Unit 2 : Visual Perception & Object Recognition
Process Map
(Expertise) Subordinate vs. Superordinate processing.
Four Stages of Object Recognition
1. Early visual processing (determined by EPP signals) 2. Grouping of visual elements (e.g., depth cues, figure-ground segmentation). 3. Matching grouped visual description onto a representation of the object stored in the brain. 4. Attaching meaning to the object (e.g., retrieved from semantic[verbal] memory)
Two Suggestions of Object Constancy
1. The brain stores objects in a single viewpoint (principal axis) - object recognition involves view normalization. 2. Stored structural descriptions are accessed by matching feature-by-feature.
Fovea
A small depression in the retina of the eye where visual acuity is highest. The center of the field of vision is focused in this region, where retinal cones are particularly concentrated.
Rod Cells
A type of photoreceptor specialized for low levels of light intensity, such as those found at night.
Topographic Representation
Eccentricity mapping : House activate peripheral visual field representations whereas faces and letters activate central representations.
Category Specificity
In neural level : - Particular object-selective responses found in the ventral occipital and temporal regions (anterior and lateral to early retinotropic cortex). - fMRI studies shows greater brain activity when subjects view pictures of objects (e.g., object, animals, houses, etc) in comparison to when they view pictures of textures, noise of scrambled objects. - Three related brain area. - Category-specific modules in the ventral visual pathway.
Dan Simon
Monkey business illusion
Kravits
The figure shows that the receptive field size increases as the visual process goes to higher order, and the brain cares more about the big picture.
Visual Field
Visual Space. The total area in which objects can be seen in the side (peripheral) vision as you focus your eyes on a central point. *Objects in the right side of space (right visual field) fall on the left side of the retina of both eyes and project to the left lateral geniculate nucleus.
Zeki
[Color Perception] (1983) V4 in the ventral extrastriate cortex.
Ungerleier & Mishkin
(1982) The "where" pathway : V1/V2 -> V5 -> Posterior Parietal Cortex (PPC)
Rizzolatti
(1987) Premotor Theory of Attention : - The orienting of attention is nothing more than the preparation for action. - Covert attention is an action plan (e.g., saccade) that is prepared but not executed. - Evidence came from recording and stimulating neurons in the frontal eye fields (FEF).
Milner & Coodale
(1995) The dorsal stream is called "how and action" pathway. It brings together different types of spatial representation needed for action (e.g., integrating visual space with body space).
Murray
(2006) Activation in V1 tracks perceived size rather than the actual size.
Bisley & Goldberg
(2010) Lateral Intra-Parietal Area (LIP) Electrophysiology studies found neurons in LIP show: - Motor properties (generates saccades) - Sensory properties (respond to both sound and vision), enables sounds to be remapped to eye-centered coordinates. - Codes a spatial "salience map" - highlighting the most behaviorally relevant stimuli - Associated with both covert and overt orientation.
Balint's Syndrome
(Bilateral parietal damage) losing space : seeing one object at a time. - Inability to perceive more than one object at a time (Simultanagnosia) - Other problems : Optic ataxia and optic apraxia - but basic visual abilities seem normal (e.g., visual capacity, color perception, contrast sensitivity) - When is an object an object?
Covert Attention
(Covert Orienting) The movement of attention from one location to another without moving the eyes/body.
Frontal Eye Field
(FEF) Part of the frontal lobes responsible for voluntary movement of the eyes.
Lateral Geniculate Nucleus
(LGN) The thalamic station in the retinocortical projection and the gateway to the visual cortex. It consists of 6 laters (3 layers in each side for each eye) and the different layers consist of either M or P cells.
Motion Induced Blindness
(MIB) A phenomenon of visual disappearance or perceptual illusions observed in the lab, in which stationary visual stimuli disappear as if erased in front of an observer's eyes when masked with a moving background.
