Veronica Mazza

Attention selection, distractor suppression and N2pc.

N2pc is generally interpreted as the electrocortical correlate of the distractor-suppression mechanisms through which attention selection takes place in humans. Here, we present data that challenge this common N2pc interpretation. In Experiment 1, multiple distractors induced greater N2pc amplitudes even when they facilitated target identification, despite the suppression account of the N2pc predicted the contrary; in Experiment 2, spatial proximity between target and distractors did not affect the N2pc amplitude, despite resulting in more interference in response times; in Experiment 3, heterogeneous distractors delayed response times but did not elicit a greater N2pc relative to homogeneous distractors again in contrast with what would have predicted the suppression hypothesis. These results do not support the notion that the N2pc unequivocally mirrors distractor-suppression processes. We propose that the N2pc indexes mechanisms involved in identifying and localizing relevant stimuli in the scene through enhancement of their features and not suppression of distractors.

Attentional selection and identification of visual objects are reflected by distinct electrophysiological responses

Lateralised ERP responses were measured over posterior visual brain regions in response to visual search arrays that contained one colour singleton. In the localisation task, responses were determined by the visual hemifield where this singleton was presented. In the discrimination task, they were determined by the singletons’ shape. While an N2pc component was elicited in an identical fashion in both tasks, a subsequent sustained contralateral negativity was consistently present at posterior sites in the discrimination task only. This dissociation demonstrates that these two activations reflect distinct visual processing stages. We suggest that while the N2pc reflects the ability of the visual system both to identify and localise a relevant stimulus in the scene, the late sustained activity reflects the subsequent in-depth analysis and identification of these stimuli.

Temporal brain dynamics of multiple object processing: the flexibility of individuation.

The ability to process concurrently multiple visual objects is fundamental for a coherent perception of the world. A core component of this ability is the simultaneous individuation of multiple objects. Many studies have addressed the mechanism of object individuation but it remains unknown whether the visual system mandatorily individuates all relevant elements in the visual field, or whether object indexing depends on task demands. We used a neural measure of visual selection, the N2pc component, to evaluate the flexibility of multiple object individuation. In three ERP experiments, participants saw a variable number of target elements among homogenous distracters and performed either an enumeration task (Experiment 1) or a detection task, reporting whether at least one (Experiment 2) or a specified number of target elements (Experiment 3) was present. While in the enumeration task the N2pc response increased as a function of the number of targets, no such modulation was found in Experiment 2, indicating that individuation of multiple targets is not mandatory. However, a modulation of the N2pc similar to the enumeration task was visible in Experiment 3, further highlighting that object individuation is a flexible mechanism that binds indexes to object properties and locations as needed for further object processing.

An electrophysiological assessment of distractor suppression in visual search tasks

We investigated whether the N2pc is unequivocally linked to distractor-suppression mechanisms, as is commonly assumed. According to the distractor-suppression account of the N2pc, no suppression, and thus no N2pc, should occur when homogeneous distractors help in selecting the target, such as when the target feature is unpredictable. Participants performed a simple detection or a finer discrimination on a singleton target, which had either a variable or a constant color. Contrary to the distractor-suppression account, an N2pc was present for both the variable and the constant conditions, and for both tasks. Additionally, target feature consistency correlated with earlier N2pc onsets relative to variable blocks. Both results indicate that the N2pc is not unequivocally linked to distractor-suppression mechanisms, but may index mechanisms involved in identifying and localizing relevant stimuli through enhancement of their features.

Brain dynamics of attention and working memory engagement in subitizing

This study addressed the engagement of attention and working memory, as inferred from electrophysiological measurements, in the processing of small sets of objects. We recorded N2pc and CDA, two lateralized components of the EEG signal associated respectively with individuation and visual working memory, while participants enumerated a variable number (1-9) of uniquely colored targets among distractors. Behavioral results showed a clear subitizing effect, with lower error rates for smaller (1-3 targets) than larger sets. ERP results showed that both N2pc and CDA amplitudes increased as a function of target numerosity up to approximately three targets. However, individual differences in the enumeration efficiency were correlated only with the individual variation in the N2pc modulations. The results suggest that the constraints of the attentional individuation system play a significant role in the occurrence of the subitizing phenomenon.

