VISION PDF

# The Student's Guide to Cognitive Neuroscience ## Geniculostriate Pathway - Number of different pathways from eye to brain - Main route terminates in primary visual cortex (V1) - Route called geniculostriate pathway because it goes via lateral geniculate nucleus (LGN) and terminates in striate co...

# The Student's Guide to Cognitive Neuroscience ## Geniculostriate Pathway - Number of different pathways from eye to brain - Main route terminates in primary visual cortex (V1) - Route called geniculostriate pathway because it goes via lateral geniculate nucleus (LGN) and terminates in striate cortex (another name for V1) ## Lateral Geniculate Nucleus - Contains six layers, three for each eye. - Cells have a center-surround receptive field. - Respond to differences in light across their receptive field (e.g., presence of light in center, absence in surround) ## Primary Visual Cortex (V1) - Extracts basic information from the visual scene (e.g., edges, orientations, wavelength of light). - This information is used by later stages of processing 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 into a line). ## Cells of Primary Visual Cortex (V1) - Simple cells may derive their response by combining the responses of several LGN center-surround cells. - Simple cells respond to different orientations. - Complex cells may be derived by combining the responses of several simple cells. - Complex cells respond to orientation too, but have larger receptive fields and require stimulation on their entire length (simple cells also respond to points of light). - Hypercomplex cells (outside V1) may be derived by combining the responses of several complex cells. - Unlike complex cells, they are sensitive to length as well as orientation. ## Spatial Arrangement of Primary Visual Cortex (V1) - Retinotopic organization - the spatial arrangement of light on the retina is retained in the response properties of V1 neurons (except inverted). - Damage to parts of area V1 results in blindness for the corresponding region of space (e.g., hemianopia) ## Cortical and Sub-Cortical Vision - Damage to geniculo-striate route impairs conscious vision, but other aspects of vision spared (blindsight). ## Blindsight - Damage to V1 leads to a clinical diagnosis of blindness (the patient cannot consciously report objects presented in this region of space). - However, the patient is still able to make some visual discriminations in the "blind" area (e.g., orientation, movement direction) — called blindsight. - This is because there are other routes from the eye to the brain. - The geniculostriate route may be specialized for conscious vision but other routes act unconsciously. ## Area V4 and Area V5/MT - PET study: Zeki et al. (1991) - Colored images ("Mondrians") compared to grayscale equivalents. - Area V4 active, conclude this region is specialized for color. - Moving dots compared to static dots. - Area V5/MT active, conclude this region is specialized for visual movement. ## Color Perception and Area V4 - Why does the brain need a specialized color center given that the retina is sensitive to different wavelengths of light? - The problem is that wavelength depends on the composition of the light source (e.g., daylight, electric light) as well as the color of an object. - Area V4 tries to compute the color of the object taking into account variations in lighting conditions. - This is called color constancy. ## Movement Perception and Area V5/MT - Cells in V5/MT do not respond to color but 90% of them respond to particular directions of movement. - Patients with bilateral damage to this region see the world in a series of still frames. - They are said to be akinetopsic. - The patients can detect movement in other senses (e.g., hearing, touch). ## Computer Object Recognition A diagram of an image fed as input to an artificial neural network. The image is of a hummingbird. The neural network has an input layer, hidden layers, and an output layer. The output layer labels the image as dog, bird, or cat. ## Beyond Visual Cortex - Visual cortex (striate and extrastriate) extracts basic visual information - colors, movement, shapes, edges. - In order for this information to be used it needs to make contact with other types of information: - Where the object is in space (and this can't be computed from the retinal image alone). - What the object is. ## A Model of Object Recognition (cont.) - Four broad stages: 1. Early visual processing (color, motion, edges etc.) 2. Grouping of visual elements (Gestalt principles, figure-ground segmentation) 3. Matching grouped visual description onto a representation of the object stored in the brain (called structural descriptions) 4. Attaching meaning to the object (retrieved from semantic memory) - The first stage was considered in the previous lecture. ## Lateral Occipital Complex (LOC) and Shape Perception - Responds to viewing objects>textures, but real and made-up objects same (Malach et al., 1995). - Unaffected by occlusion so may compute a Gestalt object shape (Kourtzi & Kanwisher, 2001). - TMS disrupts matching by shape (Chouinard et al., 2017). ## Seeing Parts But Not Wholes: Integrative Agnosia - Disorders in object recognition are called agnosia. - Many different types of agnosia that can broadly be divided into disorders of perception (apperceptive agnosia) or meaning (associative agnosias). - Integrative agnosia is a type of apperceptive agnosia in which grouping principles are disrupted. - This prevents stored knowledge of objects being accessed, but does not prevent the patient from seeing basic visual elements (computed in stage 1). ## Routes to Object Constancy - Object constancy achieved by mapping a potentially infinite number of visual depictions on to a finite set of stored descriptions of the structure of objects. - **View-invariant:** Brain stores objects across multiple viewpoints. Object constancy by matching feature-by-feature. Role of inferior temporal and LOC. - **View-dependent:** Brain stores objects in a single viewpoint (the principal axis). Object constancy may involve mental rotation. Role of inferior temporal and parietal. ## Neural Substrates of Object Constancy - Monkey cells in IT (inferotemporal) cortex respond to very particular object attributes (e.g., corners, shapes) but are less concerned with where they are located in space (Gross, 1992). - These are ideal conditions for computing object constancy. - fMRI in humans shows that inferotemporal regions respond to the same object presented in different sizes — left region is insensitive to viewpoint but right region is viewpoint sensitive. - This is consistent with two different routes to object constancy. ## A Model of Object Recognition A diagram depicting the model of object recognition, showing the lateral occipital complex (LOC), fusiform/inferotemporal, and right parietal, and the ventral stream ("what").