Hemispatial Neglect
(Right parietal damage) A deficit in attention to and awareness of one side of the field of vision is observed. It is defined by the inability of a person to process and perceive stimuli on one side of the body or environment, where that inability is not due to a lack of sensation. - Patiens still activate occipital regions in contralateral side of the "neglected" visual field - They are often able to detect objects on the visual field if cued there - Affects auditory and tactile judgements as well as vision (e.g., sound on left are mislocalized but still heard) - Phenomenon of visual extinction suggests different perceptual representations are competing for attention (and visual awareness).
Hypercomplex Cells
(outside V1) derived by combining the responses of several complex cells.
What do sensory receptor cells in retina (photoreceptors) do?
- Collect and filter the incoming sensory stimuli - Convert light stimulation into neural signals by specialized receptor cells, leading to a change in the membrane potential of the receptor cell. (e.g., rods and cones in retina).
Biased Competition Model
- Objects in the visual field compete for processing within a network of cortical visual areas. - Objects may be processed in parallel, but the information available about any given object will decline as more and more objects are deed to receptive fields. - To solve the problem -> relevant objects need to bias the competition in their favor; i.e., selective attention or visual spatial attention.
Human Eye
- Processes photons with wavelengths from about 400nm to 700nm. - Can process wide range of stimulus intensities - a few tens of photons per mm2 to a billion or more - The great range of responsiveness of the sensory system allows humans to function well in widely varying circumstances. (especially because humans only process very narrow range of light wavelengths).
What Happens Beyond V1?
- Receptive field sizes of neurons increase - Visual topography becomes coarser - Neurons become specialized for higher-order features - Neurons' response latencies (when does the neuron fire) increase.
Dorsal Streams for Spatial Cognition
- The "where" pathway: V1/V2 -> V5 -> Posterior Parietal Cortex (PPC) - Also called the "how and action" pathway : brings together different types of spatial representation needed for action. (e.g., integrating visual space with body space) - Parietal neurons respond more to behavioral saliency (e.g., abrupt onset) rather than mere presence in the receptive field (exogenous). - Parietal neurons respond more to stimuli that are task relevant (endogenous). - Other parietal neurons integrate visual spatial information with postural information - essential for locating objects in space. - Patients with damage to the right parietal lobe have more severe problems (than those with left parietal damage) and this particularly affects left side of space (hemispatial neglect) - TMS over patieral lobes impairs conjunction searches but not pop-out searches.
Premotor Theory of Attention
- The orienting of attention is nothing more than the preparation for action. - Covert attention is an action plan (e.g., saccade) that is prepared but not executed - Evidence came from recording and stimulating neurons in the frontal eye field (FEF)
What Happened to Neglected Information?
- Ventral stream may continue to process neglected objects up to the stage of object recognition and possibly semantics - Degraded line drawings presented in neglected field show subsequent priming effects when attended. - Burning house experiment - neglected information is coded to a level that supports meaningful judgements. - Compare neglect and blindsight.
Two Properties of Sensory Processing
1. Adaptation : the continue resetting of the sensitivity of the system according to ambient condition. (i.e., the temporary change in sensitivity or perception when exposed to a new or intense stimulus, and the lingering afterimage that may result when the stimulus is removed. - e.g., staring at pink dots results in seeing green dots). 2. Acuity : precision of perception (i.e., fitness of discrimination) - It is determined at the retinal level by the distribution of receptors, such as different receptor density and receptor types across the retina.
Two Points of Adaptation
1. Adaptation fo stimulus intensity - light/dark adaptation. 2. Fast and Slow Adaptation - light onset (dark -> bright) - fast, offset (bright -> dark) - slow , and persistence.
Is Face Recognition Learned or Innate?
1. An experiment demonstrated that face-deprived monkeys spent more time looking at people's hands. (controlled, non face deprived monkeys look at people's face) => Face maybe special through learning, not hard-wired.
