Perception-1-24_compressed.pdf PDF

# Unit 2: Perception ## Perception - mental function which a person or animal can obtain information from the world and organisms, starting with a physical interaction between the organism and some type of energy at sensory receptors (electromagnetic energy, waves,..) ## A Perceptual Experience...

# Unit 2: Perception ## Perception - mental function which a person or animal can obtain information from the world and organisms, starting with a physical interaction between the organism and some type of energy at sensory receptors (electromagnetic energy, waves,..) ## A Perceptual Experience - Is a subjective experience, by means of which we can be aware of our body and our environment, that thus, we obtain the information to interact with other people and things. - Results from complex, unobservable processes (the perceptual process), in which unconscious operations change the original data obtained by our sensory system to create a mental representation. ## The Perceived Object - Can be: - **External, simple, a living thing...** - **Internal, a complex thing, a non-living thing...** ## Diverse Modalities - Visual, auditory, haptic. - With differences but with similarities in function (they all turn energy into electricity and all the information goes to the brain). ## "Pure" Experimental Task - Focuses only on perception, with no other cognitive processes involved. - **Detection:** A light stimulus that appears in a random moment in a random time and for example, press a button when seen. - **Discrimination:** Based on "if… you… if… you…" - **Comparison:** Two stimuli at the same time (or first one and compare them by magnitude). - **Recognition:** Involves perceiving a stimulus and memory and identification (you didn't perceive it before, but you know it's ex. ring). ## Distal Stimulus vs. Proximal Stimulus - **Distal Stimulus:** The perceived object. - **Proximal Stimulus:** The stimulus energy. A necessary condition but it’s non-sufficient and unstable. ## Environmental Stimulus vs. Attended Stimulus - **Environmental Stimulus:** The distal stimulus, all stimulus captured - **Attended Stimulus:** The stimulus to which we are paying attention. ## The Perceptual Process - **Stimulus** 1. Environmental stimulus 2. Attended stimulus 3. Stimulus on the receptors - **Electricity** 1. **Transduction:** Turning one form of energy (the one we receive) into another type (electricity). 2. **Transmission:** The electrical responses activate more neurons, which makes this electrical information travel from the sensory receptors to the brain. - **Sensory Nerves** 3. **Processing:** Interaction between neurons that transforms those signals into the perception of the perceived object (it's a representation, it is usually very different from the original signal). - Processing, integration, interaction. ## Experience and Action 1. **Perception:** Concious sensory experience, achieved when the electrical signals are transformed into the experience of perceiving. 2. **Recognition:** Ability to place the perceived into a category that gives it meaning. 3. **Action:** Includes motor activities to concentrate in the perceived. ## Knowledge - "Visual perceptions are unconscious interferences created from sensory data and our previous knowledge" - Hermann von Helmholtz. ## Inferences - Perception is only indirectly related to the world or our organism. It is inferred from partial sensory data, we used our previous knowledge to give data a meaning. - **Outside perception, but crucial for the process.** - **The process that makes us perceive** - **Stimulus** - **Transmission** - **Transduction** - **Electricity** - **Processing** - **Simultaneous process** - **Perception** - **Recognition** - **Action** - **Experience and action** - **Knowledge** ## Information Processing - **Top-Down Processing:** Processing information based on knowledge. (Ex. These images are going to show cows, and then perceive). - **Bottom-Up Processing:** Based in the incoming data (stimulation of the receptors) to reach knowledge. Automatic, fast, rigid. (Ex. What do you see?) - Once these bottom-up data triggers the sequence of steps of the perceptual process (reach knowledge), top-down processing come into play as well. ## Activity - "d’": The harder the tasks are, the lower the ‘d’ will be. It takes into account all. - If C is close to 0, it's neutral. - If positive, it's conservative. - If negative, it's liberal. - **Present** - **Yes:** Hit - **No:** Omission - **Absent** - **Yes:** False alarms - **No:** Correct rejections ## Two Main Approaches in the Study of Perception: 1. **Psychophysical:** (Measure relationship stimuli-behaviour) - **Physiological:** Interested in stimuli-cerebral events 2. **Physiological** - **Physiological processes** - **Stimuli** ## Reaction Time - The time it takes to respond to a stimulus. The faster the response (behaviour), the faster the processing. - Beginning stimulus - beginning of response ## Measuring Perception - **Techniques, instruments, and procedures to measure perception and derive mathematical functions that represent the relationship between the experiential aspects of perception and the physical characteristics of the stimulus.