Visual Coding and Retinal Receptors Reception-absorption of physical energy by receptors Transduction-the conversion of physical energy to an electrochemical pattern in the neurons Coding- one-to-one correspondence between some aspect of the physical stimulus and some aspect of the nervous system activity
Visual Coding and Retinal Receptors From Neuronal Activity to Perception coding of visual information in the brain does not duplicate the stimulus being viewed General Principles of Sensory Coding Muller and the law of specific energies-any activity by a particular nerve always conveys the same kind of information to the brain Qualifications of the Law of Specific Energies the rate of firing or pattern of firing may signal independent stimuli timing of action potentials may signal important information indicating such things as movement the meaning of one neuron depends on what other neurons are active at the same time
Visual Coding and Retinal Receptors The Eye and Its Connections to the Brain Pupil-opening in the center of the eye that allows light to pass through Lens-focuses the light on the retina Retina-back surface of the eye that contains the photoreceptors The Fovea-point of central focus on the retina The Route Within the Retina photoreceptors-rods and cones bipolar cells-receive input from rods and cones ganglion cells-receive input from bipolar cells optic nerve-made up of axons of ganglion cells blind spot-the point where the optic nerve leaves the eye
Figure 6.2 Cross section of the vertebrate eye Note how an object in the visual field produces an inverted image on the retina.
Figure 6.4 Visual path within the eyeball The receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain.
Figure 6.6 Two demonstrations of the blind spot of the retina Close your left eye and focus your right eye on the o in the top part. Move the page toward you and away, noticing what happens to the x. At a distance of about 25 cm (10 inches), the x disappears. Now repeat this procedure with the bottom part. At that same distance what do you see? Animation
Rods abundant in the periphery of the retina best for low light conditions see black/white and shades of gray Cones abundant around fovea best for bright light conditions see color Visual Receptors: Rods and Cones
Transduction Both Rods and Cones contain photopigments (chemicals that release energy when struck by light) 11-cis-retinal is transformed into all-trans-retinal in light conditions this results in hyperpolarization of the photoreceptor the normal message from the photoreceptor is inhibitory Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells
Color Vision The Trichromatic (Young-Helmholtz) Theory we perceive color through the relative rates of response by three kinds of cones, each kind maximally sensitive to a different set of wavelengths The Opponent-Process Theory we perceive color in terms of paired opposites The Retinex Theory When information from various parts of the retina reaches the cortex, the cortex compares each of the inputs to determine the brightness and color perception for each area
Figure 6.12 Possible wiring for one bipolar cell Short-wavelength light (which we see as blue) excites the bipolar cell and (by way of the intermediate horizontal cell) also inhibits it. However, the excitation predominates, so blue light produces net excitation. Red, green, or yellow light inhibit this bipolar cell because they produce inhibition (through the horizontal cell) without any excitation. The strongest inhibition is from yellow light, which stimulates both the long- and medium-wavelength cones. Therefore we can describe this bipolar cell as excited by blue and inhibited by yellow. White light produces as much inhibition as excitation and therefore no net effect. (Actually, receptors excite by decreasing their usual inhibitory messages. Here we translate that double negative into excitation for simplicity.)
Color Vision Deficiency Color Vision Deficiency-inability to perceive color differences Generally results from people lacking different subsets of cones
Neural Basis of Visual Perception An Overview of the Mammalian Visual System Rods and Cones synapse to amacrine cells and bipolar cells Bipolar cells synapse to horizontal cells and ganglion cells Axons of the ganglion cells leave the back of the eye The inside half of the axons of each eye cross over in the optic chiasm Pass through the lateral geniculate nucleus Transferred to visual areas of cerebral cortex
Processing Visual Stimuli Mechanisms of Processing in the Visual System Receptive Field-the part of the visual field to which any one neuron responds They have both excitatory and inhibitory regions Lateral Inhibition-the reduction of activity in one neuron by activity in neighboring neurons Heightens contrasts-those receptors just inside the border are most excited and those outside the border are the least responsive
Figure 6.16 Receptive fields The receptive field of a receptor is simply the area of the visual field from which light strikes that receptor. For any other cell in the visual system, the receptive field is determined by which receptors connect to the cell in question.
