Sensation & Perception

1. Sensation & Perception • Sensory Branding: in 1946 Donald Deskey recognized the power of perception when he grabbed the attention of consumers by using eye-catching colours and a striking design on the first box of Tide. A simultaneous exploitation of all the senses. • Systems approach: remember that the key element of our eyes--the retina--is an extrusion of brain matter, this chapter looks at how physical energy in the world around us is encoded by our senses, sent to the brain, and enters conscious awareness. • In prosopagnosia--an inability to recognize the faces of familiar people, typically as a result of damage to the brain--damage to the visual processing centres in the brain can interfere with the interpretation of information coming from the eyes. • Memorize Table 4.1 (Transduction) for the next exam. Note taste & smell are linked. • An example of transduction: vibrations from a guitar string cause changes in air pressure that propagates through space to the listener's ears. • In psychophysics, we attempt to quantify a persons private or subjective perceptions.

2. Measuring Thresholds • Memorize Figure 4.1 (Absolute Thresholds) for the next exam. • The simplest quantitative measurement is the absolute threshold (or boundary), the minimum intensity needed to just barely detect a stimulus in 50% of the trials. • the just noticeable difference is the minimum change in a stimulus that just barely be perceived (note the lack of mechanical exactitude). • people tend to be more sensitive to the range of tones corresponding to human conversation. • the jnd is not a fixed quantity, rather it depends on how intense the stimuli being measured are and on the particular sense being measured. See Table 4.2 • sensory signal face a lot of competition or noise (hence the importance of signal-to-noise ratio). • competing stimuli may come from either the internal (moods, memories, motives, muscle tension) and external (other sights, sounds & smells from the external world). • signal detection theory is a response to a stimulus depending both on the person's sensitivity to the stimulus in the presence of noise and the person's response criterion. • if the sensory evidence exceeds the criterion, the person responds. Fig. 4.2 • signal detecton theory offers a practical way to choose among criteria that permit decisions makers to take into account the consequences of hits, misses, false alarms, and correct rejections.

3. Sensory Adaptation • Sensory adaptation: sensitivity to prolonged stimulation tends to decline over time as the organism adapts to current conditions. • sensory adaptation is generally due to neural fatigue; the cells that are firing to signal the stimulus get tired after a time and stop firing. • if the cells have a chance to rest, they will signal again as strongly as ever. • This renewed firing is modified by habituation, cell response fading with time as a result of learning.

4. Eye To Brain • For this course, we will begin with the retina, the light-sensitive tissue lining the back of the eyeball. It is not a photographic film; the first layer is the retina surface leading the optic nerve. Accommodation is the process by which eye musculature maintains a constant clear image on the retina. • The actual light-sensitive and light-computer cells are behind this surface. • Memorize Fig. 4.6 (Close-up of the retina) for the next exam. • the fovea is the area of greatest visual acuity, where most colour-sensitive cones are located. Fine detail is correlated to colour. Note that each rod has its own pathway from bipolar to cell to the retinal ganglion cells, while rods are clustered together . • Six million cones detail colour, operate under daylight conditions, and allow us to see fine details; rods become active under low-light conditions for night vision. • Because rods contain the same photopigment, they provide no information about colour and only respond to shades of gray. 120 rods are evenly distributed around the retina, except for the fovea, where vision is clearest and there are no rods at all. • The distribution of cones directly affects visual acuity and why peripheral objects are not so clear. • The bipolar cells collect neural signals from the rods & cones,transmitting them to the outermost layers of the retina, where retinal ganglion cells organize the signals and send them to the brain. • These bundles form the optic nerve; the area of exit is called the blind spot, the location in the visual field that produces no sensation on the retina.

5. Perceiving Colour • Cones are responsible for colour vision, and come in three types, red, green and blue. • Colour perception arises from the combination of these three basic elements in the retina. • Note that in mixing light, not pigments, all three together produce white, not black. • A genetic disorder where one of the cone types is missing results in a colour deficiency. • Cones tire out after firing, and this creates an afterimage of the opposed colour. Blue-sensitive cells work against yellow-sensitive; green-sensitive against red-sensitive. • Half of the axons in the optic nerve code information to the right visual field, whereas half code to the left visual field. • The optic nerve travels from each eye to the lateral geniculate nucleus located in the thalamus. From there the signal travels to the V1 area of the occipital lobe, the primary visual cortex. • Memorize Fig. 4.11 (Visual Pathway from Eye to Brain) for the next exam. • Area V1 is specialized for encoding edge orientation. It contains populations of neurons each tuned to respond to edges oriented at each position of the visual field. • Fig. 4.12 (Single Neuron Feature Detectors). • Fig. 4.13 Visual Streaming.