** ## Classical Psychophysical Methods - To determine perceptual thresholds. - **Absolute Threshold (Detection):** Smallest detectable stimulus intensity. - If a stimulis is detected 50% of the time, it's considered the threshold of the stimulus. - **Lower Threshold:** Higher sensitivity. ## Method of Constant Stimuli - There is not order of intensity. ## Method of Limits - Increasing intensity until the person perceives it or in reverse (after making it different times the average is made). ## Method of Adjustment - Very similar to the method of limits, but in this one, we start with an intensity and change it increasingly, not separately (not so accurate). ## Just Noticeable Difference (JND) - The amount of the increase or decrease in the stimulus needed to notice change. - It can be measured using the classic psychophysical methods, considering a model or norming stimulus as a reference. - **It depends on what you already have:** The greater the stimulus intensity presented, the larger the change needed to notice a difference. - **Weber's law**: - **$JND = KI$** - $I$ = Intensity of the Norming Stimulus - $K$ = Weber's fraction (constant) – different for each case. ## Signal Detection Theory - Used when needed to select a person (better if tested both, thresholds and sensitivity.). - **Sensitivity:** Ability to detect the stimulus, or the ability to discriminate between different stimulus. - **They are considered decision tasks** - The signal is hard to detect. - Noise is always present. ## Signal - Hit - False Detection - **No Signal** - Miss - Correct rejection - **d’:** Sensitivity response criteria. ## Applications - Assessment of human ability in diagnostic techniques using images (ex. x-rays). - Assessment of human ability in military systems, transport (ex. radars). - Assessment of human ability in quality control in industry. - Assessment of accuracy in testimony (faces, voices). - Testing the effectiveness of techniques to assess personal selection. ## Estimation of the Magnitude of Perception - A model or reference is given to measure the rest of stimulus. - Different from the rest: Clearly perceived stimulus. - It measures the physical perceived magnitude (ex. pain)- stimulus intensity - **Pain-expansion:** - **$P = K * S^n $** - $s$= physical intensity of stimulus - $k$= constant - $P$ = number we give to represent perception - We can predict if something is painful/appreciate the difference - If $N=1$, it is a linear relationship ## Visual Perception - Light is focused onto the retina: Accomodation - Light: electromagnetic energy, but we can only see a reduced part of it, thanks to the eye. - **Eye Anatomy** - **Ciliary Body** - **Suspensory Ligament** - **Cornea** - **Anterior Chamber** - **Pupil** - **Iris** - **Lens** - **Vitreous Body** - **Sclera** - **Choroid** - **Retina** - **Optic Nerve** - **Macula** **Parts of the Eye** - **Macula:** A part of the retina responsible of central vision that contains the fovea. - **Lens:** Bends and focuses light, changing its shape depending on how distant the object is. - **Pupil:** Controls the light that enters the eye. - **Retina:** In the back of the eyeball, converts the light into electrical signals and send them to the brain. - **Optic Nerve:** Transmits the information to the retina to the brain. ## Accomodation - If the object is far, the lights of rays are nearly parallel and eyes don’t need as much refraction for them to focus. - If the object is near, the light rays diverge and need more refraction. For this, the eye tightens the ciliary muscles, allowing the lens to become rounder. ## Retina - Red of network of cells including visual receptors. - Layer that has an interruption at the point (blind spot) where the optic nerves exit the eyes. - The brain fills up the blind spot (top-down mechanism) with the same information of the surrounding area. - **Light has to enter the retina (passing ganglion cells, amacrine cells, bipolar cells, and horizontal cells) in order to stimulate the rods and cones (the only ones that react to light) which react with electrical responses that affect/ influence bipolar cell and horizontal cells and their reaction makes ganglion cells also react.** - It enters by the left but the visual receptors are in the right (inner part) in order to protect them. - **Amacrine Cell** - **Ganglion Cell** - **Bipolar Cell** - **Rod** - **Horizontal Cell** - **Cone** - **Light** ## The Fovea - The fovea (center of the macula) corresponds to 0°. - There are no rods in the fovea. - About six million cones in each retina and one hundred and twenty million rods. ## Cones - Allows to discriminate differences on color, adapt quickly to darkness (7 min). ## Rods - More sensitive, our vision depends on them (they react with little light.). - **Rods** - **Sensitivity of light:** ↓ - **Speed adapt to darkness:** fast - **Cones** - **Sensitivity of light:** ↑ - **Speed adapt to darkness:** slow ## Dark Adaptation - About 20 min to adapt for us to reach our max level of vision (in darkness). ## Why Are Rods More Sensitive? - Because they convert. **Convergence (number of cells decreases as we go to the interior)**. ## Rods are More Sensitive to Light in Short-Wavelength Range - Whereas cones are more sensitive to light in the medium-wavelength range. - If the light is green/yellow, it will be perceived easily, better than red or blue (low light). - In darkness, since we are using rods, the best light is blue. ## The Purkinje Shift 1. Our perception when the stimuli are well lit, they are similar in brightness. 