Figure 6.17 Blocks on a surface of gelatin, analogous to lateral inhibition Each block pushes gelatin down and therefore pushes neighboring blocks up. Blocks at the edge are pushed up less than those in the center.
Figure 6.18 An illustration of lateral inhibition Do you see dark diamonds at the “crossroads”?
Neural Basis of Visual Perception Concurrent Pathways in the Visual System In the Retina and Lateral Geniculate Two categories of Ganglion cells Parvocellular-smaller cell bodies and small receptive fields, located near fovea; detect visual details, color Magnocellular-larger cell bodies and receptive fields, distributed fairly evenly throughout retina; respond to moving stimuli and patterns In the Cerebral Cortex V1-Primary Visual Cortex-responsible for first stage visual processing V2-Secondary Visual Cortex-conducts a second stage of visual processing and transmits the information to additional areas Ventral stream-visual paths in the temporal cortex Dorsal stream-visual path in the parietal cortex
Figure 6.20 Three visual pathways in the cerebral cortex (a) A pathway originating mainly from magnocellular neurons. (b) A mixed magnocellular/parvocellular pathway. (c) A mainly parvocellular pathway. Neurons are heavily connected with other neurons in their own pathway but only sparsely connected with neurons of other pathways. Area V1 gets its primary input from the lateral geniculate nucleus of the thalamus; the other areas get some input from the thalamus but most from cortical areas. (Sources: Based on DeYoe, Felleman, Van Essen, & McClendon, 1994; Ts’o & Roe, 1995; Van Essen & DeYoe, 1995)
Neural Basis of Visual Perception The Cerebral Cortex: The Shape Pathway Hubel and Wiesel’s Cell Types in the Primary Visual Cortex Simple Cells has fixed excitatory and inhibitory zones in its receptive field Complex Cells receptive fields cannot be mapped into fixed excitatory and inhibitory zones Respond to a pattern of light in a particular orientation Hypercomplex cells (End-stopped cells) Resemble complex cells but have a strong inhibitory area at one end of its bar-shaped receptive field
Figure 6.23 The receptive field of a complex cell in the visual cortex It is like a simple cell in that its response depends on a bar of light’s angle of orientation. It is unlike a simple cell in that its response is the same for a bar in any position within the receptive field.
Neural Basis of Visual Perception The Columnar Organization of the Visual Cortex Column are grouped together by function Ex: cell within a given column respond best to lines of a single orientation Are Visual Cortex Cells Feature Detectors? Feature Detectors-neurons whose responses indicate the presence of a particular feature Shape Analysis Beyond Areas V1 and V2 Inferior Temporal Cortex (V3)-detailed information about stimulus shape (V4)-Color Constancy; Visual Attention (V5)-Speed and Direction of Movement
Neural Basis of Perception Disorders of Object Recognition Visual Agnosia-Inability to Recognize Objects Prosopagnosia-Inability to recognize faces
Neural Basis of Visual Perception The Cerebral Cortex: The Color Pathway Parvocellular to V1 (blobs) to V2, V4, and Posterior Inferior Temporal Cortex The Cerebral Cortex: The Motion and Depth Pathways Structures Important for Motion Perception Middle-temporal cortex-V5-speed and direction of movement Motion Blindness-Inability to detect objects are moving
Neural Basis of Visual Perception Visual Attention Attentional Processes can determine what is seen The Binding Problem Revisited: Visual Consciousness How are all aspects of an object brought together? Animation
Development of the Visual System Infant Vision See better in the periphery than in the center of vision Great difficulty in shifting attention
Experience and Visual Development Early Lack of Stimulation of One Eye-blindness occurs in that one eye Early Lack of Stimulation of Both Eyes-if this occurs over a long period of time, loss of sharp receptive fields is noted Restoration of Response and Early Deprivation of Vision-deprive stimulation of the previously active eye and new connections will be made with the inactive eye Uncorrelated Stimulation in Both Eyes-each cortical neuron becomes responsive to the axons from just one eye and not the other
Experience and Visual Development Early Exposure to a Limited Array of Patterns—most of the neurons in the cortex become responsive only to the patterns that the subject has been exposed to Lack of Seeing Objects in Motion-become permanently disable at perceiving motion Effects of Blindness on the Cortex-parts of the visual cortex become more responsive to auditory and tactile stimulation