Visual spatial attention is a form of visual attention that involves directing attention to a location in space. Similar to its temporal counterpart visual temporal attention, these attention modules have been widely implemented in video analytics in computer vision to provide enhanced performance and human interpretable explanation of deep learning models.
Spatial attention allows humans to selectively process visual information through prioritization of an area within the visual field. A region of space within the visual field is selected for attention and the information within this region then receives further processing. Research shows that when spatial attention is evoked, an observer is typically faster and more accurate at detecting a target that appears in an expected location compared to an unexpected location. Attention is guided even more quickly to unexpected locations, when these locations are made salient by external visual inputs (such as a sudden flash). According to the V1 Saliency Hypothesis, the human primary visual cortex plays a critical role for such an exogenous attentional guidance.Li. Z. 2002 A saliency map in primary visual cortex Trends in Cognitive Sciences vol. 6, Pages 9-16, and Zhaoping, L. 2014, The V1 hypothesis—creating a bottom-up saliency map for preattentive selection and segmentation in the book Understanding Vision: Theory, Models, and Data
Spatial attention is distinctive from other forms of visual attention such as object-based attention and feature-based attention. These other forms of visual attention select an entire object or a specific feature of an object regardless of its location, whereas spatial attention selects a specific region of space and the objects and features within that region are processed.
Measures of visual spatial attention
Spatial cueing experiments A key property of visual attention is that attention can be selected based on spatial location and spatial cueing experiments have been used to assess this type of selection. In Posner's cueing paradigm, the task was to detect a target that could be presented in one of two locations and respond as quickly as possible. At the start of each trial, a cue is presented that either indicates the location of the target (valid cue) or indicates the incorrect location thus misdirecting the observer (invalid cue). In addition, on some trials there is no information given about the location of the target, as no cue is presented (neutral trials). Two distinct cues were used; the cue was either a peripheral 'flicker' around the target's location (peripheral cue) or the cue was centrally displayed as a symbol, such as an arrow pointing to the location of the target (central cue). Observers are faster and more accurate at detecting and recognising a target if the location of the target is known in advance. Furthermore, misinforming subjects about the location of the target leads to slower reaction times and poorer accuracy relative to performance when no information about the location of the target is given.
Spatial cueing tasks typically assess covert spatial attention, which refers to attention that can change spatially without any accompanying eye movements. To investigate covert attention, it is necessary to ensure that observer's eyes remain fixated at one location throughout the task. In spatial cueing tasks, subjects are instructed to fixate on a central fixation point. Typically it takes 200 ms to make a saccadic eye movement to a location. Therefore, the combined duration of the cue and target is typically presented in less than 200 ms. This ensures that covert spatial attention is being measured and the effects are not due to overt eye movements. Some studies specifically monitor eye movements to ensure that the observer's eyes are continually fixated on the central fixation point.
The central and peripheral cues in spatial cueing experiments can assess the orienting of covert spatial attention. These two cues appear to use different mechanisms for orienting spatial attention. The peripheral cues tend to attract attention automatically, recruiting bottom-up attentional control processes. Conversely, central cues are thought to be under voluntary control and therefore use top-down processes. Studies have shown that peripheral cues are difficult to ignore, as attention is oriented towards the peripheral cue even when the observer knows the cue does not predict the location of the target. Peripheral cues also cause an allocation of attention much faster than central cues, as central cues require greater processing time to interpret the cue.
Spatial probe experiments In spatial cueing tasks, the spatial probe (cue) causes an allocation of attention to a particular location. Spatial probes have also been often used in other types of tasks to assess how spatial attention is allocated.
Spatial probes have been used to assess spatial attention in visual searches. Visual search tasks involve the detection of a target among a set of distractors. Attention to the location of items in the search can be used to guide visual searches. This was demonstrated by valid cues improving the identification of targets relative to the invalid and neutral conditions. A visual search display can also influence how fast an observer responds to a spatial probe. In a visual search task, a small dot appeared after a visual display and it was found that observers were faster at detecting the dot when it was located at the same location as the target. This demonstrated that spatial attention had been allocated to the target location.
