Sensation & Perception (L3)

Question 1

Sensation definition

Answer 1

The stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses (transduction) that are sent to the brain.

Question 2

Perception definition

Answer 2

Making ‘sense’ of what our senses tell us. This is the active process of organising and identifying the stimulus and giving it meaning.

Question 3

Steps from sensation to perception (6)

Answer 3

1. Stimulus is received by sensory receptors.
2. Receptors translate stimulus properties into nerve impulses (transduction).
3. Feature detectors analyse stimulus features.
4. Stimulus features are reconstructed into neural representation.
5. Neural representation is compared with previously stored information in the brain.
6. Matching process results in recognition and interpretation of stimulus.

Question 4

Psychophysics

Answer 4

A scientific field relating the physical characteristics of stimuli to sensory capabilities.

Question 5

Absolute threshold

Answer 5

The lowest stimulus intensity at which a stimulus can be detected 50% of the time.

Question 6

Psychometric function

Answer 6

A psychometric function expresses sensory capability (e.g., detection, discrimination) as a function of stimulus intensity.

Question 7

Intra-individual variability (2)

Answer 7

• Sensitivity can fluctuate within an individual
• Influenced by fatigue, expectation, significance of stimulus.

Question 8

Inter-individual variability

Answer 8

Individuals can have difference decision criteria: how certain they need to feel before reporting that they detect a stimulus.

Question 9

Difference threshold

Answer 9

Difference threshold (Just Noticeable Difference), which is the difference that can be discriminated 50% of the time.

Question 10

Weber’s Law

Answer 10

The JND is directly proportional to the intensity of the stimulus being compared.

Question 11

Discriminability

Answer 11

How small a difference between two stimulus intensities can an individual discriminate?

• It is easier to discriminate a 2g difference for 100g than for 200g objects.
• Weber’s Law

Question 12

Weber’s Constant

Answer 12

Weber’s constant (or fraction) k is the ratio of the difference threshold (JND) ∆I to the stimulus intensity I.

Question 13

General info: the eye

Answer 13

• The eye is sensitive to electromagnetic energy in the approximate range of 400-700 nm.
• Light enters the eye via the cornea.
• The pupil is the adjustable aperture that controls the amount of light entering the eye.
• The iris is the pigmented region surrounding the pupil.
• It contains muscle cells that dilate or constrict the pupil.
• the lens is an elastic structure that becomes thinner to focus on distant objects and thicker to focus on near objects.

Question 14

General info: the retina

Answer 14

• The retina contains two types of sensory cells called photoreceptors: rods and cones.
• Photoreceptors are 1/5 types of neuron in the retina, and constitute the innermost layer.
• Ganglion cells receive converging input from the other layers, and project their axons towards the CNS via the optic nerve.
• There are no photoreceptors where the optic nerve leaves the eye (blind spot).

Question 15

Rods

Answer 15

Largely colour insensitive, but more sensitive to lower intensities of light (dim light conditions).

Question 16

Cones

Answer 16

Each cell is sensitive to wavelengths in blue, green, or red bands.

Question 17

Fovea

Answer 17

• A small area in the centre of the retina that contains a high density of cones but few rods.
• The fovea represents the centre of the visual field and has the highest visual acuity (ability to see fine detail).
• When we direct our gaze at something this is called foveation.

Question 18

Colour perception

Answer 18

• Cones contain special proteins called photopigments.
• When photopigments absorb light of specific wavelengths, this causes the cells to depolarise and fire action potentials.
• Impulses generated by cone cells carry information about red, green, and blue intensities in light.

Question 19

Trichromatic Theory

Answer 19

The ration of red, green, and blue cone activity is combined downstream (ganglion cells or later) to represent an intermediate colour.
- The problem is that this theory does not explain the phenomenon of colour aftereffects.

Question 20

Opponent Process Theory

Answer 20

Information from both rods and cones are integrated by three types of ganglion cells
- The colour red/green and yellow/blue are called opponent colours.
- When a photoreceptor is activated for a while, it briefly habituates.
- When the input is removed, the opponent colour is perceived until these photoreceptors recover.

Question 21

Pathway to cortex

Answer 21

• Ganglion projections in the optic nerve terminate in the thalamus.
• Cells from the left visual hemifield project to the right cortical hemisphere, and vice versa.
• Projections deccisate to the opposite hemisphere at the optic chiasm.
• Thalamic cells then relay the optical signal to primary visual cortex (V1).

Question 22

Bottom-up Processing

Answer 22

How our brains constructs perceptions by binding together primitive representations into complex object representations.
- parts to whole

Question 23

Top-down Processing

Answer 23

The ways in which existing knowledge, expectations, emotional states, arousal etc., can bias which bottom-up signals get processed, and what representations they are assigned to.
- whole to parts.
- We tend to organise stimuli into more important (figure) and less important (ground) using a variety of features: blurriness, contrast, size, separation.

Question 24

Retinotopy

Answer 24

• Visual input projected into the retina has a topical correspondence with the objects being viewed
• Preserved in the V1 representation, although it is distorted such that the fovea is over-represented.

Question 25

Ganglion Cells

Answer 25

• Reconstruct vidual input into perceived objects.
• They specialise in detecting edges: parts of the scene were colour and/or intensity change rapidly.
• Output from ganglion cells is relayed via thalamus to V1, where it informs cells specialised for: orientation of edges, colour.
• This info is further relayed to other cortical regions, where the building blocks are combined into object representations.

Question 26

Visual processing streams

Answer 26

Dorsal stream: where/how?
- Spatial relations
- Guiding movements

Ventral stream: what?
- Object shape and identity

Question 27

Gestalt Theory

Answer 27

Suggests a number of principles by which our brains group an interpret stimuli: law of similarity, law of proximity, law of closure, law of continuity.

Question 28

Law of similarity (Gestalt theory)

Answer 28

Similar objects are grouped together.

Question 29

Law of proximity (Gestalt theory)

Answer 29

Objects are grouped together based on their proximity to one another.

Question 30

Law of closure (Gestalt theory)

Answer 30

We tend to fill in gaps in incomplete figures.

Question 31

Law of continuity (Gestalt theory)

Answer 31

We link individual elements together in patterns that make sense.