Modulating attentional load affects numerosity estimation: evidence against a pre-attentive subitizing mechanism

Traditionally, the visual enumeration of a small number of items (1 to about 4), referred to as subitizing, has been thought of as a parallel and pre-attentive process and functionally different from the serial attentive enumeration of larger numerosities. We tested this hypothesis by employing a dual task paradigm that systematically manipulated the attentional resources available to an enumeration task. Enumeration accuracy for small numerosities was severely decreased as more attentional resources were taken away from the numerical task, challenging the traditionally held notion of subitizing as a pre-attentive, capacity-independent process. Judgement of larger numerosities was also affected by dual task conditions and attentional load. These results challenge the proposal that small numerosities are enumerated by a mechanism separate from large numerosities and support the idea of a single, attention-demanding enumeration mechanism

Salience-based selection: attentional capture by distractors less salient than the target

Current accounts of attentional capture predict the most salient stimulus to be invariably selected first. However, existing salience and visual search models assume noise in the map computation or selection process. Consequently, they predict the first selection to be stochastically dependent on salience, implying that attention could even be captured first by the second most salient (instead of the most salient) stimulus in the field. Yet, capture by less salient distractors has not been reported and salience-based selection accounts claim that the distractor has to be more salient in order to capture attention. We tested this prediction using an empirical and modeling approach of the visual search distractor paradigm. For the empirical part, we manipulated salience of target and distractor parametrically and measured reaction time interference when a distractor was present compared to absent. Reaction time interference was strongly correlated with distractor salience relative to the target. Moreover, even distractors less salient than the target captured attention, as measured by reaction time interference and oculomotor capture. In the modeling part, we simulated first selection in the distractor paradigm using behavioral measures of salience and considering the time course of selection including noise. We were able to replicate the result pattern we obtained in the empirical part. We conclude that each salience value follows a specific selection time distribution and attentional capture occurs when the selection time distributions of target and distractor overlap. Hence, selection is stochastic in nature and attentional capture occurs with a certain probability depending on relative salience

An analysis of the time course of attention in preview search.

We used a probe dot procedure to examine the time course of attention in preview search (Watson and Humphreys, 1997). Participants searched for an outline red vertical bar among other new red horizontal bars and old green vertical bars, superimposed on a blue background grid. Following the reaction time response for search, the participants had to decide whether a probe dot had briefly been presented. Previews appeared for 1,000 msec and were immediately followed by search displays. In Experiment 1, we demonstrated a standard preview benefit relative to a conjunction search baseline. In Experiment 2, search was combined with the probe task. Probes were more difficult to detect when they were presented 1,200 msec, relative to 800 msec, after the preview, but at both intervals detection of probes at the locations of old distractors was harder than detection on new distractors or at neutral locations. Experiment 3A demonstrated that there was no difference in the detection of probes at old, neutral, and new locations when probe detection was the primary task and there was also no difference when all of the shapes appeared simultaneously in conjunction search (Experiment 3B). In a final experiment (Experiment 4), we demonstrated that detection on old items was facilitated (relative to neutral locations and probes at the locations of new distractors) when the probes appeared 200 msec after previews, whereas there was worse detection on old items when the probes followed 800 msec after previews. We discuss the results in terms of visual marking and attention capture processes in visual search