Effects of Feature-selective and Spatial Attention at Different Stages of Visual Processing

Abstract

We investigated mechanisms of concurrent attentional selection of location and color using electrophysiological measures in human subjects. Two completely overlapping random dot kinematograms (RDKs) of two different colors were presented on either side of a central fixation cross. On each trial, participants attended one of these four RDKs, defined by its specific combination of color and location, in order to detect coherent motion targets. Sustained attentional selection while monitoring for targets was measured by means of steady-state visual evoked potentials (SSVEPs) elicited by the frequency-tagged RDKs. Attentional selection of transient targets and distractors was assessed by behavioral responses and by recording event-related potentials to these stimuli. Spatial attention and attention to color had independent and largely additive effects on the amplitudes of SSVEPs elicited in early visual areas. In contrast, behavioral false alarms and feature-selective modulation of P3 amplitudes to targets and distractors were limited to the attended location. These results suggest that feature-selective attention produces an early, global facilitation of stimuli having the attended feature throughout the visual field, whereas the discrimination of target events takes place at a later stage of processing that is only applied to stimuli at the attended position.

Attentional Selection of Feature Conjunctions Is Accomplished by Parallel and Independent Selection of Single Features

“…As in previous similar studies (Andersen et al, , 2011a, SSVEP amplitudes showed equivalent attention effects for the different frequencies. To make SSVEPs comparable across frequencies, SSVEP amplitudes were normalized to a mean of 1.0 for each frequency and subject by dividing amplitudes by the mean over all conditions (Andersen et al, 2011a). For each stimulus (i.e., frequency), conditions were labeled according to the attentional status of the driving stimulus.…”

“…This frequency tagging enabled SSVEPs to be recorded separately and concurrently to each of the four types of stimuli. The SSVEP is a continuous oscillatory response of the visual cortex with the same frequency as the driving stimulus and whose amplitude is enhanced by attention (Morgan et al, 1996;Di Russo et al, 2002;Andersen et al, 2011b).…”

“…For example, color selection would be less effective when selection is also based on another feature as opposed to when selection is based on color alone. Whereas previous studies have examined the neural correlates of attention to single features (Saenz et al, 2002;Andersen et al, 2012) and to conjunctions of features (Anllo-Vento and Hillyard, 1996;Treue and Martínez-Trujillo, 1999;Anllo-Vento et al, 2004;Lu and Itti, 2005;Andersen et al, 2008Andersen et al, , 2011aHayden and Gallant, 2009), none has directly compared the effectiveness of selection based on a single feature with selection based on the conjunction of that same feature when conjoined with another.…”

“…On each trial, they were cued to attend to a subset of the bars defined by either a single feature (red, blue, horizontal, or vertical) or by a feature conjunction (red horizontal, red vertical, blue horizontal, or blue vertical). Frequency-tagged steady-state visual evoked potentials (SSVEPs) elicited in early visual-cortical areas were recorded to concurrently assess the allocation of attention to each of the four superimposed fields of bars (Di Russo et al, 2002;Andersen et al, 2011b). If attentional selection of feature conjunctions is achieved through parallel and independent facilitation of the constituent features (Anllo-Vento et al, 2004;, then the attentional modulation of SSVEP amplitudes for feature conjunctions would be exactly the sum of the attentional modulations of the SSVEPs to the individual single features.…”

Behavioral performance follows the time course of neural facilitation and suppression during cued shifts of feature-selective attention

“…The SSVEP is an ongoing oscillatory response elicited by a flickering stimulus that indexes neural activity related to stimulus processing continuously. It has the same temporal frequency as the driving stimulus and, importantly, its amplitude is enhanced by both spatial (12,13) and feature-selective (14)(15)(16)(17) attention. This allows us to concurrently measure the allocation of processing resources to both the attended and the unattended stimulus.…”