Multiple object individuation and exact enumeration

Exact computation of numerosity requires the selective individuation of the elements to be enumerated so that each element is counted once and only once. Such a mechanism should operate not only when the elements to be enumerated are presented in isolation but also when they are presented in cluttered scenes. To uncover the electrophysiological correlates of the level of object representation necessary for exact enumeration, we examined ERP measures during the execution of a target enumeration task. A variable number (1–4) of lateralized targets were presented with or without distracters on the target side. An early nonlateralized response (N1, 120–180 msec) was modulated by target numerosity only when presented without distracters. By contrast, the amplitudes of a lateralized and later response (N2pc, 180–300 msec) increased as a function of target numerosity both with and without distracters, reaching a plateau at three targets. We propose that the stage of processing reflected in the N2pc corresponds to the component of individuation that binds specific indexes to properties and locations and that this provides the representation type necessary for exact enumeration.

Multiple object individuation and subitizing in enumeration: a view from electrophysiology

What are the processes involved in determining that there are exactly n objects in the visual field? The core level of representation for this process is based on a mechanism that iteratively individuates each of the set of relevant objects for exact enumeration. In support of this proposal, we review recent electrophysiological findings on enumeration-at-a-glance and consider three temporally distinct responses of the EEG signal that are modulated by object numerosity, and which have been associated respectively with perceptual modulation, attention selection, and working memory. We argue that the neural response associated with attention selection shows the hallmarks of an object individuation mechanism, including the property of simultaneous individuation of a limited number of objects thought to underlie the behavioral subitizing effect. The findings support the view that the core component of exact enumeration is an attention-based individuation mechanism that binds specific features to locations and provides a stable representation of a limited set of relevant objects. The resulting representation is made available for further cognitive operations for exact enumeration.

Multiple object individuation during numerical Stroop

To investigate whether semantic information relative to numbers is retained during multiple object individuation, we recorded N2pc--a neural measure of selection/individuation--during a numerical version of the Stroop task. Participants enumerated a varying number of target digits (1 to 4) whose identity was either congruent or incongruent with their numerosity. N2pc amplitudes increased as a function of target numerosity but were little affected by congruency, suggesting that multiple object individuation is a prenumeric stage that does not encode for the number magnitude of the numerosity of the individuated objects.

Perceptual Grouping and Visual Enumeration

We used lateralized Event-Related Potential (ERP) measures – the N2pc and CDA/SPCN components – to assess the role of grouping by target similarity during enumeration. Participants saw a variable number (0, 1, 2 or 3) of same- or differently-colored targets presented among homogeneous distracters, and performed an enumeration task. Results showed that the N2pc, but not the CDA, was larger for multiple targets of identical color relative to targets of different colors. The findings are interpreted in terms of the effects of grouping on early versus late stages of multiple object processing. Within this framework, they reveal that grouping has an effect on early individuation mechanisms, while later processing mechanisms are less prone to such an influence.

Cooperative and opposing effects of strategic and involuntary attention

To assess whether working memory contents can effectively bias visual selection even when they do not represent the current target in the attention task, we recorded the ERP activity from participants performing both a memory task and, in the retention period, a visual search task. In this task, a distracter matching the memory content could be presented on the same side (congruent trials) or on the opposite side (incongruent trials) relative to the target location (Experiment 1 and Experiment 2). On some trials, only the matching distracter (but no target) was presented (catch trials, Experiment 2). Results showed that the N2pc component was modulated by the presence and location of a matching distracter. We interpret these results as evidence that the involuntary control exerted by the irrelevant memory contents coexists with the strategic mechanism related to the search target, influencing attention selection with roughly equal power. In Experiment 3, we found that the modulation of the N2pc is not strictly related to the active maintenance of the memory-target features but can also be elicited by repetition priming. Overall, these findings suggest that, together with the physical properties of the stimuli presented in the visual field, irrelevant memory contents represent a powerful class of factors that lead to involuntary attentional control.