NEUR1020 Chap.3 Adaptation, Attention and Colour Constancy
Adaptation, Attention and Colour Constancy
Perception depends on the current state of neural populations, the information selected for processing and the context used to interpret ambiguous input. This chapter predicts aftereffect direction from opponent adaptation, distinguishes physical presence from attended or reportable information, and explains why surface reflectance cannot be recovered from a local retinal signal without estimating illumination.
Each phenomenon has its own mechanism, so the chapter avoids the vague claim that the brain simply fills gaps and instead asks what was changed, selected or inferred. That diagnostic question produces different, testable predictions when the adapter, attentional target or surrounding illumination is changed.
What this chapter covers
- 01
Opponent responses and adaptation
- 02
Neutral tests and biased aftereffects
- 03
Feature, spatial and temporal attention
- 04
Conscious report versus neural processing
- 05
Illumination estimates and colour constancy
Worked example · free
Predicting a neutral test after adaptation
- +1Identify Population A as the adapted response and state that its later contribution is reduced.
- +1The neutral test now produces an unequal balance because Population B contributes relatively more.
- +1The percept is biased toward the feature coded by Population B even though the test stimulus remains neutral.
Key terms
- Adaptation
- Adaptation is a response change following prolonged stimulation of a neural population.
- Aftereffect
- An aftereffect is a biased percept produced when adaptation alters responses to a later test.
- Attention
- Attention is the selective prioritisation of information according to current tasks or goals.
- Colour constancy
- Colour constancy is the tendency to experience surface colour as relatively stable across illumination changes.
Adaptation, Attention and Colour Constancy FAQ
Why does an aftereffect point in the opposite direction?
Prolonged stimulation reduces the later contribution of the adapted population. When a neutral test activates both sides of an opponent code, the unadapted side contributes relatively more. The physical test is unchanged, but the neural response balance now favours the opposite percept.
Does missing an unattended event mean it was not sensed?
No. A failure of conscious report shows that the event was not successfully selected and reported under that task. It does not prove that no sensory or neural processing occurred. A separate measure would be needed to determine how far the unattended information was processed.
Why does colour constancy require context?
The light reaching the eye depends on both surface reflectance and illumination, so the same local signal can have different physical causes. Surrounding surfaces, shadows and global lighting help the visual system estimate illumination and infer a relatively stable surface property.
What changes during visual adaptation?
Neural populations responsive to the prolonged feature change their later contribution. The subsequent bias comes from an altered response balance, not from a physical change in the neutral test and not from a negative picture stored in the eye.
Why are task instructions essential in attention studies?
Instructions determine which feature, location or time receives priority and which response must be produced. A miss is difficult to interpret without knowing the target, competing load and report rule that shaped selection during the display.
Can two people report different colours from one image?
Yes, when an ambiguous image supports different assumptions about illumination. Identical local pixels can be attributed to different combinations of reflectance and lighting, so the inferred surface colour can differ without different physical displays.
How should these mechanisms be compared?
Ask what changed. Adaptation changes a population response, attention changes processing priority, and constancy changes the inferred cause of ambiguous input. A precise answer predicts a result from that operation instead of using one broad phrase.
Why is direction so important in an aftereffect answer?
Opponent accounts make a directional prediction. Naming the adapted population and the relatively stronger opponent response shows why the later percept shifts one way rather than merely noting that an aftereffect occurred.
Does an unattended stimulus produce no brain activity?
A failed report does not justify that conclusion. Some sensory processing may occur without successful selection or awareness. Researchers need an additional behavioural or neural measure to determine which stages were influenced by the unattended input.
How can I test a constancy explanation?
Hold the local surface signal constant while changing surrounding cues that imply illumination. If the reported colour changes with the inferred light, the pattern supports a contextual interpretation rather than a response determined only by local input.
Exam move
For aftereffects, write the adapted population and the predicted percept as a direction pair. For attention, record the target, competing information and response requirement before interpreting a miss. For constancy, separate the local signal from the inferred illumination. These three mini routines prevent one generic explanation from being applied to mechanisms that solve different problems.
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