Sensation & Perception

What’s the difference?Sensation

Detecting stimuli from the body or environment

PerceptionOrganizing sensations into meaningful patterns

StimulusForm of energy that can affect sense organs

Sensation

Sensory receptorsDetect stimuli & convert energy into neural impulsesReceptors are designed to serve very specific functions

Thresholds and Stimulus Change

ThresholdsThere is a minimum amount of any given sensation that has to be present for us to notice itAbsolute threshold

This is the minimum amount of a stimulus that is necessary for us to notice it 50% of the time

Sensory adaptationIf a stimulus is unchanging, we become desensitized to itKeeps us focused on changes, not constants

VisionElectromagnetic energy

Long wavelengths: AC circuits, radio waves, infrared raysShort wavelengths: visible light, X-rays, UV & gamma raysOther animals can see other segments of the spectrum of electromagnetic energy

Bees can see ultraviolet rays and blue-violet, but not redPit vipers can see infrared rays Dogs can’t see all the colors that humans can (no red)

Our Visible Spectrum Red Orange Yellow Green Blue Indigo

Violet

Hue determines colorDepends on length of the distance from one peak to the next on the wave

Intensity determines brightnessDepends on amplitude of the wave

TransductionThe process where the eye converts electromagnetic energy (light) into nerve impulses

The Eye

Parts of the EyeCornea

Light is initially focused by this transparent covering over the eye

PupilLight enters the eye through this opening

IrisMuscle connected to the pupil that changes its size to let in more or less lightEveryone has a unique iris (thus it is a new security technique being employed by some organizations)

Parts of the EyeLens

This flexible disk under the cornea focuses light onto the back of the eyeAccommodation

Flexibility of the lens allows eye muscles to adjust light from objects at various distances away

RetinaLight reflected from the lens is received by this sheet of tissue at the back of the eyeContains the receptors that convert light to nerve impulses

How we see color: ConesRetinal cells that respond to particular wavelengths of light, allowing us to see color

Most of our cones are located on the fovea, which gives us the sharpest resolution of visual stimuli3 types of cones, each sensitive to different light frequencies

How we see in the dark: Rods

Retinal cells that are very sensitive to light but only register shades of gray (i.e., no color)Rods are located everywhere in the retina except in the fovea

Rods allow us to see at night without strong light – this is why we see less color at night

Because of where the rods are on the retina, we see best at night without light in the periphery of our vision

Dark adaptation

Rods & Cones

Optic NerveFrom the receptor cells in the retina, the converted impulse from light is directed to the optic nerve

This is the large bundle of nerve fibers that carry impulses from the retina to the brainIt sits on the retina but contains no cones or rods, so this is where you experience a ‘blind spot’

We aren’t aware that we have a blind spot because our brain completes patterns that fall across our blind spot and because our eyes are constantly moving (‘filling’ it in)Pg 149 in book - an activity to demonstrate this

Processing of Visual Information

RetinaProcesses electrical impulses, starts to encode and analyze sensory information (at the most basic level)

Optic NerveNeurons pick up the messages from retina, transmit to the thalamus, then on to the visual cortex, then on to more specified areas

Visual perception Principles

During organisation and interpretation the visual cortex applies rules to understand the information. We tend to automatically use these without any conscious effort.

There are 3 categories:Gestalt principlesDepth principlesPerceptual constancies

Gestalt Principles of VisionFigure-ground

We recognize objects (figures) by distinguishing them from the background (ground)

ClosureWe tend to fill in gaps in a figure

SimilarityParts of an image that look alike tend to be grouped together

ProximityParts of an image that are near one another tend to be grouped together

Depth PrinciplesHow is it that we perceive a 3-dimensional world when our eyes only project a 2-dimensional image on our retinas??!Our brain uses different cues to perceive depth. There are 2 types:

Binocular depth cues (convergence, retinal disparity)Monocular depth cues (accommodation, pictorial cues – linear perspective, interposition, texture gradient, relative size, height in the visual field

Binocular depth cuesOur brain uses both eyes working together to provide information about depth and distance (this is especially important for objects that are close).

Convergence The brain detecting depth and distance from the changes in the tension in the eye muscles that occur when the two eyes turn inward to focus on an object

Retinal disparityRefers to the difference in the location of the visual image on the retina (due to the angle of the view)

Monocular depth cuesMonocular cuesOur brain also uses information from the stimulus that requires the use on only one eye.AccommodationPictorial cues

Linear perspective InterpositionTexture gradient Relative sizeHeight in the visual field

Accommodation Involves the automatic adjustment of the shape of the lens to focus an object in response to changes in how far away that object is.

Monocular – pictorial depth cues

Linear perspectiveIs the apparent convergence of parallel lines as they recede (go back ) into the distance

InterpositionOccurs when one object partially block or covers another and therefore the one behind is perceived as further away

Texture gradientThe gradual reduction of detail that occurs in a surface as it recedes into the distance

Relative sizeThe tendency to visually perceive the object that produces the larger image on the retina as being closer

Height in the visual field Refers to the location of objects in out field of vision, whereby object that are located closer to the horizon are perceived as being more distant than objects located further from the horizon

Perceptual Constancies - Vision

The image of an object on your retina can very in size, shape, and brightness

But we still continue to perceive the object as stable in size, shape and brightness

Size constancyShape constancyBrightness constancy

Size Constanc

yThe tendency to view an object as constant in size despite changes in the size of its image on the retina (as we move)

Shape Constanc

yThe tendency to see an object as retaining its form despite changes in orientation

Brightness

Constancy

The tendency to perceive an object as maintaining its level of brightness in relations to its surroundings