Attention alters appearance

Abstract

Does attention alter appearance? This critical issue, debated for over a century, remains unsettled. From psychophysical evidence that covert attention affects early vision-it enhances contrast sensitivity and spatial resolution-and from neurophysiological evidence that attention increases the neuronal contrast sensitivity (contrast gain), one could infer that attention changes stimulus appearance. Surprisingly, few studies have directly investigated this issue. Here we developed a psychophysical method to directly assess the phenomenological correlates of attention in humans. We show that attention alters appearance; it boosts the apparent stimulus contrast. These behavioral results are consistent with neurophysiological findings suggesting that attention changes the strength of a stimulus by increasing its 'effective contrast' or salience.

Figure 1

Sequence of events in a single trial. (a) Each trial began with a fixation point followed by a brief neutral or peripheral cue. The peripheral cue had equal probability of appearing on the left or right hand side, and was not predictive of the stimulus contrast or orientation. The timing of this sequence maximized the effect of transient attention and precluded eye movements. (b) Observers performed a two-by-two forced choice task: they were asked to indicate the orientation (left versus right) for the stimulus that appeared higher in contrast. In this trial, they would report the orientation for the stimulus on the right.

Figure 2

Appearance psychometric functions for experiment 1 (low contrast). (a) Percentage of responses in which observers reported the contrast of the test patch as higher than the standard, plotted as a function of the test patch’s physical contrast. Data are shown for the neutral and peripheral conditions (test cued & standard cued). The standard was 6% contrast. The horizontal line intersecting both curves indicates the contrasts necessary for the test and standard stimuli to attain subjective equality (50%). (b) Psychometric functions for control experiment 1. When transient attention is extinguished via a longer timing interval, there are no differences between when the test is cued and the neutral cue. The standard was 8% contrast. Error bars correspond to the average ± standard error (s.e.) for each condition.

Figure 3

Attentional effects on apparent contrast for individual observers. The PSEs for the peripheral condition (when either the standard or test was cued) versus the PSEs for the neutral condition (central cue). A slope of 1 (dashed line) represents similar PSEs for neutral and peripheral conditions; that is, no effect of attention on apparent contrast. However, when the test is cued, PSE’s are consistently lower, and when the standard is cued PSE’s are consistently higher, for both low-contrast (a) and high-contrast (b) stimuli.

Figure 4

Effect of covert attention on apparent contrast. (a) If you were looking at one of the four fixation points (black dot), and the grating to the left of that fixation point that was cued, the stimuli at both sides of fixation would appear to have the same contrast. With attention, a subthreshold, 3.5% test contrast stimulus appears as if it were at threshold (~6% contrast). Similarly, a cued 6% contrast standard at threshold appears as if it were a more clearly discriminable 8.5% contrast stimulus. (b) Likewise, with high-contrast stimuli when a 16% contrast grating is peripherally cued, it appears as if it were 22% contrast, and a cued 22% contrast grating appears as if it were 28%.

Figure 5

Appearance psychometric functions for experiment 2 (high contrast). (a) Percentage of responses in which observers reported the contrast of the test patch as higher than the standard, plotted as a function of the test patch’s physical contrast. Data are shown for the neutral and peripheral conditions (test cued & standard cued). The standard was 22% contrast. The horizontal line intersecting both fits indicates the contrasts necessary for the test and standard stimuli to attain subjective equality (50%). (b) Psychometric functions for control experiment 2. When transient attention is extinguished via a longer timing interval, there are no differences between when the test is cued and the neutral cue. Error bars correspond to the mean ± 1 s.e. for each condition.

Figure 6

Appearance psychometric functions for when observers report the lower contrast grating (control experiment 3). Percentage of responses in which observers reported the contrast of the test patch as lower than the standard, plotted as a function of the test patch’s physical contrast. Data shown for the neutral and peripheral conditions (test cued & standrd cued). If observers’ responses resulted from cue bias, observers would have still chosen the cued stimulus a higher proportion of times. However, observers were less likely to choose the cued stimulus as appearing lower in contrast. These results confirm that attention increases the apparent contrast of a cued stimulus.