THE VISUAL SYSTEM: EYE TO CORTEX Outline 1. The Eyes a. Structure b. Accommodation c. Binocular Disparity 2. The Retina a. Structure b. Completion c. Cone & Rod Vision d. Eye Movements 3. Visual Transduction by Rhodopsin 4. From Retina to Primary Visual Cortex

THE PERCEPTION OF CONTRAST AND COLOR Outline 1. Contrast: The Perception of Edges a. Lateral Inhibition: The Physiological Basis of Contrast Enhancement 2. Brightness-Contrast Detectors in the Mammalian Visual System a. Mapping Receptive Fields b. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway c. Simple Cortical Cells d. Complex Cortical Cells e. Hubel and Wiesel's Model of Striate-Cortex Organization 3. Seeing Color a. Component Theory b. Opponent Process Theory

Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway Most retinal ganglion cells, lateral geniculate nucleus neurons, and the neurons in lower layer IV of the striate cortex have similar receptive fields They are circular They are smaller in the fovea area (larger in the periphery) Indicative of greater acuity in the fovea They are monocular They have both an excitatory and an inhibitory area separated by a circular boundary. Neurons in these regions have 2 patterns of responding: (1) on firing (2) inhibition followed by off firing

Simple Cortical Cells Respond best to bars or edges of light in a particular location in the receptive field and in a particular orientation (e.g., 45 degrees) All simple cells are monocular They also have on and off areas

Complex Cortical Cells Most of the cells in the striate cortex are complex cells more numerous like simple cells in that they respond best to straight-line stimuli in a particular orientation unlike simple cells in that the position of the stimulus within the receptive field does not matter the cell responds to the appropriate stimulus no matter where it is in its large receptive field Over half of the complex cells are binocular, and about half of those that are binocular display ocular dominance

Hubel and Wiesel's Model of Striate-Cortex Organization When you record from visual cortex using vertical electrode passes, you find (1) As the electrode moves up or down through the layers of the striate cortex it finds neurons that respond to stimulation from about the same location on the retina (2) simple and complex, cells that all prefer the same orientation – the cells respond to line orientations that are at the same degree Info. flows from on and off cells in lower layer IV to simple cells and then to complex cells (3) binocular complex cells that are all dominated by the same eye (if they display ocular dominance)

Hubel and Wiesel's Model of Striate-Cortex Organization When you record from visual cortex using horizontal electrode passes you find (1) receptive field location shifts slightly with each electrode advance (2) orientation preference shifts slightly with each electrode advance (3) ocular dominance periodically shifts to the other eye with electrode advances

CORTICAL MECHANISMS OF VISION Outline 1. The Parallel, Functionally Segregated Hierarchical Model of Perception 2. Cortical Mechanisms of Vision a. Scotomas and Blindsight b. Completion c. Secondary Visual Cortex and Association Cortex d. Visual Agnosia 3. Selective Attention

The Current Parallel, Functionally Segregated Hierarchical Model of Sensory- System Organization Parallel: sensory systems are organized so that information flows between different structures simultaneously along multiple pathways Explicit –– a sensory system for consciously seeing things Implicit –– a sensory system that helps us interact with objects in space. Functionally Segregated: There are divisions of labor at the different hierarchical levels Some neurons respond to motion, some to color, and so on. Hierarchical: information flows through brain structures in order of their increasing neuroanatomical and functional complexity. Primary Secondary Association

Vertemnus (1591) by Guiseppe Arcimboldo

The Vegetable Gardener (1590)

Neural Correlates? There is evidence that selective attention occurs by strengthening the neural responses to attended-to aspects and by weakening the response to others. Anticipation of a stimulus increases neural activity in the same circuits affected by the stimulus itself. When attempting to recognize faces - ventral stream is activated Does this face belong to the same person? face recognition activates the ventral visual pathway – is this face the same person you saw before? fMRI When attending to location - dorsal stream is activated. Is this face in the same position as the last face you saw? face position activates the dorsal visual pathway – is this face in the same location as the face you saw before? fMRI

Primates Recorded from neurons in the prestriate area that is part of the ventral stream that are particular responsive to color. Some cells responded to red; some to green Monkeys were taught to perform a task that required attention to a red cue. The activity of red neurons was increased, whereas the activity of green neurons was reduced