Sensation and perception

What is sensation?

Detection of simple properties

What is perception?

Interpretation of sensory signals

As the brain cannot sense the external world, what does it rely on?

Signals recieved from sense organs via afferent nerves

What are the two things the sensory system needs in order to send signals to the brain?

1. A biological mechanism for transplanting physical attributes into eletrical signals (receptors)

2. Mechanism for conveying this information to CNS

What happens during neuron computing?

1. Neurons transmit brief eletrical pulses with fixed amplitude and duration

2. Make excitatory or inhibitory connections with each other to create networks with one-to-many and many-to-one linkages

Where does the cell body deliver to electrical impulse to?

Another neuron

What determines the intensity of a neuron?

The frequency of the firing rate

What are the two receptors in the retina of the eye and what are they?

Rods and cones- modified neurons containing photosensitive pigment (rhodopsin)

What light do rods and cones function in?

Rods- low lights

Cones- bright lights

What are cones in the retina tuned to detect?

They are colour-tuned with peak sensitivity to either red, green, or blue wavelengths

How does cone output depend on light?

Cone output depends on the wavelength of light- each cone type responds best to specific wavelengths

How is a single wavelength of light coded by cones?

A single wavelength is uniquely coded by the specific pattern of activation across three types of cones

How does the principle of colour coding apply to mixtures of wavelengths?

The pattern of cone activation extends to mixed wavelengths, forming the basis of colour mixing

What is white light?

A mixture of all visible wavelengths at the same intensity

How is the perception of white light coded by cones?

It is perceived when there is equal output from the red, green and blue cones

How many rods and cones does the retina contain?

Around 120 million rods and 7 million cones

How is the optic nerve formed?

From axons of approximately one million ganglion cells

What is the overall role of the retina in vision?

Performs local computations through a network of connections between photoreceptors and optic nerve

How do photoreceptors and bipolar cells respond to illumination?

Their response to light is graded meaning it varies in size depending on the intensity of illumination (not all-or-none)

What is the function of amacrine and horizontal cells in the retina?

They combine and contrast signals from adjacent photoreceptors, helping to sharpen and refine visual information

Which cells generate action potentials in the retina?

Ganglion cells generate action potentials and their axons from the optic nerve

What did Hubel and Wiesel 1962 investigate?

They used single cell recording techniques (by inserting a microlectrode) to investigate the visual cortex of the cat

What did Hubel and Wiesel (1962) find)

They discovered distinct cell types categorised as simple and complex cells, and mapped out their receptive fields

How was feature detection studied in neurophysiological experiments?

By monitoring the eletrical responses of a single cell when bright lines in different orientations were projected onto a small area of the retina

What did these experiments reveal about certain neurons?

Particular neurons are selectively active in response to specific stimulus at a specific orientation- these are called feature detectors

Does the retina function as a simple light detector?

No, retinal interconnections allow for early image ‘cleaning’ and feature extraction

Where does retinal input ultimately feed into?

The visual cortex

How is the visual cortex organised?

Hierachically, with separate structures for processing shape, colour, position, motion etc.

How are features processing structures organised in the visual cortex?

In layers

What do early stages of visual cortical processing detect?

Elementary features like edges and lines, via simple cells

How do simple cells operate?

Each simple cell responds to a specific line or edge in a particular region of the retina

Are animals consciously aware in these experiments?

No, conscious awareness isn’t required; raises ethical questions on animal welfare

What processes are involved in perception

Requires computational processing of sensory data including:

1. Segmentation and object recognition

2. Construction of a 3D representation

Are perception processes conscious?

Mostly automatic or innate, with limited awareness of raw sensory input

What is segmentation in vision?

Grouping features that belong to the same object

What is figure-ground perception?

Distinguishing an object (figure) from its background (ground), essential for recognition

Can figure-ground distinction be ambiguous?

Yes (e.g. Rubin Vase illusion) but the brain must always decide on one interpretation

What do Gestalt principles of grouping explain?

How visual elements are automatically and innately grouped to form coherent figures

How does the visual system perceive depth from 2D images?

It automatically constructs a 3D world interpretation

What does the Necker Cube demonstrate?

The brain’s automatic construction of 3D perception from 2D images, even when ambiguous

What are 4 depth perception cues?

1. Relative height

2. Relative size

3. Perspective convergence

4. Texture gradient

What did Gibson and Walk (1960) investigate?

To invesitgate whether depth perception is innate or learned in infants using a visual cliff apparatus to create the illusion of a drop-off.

What did Gibson & Walk find about infants and depth perception?

Most infants avoided the “deep” side, showing early depth perception abilities

How did newborn animals behave on the visual cliff?

Newborn goats immediately avoided the deep side, suggesting innate depth perception

What is perceptual constancy?

The brain’s assumption that objects are stable and unchanging, depite changing sensory input

How does the brain perceive true object size?

It computes true size by factoring in view conditions and distance

What happens when objects of the same image appear at different distances?

It creates visual illusions due to perceptual processing of real size

How does the brain’s interpretation influence size judgements?

Judgements are based on perceived real-world distance and size, not just raw image size

What is brightness constancy?

The brain compensates for changes in illumination perceive consistent object brightness

What does the checker shadow illusion demonstrate?

Our brain automatically corrects for unseen light source effects, adjusting perceived brightness