Voluntary attention enhances contrast appearance

Abstract

Voluntary (endogenous, sustained) covert spatial attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. We asked whether voluntary attention affects the subjective appearance of contrast--a fundamental dimension of visual perception. We used a demanding rapid serial visual presentation (RSVP) task to direct endogenous attention to a given location and measured perceived contrast at the attended and unattended locations. Attention increased perceived contrast of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task at the cued location. We ruled out response bias as an alternative account of the pattern of results. Thus, this study establishes that voluntary attention enhances perceived contrast. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.

Figure 1

Schematic depiction of the trial sequence in the main and control condition. For ease of illustration, the cues are depicted as a brightening of the horizontal arm of the fixation cross, whereas in the actual experiments, they were presented as a thickening of the horizontal arm. Also for illustrative purposes, the tilt of the Gabors is exaggerated.

Figure 2

(A) Detection thresholds for individual observers and group average (last column). Error bars are s.e.m. across six measurements for individual thresholds, and across 9 observers for the average threshold. (B) RSVP performance in d' units combined for the main and control condition. Error bars are within-subject standard errors calculated using the method of (Loftus & Masson, 1994).

Figure 3

(A) Appearance psychometric functions for the main condition, plotting the proportion of trials on which observers chose the test stimulus to be of higher contrast as a function of its physical contrast. (B) Point of subject equality (PSE) values for the three cue types in the main condition. Error bars are s.e.m. calculated as in Fig. 2B. (C) Scatter plots of individual observer's PSEs, plotting the test cued PSE (circles) and standard cued PSE (cross) against the neutral PSE. (D-F) Same data for the control condition.

Figure 4

Orientation discrimination performance for the main (A) and control (B) condition. Error bars are s.e.m. calculated as in Fig. 2B. The percentage of trials when observers selected the standard stimulus for cued, uncued (i.e., when the test stimulus was cued), or neutral (neither stimulus was cued) condition are shown at the bottom.