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. 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.
… excerpt ends here. Continue reading the full article.
