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Chapter 4 - The Visual Cortex G8 Ch 4 Outline Following the Signals from Retina to Cortex The Visual System Processing in the Lateral Geniculate Nucleus Receptive Fields of LGN Neurons Information Flow in the Lateral Geniculate Nucleus Organization by Left and Right Eyes Organization as a Spatial Map Receptive Fields of Neurons in the Striate Cortex Do Feature Detectors Play a Role in Perception? Selective Adaptation and Feature Detectors Grating Stimuli and the Contrast Threshold Selective Rearing and Feature Detectors Maps and Columns in the Striate Cortex Maps in the Striate Cortex Columns in the Striate Cortex Location Columns Orientation Columns Ocular Dominance Columns Hypercolumns How is an Object Represented in the Striate Cortex Streams: pathways for What, Where, and How Streams for Information About What and Where Streams for Information about What and How The Behavior of Patient D.F. The Behavior of People Without Brain Damage Modularity: Structures for Faces, Places, and Bodies Face Neurons in the Monkey’s IT Cortex Areas for Faces, Places, and Bodies in the Human Brain Something to Consider: How Do Neurons Become Specialized How Neurons Can Be Shaped by Experience The Visual Cortex - 1 4/29/2017 Striate cortex, area V1: About 250 million neurons (out of the 15-30 billion in the cortex). Recall: The cortex is a sheet of neurons that covers the rest of the brain. Side view of the cortex, with some of the possible sources of input 1, It used to be thought that the cortex was made up of 6 layers of cells. We now know that what used to be thought of as Layer 4 are actually 4A, 4B, 4Cα, and 4Cβ. The Visual Cortex - 2 4/29/2017 The Cortical Map of the Visual Field (From Wolfe, Kluender, & Levi) Note that what is in the center of the visual field ultimately projects to the leftmost side and rightmost side of the occipital lobe. The left and right peripheries of the visual field are projected to the area between the hemispheres. The Visual Cortex - 3 4/29/2017 Visual Image Retinal Images Cortical “Images” Peripheral Foveal Foveal The Visual Cortex - 4 4/29/2017 Details of the representation The cortex is organized as Hypercolumns Hypercolumn: A 1 mm2 are of cortex receiving input from a small area on the retina. Stimulation of a small area of the retina leads to activity in the hypercolumn representing that area. It’s called a column because it is collection of columns of cells, containing all 6 layers of the cortex. It’s called a hypercolumn because it contains multiple individual columns, each one devoted to processing a the visual stimulus in a different way. Hypercolumns are analogous to states – each state has multiple subentities – counties, cities, towns – that perform different functions. Adjacent small areas of the retina are represented by adjacent hypercolumns. The hypercolumns form a distorted retinotopic map. The cells within each hypercolumn have specific receptive fields in the retina – the area in the retina whose simulation leads to activity of those cells. That is, each cell in each hypercolumn is “looking” at the activity in a unique part of the retina. Cells in different hypercolumns look at different parts. Those in adjacent hypercolumns look at adjacent parts of the retina. Hypercolumns Cortex The Visual Cortex - 5 4/29/2017 Cortical magnification G8 p. 82-84 Each small area of the retina is represented by a 1 mm2 cortical hypercolumn. Receptive fields in the fovea of the retina are smaller than those in the periphery. Regardless of the size of the retinal area, each such area is represented by a same-sized 1 mm2 cortical area. This phenomenon is called cortical magnification, illustrated below. In the figure, each circle in the retina represents a collection of ganglion cell receptive fields. Each circle in the cortex represents a collection of neurons that receive information from the retinal ganglion cells. Recent evidence reported suggests that the foveal area is actually allocated more cortical neurons than the peripheral area. Retina Ganglion cell receptive fields Cortex Periphery of retina 1 mm2 Fovea Magnification Periphery of retina Note that cortical magnification means that relatively more hypercolumns are involved in processing information from the foveal region than from other regions. The Visual Cortex - 6 4/29/2017 Types of cortical cells. Layer 4 Layer 4 cells have circular receptive fields, similar to those of the LGN cells that drive them. Other layers In other layers, the neurons have receptive fields that are not simply circular. Simple cells. These cortical cells respond only to bars of light or slits of darkness located in a specific place in the visual field. 1) located in a particular place in the visual field and 2) have a particular orientation. If the bar or slit is moved to a different location, the neuron quits firing. If the orientation of the bar or slit is changed, ditto. Consider the following. Each circle represents the visual field under a different stimulation. \ / Receptive field Receptive field Receptive field / Yippee! Cortical neuron Neuron responds because the stimulus is in its receptive field with correct orientation Ho hum. Cortical neuron Neuron does not respond because although the stimulus has the correct orientation, it’s not in the neuron’s receptive field. The Visual Cortex - 7 Ho hum. Cortical neuron Neuron does not respond because although the stimulus is in its receptive field, the stimulus does not have the appropriate orientation. 4/29/2017 Possible wiring input to simple cortical cells Here’s how the simple cortical cell receptive fields might be created. Schematic of “wiring diagram” of simple cortical cells. Ganglion Cells in retina Cortical cells Complex cells These cells respond to bars of light or slits of darkness, as do simple cells. But they respond best when the bar or slit moves within a certain area of the visual field. Many respond best to a particular direction of movement. The figure below attempts to illustrate the stimulus for a complex cell “looking” for movement of a particularly oriented bar moving from left to right across the visual field. // /// End-stopped cells (hypercomplex) These cells respond to moving lines of a specific length (hence the term, end-stopped). Some also respond to moving corners or angles. Play VL 4.2 “Visual Cortex of the cat” here – about 20 min. The Visual Cortex - 8 4/29/2017 More on the Hypercolumns G8 p 84 As stated earlier, it has been discovered that corresponding to each small area on the retina is an approximately 1 mm2 area in the striate cortex. These areas are called hypercolumns. Within each hypercolumn are individual-neuron columns which receive specific characteristics of the visual stimulus. Orientation column: A column of cells within a hypercolumn at the same location in each of the six layers, all of which respond to the same orientation of a visual stimulus. (Layer 4 excluded). Adjacent columns respond to slightly different orientations. Within each hypercolumn are 1000s of orientation columns covering virtually all possible orientations. Hypercolumn 2 mm Orientation column The Visual Cortex - 9 4/29/2017 Ocular Dominance columns. Ocular Dominance column: A column of cells at the same location across the six layers, all of which have the same eye preference. Some columns respond only to stimulation of left eye Some more to left eye stimulation than to right Some equally to stimulation of either eye Some more to right eye stimulation than to left Some only to right eye And all gradations between. Blob columns. Blob column: A collection of cells at the same location across the six layers, all of which respond to the same wavelength of light. Hypercolumn Summary Columns within each 1 mm2 area are categorized in 3 ways Orientation preference Eye preference Wavelength preference The Visual Cortex - 10 4/29/2017 What’s it all mean? Cells in the visual cortex respond to complex features – they’re feature detectors. 1) As we’ve said before, edges are probably more important for us than homogenous fields. So immediate processing of the incoming stream of visual information for edges seems to be a smart thing to do. 2) Extracting features with edges may be the most efficient way of enabling the processing of the visual world as it continually changes around us. Most of the things in the world that are important for us are defined by combinations of edges. The Visual Cortex - 11 4/29/2017