Five Visual Perceptions
1. Brightness 2. Color 3. Motion 4. Form (shape) 5. Depth
Two Features of Color Perception
1. Color Constancy 2. Color Contrast
Two Main Brain Regions Related to Spatial Attention
1. Dorsal Stream for Spatial Cognition 2. Lateral Intra-parietal Area (LIP)
Effects on Attention on Neural Responses to Visual Stimuli
1. ERPs : Enhanced occipital P1 and N1 - increased "gain" for processing targets 2. Imaging studies : Increased acticity in the occipital areas contralateral to the attended side. 3. Electrophysiology in Monkeys i) In V4, neuronal responses depend on the locus of attention within the recap ii) A neuron's orientation sensitivity is modulated by attention: revealed by the sharpness of the turning curves of single neurons in V1. 4. Attention can be selectively directed to one of the two overlapped objects.
Two Types of Visual Spatial Attention
1. Exogenous attention. 2. Endogenous attention.
Three Models of Attention
1. Feature Integration Theory 2. Biased Competition Model 3. Premotor Theory of Attention
Subcortical Sensory Processing
1. Feedforward (one level to different level), feedback (from brain to cortical), and lateral information processing: for filtering and sharpening the information received at each processing stage. 2. Thalamus (e.g., "dorsal LGN" for the visual system) - 2 information processing streams: motion pathway vs. detail/color pathway - with distinct population of cell types in different layers (magnocelluar [M] vs. Parvocellular [P]) 3. Superior colliculus: Blindsight.
Three Regions Related to Category Specificity
1. Lateral Occipical Cortex (LOC) : Object, Places, and Face. (most lateral) 2. Fusiform Gyrus (FFA) : Face 3. Parahippocampal Gyrus (PPA) : Places (most medial)
Two Basic Phenomena of Visual Spatial Attention
1. Limited capacity for processing information 2. Selectivity or the ability to filter out unwanted information and focus on task relevant information. (e.g., spotlight metaphor, overlapping pictures of two images, we can focus on either of them, MIB)
Four Possible Accounts for Category-specific Modules in the Ventral Visual Pathway.
1. Limited category-specific modules and one region for general object recognition. 2. Process map (expertise) - subordinate versus superordinate processing 3. Topographic representation - eccentricity mapping : House activate peripheral visual field representations whereas faces and letters activate central representations. 4. Object representation in the form of a distant pattern of responses across all ventral cortical areas.
How Do We Test Biased Competition Model?
1. Need to create experimental settings mimicking our typical visual experience - one or more objects in a visual scene that are relevant to current behavior, along with others that are not 2. Evidence came from recording parietal neurons
Common Paradigms Attention
1. Posner's spatial cueing paradigm 2. Non-spatial attentional selection : Object-based attentional processes - modulate activity in different parts of the ventral pathway. (i.g., Two overlapping images of a house and a face activates different part of the ventral pathway depending which picture the participant is focusing on.) 3. Time-based attentional processes: Attentional blink - attentional related bottleneck in processing sensory input. 4. Change and inattentional blindness- when large changes in scenes go unnoticed (The monkey business illusion)
Conclusion from Neuroimaging and Agnosia Case Studies
1. Provides some evidence for the fundamental dissociation between the dorsal and ventral streams. 2. Visual experience is not unified. 3. Distinct syndromes of visual disturbance observed. (Some case study shows that a person still recognize different sheeps' face but not humans') 4. The ventral temporal areas for object recognition are not uniform -> Asymmetry or heterogeneous in functions of occipital and temporal cortical areas.
Three Types of Spatial Representation
1. Retinocentric space 2. Egocentric space 3. Allocentric space
Spatial Representation
1. Space is not represented as a single entity 2. Space in brain exists in many forms. (e.g., finding the nearest bank[larger] vs. picking up a pencil[local])
Why Are Faces Special?
1. Task Difficulty? People are not usually dealing with other objects. e.g., monkeys looked the same for the first year, but later they were all recognizable in the third year. 2. Holistic/Configural processing? People usually don't pay much attention to other objects. 3. Visual edxpertise? 4. Domain-specificity?
