Global facilitation of attended features is obligatory and restricts divided attention

Abstract

In many common situations such as driving an automobile it is advantageous to attend concurrently to events at different locations (e.g., the car in front, the pedestrian to the side). While spatial attention can be divided effectively between separate locations, studies investigating attention to nonspatial features have often reported a "global effect", whereby items having the attended feature may be preferentially processed throughout the entire visual field. These findings suggest that spatial and feature-based attention may at times act in direct opposition: spatially divided foci of attention cannot be truly independent if feature attention is spatially global and thereby affects all foci equally. In two experiments, human observers attended concurrently to one of two overlapping fields of dots of different colors presented in both the left and right visual fields. When the same color or two different colors were attended on the two sides, deviant targets were detected accurately, and visual-cortical potentials elicited by attended dots were enhanced. However, when the attended color on one side matched the ignored color on the opposite side, attentional modulation of cortical potentials was abolished. This loss of feature selectivity could be attributed to enhanced processing of unattended items that shared the color of the attended items in the opposite field. Thus, while it is possible to attend to two different colors at the same time, this ability is fundamentally constrained by spatially global feature enhancement in early visual-cortical areas, which is obligatory and persists even when it explicitly conflicts with task demands.

Figure 1.

Schematic illustration of the time course of an individual trial. Stimulus size in visual angles is indicated by numbers and arrows that were not present in the experiment.

Figure 2.

Experiment 1. A, Stimulus display and schematic illustration of assignment of flicker frequencies to stimuli, together with SSVEP waveforms obtained from a single subject by moving window time-domain averages. All dots were in constant random incoherent motion, and each of the four fields of dots flickered at a specific frequency: blue left, 7.5 Hz; red left, 10 Hz; blue right, 12 Hz; red right, 8.6 Hz. B, Spline-interpolated isocontour voltage maps of SSVEP amplitudes averaged over all subjects and attentional conditions for each of the four frequencies. Electrodes used for the analysis are indicated by larger dots. C, D, Grand-average amplitude spectrum for attend-same (C) and attend-opposite conditions (D). SSVEP amplitudes at all four frequencies show clear enhancement with attention in attend-same conditions. This selectivity was absent in the attend-opposite conditions, where the attended color on one side was the same as the unattended color on the opposite side.

Figure 3.

Experiment 2. A, Stimulus display and schematic illustration of assignment of flicker frequencies to stimuli, with SSVEP waveforms obtained from a single subject by moving window averages. All dots were in constant random incoherent motion, and each of the four fields of dots flickered at a specific frequency: violet left, 7.5 Hz; orange left, 10 Hz; cyan right, 12 Hz; magenta right, 8.6 Hz. B, Spline-interpolated isocontour voltage maps of SSVEP amplitudes averaged over all subjects and attentional conditions for each of the four frequencies. Electrodes used for the analysis are indicated by larger dots. C, Grand-average amplitude spectrum for attend-different conditions in which either orange and magenta or violet and cyan were attended. D, Same as C, but for conditions in which orange and cyan or violet and magenta were attended. Amplitudes at all four frequencies show clear enhancement by attention (C, D). Note that in half of all trials, the assignment of colors to sides and frequencies was swapped (cyan left, 7.5 Hz; magenta left, 10 Hz; violet right, 12 Hz; orange right, 8.6 Hz). These conditions showed a corresponding pattern of results and are included in the statistical analyses.

Figure 4.

Normalized SSVEP amplitudes. Amplitudes were collapsed across frequencies after normalization for both experiments. Attend-same and attend-different conditions show clear attentional enhancement of SSVEP amplitudes, whereas attend-opposite conditions do not. Error bars represent the SEM.

Figure 5.

ERPs to targets and distractors. Difference waveforms formed by subtracting ERPs elicited by luminance decrement distractors (unattended) from those elicited by physically identical targets (attended), averaged over electrodes Pz and POz. Onsets of significant target-distractor differences are indicated by short vertical lines. Attentional modulation of the P3 component was the strongest and occurred earliest for attend-same conditions, and was latest and smallest for attend-opposite conditions. Attend-different conditions (Experiment 2) showed intermediate values. This pattern is consistent with behavioral performance as measured by sensitivity d′ and reaction time (Table 1).