Lecture 2: Early Visual Processing Read Gazzaniga Chapter 5, Date Topic Jan 19 Overview of the course: understanding human actions Jan 21 The Perception Action Cycle: some examples. (Guest lecture: Dr Matthis) Jan 26 Early Visual Processing: the eye and visual cortex Jan 28. Central visual processing: dorsal and ventral streams, posterior parietal cortex. (Consequences of stroke in these areas) Feb 2 Walking and catching: class experiment. ( one versus two eyes ) Visual Perception: what do we want to explain? How do we get visual information from the world and use it to control behavior? What neural processes underlie visually guided behavior? Different levels: anatomy, neurophysiology, behavior The eye: turning light into visual signals. Many basic features of visual perception are determined by the structure and function of the eye. The Eye and Retina 18mm 0.3mm = 1 deg visual angle xa tan(a/2) = x/d a = 2 tan - 1 x/d Visual Angle d 1 diopter = 1/focal length in meters 55 diopters = 1/.018 A small eye rotation translates into a big change in visual angle Fundus of the right eye of a human Origin of red-eye effect Most of the refraction is done by the cornea. Presbyopia: stiffening of the lens with age. Types of refractive error Snellen Chart M= magnocellular, P= Parvocellular light Bipolar cells Photoreceptors Ganglion cells Rods and cones Night vision Daytime vision Photoreceptor density across the retina Cone Photoreceptors are densely packed in the central fovea Foveal cones are about 0.5 min arc Snellen Chart at 20/20 D Foveal cone diameter is approximately the width of the lines Hundreds of rods converge onto one bipolar cell Little or no convergence in cones cones rods rod bipolar cone bipolar Convergence leads to greater sensitivity (cf catchment area) Convergence: many rods converge onto a single rod bipolar cell, and several cones converge onto a diffuse bipolar cell. This allows the signal to be amplified. M= magnocellular, P= Parvocellular light Bipolar cells Photoreceptors Ganglion cells Visual Acuity matches photoreceptor density Relative visual acuity Receptor density Blue, green, and red dots represent the three different cone types, of a living human being in a patch of retina near the fovea. Each cone type has a different spectral sensitivity profile. Color appearance results form the different pattern of activity across the 3 cone types. Color Vision is a consequence of having 3 different cone types Consequences of Macular Degeneration scotoma The macular is the region around the central fovea. Macular degeneration is a disease of aging but may also occur in the juvenile form. Fundus of a patient with retinitis pigmentosa Retinitis is an inherited disease that progresses through early adulthood and leads to blindness. Gene therapy treatments are being developed. (May co-occur with deafness.) Other eye diseases: Glaucoma high intra-ocular pressure Diabetic retinopathy Retinal detachment. Photoreceptor response to light is sluggish This is why your computer monitor doesnt flicker. Photoreceptor response to light Cones are much faster than rods dont play tennis at dusk! The receptive field of a cell is the region in space where light leads to a response in the cell. Center-surround organization of receptive fields means that light in the center and light in the surrounding region have opposite effects Retinal ganglion cell receptive fields: how signals are organized after the receptors. Retinal ganglion cell receptive fields ON and OFF center cells (What are they for?) Low sensitivity to blurred images leads to fading Light Adaptation The light levels the eye can respond to covers a range of However, the ganglion cells can only respond between 1 and 200 impulses per second (approx). Therefore the retina must adjust its sensitivity so that it can signal meaningful changes around a mean level. This adjustment is called light adaptation. The corresponding changes when light level decreases is called dark adaptation Pupil can constrict to reduce the amount of light entering the eye. demo Major transformations of the light signal in the retina: 1. Anatomical organization of photoreceptors provides high acuity in fovea with rapid fall-off in the periphery. (photoreceptor density) 2. Color Vision: 3 cone photoreceptors types. 3. Light adaptation sensitivity regulation - adjustment of operating range to mean light level. (Light level range, ganglion cells, 10 2 range.) 4. Sluggish response reduced response flickering lights Temporal summation/integration a strong 1 msec flash is equivalent to a weaker 50 msec flash. 5. Convergence of photoreceptors onto ganglion cells also leads to acuity limitations in the peripheral retina. (1 cone per midget cell in fovea) Projections of visual signals from retina to visual cortex The mapping of objects in space onto the visual cortex Visual consequences of lesions at different locations in the visual pathway. contralateral hemianopsia (cf Homonymous hemianopia) Foveal sparing Eye movements Retinotopic Organization and Cortical Magnification The brain uses more physical space for signals from the fovea than the periphery Adjacent points in the world project to adjacent points in cortex Signals from each eye are adjacent in LGN but remain segregated in different layers. Convergence occurs in V1. Two kinds of cells in retina project to different layers in LGN M=magno=big P=parvo=small K= konio Lateral Geniculate Nucleus In the thalamus Primary Cortical Sub-divisions Visual cortex is a layered structure (6 layers). LGN inputs arrive in Layer 4. Layers 2,3 output to higher visual area., Layers 5,6 output to sub-cortical areas (eg superior colliculus) Massive feedback projection from layer 6 to LGN 800 lb gorilla. Also extensive feedback inputs from extra-striate cortex) Cells in V1 respond to moving or flashing oriented bars. Little response to diffuse flashes or steady lights. (Note this established the paradigm for neurophysiol investigation cf natural vision) LGN cells have circular receptive fields, like retina. Not clear what the role of the LGN is. (Murray Sherman gates input to cortex) Oriented cells emerge in V1, probably composed Of appropriately aligned LGN cells as shown. (Usrey dual cell recordings) Cells in V1 are organized into columns. Orientation preference gradually changes as one progresses across cortex. Cells at different depths Have same orientation preference. Binocular convergence: Cells respond more or less to R and L eye inputs. Ocular dominance varies smoothly across cortical surface orthogonal to orientation variation Note: ocular dominance not equal to disparity sensitivity (stereo). Orderly anatomical organization in V1 Regular large scale organization of orientation preference across cortical surface. Does this simplify signal processing? What is V1 doing? Early idea: edge detectors basis for more complex patterns Later ( s) spatial frequency channels any spatial pattern can be composed of a sum of sinusoids Late 90s to now: Main idea about V1 is that it represents an efficient recoding of the information in the visual image. Images are not random. Random images would require point-by-point representation like a camera. Images have clusters of similar pixels and cells designed to pick this up. Cells extract information about spatial variation at different scales (clusters of different sizes). Can think of receptive fields as basis functions (an alphabet of elemental image components that capture clustering in local image regions)