Approaches to Further Study of Object Representation in the Human Brain
1. Test object constancy and invariance to external viewing conditions. i.e., perception-related activity should be relatively insensitive to the i) Precise physical cues - the same objects can be defined by luminance, texture, motion, format (realistic pictures vs. line drawings) and illusionary contours. ii) Viewing conditions - object size, rotation, location, motion, luminance conditions. => Different pictures used to study this. In MIT, objects are presented as within different circumstances and from diverse viewpoints. (Object presented with black or white background doesn't test properly) 2. Rule out other processes that could affect brain activity during object viewing : arousal (e.g., caffeine), figure-ground segmentation and surface extraction among others. 3. Determine the relationship between changes in the brain activation, task demand and subject performance (e.g., manipulate visibility, binocular rivalry, recognition threshold) 4. Determine learning effects - how experience affects the activity of these regions? How quickly does the brain "learn?"
Two Broadly Different Types of Attention
1. Visual Spatial Attention 2. Object-based attention (Feature-integration theory)
Refutation of Marr's Theory
1. Visual stimuli are inherently ambiguous and context dependent. 2. Retinal images cannot uniquely specify the physical objects in a scene. 3. The brain must makes assumptions and guesses (e.g., there is a single light sources, which comes from above) 4. The outcome is that the perception does not correspond with physical reality (but rather probabilistic). <=Visual Abstraction.
Integrative Agnosia
A Failure to integrate parts into whole in visual perception - HJA showed difficulties in using perceptual grouping mechanisms to translate his intact perception of lines into more complex visual descriptions. (I.e., he was able to copy from a picture and draw from a memory but cannot tell if the object is real or not and the name of the objects -e.g., carrot)
Agnosia
A Failure to understand the meaning of objects. - Apperceptive - Associative - Integrated - Object Orientation
N170
A component of the event-related potential (ERP) that reflects the neural processing of faces.
Achromatopsia
A failture to perceive color (the world appears in grayscale), not to be confused with color blindness (deficient or absent types of cone cell). *Although these individuals fail to see color, their retina and their V1 cells still respond to different wavelengths of light. => Visual perception is constructed at the "cortical level".
Inattentional Blindness
A failure to be aware of a visual stimulus because attention is directed away from it.
Akinetopsia
A failure to perceive visual motion. Patient LM with bilateral damage including the MT+ area sees the world in a series of still frames. - e.g., when tea is poured, the liquid appears "frozen" - difficulty in following speed and crossing streets (cannot tell if the car is approaching or not) - She can still distinguish some body motion in the dark (e.g., point-light walking)
Receptive Field
A region of a retina that, when stimulated, changes the response of a visual neuron.
V4
A region of extrastriate cortex associated with color perception.
V5/MT
A region of extrastriate cortex associated with color perception.
Illusory Conjunctions
A situation in which visual features of two different objects are incorrectly perceived as being associated with a single object.
Blindsight
A symptom in which the patient reports not being able to consciously see stimuli in a particular region but can nevertheless perform visual discriminations (e.g., long, short) accurately. => Other pathway can interpret even though the main pathway of perceiving is blocked.
Feature-integration Theory
A theory of attention suggests that when perceiving a stimulus, features are "registered early, automatically, and in parallel, while objects are identified separately" and at a later stage in processing.
Topography
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system.
Figure-ground Segmentation
A type of perceptual grouping which is a vital necessity for recognizing objects through vision. In Gestalt psychology it is known as identifying a figure from the background. (e.g., a wine glass vs. two side views of human faces.)
Cone Cells
A type of photoreceptor specialized for high levels of light intensity, such as those found during the day, and specialized for the detection of different wavelengths (color).
Motion Aftereffect
A visual illusion experienced after viewing a moving visual stimulus for a time (tens of milliseconds to minutes) with stationary eyes, and then fixating a stationary stimulus. The stationary stimulus appears to move in the opposite direction to the original (physically moving) stimulus. (e.g., Waterfall illusion - Buddha) Why? The neurons perceive one direction and the other neurons that perceive the other direction is suppressed, then suddenly it released.
