Perception, attention & awareness

We seem to perceive everything in the visual scene around us in a single, integrated visual percept. In reality, we perceive only selected aspects of the scene, focusing on different aspects at different times. 

Focused attention may be necessary for even a clearly visible stimulus to be consciously reported and stored in visual short-term memory. This is demonstrated by inattentional blindness. It is a lack of attention that isn't associated with any vision defects or deficits; it is whereby an individual fails to recognise an unexpected stimulus that is in plain sight. A classic study that investigated inattentional blindness is the gorilla study. A video shows an ordinary scene, and at one point in the video a gorilla walks through the scene. Many participants fail to notice the obvious but unexpected gorilla.

Blindsight patients have a hemianopia - an area of blindness in the visual field due to damage in the primary visual cortex

Backward-masked (unconscious) visual words evoke brain activity confined to the visual word recognition areas of the cortex. Identical conscious words evoke widely distributed activity in the parietal and prefrontal cortex. Conscious sensory input evokes far wider and more intense brain activity than identical unconscious input.

NCC: bottom-up activation - if the stimulus is too weak, the activation it causes will be insufficient to trigger a further activation, i.e representation of the stimulus will be subliminal. Availability of attention to allow extension of activation to higher cortices - if attention is temporarily withdrawn, bottom-up activation will not spread to higher association cortices and will not result in conscious awareness, i.e representation of the stimulus will be pre-conscious.
We cannot report stimuli that have only undergone preconscious processing. We might have been briefly aware of them but have forgotten them so quickly that we cannot remember the stimuli and so cannot report them

The neural correlates of consciousness (NCC): the neural states that are necessary and sufficient for conscious experience. When a participant is presented with a stimulus, there should be a measurable difference in the neural activity when they are aware of the stimulus and then unaware of the stimulus. We can find the NCC by comparing conditions where the stimulus presented is identical but conscious perception changes.

Binocular rivalry experiments can help us find the NCC. In these experiments perception alternates spontaneously between each monocular image (blue and red patterns). Participants indicate which pattern they are conscious of over time. fMRI response show some regions with higher signals during perception of the red pattern and other regions with higher signals during perception of the blue pattern. It can predict which pattern the subject is conscious of by measuring when fMRI response to each pattern is dominant.

Most experiments show that one neural correlate of attention is enhanced firing. If a neuron has a certain response to a stimulus when one is not attending to the stimulus, then when they do attend to the stimulus, the neuron's response will be enhanced even if the physical characteristics of the stimulus remain the same

Feature integration theory is a theory of attention developed in 1980 by Anne Treisman and Garry Gelade. It suggests that when perceiving a stimulus, features are "registered early, automatically, and in parallel, while objects are identified separately" and at a later stage in processing.

According to Treisman, the first stage of the theory is the preattentive stage. During this stage, the different parts of the brain automatically gather information about basic features (colours, shape, movement) that are found in the visual field.

 The second stage is the focused attention stage, where the individual features of an object combine in order to perceive the whole object. In order to combine the individual features of an object, attention is required and selection of that object occurs within a "master map" of locations. The master map of locations contains all of the locations in which features have been detected, with each location in the master map having access to the multiple feature maps. When attention is focused at a particular location on the map, features currently in that position are attended to and stored in "object files". If it is familiar, associations are made between the object and prior knowledge, resulting in identification of that object.

Bottom-up processing is also known as stimulus-driven attention, or exogenous attention. These describe attentional processing which is driven by the properties of the objects themselves. Some processes, such as motion or a sudden loud noise, can attract our attention in a pre-conscious, or non-volitional way. We attend to them whether we want to or not. These aspects of attention are thought to involve the parietal and temporal cortices, as well as the brainstem. Bottom-up mechanisms are thought to operate on raw sensory input, rapidly and involuntarily shifting attention to salient visual features of potential importance. It starts with features and builds up into a complete perception

Top-down processing is also known as goal-driven, endogenous attention, attentional control, or executive attention. This aspect of our attentional orienting is under the control of the person who is attending. It's mediated primarily by the frontal cortex and basal ganglia as one of the executive functions. Top-down mechanisms implement our longer-term cognitive strategies, biasing attention toward coloured spots if we're hungry, or toward sudden movements and quadrupedal shapes if we fear a predator.

If we can be conscious of only those few objects we can focus our attention on at any one moment, why does it seem to use that we have a visual sensation of the entire scene and its visual qualities?

What we see is an incredible illusion. Distributed attention across the scene gives us a fleeting awareness of primitive visual features, whereas focused attention gives us awareness of integrated meaningful objects in short-term memory that we can report.

We cannot report stimuli that have only undergone preconscious processing. We may have briefly been aware of them, but have forgotten them so quick that we can't remember the stimuli and thus cannot report it. However, even when a stimulus generates no report or conscious awareness, it can influence aspects of behaviour. In one study, a cross was shown on the screen to focus attention, and then the word flake was flashed on the sceen. After, participants are presented with two letters 'FL' and 3 blank spaces to complete the word. Flake is the word that 'comes to mind' in 40% of participants who've taken part in the cross experiment but reported not seeing anything other than the cross. Only 4% of participants completed the word as flake when they had not taken part in the earlier cross experiment.