CHAPTER 10 Vision and visual perception Form Vision

Pathway to the Brain Basic pathway: –Rods or cones  bipolar cells  ganglion cells –  optic nerve  optic chiasm  optic tracts (L and R) –  lateral geniculate nuclei of thalamus –Optic radiations leave LGN  occipital cortex –Several areas of occipital cortex of processing: Areas 17, 18 and 19

Two kinds of pathways Ambient pathway: – Spatial vision pathway –Where things are, not what they are –Magnocelluar pathway Focal vision – Object or form vision pathway –What things are, but not where they are –Parvocellular pathway Damage to either or both pathways: Cortical blindness –Damage to brain, not eyes or even optic tract –Typically lose form vision first, then all of it –Very rare to just lose ambient pathway

Visual information follows two routes from the retina through the brain: – Parvocellular system : Focal or form vision – Magnocellular system : spatial vision or ambient pathway Parvocellular ganglion cells : located mostly in the fovea. –These cells have circular receptive fields –Receptive fields are small and color opponent –These smaller receptive fields are best for discrimination of fine detail and color. –Thus this is the form pathway The Perception of Movement and distance

Magnocellular ganglion cells – have a larger circular receptive fields – for example: Brightness opponent respond only briefly to stimulation. specialized for brightness contrast and for movement –This is the spatial pathway. Parvocellular and magnocellular pathways travel to –The lateral geniculate nucleus or LGN –Then to the primary visual cortex, also known as V1. –From there they begin to travel different paths Perception of Objects, Color, and Movement

But: Two pathways are highly interconnected Form pathway : –Parvocellular system = ventral stream that flows from the visual cortex into the temporal lobes –Ventral stream = what of visual processing Spatial pathway : –magnocellular system = dorsal stream from the visual cortex to the parietal lobes. –Dorsal stream = where of visual processing The Perception of Objects, Color, and Movement

Beyond V1: the ventral stream –Passes through V2 and into V4, –Mostly concerned with color perceptio n –Fine vision….important to make fine perceptual distinctions –Remember, this is form vision. Then projects to the inferior temporal cortex, – L ower boundary of the temporal lobe. –This area shows a remarkable specialization for object recognition. –Must link visual information to auditory information: connect verbal label to object form! Further processing of form vision

Dorsal stream (magnocellular/spatial) proceeds to – V2 through to V5, also known as Medial temporal or MT It is on the middle temporal gyrus of the temporal lobe –Neurons there have strong directional sensitivity –Contributes to the perception of movement and distance The dorsal stream then travels to posterior parietal cortex –located just behind the somatosensory cortex. –Primary role: locate objects in space –Significant behavioral implications of this function VERY important Further processing of spatial vision

Visual pathways rely on inhibition A retinotopic map exists in the visual cortex –adjacent retinal receptors activate adjacent cells in the visual cortex. For example: Form vision is able to –detect an object’s boundaries and features (such as texture) –Differs from spatial location (that is spatial vision!). –Relies much more heavily on stimulation of cones –Relies on lateral inhibition to detect “edges” Lateral inhibition, –Each neuron’s activity inhibits the activity of its neighbors –In turn, its activity is inhibited by them. –The inhibition is delivered by horizontal cells to nearby synapses of receptors with bipolar cells.

Form Vision and inhibition In on center cells –light in the center increases firing –light in the off surround reduces firing below resting levels. Other ganglion cells have an off center and an on surround function. –Work the opposite –Off center: light in surround increases firing; center reduces firing –On surround: light in surround increases firing Why: think of pixels: which pixel is triggered helps build the visual image in the visual field

Form Vision: Many specialized neurons Simple cells –respond to a line or an edge that is at a specific orientation –May also respond to specific place in the visual field on the retina –This would be like an individual pixel being turned on or off. Complex cells: –continue to respond when a line or edge moves to a different location. –Activating a “series of pixels” –Allows processing of movement and change

Form Vision According to Spatial Frequency Theory –Visual cortical cells “do” a Fourier frequency analysis of the luminosity variations in a scene. –Visual cortical cells can detect not just edges but gradations of change. –Can stimulate individual “pixels”, –Can stimulate individual or groups of pixels at various degrees –Can stimulate a moving series or group of pixels Allows the brain to detect movement, shape, color, etc.

Perception of Objects, Color, and Movement Modular processing –Segregation of the various components of processing into separate locations. –Processing of form is conducted in modules or groupings Hierarchical processing: –Lower levels of the nervous system analyze their information and pass the results on to the next higher level for further analysis. –We process visual information in layers Maybe: Distributed vs. localized processing : –Some neuroscientists reject modular notion, –Argue instead that any visual function is instead distributed –E.g., occurs across a relatively wide area of the brain and may occur simultaneously

Agnosias: inability to process sensory information –Object agnosia: impaired ability to recognize objects. –Prosopagnosia : inability to visually recognize familiar faces. –Both object agnosia and prosopagnosia are caused by damage to the inferior temporal cortex (part of the ventral stream). Fusiform face area –Face-responsive neurons are often intermingled with object-responsive neurons, –Part of the fusiform gyrus on the underside of the temporal lobe is critical for face recognition –Hence, the name! Facial recognition is innate! Cortical damage: several specific forms

Blindness is not just “being blind” –Depends on type of damage –Depends on degree of damage Most often: damage to focal or form vision –Cannot see individual shapes, colors or refined vision –Can still move around in environment Movement agnosia : inability to perceive movement. –Very debilitating –More critical to be able to perceive distance and movement Typically, damage form vision or all of vision –Very rare to have form vision but no spatial vision –Loss of spatial vision = lack of ability to move in environment Other Vision disorders

Blindsight: Patients blind due to damage to V1 –Can locate and track the movement of objects; discriminate colors, –But say they are guessing, unaware of abilities. Color agnosia: loss of the ability to perceive colors –Depends on color constancy –Color constancy: critical behavior : The ability to recognize the “natural color “ of object despite of illuminating wavelength If could not, objects would seem to change colors as the sun shifted position through the day or as we went indoors into artificial light Other Vision disorders

Function of posterior parietal cortex: – combines input from visual, auditory, and somatosensory areas –helps the individual locate objects in space –Helps orient the body in the environment. Damage here impairs several abilities: –reaching for objects –often produces sensory neglect –patient ignores visual, touch, and auditory stimulation on the side opposite the injury. Neglect not due to any defect in visual processing –Due to deficit in attention TO visual stimuli. Sensory neglect

Many researchers have wondered: – where is all the information about a visual object brought back together?. –Must put visual, auditory, kinesthetic information together to form “concept” of an object, event or activity Ultimate understanding of an object appears to occur in part of superior temporal gyrus –Area receives input from both visual and kinesthetic neural streams Two other areas also important: – Parietal cortex : where damage causes sensory neglect. – Frontal areas where both visual streams eventually converge. Most likely: redundant processing involving all three areas Why such redundancy for our perceptual systems? Where is all this processing finally put together?