Visual Constancy
According to single neuron recording of a monkey, the brain is still able to tell no matter then source of the images is vague. I.e., we can still recognize a person's face even it's very blurry.
Random Dot Motion Experiment
An experiment demonstrated how these motion related area in the brain (MT+/V5) perceive motion in which they show bunch of random dots moving in different direction and the participants had to choose which direction it is going.
Spatial Cueing Paradigm
An experiment demonstrated that participants are faster at detecting the target in the cued location if the target appears soon after the cue (facilitation) but are slower if the target appears much later (inhibition).
Point-light Walking
An experiment for participants to determine if they can detect biological motion. - People are generally distinguish if it's animal or humans, and even the gender of the walker. - Autism with patients have problem detecting this.
Saliency Map
An image that shows each pixel's unique quality. The goal of a saliency map is to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze.
Hermann Grid
An optical illusion characterized by "ghostlike" grey blobs perceived at the intersections of a white (or light-colored) grid on a black background. It is due to : When we look at the white spot, we use our fovea, at which the highest density of cells, so that most of the receptive field is covered. However, if we shift our eye to the side, then the junctions will locate at the periphery, your receptive field now is covering the bigger area.
Object Constancy
An understanding that objects remain the same, irrespective of differences in viewing condition. It is achieved by: Mapping a potentially infinite number of visual depictions on to a finite set of stored descriptions of the structure of objects. It is processed by: Neural level: - Neurons in the monkey inferotemporal cortex respond to very particular object attributes (e.g., corners, shapes) but are less concerned with where they are located in space. => Ideal conditions for computing object constancy. - fMRI in humans shows that IT areas respond to the same object presented in different sizes and viewpoint. -- Left region is insensitive to viewpoint/size but right region is viewpoint sensitive.
Endogenous Attention
Attentional processing which is driven by the goals of the perceiver. (i.e., top-down or voluntary attention)
Exogenous Attention
Attentional processing which is driven by the properties of the objects themselves. (i.e., bottom-up processing, or stimulus-driven attention)
Binocular Rivalry
Bi-stable dissimilar percepts originated from the same visual stimulus in space. (e.g., Young and old lady) A phenomenon of visual perception in which perception alternates between different images presented to each eye. => Since images are not merged into one stable image, this suggests that binocular neurons do not simply combine the two retinal images.
Line Orientation Selectivity
Can be found in: - V1 neurons - Neurons in M channel-simple cell)
Photoreceptors
Cells that exist in retina. There are Rod cells and Cone Cells.
Magnocellular Layers
Consist of larger cell bodies that are sensitive to movement and respond to larger areas of visual field.
Parvocellular Layers
Consist of small cell bodies that are respond to detail and are concerned with color vision.
Hemianopia
Cortical blindness restricted to one half of the visual field (associated with damage to the primary visual cortex in one hemisphere. Damage to parts of V1 results in blindness for the corresponding region of space. <= Retinotopic Partial - Scotoma (dot) 1/4 - Quadrantanopia
Simple Cells
Derive response by combining the responses of several LGN cells they respond to different orientations (and length)
Complex Cells
Derive response by combining the responses of several simple cells they respond to different orientation, but have larger receptive fields and require stimulation on their entire length.
Membrane Potential
Differences in the concentrations of ions on opposite sides of a cellular membrane.
Lateral Intra-parietal Area (LIP)
Electrophysiology studies found neurons in LIP shows: - Motor properties (generates saccades) - Sensory properties (respond to both sound and vision), enables sounds to be remapped to eye-centered coordinates - Codes a spatial "salience map" - highlighting the most behaviorally relevant stimuli - Associated with both covert and overt orientation.
