Chapter 6 Vision

Introduction Sensory receptors – a specialized neuron that detects a particular category of physical events Sensory transduction – the process by which sensory stimuli are transduced into slow, graded receptor potentials

The Stimulus The perceived color of light is determined by 3 dimensions: –Hue – the dominant wavelength –Saturation - purity –Brightness - intensity

Anatomy of the visual system Eyes – –Suspended in the orbits of the skull –Held in place by 6 extraocular muscles –Retina – the neural tissue and photoreceptive cells located on the inner surface of the posterior position of the eye –Fovea – the region of the retina that mediates the most acute vision; color sensitive cones constitute the only type of photoreceptor here

Anatomy of the visual system Eyes –Photoreceptor – one of the receptor cells of the retina; transduces photic energy into electrical potentials Rod – sensitive to light of low intensity Cones – maximally sensitive to one of 3 different wavelengths of light and hence encodes color vision –Optic disk – the location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot –3 types of movements: Vergence movements – the cooperative movements that keep both eyes fixed on the same target Saccadic movements – rapid, jerky movements of the eyes used in scanning a visual scene Pursuit movement – the movement made to maintain an image of a moving object on the fovea

Photoreceptors Each photoreceptor consists of an inner segment and an outer segment, which contains several hundred lamellae (thin plate of membrane)

Visual info transduction Photopigments are special molecules embedded in the lamellae (e.g. rhodopsin) and consists of 2 parts: –Opsin – protein –Retinal – lipid When a molecule of rhodopsin is exposed to light it breaks into its two parts, and this causes a change in the membrane potential of the photoreceptor, which changes the firing rate of glutamate

Visual info transduction The membrane of photoreceptors is different from others – the cation channels (sodium and calcium) are normally open In the dark these ion channels are open, and so the photoreceptors continually release glutamate When light strikes the photopigment, the G protein transducin is activated, which then activate the enzyme phosphodiesterase which closes the ion channels; this lowers the rate of glutamate release Light hyperpolarizes the photoreceptor and then depolarizes the bipolar cell

Connections between eye and brain The axons of the retinal ganglion cells project through the optic nerves, cross over through the optic chiasm to the dorsal lateral geniculate nucleus (LGN) The LGN consists of 6 layers of neurons: the inner 2 layers are called the magnocellular layers and the outer 4 are called the parvocellular layers The cells in the LGN project through the optic radiations to the primary visual cortex (aka striate cortex) Diagram of visual pathways

Coding of light and dark The receptive field of a neuron in the visual system is the part of the visual field that an individual neuron “sees”, i.e. the part in which light must fall for the neuron to be stimulated 2 major types of retinal ganglion cells, ON center and OFF center cells –ON center cells are excited by light falling in the center of the field (center), and inhibited by light falling in the surrounding field (surround) –OFF center cells are excited by light in the surround, and inhibited by light in the center

Coding of light and dark The center-surround organization of the retinal ganglion cells enhances our ability to detect the outlines of objects

Coding of color The retinas of humans, Old World monkeys and apes contain 3 different types of cones which provide us with an elaborate form of color vision All visible colors (for humans at least) can be mixed from the 3 main colors: red (long), green (medium), and blue (short); due to the wavelengths absorbed by the 3 different cones (trichromatic theory) Genetic defects can cause one or more of the 3 types of cones to not function properly, resulting in either protanopia, where red and green are confused because the red cones respond to green; deuteranopia, where where red and green are confused also, but because the green cones respond to red; or tritanopia, where they lack blue cones

Anatomy of the striate cortex Consists of 6 principle layers, arranged in bands parallel to the surface The striate cortex of one hemisphere contains info from the contralateral visual field Approx. 25% of the striate cortex is devoted to anlaysis of info from the fovea Neurons in the visual cortex selectively respond to specific features of the visual world, not just to light

Orientation and Movement Simple cell – an orientation-sensitive neuron whose receptive field is organized in an opponent fashion Complex cell – a neuron that responds to the presence of a line segment with a particular orientation located within it s receptor field, especially when the line moves perpendicularly to its orientation Hypercomplex cell – a neuron that responds to the presence of a line segment with a particular orientation that ends at a particluar point within the cell’s receptive field

