked “redder”, “greener” or “bluer”). Participants consistently reported the orientation of the attended oval, which indicates that it was perceived as being more colorful than the unattended one. Even if the attended oval was lower in saturation than the unattended one, the two seemed indistinguishable. These results were replicated in a later study with ADHD subjects, which further supports their robustness (Kim et al. 2014).

To be sure, it is plausible that these increases in perceived saturation underlie an increase in the determinacy of color perception, as per claim (1). However, this might not extend to the conceptualization of determinacy in terms of range contents proposed in (2), the claim that attentive color experiences have narrower range contents than inattentive color experiences, and its derived claims (3) and (3’). To see why, consider Fig. 4.

A simplified visualization of the difference between changes in saturation and changes in range contents. Increasing saturation of a color could only move the same range of contents along the scale, without narrowing it down

The figure illustrates that increasing perceived saturation and narrowing down the range of saturation values represented are conceptually distinct. Increasing perceived saturation does not need to change the range of values represented. It could just move the range towards higher values in the scale. Think again of the blue car. Covertly directing our attention to it might increase its perceived saturation, resulting in a more saturated blue percept. But if we perceived the car as having, for example, 10 possible shades of blue, perceiving the car as more saturated need not narrow the range to, say, 3 possible shades. The range could be still 10 shades wide; but now, the shades are all a bit more saturated.

It is possible that attention does both things, that is, narrowing down the range of attributed saturation values, and shifting this range towards the more colorful end of the scale. But at this point our evidence is inconclusive. Hence, claim (3’ii) remains unsupported. The remaining possibility is now (3’iii), the claim that attention narrows down the ranges of values represented along the brightness dimension.

The brightness dimension (also called lightness, brightness, or value, with slightly different meanings) consists in the property of color related to luminance – that is, how light the color appears (as in light blue versus dark blue). Two studies by Tse (2005) and Tsal and colleagues (1994) explore the effects of attention on brightness perception.

Tse (2005) observed that covert attention to one of three overlapping gray disks, presented on a white background, makes the disk appear darker. The effect is as if this disk was popping out to a foreground plane, while the unattended disks seem to recede into a background plane. Although the effect disappears with a black background, it reappears in an inverse luminance version of the display. On this version, the attended disk looks brighter than the unattended ones. See Fig. 5 for illustration.

Adapted from Tse (2005)

Attentional modulations of brightness. Left: Attended disc appears darker. Middle: This effect is not present with a dark background. Right: With dark background and reversed luminance, the attended disc appears brighter.

Tsal and colleagues (1994) further explored the interactions between the attentional effects on perceived brightness and stimulus background. They presented participants with gray squares on a white background and asked them to report the square’s brightness, by matching it to one of four squares, with Square 1 being the lighter one and Square 4 being the darker one. Attention was directed to either side of the display with endogenous and exogenous cues. They found that participants made more “dark” errors (i.e., reporting Square 2 as Square 3) for unattended squares (that is, squares from which attention was diverted away with an invalid cue). For attended squares, in turn, the proportion of “light” errors (i.e., reporting Square 2 as Square 1) was higher. These results suggested that participants were perceiving unattended squares as darker and attended squares as brighter.

Tsal and colleagues then conducted a follow-up study where they assessed whether attention was in fact “brightening” the stimuli or rather doing something else, like reducing the contrast between stimulus and background. To test this, they presented participants with the same set of squares, but this time on a black background. This condition had the opposite pattern of responses: participants made more “light” errors for unattended squares (which suggests that unattended squares were perceived as brighter) and made more “dark” errors for attended squares (which suggests that attended squares were perceived as darker). Due to these findings, the experimenters concluded that attention affects perceived brightness indirectly, that is, by removing processing resources from the background and concentrating them on the stimulus, so that the effect of the background was lessened (for example, a white background makes stimuli look darker, but if the effect of the background is lessened, the stimulus will look brighter).

Although the studies by Tse on the one hand and Tsal and colleagues on the other hand point in opposite directions, the common finding to the two of them is that attention modulates brightness perception.Footnote 26 Do these modulations support the view that attentive color experiences represent narrower ranges of values along the brightness dimension?

The short answer is, again: No, for familiar reasons. Evidence that attention shifts perceived brightness to the lighter or darker ends of the scale need not be evidence that a narrower range of brightness values is represented – although, as in the case of saturation, these shifts could still contribute to the representation of a narrower range of color values, in some other way.

Regarding the latter point, note that brightness is unlike saturation in one important aspect: whereas increased saturation may plausibly contribute to increasing the determinacy of color perception, increased brightness does not always do so. There is indeed an ideal level of brightness in which we are able to discern the greatest number of possible shades of color. However, that level is not towards the higher values on the brightness axis (see Fig. 3 again), but rather towards its middle portion. As an example, when we display a colored picture on the computer and increase or decrease the brightness of the display, at some point we become able to easily discriminate, for instance, orange from red. When we decrease the brightness substantially, it becomes impossible to discriminate between some colors. But the same would happen if we could increase the brightness of the display above the maximum brightness (which our displays do not allow). Too much brightness blurs the distinctions between colors. Some shades, for example of yellow, would be impossible to discriminate. Hence, if we want to say that attention increases determinacy by increasing brightness, we must show that attention helps shifting the values of perceived brightness closer to the ideal level where one value is more discriminable from another.Footnote 27

To be sure, the studies we just cited could indeed provide evidence that attention works in this way: depending on contextual conditions, like background color, attention might increase or decrease perceived brightness, which might in turn contribute to the representation of more determinate contents, including color contents. Still, as before, there is a conceptual gap between this kind of shift and a change in the width of the range of values represented.

If the considerations we offer in this section are correct, our current evidence on the effects of attention on perceived saturation and brightness does not unequivocally support the view that attentive experiences have narrower range contents along any of these dimensions. Hence, claims (3’ii) and (3’iii) remain unsupported: our current evidence does not make clear whether attentive color experiences represent narrower ranges of saturation or brightness values. Consequently, (3’’) also remains unsupported: it is not clear whether attentive color experiences represent narrower value ranges along a subset of the HSB space. Hence, the modest strategy also rests on shaky grounds.

To recap, the modest and the ambitious strategies are ways of defending (3), the claim that attentive color experiences carve out a smaller region of the three-dimensional HSB space. Since none of these two strategies is empirically well-supported, claim (3) remains unsupported as well: for all we know, attentive color experiences might not carve out a smaller region of the HSB space.

Now, recall that claim (3) was proposed as a plausible specification of (2), the claim that attentive color experiences have narrower range contents than inattentive color experiences. If carving out a smaller region of the HSB space is the best way (or the only way) to understand what it is to have narrower range contents along the color dimension, then claim (2) also remains unsupported: for all we know, attentive color experiences might not have narrower range contents along the color dimension.

Finally, recall that claim (2) is proposed as a plausible specification of (1), the claim that attentive color experiences have more determinate color contents. If having narrower range contents along the color dimension is the best way (or the only way) to understand what it is to have more determinate color contents, then claim (1) remains unsupported as well: for all we know, attentive color experiences might not have more determinate color contents than inattentive color experiences.