The Primary Visual Processing Pathway
Eye (Retina) -> Thalamus (LGN) -> Striate Ctx -> Extrastriate Ctx -> Association Ctx Eye (Retina) : Retinal Ganglion Neurons (Cones and Rods) Thalamus (LGN) : Parvocellular (color, detail), and Magnocellular (motoin) Striate Ctx : V1 (Primary Visual Cortex) Extrastriate Ctx V4 (color), and V5/MT(medial temporal lobe) (motion) Association Ctx : IT (Inferotemporal Ctx) and PPC (Posterior Patieral Ctx)
Grill-Spector
Fig 2. Different area of the brain activates by seeing different types of objects. - Red: Faces - Green: Places - Blue : Places & Objects Fig 3. Comparing earlier areas of the brain (V1 and V4) to later/higher areas of the brain (FFA and PPA) => V1 and V4 do not vary by the subject, whereas FFA does. *There is some account for other detail discrimination ability towards other objets in FFA. (not just the human face).
Bionic Eye
Glasses with a camera that has implanted chip that sends visual information to the brain.
Prosopagnosia
Impairments of face processing that do not reflect difficulties in early visual analysis (also used to refer to an inability to recognize previously familiar faces).
Semantic Memory
Knowledge. A portion of long-term memory that processes ideas and concepts that are not drawn from personal experience. It includes things that are common knowledge, such as the names of colors, the sounds of letters, the capitals of countries and other basic facts acquired over a lifetime.
Retinal-Geniculate-Striate Pathway
Largest visual pathway in the brain. The visual stimulation from retina goes to the primary visual cortex (striate cortex) via the lateral geniculate nuclei (LGN) of the thalamus.
Allocentric Space
Location of object relative to each other. (distance between Lisa and professor)
Engocentric Space
Location of objects relative to the body. (e.g., turning head - eye moves with head)
Retinocentric Space
Locations on sensory surface (e.g, reina, 20 degrees to the right)
Distributed Object-form Topography
Object representation in the form of a distant pattern of responses across all ventral cortical areas.
Cortical Sensory Processing
Perception is considered to happen in the cortex (i.e., reached conscious awareness) 1. The primary visual cortex (V1) 2. Association cortexes ("higher-order" areas) - Occupy most of the cortical surface - Integrate information within across sensory modalities - Linking sensory systems and cognitive systems (probably through feedback influences) e.g., Expectations.
V1
Primary Visual Cortex (or Striate Cortex) : The first stage of visual processing in the cortex; the region retains the spatial relationships found on the retina and combines simple visual features into more complex ones. - It extracts basic information from the visual scene (e.g., edges, orientations, wavelength of light) - Its information is used by later stages of processing (e.g., V4, V5, etc) to extract information about shape, color, movement, etc. - Single-cell recordings by Hubel and Wiesel lead to a hierarchical view of vision in which simple visual features (e.g., points of light) are combined into more complex ones (e.g., adjacent points of light may combine to a line)
Center-surrond Receptive Field
Receptive structure found in : - Retinal Ganglion Cells (sole source of visual input) - LGN cells - Layer 4 in V1 Due to its antagonistic aspect, RG and LGN cells respond mostly to differences illumination across their receptive fields. => Results in enhanced or exaggerated contrast at borders (luminance boundaries)
Visual Hierarchy
Refers to the fact that our visual stimulation is processed and interpreted though different stages in our brain. Retina -> LGN -> Cortex
Color Contrast
Same color can be perceived differently depending on the background.
Sensory Transduction
She conversion of a sensory stimulus from one form to another.
Posner
Spatial Cueing Paradigm : An experiment demonstrated that participants are faster at detecting the target in the cued location if the target appears soon after the cue (facilitation) but are slower if the target appears much later (inhibition).
Pop-out
The ability to detect an object among distractor objects in situations in which the number of distractors presented is unimportant.
Resolution of Problems of Visual Perception
The brain creates "shapes" from incomplete data, based on the context. <- This is important for survival. - Cortical activity is more concerned with percepts than with physical properties of light (e.g., absolute intensity, wavelengths) - Percepts are generated empirically, according to context and past experience.