The use of multiple tasks simultaneously in an experiment can also demonstrate the generality of spatial attention, as allocation of attention to one task can influence performance in other tasks. For example, it was found that when attention was allocated to detecting a flickering dot (spatial probe), this increased the likelihood of identifying nearby letters.
Distribution of spatial attention The distribution of spatial attention has been subject to considerable research. Consequently, this has led to the development of different metaphors and models that represent the proposed spatial distribution of attention.
Spotlight metaphor According to the 'spotlight' metaphor, the focus of attention is analogous to the beam of a spotlight. The moveable spotlight is directed at one location and everything within its beam is attended and processed preferentially, while information outside the beam is unattended. This suggests that the focus of visual attention is limited in spatial size and moves to process other areas in the visual field.
Zoom-lens metaphor Research has suggested that the attentional focus is variable in size. Eriksen and St James proposed the 'zoom-lens' metaphor, which is an alternative to the spotlight metaphor and takes into account the variable nature of attention. This account likens the distribution of attention to a zoom-lens that can narrow or widen the focus of attention. This supports findings that show attention can be distributed both over a large area of the visual field and also function in a focused mode. In support of this analogy, research has shown that there is an inverse relationship between the size of the attentional focus and the efficiency of processing within the boundaries of a zoom-lens.
Gradient model The Gradient Model is an alternative theory on the distribution of spatial attention. This model proposes that attentional resources are allocated in a gradient pattern, with concentrated resources in the centre of focus and resources decrease in a continuous fashion away from the centre. Downing conducted research using an adaptation of Posner's cueing paradigm that supported this model. The target could appear in 12 potential locations, marked by boxes. Results showed that attentional facilitation was strongest at the cued location and gradually decreased with distance away from the cued location. However, not all research has supported the gradient model. For example, Hughes and Zimba conducted a similar experiment, using a highly distributed visual array and did not use boxes to mark the potential locations of the target. There was no evidence of a gradient effect, as the faster responses were when the cue and target were in the same hemifield and slower responses when they were in different hemifields. The boxes played an important role in attention as a later experiment, used the boxes and consequently found a gradient pattern. Therefore, it is considered that the size of the gradient can adjust according to the circumstances. A broader gradient may be adopted when there is an empty display, as attention can spread and is only restricted by hemifield borders.
Splitting spatial attention It is debated in research on visual spatial attention whether it is possible to split attention across different areas in the visual field. The 'spotlight' and 'zoom-lens' accounts postulate that attention uses a single unitary focus. Therefore, spatial attention can only be allocated to adjacent areas in the visual field and consequently cannot be split. This was supported by an experiment that altered the spatial cueing paradigm by using two cues, a primary and a secondary cue. It was found that the secondary cue was only effective in focusing attention when its location was adjacent to the primary cue. In addition, it has been demonstrated that observers are unable to ignore stimuli presented in areas situated between two cued locations. These findings have proposed that attention cannot be split across two non-contiguous regions. However, other studies have demonstrated that spatial attention can be split across two locations. For example, observers were able to attend simultaneously to two different targets located in opposite hemifields. Research has even suggested that humans are able to focus attention across two to four locations in the visual field. Another perspective is that spatial attention can be split only under certain conditions. This perspective suggests that the splitting of spatial attention is flexible. Research demonstrated that whether spatial attention is unitary or divided depends on the goals of the task. Therefore, if dividing attention is beneficial to the observer then a divided focus of attention will be utilised.
One of the main difficulties in establishing whether spatial attention can be divided is that a unitary focus model of attention can also explain a number of the findings. For example, when two non-contiguous locations are attended to, it may not be that attention has been split between these two locations but instead it may be that the unitary focus of attention has expanded. Alternatively, the two locations may not be attended to simultaneously and instead the area of focus is moving quickly from one location to another. Consequently, it appears very difficult to prove undoubtedly that spatial attention can be split.
Deficits in visual spatial attention
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