2. What is visual selection?
Imagine a situation in which the visual system is confronted with two different objects….Within the visual system, these two objects
compete and question is which object wins this competition and drives neurons throughout the visual system forming an ensemble of neurons that represent this one single object. In line with the biased competition model attention biases these competitive interactions such that attended stimuli receive priority over unattended stimuli.
Attentional effects on resolving this competition are the result of bottom-up and top-down factors. The bottom-up signal depends on the (acquired) properties of the stimulus field.
Objects that are highly salient and stand out from the background may immediately receive attention priority. Indeed, it is likely that before top-down influences can have an effect, the visual system is biased towards salient stimuli that resolve the competition simply on the basis of the bottom-up input. Bottom-up signals can also be acquired through inter-trial priming, conditioning or reward contingencies. We consider these effects to be bottom-up because the effects cannot be counteracted by volitional top-down control.
top-down & bottom-up control to a location results in a greater neuronal sensitivity (i.e., a decreased threshold).
Another way to bias the competition within the visual system is through top-down volitional feedback signals that depend on the goals, intentions and expectations of the observer. For example, directing attention in a volitional way to a specific location in space increases the sensory gain for features at that location and appears to alter the apparent stimulus contrast. These results imply that the directing attention is endogenous and is often referred to as goal-driven selection.
As a metaphor visual attention has been compared to a spotlight that “selects” parts of the visual world around us. Visual attention allows people to select information that is relevant for their ongoing behavior. For several decades, there has been agreement that there are two functionally independent stages of visual processing. An early visual stage, sometimes referred to as pre-attentive operating in parallel across the visual field and a later stage often referred to as attentive that can deal with only one (or a few items) at the same time.
Even though the dichotomy between these two stages is not as strict as originally assumed, in almost all past and present theories of visual attention this basic architecture is more or less still present. Given the two-stage framework, it is generally assumed that visual selection depends principally on the outcome of the early stage of visual processing. Processing occurring during the initial wave of stimulation through the brain, determines which element is selected and is passed on to the second stage of processing.
In line with the two-stage approach, passing on an item to the second stage of processing implies that this item has been selected for further processing. According to this notion, from all objects that are present in the visual field (and are available at the early preattentive stage of processing), each time an object is passed on the final stage of processing, it will affect decision making and responding. This passing on from the initial stage of preattentive processing to attentive processing is what is considered to be visual selection.
The selection of one object out of many objects that are available during the initial preattentive stage of parallel processing is what we consider visual selection. Note that in some conditions the preattentive, parallel stage of processing plays basically no role. In that case there is no salience calculation across the visual field and an object is selected purely on the basis of spatial