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Feature-based and spatial attention may be based on similar mechanisms.

Spatial attention does not seem to affect the selectivity of visual neurons—just the vigour of their response.

Spatial visual attention increases the activity of neurons in the visual cortex whose receptive fields overlap the attended region.

Spatial and feature-based visual attention are additive: together, they particularly enhance the activity of any neuron whose receptive field encompasses the attended region, contains a stimulus with the attended feature, and prefers that feature.

Born et al. provided evidence which shows that preparing a saccade alone already enhances visual processing at the target of the saccade: discrimination targets presented before saccade onset were identified more successfully if they were in the location of the saccade target than when they were not.

Activation build-up in build-up neurons is modulated by spatial attention.

Spatial attention raises baseline activity in neurons whose RF are where the attention is even without a visual stimulus (in visual cortex).

Spatial attention can enhance the activity of SC neurons whose receptive fields overlap the attended region

Task-irrelevant cues in one modality can enhance reaction times in others—but they don't always do that. Instances of this effect have been implicated with exogenous attention.

Task-irrelevant auditory cues have been found to enhance reaction times in others. visual cues, however, which cued visual localization, did not cue auditory localization.

Fixating some point in space enhances spoken language understanding if the words come from that point in space. Fixating a visual stream showing lips consistent with the utterances, this effect is strongest, but it also works if the visual display is random. The effect is also enhanced if fixation is combined with some form of visual task which is complex enough.

Fixating at some point in space can impede language understanding if the utterance do not emanate from the focus of visual attention and there are auditory distractors which do.

Traditionally, visual attention is subdivided into feature-based attention and spatial attention. However, spatial is arguably only one cue out of possibly a number of cues and possibly only a special case.

Bertelson et al. did not find a shift of sound source localization due to manipulated endogenous visual spatial attention—localization was shifted only due to (the salience of) light flashes which would induce (automatic, mandatory) exogenous attention.

A localized visual stimulus can shorten the response to a target stimulus if it appears near and shortly after the first stimulus.

It can lengthen the response time if the target stimulus appears somewhere else or too late.

A localized visual stimulus can lengthen the response time to a target if the target stimulus appears somewhere too late after the first stimulus.

This is called `inhibition of return'.

Bell et al. make it sound like enhancement in SC neurons due to exogenous, visual, spatial attention is due to residual cue-related activity which is combined (non-linearly) with target-related activity.

If enhancement in SC neurons due to exogenous, visual, spatial attention is due to residual cue-related activity which is combined (non-linearly) with target-related activity, then that casts an interesting light on (the lack of) intra-modal enhancement:

The only difference between an intra-modal cue-stimulus combination and an intra-modal stimulus-stimulus combination lies in the temporal order of the two. Therefore, if two visual stimuli presented in the receptive field of an SC at the same time) neuron do not enhance the response to each other, then the reason can only be a matter of timing.

In an fMRI experiment, Fairhall and Macaluso found that attending (endogenously, spatially) to congruent audio-visual stimuli (moving lips and speech) produced greater activation in SC than either attending to non-congruent stimuli or not attending to congruent stimuli.

Visual spatial attention

  • lowers the stimulus detection threshold,
  • improves stimulus discrimination,

With two stimuli in the receptive field, one with features of a visual search target and one with different features

  • increases average neural activity in cortex compared to the same two objects without attending to any features
  • decreases average neural activity if spatial attention is on the location of the non-target compared to when it is on the target.

The fact that average neural activity in cortex is decreased if spatial attention is on the location of a non-target out of a target and a non-target compared to when it is on the target supports the notion that inhibition plays an important role in stimulus selection.

Desimone and Duncan argue that spatial information about a search target can be part of the attentional template fitted against all potential targets in the visual display as any other object feature.