2. 2-3 min after switching the light: grey, similar in brightness. 3. 20 min later the red one is darker = Purkinje shift. - We are less sensitive to red in dark conditions. ## Visual Acuity - Distinguish a figure from the background. - More convergence of rods= less visual acuity (possibilities to discriminate). - **Cones** - **Specialized Into:** Bright light - **Rods** - **Specialized Into:** Low light - **Sensitivity:** ↓ - **Visual acuity:** ↑ - **Sensitivity:** ↑ - **Visual acuity:** ↓ ## Degeneration of Different Parts of The Retina - **Macular Degeneration:** You miss information of where you are looking at. - **Retinis Pigmentosa:** You miss information from the peripheral area. Normally ends up in blindness. ## Some Perceptual Phenomena Related to Lateral Inhibition. - Some cells can pass information in a lateral way = inhibit information. - When looking at a certain point, this point is the fovea and the rest is the retina. ## Optic Nerve, LGN of the Thalamus, Visual Receiving Area (Striate Cortex), Extrastriate Cortex - **The optic nerve transmits visual information from the eye to the brain**. - **Flow top-down** - **The LGN of the thalamus regulates the flow of neural information from the retina to the visual cortex, and it receives cortical signals that may mudulate the visual signals.** - **The visual receiving area in the cortex (=primary visual cortex= striate context=VI = Broadman area 17) has “feature detectors”(have preferences for some stimulus, a lot more specialised).** - The extrastriate cortex is responsible for the high processing of visual signals, and their dispatch to other parts of the brain. ## Retinotopic Maps - The neurons in the LGN of the thalamus, and also in the cortex, process visual information according to retinotopic maps in a way that they respect where information is outside. - Perfect correspondence of the exterior according to where they are in the space. - However, in the striate cortex, the number of neurons that process visual information from the fovea is much higher than the expected= cortical magnification factor (to process with the higher detail). ## Receptive Fields of Ganglion Cells, LGN Cells, Cortical Cells. - Neural convergence is based on the fact that the firing rate of the A neuron depends on the amount of the light stimulating the receptors 1 and 2. - **Low Firing Rate** ---- - **--- High Firing Rate** ## Then, We Can Introduce The Receptive Field of A Neuron - The set of sensory receptors that influence the firing rate of this neuron. - RRRRRR - RR - 6 - N - **Type of Cell** - **Optic nerve fiber (ganglion cell):** Center-surround receptive field. Responds best to small spots, but will also respond to other stimuli. - **Lateral geniculate:** Center-surround receptive fields very similar to the receptive field of a ganglion cell - **Simple cortical:** Excitatory and inhibitory areas arranged side by side. Responds best to bars of a particular orientation. - **Complex cortical:** Responds best to movement of a correctly oriented bar across the receptive field. Many cells respond best to a particular direction of movement. - **End-stopped cortical:** Responds to corners, angles, or bars of a particular length moving in a particular direction. ## We Have "Features Detectors" in The Visual Cortex - Neurons that only activate with a certain stimulus, type of moviment,etc. ## Linking Neural Processing and Perception: Role of Feature Detectors in Selective Adaptation, and in Selective Rearing Experiments - **Evidence from experiments of selective adaptation to a visual feature** - **Ex. Adaptation to vertical orientation:** - Stimuli: Gratings varying in orientation: - 40º 20º 0º 20º 40º - We are better to detect contrast is vertical or horizontal than obliq. ## Experimental Procedure 1. Determinate the contrast threshold at each orientation. 2. Adapt the person to the vertical grating, using a high contrast one. 3. Repeat 1. - The threshold raises = If we get adapted to something, we notice less the stimuli/effect. ## The Role of Feature Detectors in Perception - **Ex. Evidence from selective rearing experiments with kittens.** - A new born cat is putted in a box with vertical strays painted on it. - Makes the cat unable to detect the vertical strays and is not-reversible. ## Activity - The selective adaptation experiments indicate that when we have been adapted to a specific visual feature, the detection of this feature for some time is impaired. - The selective rearing experiments indicate that when an animal is deprived of a specific visual feature, the detection of this feature improves. ## Pathways of What and Where/How - **If temporal lope is removed, object discrimination (shape) is inhibited.