6. Visual Streaming • The ventral stream travels across the occipital lobe into the lower levels of the temporal lobe, including brain areas that represent an object's shape and identity. • The dorsal stream travels up from the occipital lobe to the parietal lobe (including some of the middle and upper levels of the temporal lobes) connecting with brain areas the identify the location and motion of an object. • The two pathways were discovered when studying visual form agnosia. When researches asked patient D.F. to orient her hand to match the angle of the slot in the testing apparatus, she was unable to comply. However, when asked to insert a card into the slot at various angles, she accomplished the task perfectly. • Conversely, damage to the parietal lobe (a section of the dorsal stream) causes difficulty using vision to guide reaching and grasping, though they can recognize what objects are. • recent fMRI research indicates that some regions within the dorsal stream are sensitive to properties of an object's identity, responding differently to line drawings of the same object in different sizes or viewed from different perspectives. This may be how the ventral and dorsal streams exchange information about what and where. • Memorize Fig. 4.13 (Visual streaming) for the next exam.

7. Perceptual Mistakes • The binding problem: how features are linked together so that we see unified objects in our visual experience rather than free-floating or uncombined features. • Fig. 4.15 Illusory Conjunctions occur whenever features such as colour or shape from multiple objects are combined incorrectly. These illusions look real to the participants, who were just as confident they had seen them as they were about the actual coloured letters they perceived correctly. • Illusory conjunctions are theorized to occur because of feature integration. Focused attention is not required to detect the individual features that comprise a stimulus, such as colour, shape, size & location, but is required to bind those individual features together. • In the experiment being considered, participants were required to process the digits that flank the coloured letters, thereby reducing attention to the letters and allowing illusory conjunctions to occur. • Because binding involves linking together features processed in distinct parts of the ventral stream at a particular location, it also depends critically on the parietal lobe in the dorsal stream. • Recent studies suggest that damage to the upper and posterior portions of the parietal lobe is likely to produce severe problems attending to spatially distinct objects.

8. Recognizing Objects by Sight • Why is it that we can recognize the letter 'G' in hundreds of different fonts? • In general object recognition proceeds fairly smoothly, largely due to feature detectors. • Modular view: specialized brain areas detect and represents faces, houses and body parts. • using fMRI in healthy young patients, a subregion in the temporal lobe was found that responds most strongly to faces, whereas a nearby area to buildings and landscapes. • Distributed view: a pattern of activity across multiple brain regions identifies a viewed object. • Both views are necessary for a full understanding. • Quiroga et. al. (2005): electrodes were placed in the temporal lobes of epileptics. These volunteers were then shown pictures of faces and objects, and their neural responses were recorded. • Neurons in the temporal lobe responded to specific objects viewed from multiple angles, and people wearing different clothing and facial expressions. • Neurons also responded to words corresponding to specific objects. This led to the conclusion of perceptual constancy: even as aspects of sensory signals change, perception remains constant.

9. Principles of Perceptual Organization • Fig. 4.16 (Perceptual Grouping Rules). • Fig. 4.17 (Ambiguous Edges). A reversible figure-ground relationship here shows up in fMRI as greater activity in the face-selecting region of the temporal lobe. • Image-based versus parts-based object recognition: too many templates! The more likely solution is the dynamic assembly (deconstruction and re-construction) of geometric primitives, or geons. • Part-based does have a major limitation. It allows for objects only at the level of categories and not for individual objects, so it does not clearly explain the difference between a friend's and a strangers' face. • Fig. 4.19 (Familiar Size and Relative Size) Monocular Depth Cues are aspects of a scene that yield information about depth when viewed with only one eye. • Fig. 4.19 (Pictorial Depth Cues). Binocular vision is caused by retinal disparity, or the overlap of the left and right visual fields. • Fig. 4.22 (The Amazing Ames Room). Only monocular cues available, so our brains, conditioned to right-angled buildings, make incorrect assumptions about the size of the two people. • Motion perception is caused by the change in retinal stimulus; a region in the temporal lobe (part of the dorsal stream) is specialized for the visual perception of motion.

10. Limitations of Attention • Apparent motion: the perception of movement as a result of alternating signals appearing in rapid succession in different locations. Video technology and animation depend on this, it is also called visual capture, for humans 24 frames per second. • Fig. 4.32 (Change Blindness): failure to detect changes in a scene. • Inattentional blindness: a failure to perceive objects that are not the focus of attention. • Using smartphones draws on focused attention, resulting in increased inattentioal blindness. • Our conscious experience of our visual environment is restricted to those features or objects selected by focused attention. • We will discuss hallucinations from a whole new point of view in Web Article Four.