Retina
The internal surface of the eyes that consists of multiple layers. Some layers contain photoreceptors that convert light to neural signals, and others consist of neurons themselves.
Overt Orienting
The movement of attention accompanied by movement of the eyes or body.
Color Constancy
The perceived color of objects remains relatively constant under varying illumination conditions.
Attentional Blink
The phenomenon that the second of two targets cannot be detected or identified when it appears close in time to the first. (i.e., people cannot detect the second target if it was presented right after the first target. => The first target take over limited attentional capacity which leading to an apparent blindness of a subsequent target.)
Attention
The process by which certain information is selected for further processing and other information is discarded - e.g., spotlight metaphor and cocktail party effect.
Superior Colliculus
The rostral (front) bump on the lateral (side) part of the midbrain.
Retinotropic Organization
The spatial arrangement of light on the retina is retained in the visual cortex - i.e., spatial selectivity of visual neurons vary across V1 - Cortical magnification and Columnar organization.
Object Perception
The subjective visual experience mostly about - Whole objects. - Conspecies objects (e.g., faces) and meaningful symbols are of particular significance. Engaged with: The ventral visual pathway : V1/V2 -> Ventral occipital (V4) -> Inferotemporal (IT)
Depth Perception
The subjective visual experience of a three-dimensional world from two-dimensional retinal image. *Binocular disparity (integration) & behavioral significant : Tool manipulation. (.e.g, putting a thread in a needle is hard when you only have one eye open). => Evolutionary significant for primates in order to use tools. It is processed by: Neural level : - Binocular cells in V1 (complex cells) receive the monocular inputs from LGN. - These neurons in V1 and beyond (e.g., V3A) are tuned to specific disparities. - Binocular Rivalry (e.g., Young and old lady)
Motion Perception
The subjective visual experience of a visual stimulus that shift in space. It is perception of speed and direction. It is processed by: - Medial Temporal+ (V5) (fMRI and TMS evidence) and Medial superior temporal (MST) areas (e.g., Random dot motion experiments). - Neurons in MT coes both stimulus heading direction and speed *There is separate mechanism for biological motion.
Form Perception
The subjective visual experience of the geometrical (or structure) characteristics of a visual stimulus (e.g., length of lines, their orientation, and the angles they make). (e.g.2, Front sphere and back sphere perceived as different size.) It is necessary to distinguish among shapes. It is processed by: Activation in V1 tracks perceived size rather than the physical size of an object.
Color Perception
The subjective visual experience of the hue and saturation of a visual stimulus - the relative amount of light energy at short, medium and long wavelengths. It's processed by: Retinal level : - Three types of cones. Each cone opsin has a different absorption spectrum. - Opponent color processing operates in a push-pull fashion: red/green, blue/yellow, black/white. Cortical level : V4 in the ventral extrastriate cortex. *Context and experience dependent
Brightness Perception
The subjective visual experience of the overall amount of light. (i.e., intensity). It uses to : - detect constrast boundaries (i.e., edges - able to read letters) - interprete surface (shading, reflectance, illumination, transparency). It is processed by : Retinal level : increasing neural firing rate with increasing light intensity at the retinal level. - Antagonistic nature of the center/surround structure. (but first, adaptation occurs). Cortical level : - Neurons in V1 and beyond respond only weakly to changes in stimulus intensity. - They are more responsive to the light/dark contrast regions in the stimulus (i.e., edges) *Illumination, reflectance, and transmittance of light stimulus are conflated in the retinal image. *has odd relationship to luminance - Simultaneous brightness contrast effects: The brightness of a patch is perceived differently depending on the background luminance. (e.g., the gray box inside different colors look different although they are the same color.) <- The illusion is affected by both the prior knowledge and retinal structure.
Biological Motion
The visual system's ability to perceive object movement by connecting a few small, individual stimuli.
Limited Category-specific Module
There is only one region for general object recognition. (Kanwisher)
Significance of Adaptation
To compensate neuron's signaling limitations, allowing useful perception over the full range of environmental conditions.
