Sensation & Perception Midterm 1

perception

our link and access to the world, construction of our reality

result of available physical energy in the world, sensitivities of our sense organs, and information processing in the brain

includes sense of 3D space/distance, sounds/voices, tactile (touch) sensations

perception informs an organism about

what or who is in its environment, where it is, and how to interact with it

evolutionarily significant actions - free from predators, hunt/gather food, find mates, navigate

naive realism

the world is simply as it appears, physical (measured) and perceptual (as we see it) world are the same thing, I see the apple as round and red because it is round and red

problems:

• we are sensitive to a small fraction of the physical energy around us - visible light is small part of electromagnetic spectrum, ears cannot detect low frequency <20 Hertz or high frequency >22,000 Hertz sounds, we cannot perceive electric or magnetic fields

• different animals perceive the world differently - sharks sense electrical trails left by prey, trout can navigate using Earth’s magnetic field, bats navigate using high frequency Sonar sounds and sensing patterns of echoes

multistability

changes in perception with no change in physical stimulus

Ex - continuing to stare at Necker cube and after a while a different face appears to be in front, rabbit/duck illusion

subjective idealism

the world exists only as a result of perception, its all in the mind, no world is independent of someone’s perception (Bishop Berkeley), supported by brain in vat problem

problems - leads to solipsism (only my mind exists), has difficulty explaining regularities in perception

John Locke observation

if you first dip hands in hot water than in lukewarm water you will think the same water feels much cooler – warmth is property of perceivers' state, not a property of the water itself 

primary qualities - intrinsic properties of objects (motion, shape, bulk)

secondary qualities - subjective properties of the perceiver (warmth, color, sound, taste), all qualities are secondary to subjective idealists, not scientifically useful (how to unify the study of mind with other sciences) 

visual perception

light and its interaction with objects, structure and function of the eye, and information processing in the eye and brain

over 1/3 of cortex in monkeys and humans is involved

input is image - 2D array (matrix/spread) of light intensities, unstructured

output is representation of 3D environment, 3D shapes of objects, relative locations, material properties, identity

unsolved problem of 2D to 3D

how can any system (brain, ai) transform 2D arrays of light and dark into 3D objects and environments 

• angle – image angle vs perceived angle 

• shape – image shape (2D trapezoid) vs perceived shape (3D rectangle) 

• size – image size vs perceived size (Shepard’s monster)

given 2D image could have been projected by many different 3D structures.

input and output of human vision

image produced on back of eyeball in retina, where 3D scene you are looking at is distal stimulus and 2D retinal image is proximal stimulus

Shepard’s illusion of 2 monsters - they have the same size at the level of the perceptual “input,” but different sizes at the level of the perceptual “output”

fundamental problem of perception

every proximal stimulus us consistent with many different distal stimuli, each image is consistent with many different scene interpretations

optics

mapping from 3D scene to projected image on retina 

inverse optics is mapping from projected image on retina to 3D scene 

perception involves construction

governed by strict rules, shape, color, size are attributes computed by the brain, starting with the pattern of light on the retina

even though we can’t consciously control our perception in most cases, it relies on biological constructive processes in the central nervous system

approaches to study of perception

detailed circuitry, input-output relations (can feed different inputs to see what output – information processing)

• Theoretical (computational and evolutionary) - what problem is it solving 

• Psychological (behavioral) - what strategy is it adopting on higher level (algorithm) 

• Biological (neuroscience) - hardware implementation 

Ex – cash register – does addition because that is how prices combine, representation is decimal and algorithm is rules for adding, hardware is mechanical/electronic

psychophysics

study of relationship between physical world and psyche, founded by Gustav Fechner 

• physical variable (intensity of light) vs perceptual attribute (brightness) 

absolute threshold

minimum threshold needed to evoke a sensation

difference threshold

minimum change in intensity that leads to a noticeably different stimulus, boundary between two stimuli looking the same or different 

Weber’s law – the difference threshold is not a fixed value, but is proportional to stimuli intensity, smaller Weber fraction = more sensitive   

• delta I = K times I → I = original intensity, delta I = change in I, K = fixed ratio (Weber fraction) 

Each difference threshold corresponds to a just noticeable difference (JND) on y axis (perceptual axis)

Fechner’s psychological methods

method of constant stimuli - fixed stimuli in random order

method of limit - fixed stimuli in up or down sequence

method of adjustment - adjusted by observer

method of constant stimuli

fixed set of stimuli, range from undetectable to easily detectable, presented multiple times in random order, respond yes or no → plot percentage of detections

intensity of stimuli vs percent detections (yes responses) - ideal case is that detection is 0 until absolute threshold where detection rate jumps to 0. what actually happens is a gradual increase of yes responses and absolute threshold is at 50% (so not due to chance)

method of limits