** - **If the parietal lobe is removed, the landmark discrimination (which one is closer, etc.) in inhibited.** - **Where Flow** - **Dorsal Pathway:** - Parietal lobe - **Ventral Pathway:** - Temporal lobe - **Occipital Lobe (Primary Visual Receiving Area).** - **Two types of ganglion cells that send information to different parts of the thalamus:** - **V1** - **Magno LGN** - **M-Ganglion cell** - **Parvo LGN** - **P-Ganglion Cell** ## Ventral Pathway - Places (scenes). - Cells specialized in different shapes, one for almost any shape that we have learned before (and depending on the orientation). ## Visual Perception of Objects and Scenes - **Objects, scenes, and objects in scenes:** - If we are shown a scene with objects that are normally but not in the typical configuration (very disorganized), we won't be able to identify it quickly. - 3- We can focus attention. ## Problems That Have to be Solved by Our Visual System in order to Perceive Objects - Depending in the visual perception, we can identify different things = the stimulus on the retina can be ambiguous. - Objects can be hidden or blurred but with partial data, still can be perceived. - Objects can be viewed from different angles. ## First Attempts to Explain Object Perception: Structuralism vs Gestalt - **Structuralism:** - Object perception is built up of sensations, adding up to create a perception. - Couldn't explain: - **Apparent movement** - **Illusory contours** - **Bistable figures** - **Gestalt:** - The whole is different from the sum of its parts. Object perception is a result of perceptual organization. ## Apparent Movement: Stroboscopic Movement - If structuralism was true, we shouldn’t feel the sensation of movement because it’s not real. - **A:** Flash line on the left. - **50ms of darkness** - **B:** Flash line on the right. - **A B:** Perception- movement from left to right. - We perceive only one light moving from side to side. ## Illusory Contours - Contours (are crucial) = parts of the seen in which there is a clear difference in light or color. - Our need is so big that sometimes we create the contours. ## Bistable Figures - **Stimuli that can be interpreted according to two different, mutually exclusive perceptions (we can switch between them, but can’t perceive in the same time).** ## "Laws" of Gestalt Approach - **Heuristic of Perceptual Organization:** How elements in a scene tend to group together. - **Pragnaz:** Pattern is seen for the resulting structure to be the simplest option. - **Proximity:** Close things are grouped together. - **Similarity:** Similar things are grouped together (color, shape, ..). - **Good continuation:** Things that, when connected, results in contours that follow smooth paths are grouped together. - **Common fate:** Things that move together are grouped. - **Closure:** Fragmentary stimuli are completed to perceive a single unit. - **Common region** - **Connectedness:** ## Heuristics of Figure-ground Segregation: Determining What Part of the Environment is the Figure so That It "Stands Out" - **There are no correct interpretations.** - **Different memories and experiences influence people. If something has meaning to someone, it will move their attention.** >- **Top-down processing.** **Figures are More "Thinglike" and More Memorable.** - Figures are seen as being in front of the ground and the ground as a uniformed material that extends behind. - The contour separating the figure from the ground appears to belong to the figure. - Normally are: - In the lower part of the display. - Symmetric. - Relatively small area. - Convex. - Orientated vertically or horizontally rather than obliquely. ## Recognition-by-Components Theory (Irving Biderman, 1987) - The brain breaks down complex objects into simpler, three-dimensional shapes that can be recognized regardless of the object’s orientation or position = geon. - Each geon has a unique set of non-accidental properties so that it can be discriminated from other geons from most points of view. - **Viewpoint invariance:** Recognition is largely unaffected by the viewpoint from which an object is seen. As long as enough geons are visible, the object can be identified. - **Structural Information:** Objects are recognized not just by their appearance, but but by the spatial arrangement of their geons. - **Importance of edges:** human visual system pays special attention to the edges where geons intersect, which helps in determining the shape and structure of objects. ## Perception of Scenes and Objects in Scenes - Scene: a view of the real-world environment that contains: - Background elements - Multiple objects that are organized in a meaningful way relative to each other and the background. - The perception of the gist of a scene is based on the global (holistic) image features: - Degree of naturalness. - Degree of openness. - Degree of roughness. - Degree of expansion. - Color. - **Global Image Features Are:** - **Rapidly perceived.** - **Related to the scene’s structure and spatial layout.** - **In most cases, they are regularities in the environment- physical or semantic regularities - that we have learned from our perceptual experiences.