The senses are:

Vision is the primary sense in humans. Blindness is a much more serious disability than in any of the other senses. The human also does a lot of analysis of vision. A huge proportion of our brain is devoted to vision.

Perception is very subjective.

• Sensation: a translation of the external physical environment into a pattern of neural activity ( by a sensory organ ).
• Perception: analysis of this neural activity to understand the environment and guide behavior. Or: the subjective conscious experience of the outside world.

Sensation and perception:

• Depends on physical property of the world
• Limited by the physical properties of our sensors

E.g.: the human eye is sensitive to only a spectrum of wavelengths.

S&P have evolved to help us survive and reproduce, so they are:

• Optimized for useful representations of the environment
• Influenced by interpretation: context and experience
• Dependent on limited resources of attention and awareness

How to study perception experimentally:

• Change the physical environment of a human or animal
• Measure the resulting behavior or
• Measure the resulting change in neural activity

Psychological approach:

• Quantitative measurements of behavior resulting from perception
• Psychophysics: the scientific study of the relation between stimulus and sensation

Just noticeable difference (JND). Depends on the stimulus and the amount of it.

Weber-Fechner law: detectable difference increases with average stimulus intensity. It describes the relationship between a physical intensity and its perceived intensity.

How to determine a perceptual threshold: 2-alternative forced choice. Method of constant stimuli: present trials with extent of differences randomized from one trial to the next. Plot detection probability against extent of difference. Steepest point of curve: largest change to the smallest change in stimulus (75%)

Slope: change from missing to perceiving the difference.

More efficient: make the next trial harder for a correct answer and easier for an incorrect answer.

Biological approach:

• What are perceptions neural substrates?
• Correlate a neural activity measure with a change in the presented stimulus or the animal’s behavior
• Neural activity is either:
  
  • Spiking activity           -          Action potentials
  • Synaptic activity         -          Synaptic potentials
  • Metabolic activity      -          Oxygen & glucose consumption

These are closely related

Spikes are often seen as the gold standard of neural activity. Spikes are very small changes, so they must be measured directly from the neuron. These recordings are invasive inside the brain of an animal or human.

Measuring synaptic activity:

• Smallest: local field potential.

Oscillations: interaction between excitatory and inhibitory neuron groups.

Why use EEG?

• Advantages
  • Cheap
  • High temporal resolution
  • Moves with the subject
  • Silent
• Disadvantages
  
  • Poor spatial resolution
  • Poor signal-to-noise ratio
  • Only senses activity near the scalp (cortex)
  • Slow to set up

How does MRI work?:

• T1 and T2 weighted image.

Deoxygenated blood causes signal loss. It affects T2*. Blood response follows neural activity. This overcompensates: it does not compensate accurately.

So oxyhemoglobin concentration increases due to increased blood flow. This reduces the signal loss. This is the BOLD signal:

Neural activity:

• MUA (multi unit activity): action potentials = neural output
• LFP (local field potentials): slow electrical signals and sub-threshold activity, including synaptic activity and voltage-dependent membrane oscillations. = neural processing

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