Debate over role of attention
One major dividing line between theories that explain natural scene perception is the role of attention. Some theories maintain the need for focused attention, while others claim that focused attention is not involved.
Attention is the behavioral and cognitive process of selectively concentrating on a discrete aspect of information, whether deemed subjective or objective, while ignoring other perceivable information. It is a state of arousal. It is the taking possession by the mind in clear and vivid form of one out of what seem several simultaneous objects or trains of thought. Focalization, the concentration of consciousness, is of its essence. Attention has also been described as the allocation of limited cognitive processing resources.
Focused attention played a partial role in early models of natural scene perception. Such models involved two stages of visual processing. [2] According to these models, the first stage is attention free and registers low level features such as brightness gradients, motion and orientation in a parallel manner. Meanwhile, the second stage requires focused attention. It registers high-level object descriptions, has limited capacity and operates serially. These models have been empirically informed by studies demonstrating change blindness, inattentional blindness and attentional blink. Such studies show that when one's visual focused attention is engaged by a task, significant changes in one's environment that are not directly pertinent to the task can escape awareness. It was generally thought that natural scene perception was similarly susceptible to change blindness, inattentional blindness and attentional blink, and that these psychological phenomena occurred because engaging in a task diverts attentional resources that would otherwise be used for natural scene perception.
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed and one of the color appearance parameters of color appearance models. Brightness refers to an absolute term and should not be confused with Lightness.
Change blindness is a perceptual phenomenon that occurs when a change in a visual stimulus is introduced and the observer does not notice it. For example, observers often fail to notice major differences introduced into an image while it flickers off and on again. People's poor ability to detect changes has been argued to reflect fundamental limitations of human attention. Change blindness has become a highly researched topic and some have argued that it may have important practical implications in areas such as eyewitness testimony and distractions while driving.
Evidence against the need for focused attention
The attention-free hypothesis soon emerged to challenge early models. The initial basis for the attention-free hypothesis was the finding that in visual search, basic visual features of objects immediately and automatically pop out to the person doing the visual search. [3] Further experiments seemed to support this: Potter (as cited by Evans & Treisman, 2005) showed that high-order representations can be accessed rapidly from natural scenes presented at rates of up to 10 per second. Additionally, Thorpe, Fize & Marlot (as cited by Evans & Treisman) discovered that humans and primates can categorize natural images (i.e. of animals in everyday indoor and outdoor scenes) rapidly and accurately even after brief exposures. [3] The basic idea in these studies is that exposure to each individual scene is too brief for attentional processes to occur, yet human beings are able to interpret and categorize these scenes.
Visual search is a type of perceptual task requiring attention that typically involves an active scan of the visual environment for a particular object or feature among other objects or features. Visual search can take place with or without eye movements. The ability to consciously locate an object or target amongst a complex array of stimuli has been extensively studied over the past 40 years. Practical examples of using visual search can be seen in everyday life, such as when one is picking out a product on a supermarket shelf, when animals are searching for food amongst piles of leaves, when trying to find your friend in a large crowd of people, or simply when playing visual search games such as Where's Wally? Much previous literature on visual search used reaction time in order to measure the time it takes to detect the target amongst its distractors. An example of this could be a green square amongst a set of red circles. However, reaction time measurements do not always distinguish between the role of attention and other factors: a long reaction time might be the result of difficulty directing attention to the target, or slowed decision-making processes or slowed motor responses after attention is already directed to the target and the target has already been detected. Many visual search paradigms have therefore used eye movement as a means to measure the degree of attention given to stimuli. However, vast research to date suggests that eye movements can move independently of attention, and therefore eye movement measures do not completely capture the role of attention.
Weaker versions of the attention-free hypothesis have also been targeted at specific components of the natural scene perception process instead of the process as a whole. Kihara & Takeda (2012) limit their claim to saying that it is the integration of spatial frequency-based information in natural scenes (a sub-process of natural scene perception) that is attention free. [4] This claim is based on a study of theirs which used attention-demanding tasks to examine participants' abilities to accurately categorize images that were filtered to have a wide range of spatial frequencies. The logic behind this experiment was that if integration of visual information across spatial frequencies (measured by the categorization task) is preattentive, then attention-demanding tasks should not affect performance in the categorization task. This was indeed found to be the case.
In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance. The SI unit of spatial frequency is cycles per meter. In image-processing applications, spatial frequency is often expressed in units of cycles per millimeter or equivalently line pairs per millimeter.
More recent evidence reasserting the need for focused attention
A recent study by Cohen, Alvarez & Nakayama (2011) calls into question the validity of evidence supporting the attention-free hypothesis. They found that participants did display inattentional blindness while doing certain kinds of multiple-object tracking (MOT) and rapid serial visual presentation (RSVP) tasks. [5] Furthermore, Cohen et al. found that participants' natural scene perception was impaired under dual-task conditions, but that this dual-task impairment happened only when participants' primary task was sufficiently demanding. The authors concluded that previous studies showing the absence of a need for focused attention did not use tasks that were demanding enough to fully engage attention.
Rapid serial visual presentation is an Paradigm used by psychologists to study an attentional phenomena named "the attentional blink". The RSVP tasks asks a participant to observe a continuous presentation of multiple separate visual images or objects that appear in rapid succession (very short duration of exposure time. Two targets are shown during each presentation. In the classic setup, the first target is shown, and then a second one comes after it in either two spots or 8 spots later. The other visual stimuli shown between target 1 and target 2 are known as distractions. An example of a distractor is a color change in the entire display, a post-mask or it can be letters that are among the numbers.
A dual-task paradigm is a procedure in experimental (neuro)psychology that requires an individual to perform two tasks simultaneously, in order to compare performance with single-task conditions. When performance scores on one and/or both tasks are lower when they are done simultaneously compared to separately, these two tasks interfere with each other, and it is assumed that both tasks compete for the same class of information processing resources in the brain.
In the Cohen et al. study, the MOT task involved viewing eight black moving discs presented against a changing background that consisted of randomly colored checkerboard masks. Four of these discs were picked out and participants were instructed to track these four discs. The RSVP task involved viewing a stream of letters and digits presented against a series of changing checkerboards, and counting the number of times a digit was presented. In both experiments, the critical trial involved a natural scene suddenly replacing the second last checkerboard, and participants were immediately afterwards asked whether they had noticed anything different, as well as presented with six questions to determine whether they had categorized the scene. The dual-task condition simply involved participants performing the MOT task mentioned above and a scene-classification task simultaneously. The authors varied the difficulty of the task (i.e. how demanding the task was) by increasing or decreasing the speed of the moving discs.
