Spatial and feature-based effects of exogenous cueing on visual motion processing

Similar Expectation Effects for Immediate and Delayed Stimulus Repetitions

A prior cue or stimulus allows prediction of the future occurrence of an event and therefore reduces the associated neural activity in several cortical areas. This phenomenon is labeled expectation suppression (ES) and has recently been shown to be independent of the generally observed effects of stimulus repetitions (repetition suppression, RS: reduced neuronal response after the repetition of a given stimulus). While it has been shown that attentional cueing is strongly affected by the length of the cue-target delay, we have no information on the temporal dynamics of expectation effects, as in most prior studies of ES the delay between the predictive cue and the target (i.e., the inter-stimulus interval, ISI) was in the range of a few hundred milliseconds. Hence, we presented participants with pairs of faces where the first face could be used to build expectations regarding the second one, in the sense that one gender indicated repetition of the same face while the other gender predicted the occurrence of novel faces. In addition, we presented the stimulus pairs with two different ISIs (0.5 s for Immediate and 1.75 or 3.75 s for Delayed ISIs). We found significant RS as well as a reduced response for correctly predicted when compared to surprising trials in the fusiform face area. Importantly, the effects of repetition and expectation were both independent of the length of the ISI period. This implies that Immediate and Delayed cue-target stimulus arrangements lead to similar expectation effects in the face sensitive-visual cortex.

In search of exogenous feature-based attention

Visual attention prioritizes the processing of sensory information at specific spatial locations (spatial attention; SA) or with specific feature values (feature-based attention; FBA). SA is well characterized in terms of behavior, brain activity, and temporal dynamics-for both top-down (endogenous) and bottom-up (exogenous) spatial orienting. FBA has been thoroughly studied in terms of top-down endogenous orienting, but much less is known about the potential of bottom-up exogenous influences of FBA. Here, in four experiments, we adapted a procedure used in two previous studies that reported exogenous FBA effects, with the goal of replicating and expanding on these findings, especially regarding its temporal dynamics. Unlike the two previous studies, we did not find significant effects of exogenous FBA. This was true (1) whether accuracy or RT was prioritized as the main measure, (2) with precues presented peripherally or centrally, (3) with cue-to-stimulus ISIs of varying durations, (4) with four or eight possible target locations, (5) at different meridians, (6) with either brief or long stimulus presentations, (7) and with either fixation contingent or noncontingent stimulus displays. In the last experiment, a postexperiment participant questionnaire indicated that only a small subset of participants, who mistakenly believed the irrelevant color of the precue indicated which stimulus was the target, exhibited benefits for valid exogenous FBA precues. Overall, we conclude that with the protocol used in the studies reporting exogenous FBA, the exogenous stimulus-driven influence of FBA is elusive at best, and that FBA is primarily a top-down, goal-driven process.

Attentional cueing by cross-modal congruency produces both facilitation and inhibition on short-term visual recognition

The attentional modulation of performance in a memory task, comparable to the one obtained in a perceptual task, is at the focus of contemporary research. We hypothesized that a biphasic effect (namely, facilitation followed by inhibition) can be obtained in visual working memory when attention is cued towards one item of the memorandum and participants must recognize a delayed probe as being identical to any item of the memorandum. In every trial, a delayed spiky/curvy probe appeared centrally, to be matched with the same-category shape maintained in visual working memory which could be either physically identical (positive trials) or only categorically similar (negative trials). To orient the participant's attention towards a selected portion of a two-item memorandum, a (tzk/wow) sound was played simultaneously with two lateral visual shapes (one spiky and one curved). Our results indicate that an exogenous attentional shift during perception of the memorandum, induced by a congruent audio-visual pairing, first facilitates and then inhibits the recognition of a cued item (but not of a non-cued item) stored in visual working memory. A coherent pattern of individual differences emerged, indicating that the amount of early facilitation in congruent-sound trials was negatively correlated with recognition sensitivity in no-sound trials (suggesting that the inverse effectiveness rule may also apply to memory) and positively correlated with later inhibition, as well as with the self-reported susceptibility to memory failures.