Spatial Frequency Neurons in the primary visual cortex respond best to sine-wave gratings, which are a series of straight parallel bands varying continuously in brightness according to a sine-wave function, along a line perpendicular to their lengths A sine-wave grating is designated by its spatial frequency, or the relative width of the bands, measured in cycles per degree of visual angle The most important visual information is that contained in low spatial frequencies

Retinal disparity Binocular vision (i.e. from 2 eyes) provides a vivid perception of depth through the process of stereoscopic vision, or stereopsis Most neurons in the primary visual cortex are binocular, they respond to info of either eye The cells respond most vigorously when each eye sees a stimulus in a slightly different location, called retinal disparity

Color In the striate cortex, info from color-sensitive ganglion cells is transmitted through to special cells grouped into cytochrome oxidase (CO) blobs

Modular organization of the striate cortex The striate cortex is divided into ~2500 modules, each containing ~150,000 neurons The neurons in each module are devoted to the analysis of various features contained in one very small portion of the visual field, and combine together to form a complete whole Info from the layers of the LGN project to the different layers of the modules The modules consist of 2 segments, each surrounding a CO blob –The neurons in the CO blobs are sensitive to color and low spatial frequencies –The neurons outside of the blobs are sensitive to orientation, movement, spatial freq, texture and binocular disparity

Role of the visual association cortex Two streams of visual analysis –Visual info receive from the striate cortex is analyzed in the visual assc cortex –Neurons in the striate cortex project to the extrastriate cortex, which surrounds the visual assc cortex –The primate extrastriate cortex consists of several specialized regions that respond to particular features of a visual stimulus –Contains 2 streams of analysis: Dorsal stream – a system of interconnected regions of visual cortex involved in the perception of spatial location, beginning with the striate cortex and ending with the posterior parietal cortex Ventral stream – a system of interconnected regions of visual cortex involved in the perception of form, beginning in the striate cortex and ending in the inferior temporal cortex

Perception of color Color constancy – the relatively constant appearance of the colors of objects under varying lighting conditions; the visual system compensates for the source of light when process visual information about colors Achromatopsia – inability to discriminate among different hues; caused by damage to the visual assc cortex

Analysis of form In primates the recognition of visual patterns and identification of particular objects take place in the inferior temporal cortex, located at the end of the ventral stream on the ventral part of the temporal lobe –Analyses of form and color are put together here and perceptions of 3D objects and backgrounds are achieved –Consists of 2 major regions: TE and TEO Damage to the human visual assc cortex can result in a visual agnosia, which results in an inability to perceive or identify a visual stimulus –Apperceptive visual agnosia – failure to perceive objects, even though visual acuity is normal (e.g. cannot name an object by looking at it, but can if allowed to touch it) –Prosopagnosia – failure to recognize particular people by the sight of their faces (i.e. can recognize by voice, hair color, etc.) Fusiform face area – region of the extrastriate cortex located at the base of the brain; involved in perception of faces and other complex objects that require expertise to recognize Associative visual agnosia – inability to identify objects that are perceived visually, even though the form of the perceived object can be drawn or matched with similar objects; appears to involve difficulty in transferring visual info to verbal mechanisms

Perception of movement Area V5 of the extrastriate cortex contains neurons that respond to movement, and show directional sensitivity An area adjacent to V5 receives info from V5 about movement and respond to more complex movement features, such as radial, circular or spiral motion –Optic flow – the complex motion of points in the visual field caused by relative movement b/t the observer and env’t; provides info about the relative distance of objects from the observer and of the relative direction of movement Akinetopsia – inability to perceive movement, caused by damage to area V5 of the visual assc cortex

Perception of spatial location 3 subareas of the extrastriate cortex send info through area V5 to the parietal cortex, which is involved in spatial perception Damage to this area disrupts performance on tasks that require perceiving and remembering the location of objects Balint’s syndrome – caused by bilateral damage to the parieto- occipital region; includes optic ataxia (difficulty in reaching for objects under visual guidance), ocular apraxia (difficulty in visual scanning), and simultanagnosia (difficulty in perceiving more than one object at a time)