Neural measures of individual differences in selecting and tracking multiple moving objects

Abstract

Attention can be divided so that multiple objects can be tracked simultaneously as they move among distractors. Although attentional tracking is known to be highly limited, such that most individuals can track only approximately four objects simultaneously, the neurophysiological mechanisms that underlie this capacity limitation have not been established. Here, we provide electrophysiological measures in humans of the initial selection and sustained attention processes that facilitate attentional tracking. Each measure was modulated by the number of objects the subject was tracking and was highly sensitive to each individual's specific tracking capacity. Consequently, these measures provide strong neurophysiological predictors of an individual's attentional tracking capacity. Moreover, by manipulating the difficulty of these two phases of the task, we observe that the limiting factor underlying tracking capacity can flexibly shift between these two attentional mechanisms depending on the requirements of the task.

Figure 1.

A, ERP multiple-object tracking task. Participants tracked either red or green boxes while maintaining central fixation. In each experiment, the number of total objects (including distractors) was held constant while the number of target boxes varied across trials. B, ERP difference waves (contralateral − ipsilateral) for experiment 1 from the average of posterior electrode sites (PO3/PO4; P3/P4; OL/OR; T5/T6). Negative voltage is plotted upward. Note that all ERP waveforms in this and subsequent figures reflect correct trial performance. C, Mean amplitude during the selection (200–300 ms) and tracking periods (800–1200 ms) as a function of the number of target items.

Figure 2.

A, Contralateral and ipsilateral activity in response to the three tracking loads in experiment 1 across all frontal, parietal, and occipital electrodes. Waveforms were time locked to the initial appearance of targets, and motion began at 500 ms. B, Grand-averaged horizontal EOG waveforms for attend-left and attend-right trials.

Figure 3.

A, Behavioral performance in experiment 2 showing significant main effects of both area and number of items. B, Mean amplitude of CDA activity in experiment 2. Although there was a significant main effect of number of targets, area had no significant effect on amplitude of either the N2pc or the CDA.

Figure 4.

A, B, ERP difference waves for correct trials in experiment 3 divided between high-capacity (A) and low-capacity (B) individuals on the basis of a median split of tracking performance. C, D, Mean amplitude (in microvolts) of the N2pc (C) and the CDA (D) for the high- and low-capacity groups across the three target array sizes.

Figure 5.

A, B, ERP difference waves for high- and low-capacity subjects in experiment 4. C, D, Mean amplitudes of the N2pc and CDA waves across high- and low-capacity groups. There was a significant interaction between group (high/low) and number of objects for both waves (p < 0.01). E, F, Correlation between an individual's tracking capacity and the difference in amplitude (in microvolts) between one and three objects for the N2pc and the CDA. Note that tracking capacity in our single-hemifield experiments was generally two to three items: lower than most previous tracking capacity estimates, but consistent with the demonstration by Alvarez and Cavanagh (2005) of lower capacity estimates when tracking items in a single hemifield.

Figure 6.

A, B, Correlations between an individual's whole-field tracking capacity and the rise in amplitude from one to three targets for the N2pc (A) and the CDA (B). Tracking capacity was estimated by averaging behavioral performance across all set sizes (3, 4, and 5).