3 Psychophysics A study of the relationship between physical characteristics of stimuli and our psychological experience with them. Physical World Psychological World LightBrightness SoundVolume PressureWeight SugarSweetness SENSATION & PERCEPTION

Sensation: A person’s awareness of the world The detection of physical energy emitted or reflected by physical objects Occurs when energy in the external environment or the body stimulates receptors in the sense organs

Perception: The process of organizing and interpreting sensory information The process by which the brain organizes and interprets sensory information

Sense receptors Specialized cells that convert physical energy into electrical energy that can be transmitted as nerve impulses to the brain

Sensations occur via the sense organs The process by which the sense organ's (ex., nose) sensory neurons convert the physical or chemical energy (ex., skunk molecules) into electrical signals (neural impulses) which may be sent to the brain for processing (ex., unpleasant odor) is TRANSDUCTION

Sensation and perception chapter 6

Bottom-Up Processing: We process this way when we have no prior knowledge. We start at the bottom and work our way up.

10 Bottom-up Processing Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind. Letter “A” is really a black blotch broken down into features by the brain that we perceive as an “A.” Bottom up = the dalmation!

Top-Down Processing: using our past experiences and expectations to organize and interpret situations. We process this way when we have prior knowledge. We start at the top and have to work to process details.

12 Top-Down Processing Information processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations. THE CHT

Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a total mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. Note: NOT based on study done by Cambridge; scientific validity doubtful; BUT gives example of top down processing AND the importance of perceptual constancy

The Man Who Cannot Recognize Faces (Myers online video)

Different sensory modalities exist because signals received by the sense organs stimulate different nerve pathways leading to different areas of the brain. Synesthesia A condition in which stimulation of one sense also evokes another Synesthesia: The Man Who Tastes Words (Myers online video) chapter 6

1. Absolute Threshold: The minimum amount of stimulation a person can normally detect 2. Difference Threshold or “JND” (Just Noticeable Difference): the amount of change in stimulation that it takes to notice a difference Threshold: An edge or boundary

Absolute Threshold

18 Thresholds Absolute Threshold: Minimum stimulation needed to detect a particular stimulus 50% of the time. Proportion of “Yes” Responses Stimulus Intensity (lumens)

Absolute thresholds Vision A single candle flame from 30 miles on a clear night Hearing The tick of a watch from 20 feet in total quiet Smell One drop of perfume in a 6-room apartment Touch The wing of a bee on the cheek, dropped from 1 cm Taste One teaspoon of sugar in 2 gallons of water chapter 6

20 Subliminal Threshold Subliminal Threshold: When stimuli are below one’s absolute threshold for conscious awareness. Kurt Scholz/ Superstock

Difference threshold The smallest difference in stimulation that can be reliably detected by an observer when two stimuli are compared Also called the Just Noticeable Difference (JND) Less intense = smaller JND More intense = greater JND chapter 6

22 Weber’s Law Two stimuli must differ by a constant minimum percentage (rather than a constant amount), to be perceived as different. Weber fraction: k =  I/I. StimulusConstant (k) Light8% Weight2% Tone3%

Signal-detection theory A psychophysical theory that divides the detection of a sensory signal into a sensory process and a decision process chapter 6

24 Sensory Adaptation Diminished sensitivity as a consequence of constant stimulation. - Put a band aid on your arm and after awhile you don’t sense it. - after sitting in a hot tub for 10 minutes, it doesn’t feel as hot

Sensory adaptation and deprivation Adaptation The reduction or disappearance of sensory responsiveness when stimulation is unchanging or repetitious Prevents us from having to respond continuously to unimportant information Deprivation The absence of normal levels of sensory stimulation chapter 6

26 Now you see, now you don’t

Sensory Adaptation & the Eye The phenomena on these slides rely on your eye movements. You will be moving them anyway (you can never keep your eyes really still), but the demonstrations are aided by moving your display (if possible), or by scrolling the page in small steps.

The first sample on the right is the “Ouchi Illusion” (Ouchi 1977, Spillmann et al 1986). When you shake your head rapidly, or better shake the display, a central disk will segregate as a distinct object, which in addition seems to be floating atop the background.

Akiyoshi Kitaoka’sAkiyoshi Kitaoka’s image on the left is called “Out of Focus”. It also leads to a seeming shift of the central disk with respect to the surround. It is very effectively provoked by the eye movements occurring during reading. So, while you are reading this cast your “inner eye” to the left and watch for a seeming decoupling of disk and background. You may also observe that the disk floats above the background. (Image reproduced with kind permission.)

This sample called “Floating Motion” from Pinna & Spillmann (2002) also often appears very strong to me. I do not need to shake the screen, or the saccades from reading, just by exploratory eye movements over the image the centre square “decouples”. Here the background seems to move, while the central square remains in place, and seems to float on top. (Image redrawn with kind permission.)

SENSORY ADAPTATION The more your sense organ is exposed to a continuous level of stimulation (ex., wearing jewelry), the less your sense organs respond to it (ex., after a while you don't feel it) DOES NOT HAPPEN WITH INTENSE STIMUI.ATION - encourages us to focus on novelty - critical for survival (ex., if adaptation didn't occur with discomfort of sitting on rocks, might have missed real danger, like predatory animal approaching)