The Goal of the Visual System
To maintain the perceptual constancy.
Ventral Stream
V2 -> V4 -> IT A pathway extending from the occipital lobes to the temporal lobes involved in object recognition, memory and semantics.
Dorsal Stream
V2 -> V5/MT -> PPC A pathway extending from the occipital lobes to the parietal lobes involved in visually guided action and attention.
Haxby
[Category Specific Object Recognition] Distributed object-form topography : Object representation in the form of a distant pattern of responses across all ventral cortical areas. There is core system and extended system. The core system is for visual analysis which invovles : 1. Early occipital that goes onto different pathways i) STC - different aspects of faces and ii) FFA - unique identity 2. Then those core system further projects to extended system such as amygdala.
Kanwisher
[Category Specific Object Recognition] Limited category-specific modules and one region for general object recognition.
Tarr & Gauthier
[Category Specific Object Recognition] Process map (expertise) - subordinate versus superordinate processing
Malach
[Category Specific Object Recognition] Topographic representation - eccentricity mapping : House activate peripheral visual field representations whereas faces and letters activate central representations.
McAdams & Maunsell
[Effects of attention on neural responses to visual stimuli] (1999) Electrophysiology in monkeys : A neuron's orientation sensitivity is modulated by attention: revealed by the sharpness of the tuning curves of single neurons in V1.
O'Craven
[Effects of attention on neural responses to visual stimuli] Attention can be selectively directed to one of the two overlapped objects.
Mangun
[Effects of attention on neural responses to visual stimuli] ERPs : Enhanced occipital P1 and N1 - increased "gain" for processing targets (?)
Morgan & Desimone
[Effects of attention on neural responses to visual stimuli] Electrophysiology in monkeys : In V4,neuronal responses depend on the locus of attention within the recap.
Tootell
[Effects of attention on neural responses to visual stimuli] Imaging studies: Increased activity in the occipital areas contralateral to the attention side.
De Renzi
[Face Recognition] (1986) Patient failed to recognizes his own family but could do so by voice, clothes.
Zihl
[Motion Perception] (1983) Patient LM with bilateral damage including the MT+ area sees the world in a series of still frames. - e.g., when tea is poured, the liquid appears "frozen" - difficulty in following speed and crossing streets (cannot tell if the car is approaching or not) - She can still distinguish some body motion in the dark (e.g., point-light walking) => Biological Motion is separate.
Gross
[Object Constancy] (1992) Neurons in the monkey inferotemporal cortex respond to very particular object attributes (e.g. corners, shapes) but are less concerned with where they are located in space. This indicates the ideal conditions for computing object constancy.
Vuilleumier
[Object Constancy] (2002) fMRI in humans show that IT areas respond to the same object presented in different sizes and viewpoint. (ERP study about N170 failed cause they only used the front face.) [Spatial Neglect] Degraded line drawings presented in neglected field show subsequent priming effects when attended.
David Marr
[Problem of Visual Perception] (1970, 80s) His theory of vision suggested the vision as an information-processing task, leading to a representation of the visual world by neurons in the brain.
Marshall & Halligan
[Spatial Neglect] (1988) Burning house experiment - neglected information is coded to a level that supports meaningful judgements.
Hubel and Weisel
[Visual Neurophysiology] They recorded single cell and it lead to a hierarchical view of vision in which simple visual features (e.g. points of light) are combined into more complex ones (e.g. adjacent points of light may combine into a line)
Desimone & Duncun
[Visual Spatial Attention] (1995) 1. Limited capacity for precessing information. 2. Selectivity of the ability to filter out unwanted information and focus on task relevant information (e.g., spotlight metaphor) Biased competition model (1995) : - Objects in the visual field compete for processing within a network of cortical visual areas. - Objects may be processed in parallel, but the information available about any given object will decline as more and more objects are added to receptive field. - To solve the problem -> relevant objects need to bias the competition in their favor; i.e., selective attention or visual spatial attention.
