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Part I - Lectures 3, 4, & 5.doc

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University of British Columbia
PSYC 260
Julia Kam

Visual Representation Dual Visual Streams - In cortex, visual processing splits into two distinct streams, each dealing with a distinct kind of representation - “What” (ventral) stream = (A.K.A. “vision for perception” stream) occipital lobe’s visual cortex to the temporal lobe, which includes the fusiform gyrus - “How” (dorsal) stream = occipital lobe’s visual cortex to parietal lobe and other motor- related regions, and mostly detects motion and is associated with our movements and how we interact with what we are looking at - The visual representation of the “what” pathway is distinct from the visual representation of the “how” pathway Visual Agnosia (damage to the “what” stream) (in hierarchical organization) - Apperceptive agnosia (form/shape) • Characterized by the inability to recognize and perceive visual forms and shapes  Patients have trouble recognizing, copying, or discriminating between different visual stimuli o Patients are unable to copy images  Mostly caused by damage to V1 and V2 • Patient DF, who suffered from bilateral damage to the ventral stream, was unable to recognize and distinguish basic shapes and could not tell the orientation or width of an object. • When patients are able to identify objects, they do so based on inferences using colour, size, texture, and/or reflective cues to piece it together - Associative agnosia (objects) • Characterized by the inability to identify or categorize a visually presented stimulus despite adequate visual perception/recognition  Although perception is intact, the particular stimulus has no meaning (“associative” value) to the patient. The stimulus can neither be named nor linked to other personal or sensory experiences.  Patients are still able to reproduce an image through copying.  Less severe form of apperceptive agnosia • Rely on other senses (e.g. touch, smell) to identify objects - Prosopagnosia (faces) • Characterized by the inability to recognize faces  Patients recognize facial features as distinct geometric shapes but cannot piece together the different features to form a picture of the face as a whole • Usually due to damage to the fusiform gyrus in the right hemisphere • Patients compensate for inability to recognize familiar faces by recognizing other features of people’s appearance (e.g. clothing, hair) • Patient JS suffered a stroke in the occipito-temporal area  Patient JS and subjects who served as controls were shown photos of familiar faces and unfamiliar faces  After stroke, the patient was impaired in her ability to recognize family members  Recognition of family members was worse than that of celebrities, and both were worse than identifying the faces of unfamiliar people  However, patient JS displayed normal galvanic skin conductance response (SCR), which is an automatic response that the patient cannot control, to family faces  Such data mean that patient JS is showing impaired overt/explicit awareness but intact covert/implicit awareness of family facial representations - Cerebral achromatopsia • Neurological condition associated with loss of the ability to represent colour due to damage in the brain • Cone receptors are missing or dysfunctional • Vision only functional in low-light level  Vision under bright light is accompanied by a debilitating glare (an overwhelming bright light) • May not be able to perceive danger due to the inability to distinguish foreground from background Overt vs. Covert Awareness - Explicit or overt awareness concerns representations in mind/brain that you are consciously aware of. • Patients whose “what” pathway is damaged respond to visual stimuli without overt awareness - Implicit or covert awareness concerns representations in mind/brain that you are not consciously aware of. Optic ataxia - Neurological disorder wherein the ability to use visual information to guide motor movements is impaired, due to damage to the “how” visual pathway - Depth perception is not impaired, as it relies on both the “what” pathway and the “how” pathway, so patients can perceive depth of objects but cannot interact with them Motion Representation - Motion is represented in area MT, or area V5 (which appears to be part of both the “what” and the “how” pathways) - Cerebral akinetopsia • Neurological disorder associated with impairments in perceiving motion— whether objects are moving, which direction they are moving, and how fast they are moving • Perceive motion as series of snapshots of things - Blindsight • Neurological phenomena characterized by the ability to sense or “see” the movement of an object within one’s blind region of space (scotoma, which is an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal vision), despite an absence of overt awareness for what that object actually is  Patients are cortically blind and cannot consciously see due to damage to the primary visual cortex (V1, or the “striate cortex”) • Normal pathway goes from eye 
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