Cross-modal cueing of attention alters appearance and early cortical processing of visual stimuli

Abstract

The question of whether attention makes sensory impressions appear more intense has been a matter of debate for over a century. Recent psychophysical studies have reported that attention increases apparent contrast of visual stimuli, but the issue continues to be debated. We obtained converging neurophysiological evidence from human observers as they judged the relative contrast of visual stimuli presented to the left and right visual fields following a lateralized auditory cue. Cross-modal cueing of attention boosted the apparent contrast of the visual target in association with an enlarged neural response in the contralateral visual cortex that began within 100 ms after target onset. The magnitude of the enhanced neural response was positively correlated with perceptual reports of the cued target being higher in contrast. The results suggest that attention increases the perceived contrast of visual stimuli by boosting early sensory processing in the visual cortex.

Fig. 1.

Experimental procedure and behavioral results. (A) Illustration of a target display on an equal-contrast trial. The auditory cue was presented with equal probability from the left or right loudspeaker. The left-right positions of the standard and test patches also varied at random from trial to trial. (B) Probability of reporting the contrast of the test patch to be higher than that of the standard patch, averaged over all participants and plotted as a function of test-patch contrast. The probabilities are depicted for cued-test and cued-standard trials separately. The standard-patch contrast was fixed at 22%.

Fig. 2.

Grand-average ERP waveforms to equal-contrast targets. ERPs at occipital sites (PO7/PO8) were collapsed over left- and right-cue conditions and left and right hemispheres to obtain waveforms recorded ipsilaterally and contralaterally to the side of the cue. Statistically significant (P < 0.05) differences between contralateral and ipsilateral waveforms are denoted in red on the time axis. (A) Enlarged ERP positivity contralateral to the cued target was found when observers reported the cued target as being higher in contrast than the uncued target. (B) No significant differences between ipsilateral and contralateral ERP waveforms were found when observers reported the uncued target as being higher in contrast than the cued target.

Fig. 3.

Correlations between individual participants' tendencies to report the cued-side target to be higher in contrast and the magnitude of the enlarged contralateral ERP positivities recorded at occipital electrode sites (PO7/PO8, PO3/PO4) at different time intervals (120–140 ms and 180–200 ms). The tendency to report the cued-side target as being higher in contrast (x axis) is indexed by the difference between the probability of choosing the cued patch minus the probability of choosing the uncued patch on equal-contrast trials. The magnitude of the enhanced positivity (y axis) was calculated as the mean contralateral minus ipsilateral amplitude difference in the indicated time windows averaged over all equal-contrast trials for each subject.

Fig. 4.

Topographical distributions and estimated neural sources of the enlarged contralateral ERP positivities in the time interval of the P1 (120–140 ms) and N1 (180–200 ms) components. (A) Scalp topographies of the equal-contrast target ERP waveforms recorded contralaterally and ipsilaterally to the cued side. The ERP data were collapsed over cued side (left, right) and recording hemisphere (left, right) to show ipsilateral and contralateral ERP distributions on the left and right sides of the maps, respectively. (B) Topographical maps of the contralateral-ipsilateral difference waveforms, projected on the right side of the scalp (see Methods for details). (C) Localization of distributed cortical current sources underlying the contralateral minus ipsilateral ERP positivity, estimated by the LAURA algorithm. View is of the ventral cortical surface.

Fig. 5.

Enlarged P1 positivity to high-contrast test patch. (A) Grand-averaged ERP waveforms to visual displays containing a high-contrast (78%) or low-contrast (6%) test patch, recorded occipitally, contralateral to the side of the test patch (PO7/PO8). The waveforms were collapsed over cue location and recording hemisphere. Gray box denotes time interval for analysis (110–130 ms). (B) Scalp topography of the high minus low voltage difference, calculated by subtracting the ERPs elicited by displays containing a low-contrast test patch from the ERPs elicited by displays containing a high-contrast test patch. The ERP data were collapsed over test-patch side and recording hemisphere to show the voltage distributions ipsilateral and contralateral to the test flash on the left and right sides of the maps, respectively. (C) LAURA estimations of the current sources underlying the high minus low difference waveforms, illustrated on the ventral cortical surface.