Chapter 4: Sensation and PerceptionSensation and Perception Are Distinct Activities- Sensation- simple stimulation of sense organo Basic registration of light, sound, pressure, odor, taste as parts of body interact with physical world- Perception- organization, identification, and interpretation of sensation in order to form a mental representation- Eyes aren’t really seeing words, simply encoding different lines and curves on a page- Brain transforms those lines and curves to coherent mental presentation of words and concepts- Damage to visual processing centers in brain can interfere with interpretation of info coming form eyes: the senses are intact, but perceptual ability is compromised - Transduction- when many sensors in body convert physical signals from environment into encoded neural signals sent to central nervous systemPsychophysics- Structuralists, led by Wilhelm Wundt and Edward Titchener, tried using introspection to measure perceptual experiences; failed miserably- Psychophysics- methods that measure strength of stimulus and observer’s sensitivity to that stimuluso Developed by German scientist and philosopher Gustave FechnerMeasuring Thresholds- Psychophysicists being measurement process with single sensory signal to determine precisely how much physical energy is required for an observer to become aware of sensation- Absolute threshold- minimal intensity needed to just barely detect stimulus in 50% of trialso Threshold is boundary- Just noticeable difference (JND)- minimal change in stimulus that can just barely be detectedo Not fixed quantity, depends on how intense stimuli being measured are and on particular sense being measured- Weber’s law- just noticeable difference of stimulus is constant proportion despite variations in intensityo Resulted from Fechner applying, German physiologist, Ernst Weber’s insight directly to psychophysicsSignal Detection- Transition from not sensing to sensing is gradual- Signal detection theory- response to stimulus depends on both person’s sensitivity to stimulus in presence of noise and on person’s decision criteriono Observers consider sensory evidence evokes by stimulus and compare it to an internal decision criteriono Allows researchers to quantify observer’s response in presence of noiseo Proposes way to measure perceptual sensitivity (how effectively perceptual system represents sensory events)o Offers practical way to choose among criteria that permit decision makers to takeinto account consequences of hits, misses, false alarms, and correct rejectionsSensory Adaption- Sensory adaption- sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditionso Our sensory systems respond more strongly to changes in stimulation than to constant stimulationo Change in stimulation often signals need for actionVision I: How Eyes and Brain Convert Light Waves to Neural Signals- 20/20 vision refers to measurement associated with Snellen chart, named after Hermann Snellen, Dutch ophthalmologist who developed it to asses visual acuity- Visual acuity- ability to see fine detailo Smallest line of letters that typical person can read from distance of 20 feetSensing Light- Visible light is simply portion of electromagnetic spectrum that we can see- 3 properties of light waves:o Length determines it hue, what humans perceive as coloro Amplitude determines what we perceive as brightness of lighto Purity is number of distinct wavelengths that make up light Corresponds to what humans perceive as saturation, richness in color- Eyes have evolved as specialized organ to detect lighto Light that reaches eye passes through cornea (clear, smooth outer tissue)o Blends light wave and sends it through pupil- Retina- light sensitive tissue lining in back of eyeballo Behind iris- Accommodation- process by which eye maintains clear image on retina- If eyeball is too long, images are focused in front of retina, nearsightedness (myopia)- Eyeball is too short, images are focused behind retina, farsightedness (hyperopia)- 2 types of photoreceptor cells in retina contain light sensitive pigments that transduce light into neural impulseso Cones- detect color, operate under normal daylight conditions, and allow us to focus on fine detailo Rods- become active under low-light conditions for night vision More sensitive to photoreceptors than coneso Difference b/t rods and cones 120 million rods are distributed evenly around each retina except in very center, fovea- Fovea- area of retina where vision is clearest and there are no rods at all 6 million cones, which are densely packed in fovea and much more sparsely distributed over rest of retina. o Blind spot- location in visual field that produces no sensation on retinaPerceiving color- Sir Isaac Newton pointed out that color is not something “in” lighto Color is nothing but our perception of wavelengths from spectrum of visible light- Rainbow of hues and accompanying wave lengths called visible spectrum- Rods ideal for low-light vision, but bad for distinguishing colors- Cones come in 3 types:o Long-wavelength (red)o Medium-wavelength (green)o Short-wavelength (blue)- Color-opponent system- pairs of visual neurons work in opposition:o Red-sensitive cells against green-sensitive o Blue-sensitive cells against yellow-sensitiveVisual Brain- Streams of actin potentials containing information encoded by retina travel to brain along optic nerve- Area V1- part of occipital lobe that contains primary visual cortexo Specialized for encoding edge orientation- 2 visual streams project from occipital cortex to visual areas in other parts of brain:o Ventral (below) stream travels across occipital lobe into lower levels of temporal lobes and includes brain areas that represent an object’s shape and identity  A “what” pathwayo Dorsal (above) streams travel up from occipital lobe to parietal lobes, connecting brain areas that identify location and motion of object “where” pathwayo Possible to damage one while leaving other intact- Visual form agnosia- inability to recognize objects by sightVision II: Recognizing What We PerceiveAttention: “glue” that binds individual features into a whole- Binding problem- how features are linked together so that we see unified objects in our visual world rather than free-floating or miscombined features- Illusory conjunction- perceptual mistake where features from multiple objects are incorrectly