Cartesian Dualism

We've heard the theory that we can be split into the hardware of the brain and the software of the mind. This is fine as long as we don't take the analogy too far.

This theory can be linked to Cartesian Dualism, where "Descartes clearly identified the mind with consciousness and self-awareness and distinguished this from the brain as the seat of intelligence". Nobody taken seriously properly thinks like this, but it can be a useful tool.

The Principles of Psychophysics

  1. The relationship between the physical stimulus and its perceived properties is complex.
  2. Our absolute sensitivity and discrimination thresholds are limited
  3. Perception is tuned to ecologically relevant information
  4. Perception is fast, automatic, effortless and immediate
  5. Perceptual illusions are not errors, but reveal important principles of perceptual processing
  6. Sensory input is ambiguous
  7. The perceptual systems are geared towards the extraction of relative, not absolute information
  8. The sensitivity of our sensory systems is dynamic; It changes as a result of prolonged stimulation.


Transduction: The conversion of environmental energy into neural signals (via sensory receptor cells)
Sensation: The process by which organisms gather information about the environment and transmit this information to the brain for processing (neural computation).
Perception: The process by which the brain organises and interprets sensory information.
Psychophysics: The scientific study of the quantitive relationship between stimulus and sensation (physical events and psychological events). (Wikipedia describes it as "the analysis of perceptual processes by studying the effect on a subject's experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions").

Sensory Apparatus

The process of perceiving the world around us is simplified into:

  • Sampling the world
  • Neural computation (perception, decision making, motor programming)
  • Behavioural response

By looking at the behavioural response and understanding the way the brain works, we can try to piece the two together and determine what causes what.

Adaptive Behaviour

Our Central Nervous System (CNS) has evolved to make an inventory of the ecologically important contents of the world, as a way of organising and prioritising information. The CNS then generates optimal output behaviour based on the sensory input.

Our perception ("imprint") of the stimuli of the physical world is passed through to the peripheral nervous system through the process of transduction. We use psychophysics to determine how our imprint is different to reality.


Sensory Thresholds

Sensory thresholds are a theory used to determine the amount of a stimulus needed to elicit a sensation.

Detection Threshold: The smallest amount of a sensory signal that an organism can detect 50% of the time. It's a variant definition of the Absolute Threshold (below)


Absolute Thresholds

The Absolute Threshold is the lowest level at which a stimulus can be detected at all.

  • Vision: candle flame seen at 30 miles on a clear dark night
  • Hearing: tick of a watch under quiet conditions at 20 feet
  • Touch: the wing of a bee falling on your cheek from a height of 1cm
  • Taste: one tsp of sugar in 2 gallons of water/7.5L of water
  • Smell: one drop of perfume diffused into the entire volume of a 3-room apartment

Discrimination/Difference Threshold

The Discrimination Threshold is the Just Noticeable Difference between two levels of a stimulus. (i.e. how different they have to be in order to detect the change).
The 50% detection threshold is what the discrimination threshold defaults to unless otherwise specified (e.g. the 75% jnd).

Weber’s Law

Weber's Law states that the size of the JND between two stimuli is proportional to the magnitude of the stimuli. It's a constant fraction of the magnitude of the base level stimulation.

Principle 3 - Ecologically Relevant Perception

Given our sensory systems have threshold limits, it follows that these are selected by evolutionary pressure, and in some sense selected out entirely.


This explains why face recognition is much harder when not in the ecologically typical environment (e.g. face up).

Agnosia (Requirements for Visual Processing)

Associative Visual Agnosia

Associative Visual Agnosia is a condition often associated with damage to the occipital-temporal lobes, whereby sufferers can describe a scene or object, but fail to recognise it when it is shown to them.



Prosopagnosia is common in those suffering damage to the fusiform gyrus. They have difficulty recognising and interpreting faces.

Akinetopsia: Motion agnosia

Akinetopsia is a rare form of agnosia known as motion blindness; patients cannot see objects in motion, even though they have no problem with stationery forms.


Hemispatial Neglect

Hemispatial Neglect is a condition affecting those with lesions of the parietal lobe. It is characterised by a deficit of attention and awareness of one side of the visual space.


The afflicted can draw the rest of something if it's pointed out to them, they just don't attend to it or realise themselves that they've missed something.

Perceptual Illusions (Principle 5)

Perceptual illusions are often used in psychophysics as a way of revealing the principles behind visual processing.

Examples include:


  • Interpretation of ambiguous figures (e.g.:)


  • The three-dimensional structure of the world has to be reconstructed from twodimensional projections on the retina


The eye is attracted to motion, meaning that we won't notice anything off to the edge, or not involved in the perceivedly important action (often the social activity). E.g:

  • We'll miss a slow colour change
  • Flickers will distract us from shifts of items
  • Static things are rated as unimportant (motion is an indicator that something is worthy of further inspection)