** - **Top-down processing.** ## Physical Regularities - **Dominance of vertical and horizontal orientations in the environment.** - **Most light comes from above.** - **Oblique Effect:** Details of oriented visual stimuli are better resolved if horizontal or vertical rather than oblique. ## Semantic Regularities - The meaning of a scene. The characteristics associated with the functions carried out in different types of scenes. ## Face Perception - **Fusiform Face Area (FFA):** Responds only to faces. - **Amygdala (AG):** Activated by emotional aspects of faces. - **Superior Temporal Sulcus (STS):** Responds to where the person is looking and to mouth movements. - **Frontal Cortex (FC):** Activated when evaluating facial attractiveness. - **The FFA for each person was determined first by:** - Showing participants faces and non-faces - Finding the area that responded preferentially to faces - The participants did the task Stimulus+ mask and response. - Most people did a correct identification, some saw the face incorrect identification and little couldn’t identificate anything. ## Tong and Coworkers Experiment - A house image was presented to the participant’s right eye, and the face image to the left = participant’s perception alternated. - **House:** Activity on the PPA in the left and right hemispheres (red ellipses). - **Face:** Activity in FFA in the left hemisphere (green ellipse). - In order to recognise a person’s face, we use: - Their local features (shape of their eyes) - Their global features (shape of the face). ## Pawan, How the Brain Learns to See ## Colour Perception - Color is a basic feature of visual perception. - Color is a subjective property, not a physical property of things, it doesn’t exist on the physical world, it depends on how the wavelengths of light interact with our visual system. - The color we perceive results from three aspects: - **Hue:** Distinct color perceived. - **Brightness:** Intensity of the light. - **Saturation:** Purity of the color. - **Helps with:** - **Visual detection.** - **Visual organization** - **Visual discrimination** - **Visual identification** - Enhances our interaction with the environment, providing critical information for daily tasks and survival. ## Basic Elements Participating in Color Perception - **An observer who is able to see in color:** - **Light stimulating visual receptors of the observer:** - Emitted Light (Sun). - Transmission of light (air, water, transparent things). - Transmission+reflection (semi-transparent things). - Reflection and observation of light (opaque surfaces, apple). - **In all cases, the color we see depends on the characteristics of the light that stimulates the visual receptors.** ## Factors That Influence Color Vision in Humans - **Stimulus-related factors:** Wavelength, light purity, light intensity. - **Selective:** If light mostly contains some types of wavelengths = chromatic color. - **Non-selective:** The light consists of all wavelengths in similar proportions = achromatic color (white, grey, black). - **Additive color mixing (light) / substractive (pigments):** - The primary colors are red, green, and blue (RGB). When these colors of light are mixed, they combine to form other colors, the more colors you add, the closer you get to white light because the light wavelengths combine to cover the full visible spectrum. - Television screens, computer monitors, and projectors work. ## Physiological Factors in Color Perception: Cones, Opponent Neurons Cortical Mechanisms - **Trichromatic theory:** By mixing different wavelengths of light, we can create the perception of any color thanks to the cones. - Our retina have cones = photoreceptors cells: S (short waves, blue), M (middle, green), and L (long, red). - Since we only have three, we face metamerism. - We perceive the same color even when the profiles of it are very different.. - **Reflectance:** - 0.8 - 0.6 - 0.4 - 0.2 - 0 - 400 - 500 - 600 - 700 - **A** - **B** - **C** - **Wavelength (nm)** - **There are people with four cones who are able to distinguish more colors** ## Opponent-Process Theory - Based in the function of opponent neurons: work in pairs, red-green, blue-yellow, white-black, when one of the colors are stimulated, the other one is inhibited. - Phenomenological and psychophysical evidence. Afterimages, imagining mixtures of colors, estimating the percentages of different colors in a patch and color deficiencies. ## Role of The Cortex In Color Perception - **Cerebral achromatopsia indicates that the cortex is involved in color perception. Areas? Two hypotheses:** - **Color processed by a specific cortical area (color module)** - **Processed by a number of cortical areas (distribute color processing).** ## Other Factors - **Color (hue) constancy:** Changes in the spectral composition of the stimulus but our perception of the color remains the same. - **Chromatic adaptation:** we adapt to the dominant wavelength of lighting rapidly. - **Memory of color for objects:** Our previous experiences with objects and their colors. - **Surrounding light:** Comparing the light reflected by an object and the one of the surroundings (simultaneous color contrast).