Reward- and attention-related biasing of sensory selection in visual cortex

Attention to task-relevant features leads to a biasing of sensory selection in extrastriate cortex. Features signaling reward seem to produce a similar bias, but how modulatory effects due to reward and attention relate to each other is largely unexplored. To address this issue, it is critical to separate top-down settings defining reward relevance from those defining attention. To this end, we used a visual search paradigm in which the target's definition (attention to color) was dissociated from reward relevance by delivering monetary reward on search frames where a certain task-irrelevant color was combined with the target-defining color to form the target object. We assessed the state of neural biasing for the attended and reward-relevant color by analyzing the neuromagnetic brain response to asynchronously presented irrelevant distractor probes drawn in the target-defining color, the reward-relevant color, and a completely irrelevant color as a reference. We observed that for the prospect of moderate rewards, the target-defining color but not the reward-relevant color produced a selective enhancement of the neuromagnetic response between 180 and 280 msec in ventral extrastriate visual cortex. Increasing reward prospect caused a delayed attenuation (220-250 msec) of the response to reward probes, which followed a prior (160-180 msec) response enhancement in dorsal ACC. Notably, shorter latency responses in dorsal ACC were associated with stronger attenuation in extrastriate visual cortex. Finally, an analysis of the brain response to the search frames revealed that the presence of the reward-relevant color in search distractors elicited an enhanced response that was abolished after increasing reward size. The present data together indicate that when top-down definitions of reward relevance and attention are separated, the behavioral significance of reward-associated features is still rapidly coded in higher-level cortex areas, thereby commanding effective top-down inhibitory control to counter a selection bias for those features in extrastriate visual cortex.

Real color captures attention and overrides spatial cues in grapheme-color synesthetes but not in controls

Grapheme-color synesthetes perceive color when reading letters or digits. We investigated oscillatory brain signals of synesthetes vs. controls using magnetoencephalography. Brain oscillations specifically in the alpha band (∼10Hz) have two interesting features: alpha has been linked to inhibitory processes and can act as a marker for attention. The possible role of reduced inhibition as an underlying cause of synesthesia, as well as the precise role of attention in synesthesia is widely discussed. To assess alpha power effects due to synesthesia, synesthetes as well as matched controls viewed synesthesia-inducing graphemes, colored control graphemes, and non-colored control graphemes while brain activity was recorded. Subjects had to report a color change at the end of each trial which allowed us to assess the strength of synesthesia in each synesthete. Since color (synesthetic or real) might allocate attention we also included an attentional cue in our paradigm which could direct covert attention. In controls the attentional cue always caused a lateralization of alpha power with a contralateral decrease and ipsilateral alpha increase over occipital sensors. In synesthetes, however, the influence of the cue was overruled by color: independent of the attentional cue, alpha power decreased contralateral to the color (synesthetic or real). This indicates that in synesthetes color guides attention. This was confirmed by reaction time effects due to color, i.e. faster RTs for the color side independent of the cue. Finally, the stronger the observed color dependent alpha lateralization, the stronger was the manifestation of synesthesia as measured by congruency effects of synesthetic colors on RTs. Behavioral and imaging results indicate that color induces a location-specific, automatic shift of attention towards color in synesthetes but not in controls. We hypothesize that this mechanism can facilitate coupling of grapheme and color during the development of synesthesia.

The neural basis of visual attention

Visual attention is the mechanism the nervous system uses to highlight specific locations, objects or features within the visual field. This can be accomplished by making an eye movement to bring the object onto the fovea (overt attention) or by increased processing of visual information in neurons representing more peripheral regions of the visual field (covert attention). This review will examine two aspects of visual attention: the changes in neural responses within visual cortices due to the allocation of covert attention; and the neural activity in higher cortical areas involved in guiding the allocation of attention. The first section will highlight processes that occur during visual spatial attention and feature-based attention in cortical visual areas and several related models that have recently been proposed to explain this activity. The second section will focus on the parietofrontal network thought to be involved in targeting eye movements and allocating covert attention. It will describe evidence that the lateral intraparietal area, frontal eye field and superior colliculus are involved in the guidance of visual attention, and describe the priority map model, which is thought to operate in at least several of these areas.