The one reason which makes attention multifaceted in its own right is due to its main features common to various visual systems. The problem however is the advent of excessive information which requires hardcore processing. The solution then becomes even complex at instances when there is a genuine need for selection and modulation of information based on that which is most appropriate for a given visual action. Then again, the challenge is to constantly aid processing of what is most important and excluding that which is not, all this of course while sustaining alertness towards a goal-directed action. With features such as these complementing several types of visual attention, it would be ideal to categorize them individually according to the kind of information arriving from their targets.
There are two major types of information to which we are all constantly bombarded which requires selection. On one hand, the information that comes to an observers mind from various sensory modalities with intermittent time and location specific labels requiring selection and modulation is referred to as "external attention" (e.g. conversations happening around an observer). On the other hand, information which is mentally created (ongoing memory processes within the long-term and working memory, active cognitive supervision, and choice of responses) requiring selection and modulation is referred to as "internal attention" (e.g. identifying the familiarity of those talking). By having to concentrate on one specific source of input so much so that the other input is subjected to exclusion, this type of attention is referred to as "focused attention" (e.g. reading a piece of text while ignoring the vocal news bulletin over the radio). Suppose more than one source is attended at any one given time, the information which eventually gets selected is somewhat incomplete, the term referred to as "divided attention" (e.g. while following both the newspaper and the radio bulletin). The loss of information in a divided state of attention is due to the two information sources competing for common attention resources. Given the limited capacity of resources available, task(s) demanding more than what is available eventually leads to task limitations and failures. Attention process which are undertaken voluntary by nature and that which is also top-down (i.e. derived from previously acquired knowledge) is referred to as "endogenous attention". In fact, process where the visual attention system had innately captured the stimuli, generated external to an observer, in a bottom-up fashion is referred to as "exogenous attention". Some of these mentioned attentional types are also known to be further subdivided as spatial, temporal and object forms of attention deployed over both space and time. Spatial attention can be either feature or object based. Feature based attention is one which gets deployed covertly towards explicit target features (e.g. orientation, colour, etc)
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independently of their location, while object-based attention is when attention gets directed by a specific target configuration. By having to focus on a diverse set of visual information at various time points, these attention types optimize our visual system in a way the observer is able to subjectively differentiate a target stimulus from distractors (Carrasco, 2011).
The study into spatial attention was first carried out by Posner where the focus of spatial attention was described as an internal spotlight (Posner, 1980) or zoom lens (Eriksen & Yeh, 1985). This was known to intensify the visual processing of a given stimulus arriving from a limited area or spread across space, the former generally yielding an enhanced effect (Jans, Peters, & De-Weerd, 2010). Given the large volume of incoming spatial visual information from ones surroundings, spatial attention is responsible for prioritizing these "information packets" so as to guide and deploy relevant eye movements (foveal acuity) needed for perception (Rayner, 2009). Eye movements (saccades) are therefore closely coupled with spatial attention, despite the former being second to a process more complex as attention filtering. Both spatial attention and saccades functions either dependently or independently from one another, in that, spatial attention can either guide saccades in an overt manner (voluntary), or it can attend to a given location in the absence of saccades in a covert manner (involuntary). Using specific task related exogenous and endogenous cues studies have been able to modulate both overt and covert attention, thereby improving the detection and discrimination of targets from surrounding distractors (e.g. Posner, 1980; Yantis et al., 2002). The voluntary orienting of attention is achieved via a goal-directed fashion, whilst involuntary orienting occurs predominantly in a stimulus driven fashion. The temporal characteristics of the cue have been further attributed to two other forms of spatial attention, namely transient and sustained attention. A cue when presented 100 – 120 msec prior to the stimulus array (containing the target and distractors) facilitated a greater degree of involuntary attention resulting in a transient effect (i.e. known for operating over shorter durations with rapid decay, e.g. Ling & Carrasco, 2006). In contrast, a cue presented 300 msec prior to the stimulus array facilitated ample time for voluntary deployment of attention resulting in a sustained effect (i.e. known for operating over longer durations with smaller intensity, e.g. Liu, Stevens, & Carrasco, 2007). In this way, spatial attention assists in processing of incoming stimuli from attended locations while inhibiting distractors from surrounding locations. Immediately after focusing attention to a cued location, reorienting or shifting of attention thereby ensures that accurate and precise deployment of attention is carried out to a new location in space.
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Studies into visual temporal attention were demonstrated in studies employing attentional blink paradigm, with its focus concentrated on stimuli arriving at various time intervals within the same location (Jolicoeur, Sessa, Dell’Acqua, & Robitaille, 2006). Although temporal attention experience the same features as that of spatial attention, unlike the latter, temporal attention is untroubled by the impedance caused by dual tasks (Correa & Nobre, 2008). An important point to note here is that the temporal attention is restricted based on the quantity of incoming visual stimuli both in time and space (despite being fully attended). However, the way in which temporal attention conquers this constraint is by selecting task appropriate information resulting in a slowed processing rate. Studies of this type require individuals to search, retain and report one or two targets in the midst of several other distractors presented in quick succession at one common location. The ability to retain and report targets is reported to be severely inhibited in the second stage when searching for more than two targets (Broadbent & Broadbent, 1987; Shapiro, Driver, Ward, & Sorensen, 1997). The higher performance in the first stage strengthens representations of the previously seen targets into awareness required for the second stage, the process which was aided by the working memory involving the fronto-parieto- temporal pathway (Dehaene, Sergent, & Changeux, 2003).
In addition to spatial locations and various points in time, visual attention is also capable of being directed exogenously towards object features (modality specific, e.g. colour, pitch, orientation, etc). One other mechanism of attention operation in addition to that mentioned above is via feature saliency, i.e. how one given item (usually the target) is made more conspicuous than the other (distractor) defined by the extent of target- distractor similarity. The modulation towards attention of object features occurs in groups of neurons located at feature-dependent regions of the cortex (Kanwisher & Wojciulik, 2000). For example, changes in stimulus orientation (e.g. Gabor patches) during selective attention sparks a simultaneous increase in orientation processing, which goes on to further influence the contrast sensitivity in the layer four of the visual area (V4 in the extrastriate cortex, an area known for the tuning properties of orientation - McAdams & Maunsell, 1999). Further orientation processing is subsequently carried out within the visual cortex (Liu et al., 2007) and visual area V5/MT (middle temporal, a region within the extrastriate visual cortex), the latter known for its major function in motion processing during feature attention (O’Craven et al., 1997). Feature based attention on more complex stimuli (e.g. faces and objects/locations) have been observed producing significantly greater activation patterns within face (face fusiform area, FFA - within the ventral occipito-temporal cortex) and object/location-specific (para-hippocampal place area, PPA - ventromedial cortex) regions of the visual cortex (Kanwisher, McDermott, & Chun, 1997;
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Haxby et al., 1999). Unlike spatial attention, feature based attention is spatially unlimited such that features belonging to various stimuli outside the attended location undergoes significant enhancement (Martinez-Trujillo & Treue, 2004). The neural pathway governing both feature and spatial selection entirely rests on the frontal-parietal network where attention is believed to enhance efficiency by lowering noise outside the limits anticipated from the coarse visual input (Mitchell, Sundberg, & Reynolds, 2009).