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Chapter 3


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Matthias Niemeier

CHAPTER 3 SPATIAL VISION: FROM SPOTS TO STRIPES VISUAL ACUITY: OH SAY, CAN YOU SEE?  Contrast: difference in luminance between an object and the back ground, or between lighter and darker parts of the same object.  Acuity: the smallest spatial detail that can be resolved.  Cycle: for a grating, a pair consisting of one dark bar and one bright bar.  Visual angle: the angle subtended by an object at the retina  To calculate visual angle of your resolution acuity  divide size of the cycle by the viewing distance at which you could just barely make out the orientation of the gratings, then take the arctangent of this ratio.  This resolution acuity represents one of the fundamental limits of spatial vision: it’s the finest high-contrast detail that can be resolved  The limit is determined by the spacing of photoreceptors in the retina.  Sine wave grating: a grating with a sinusoidal luminance profile. Light intensity in such gratings varies smoothly and continuously across each cycle.  Aliasing: misperception of a grating due to undersampling. A VISIT TO THE EYE DOCTOR  The method for designating visual acuity was inventedin 1862 by a Dutch eye doctor, Herman Snellen.  Although 20/20 vision is often considered the gold standard, most healthy young adults have an acuity level closer to 20/15. ACUITY FOR LOW-CONTRAST STRIPES  Spatial frequency: number of times a pattern, such as a sine wave grating, repeats in a given unit of space. o Example, if you view your book from 120 cm away, the visual angle between each pair of white stripes is about 0.25 degree, so the spatial frequency of this grating is 1/0.25 = 4 cycles per degree.  Cycles per degree: the number of pair of dark and bright bars per degree of visual angle.  You might think that the wider the stripes (the lower the spatial frequency), the easier it would be to distinguish the light stripes from dark stripes but this isn’t true. Schade, Fergus Campbell and Dan Green demonstrated that the human contrast sensitivity function (CSF) is shaped like an upside down U.  Contrast threshold: smallest amount of contrast required to detect a pattern.  For a 1 cycle/degree grating to be just distinguishable from uniform gray, the dark stripes must be about 1% darker than the light stripes o Example if a light stripe reflects 1000 photons, dark stripe should reflect 990 photons). o The reciprocal of this threshold is 1/0.01 = 100.  A contrast of 100% corresponds to a sensitivity value of 1. The CSF value reaches this value at about 60 cycles/degree RETINAL GANGLION CELLS AND STRIPES  Retinal ganglion cells respond vigorously to spots of light. They also respond well to certain types of stripes or gratings.  When the spatial frequency of the grating is too low, the ganglion cell responds weakly because of the fat, bright bar of the grating lands in the inhibitory surround, damping the cell’s response.  When the spatial frequency is too high, the ganglion cell responds weakly because both dark and bright stripes fall within the receptive field center, washing out the response.  When spatial frequency is just right, with bright bar filling the center and dark bar filling the surround, the cell responds vigorously.  Thus, retinal ganglion cells are “tuned” to spatial frequency: each cell responds best to a specific spatial frequency that matches its receptive field size, and it responds less to both higher and lower spatial frequencies.  Christina Enroth-Cugell and John Robson  first to record responses of retinal ganglion cells to sinusoidal gratings.  In addition to showing that these cells respond vigorously to gratings of just the right size they discovered that the responses depend on the phase of the grating, its position within the receptive field.  In figure 3.10, when the grating has a light bar filling the receptive-field center and dark bars filling the surround, this ON- center cell responds vigorously, increasing the firing rate.  If the grating phase is shifted by 90 degrees, half the receptive field center will be filled by a light bar and half by a dark bar, and similarly for the surround. There won’t be a net difference between the light intensity in the receptive field’s center and its surround so the cell’s response rate doesn’t change from its resting rate when the grating is turned on.  A second 90 degree shift puts the dark bar in the centre and the light bars in the surround, producing a negative response  A third phase shift returns us to the situation after the first shift and the cell will be blind to the grating (other ganglion cells would respond to the 90 and 270 degree phases but not to the 0 and 180 degree phases, which is why the visual system as a whole is able to see all 4 phases equally well). THE LATERAL GENICULATE NUCLEUS  The axons of retinal ganglion cells synapse in the 2 lateral geniculate nuclei (LGNs), one in each cerebral hemisphere.  These act as relay stations on the way from the retina to the cortex.  LGN of primates is a 6 layered structure, like a stack of pancakes that has been bent in the middle (geniculate means bent).  The neurons in the bottom 2 layers are larger than those in the top 4 layers, the bottom 2 are called magnocelular layers and the top 4 are called parvocellular layers  Another way the 2 types of layers differ is that the magnocellularlayers receive input from M ganglion cells in the retina, and parvocellular layers receive input from P ganglion cells.  Magnocellular pathway responds to large, fast moving objects, and the parvocellular pathway is responsible for processing details of stationary targets.  Even more splitting takes between magno and parvo layers. Between these 2 layers we find layers consisting of koniocellular cells. Each layer is involved in different aspects of processing. o Example, one layer is specialized for relaying signals from the S cones and may be part of a “primordial” blue-yellow pathway.  1. Left LGN receives projections from the left side of the retina in both eyes, and the right LGN receives projctions from the right side of both retinas.  2. Each layer of the LGN receives input from one or the other eye.  Layers 1, 4, and 6 of the right LGN receive input from the left (contralateral) eye, while layers 2, 3, and 5 get their input from the right (ipsilateral) eye o Contralateral means opposite side of the body (or brain) o Ipsilateral means same side of the body (or brain)  Each LGN layer contains a highly organized map of a complete half of the visual field. Figure 3.12 shows how objects in the right visual field are mappedonto different layers of the left LGN (the right side of the world falls on the left side of the retina, whose ganglion cells project to the left LGN).  This ordered mapping of the world onto the visual nervous system, known as topographic mapping, provides us with a neural basis for knowing where things are in space.  LGN neurons have concentric receptive fields similar to retinal ganglion cells: they respond well to spots and gratings.  Then why don’t the ganglion cell axons simply travel directly back to the cerebral cortex? o LGN isn’t merely a stop on the line from retina to cortex, there are many connections between other parts of the brain and the LGN o There are more feedback connections from the visual cortex to the LGN than from the LGN to the cortex o LGN is a location where various parts of the brain can modulate input from the eyes.  Example, LGN is part of a larger brain structure called the thalamus. When you go to sleep, the entire thalamus is inhibited by circuitry elsewherein the brain that works to keep you asleep  If you were sleeping with your eyes open you wouldn’t see anything in a dimly lit room because input would go from your retinas to your LGNs, but the neural signals would stop there before reaching the cortex.  Bright lights and alarms will be perceived and cause you to wake up because thalamic inhibition isn’t complete. STRIATE CORTEX  The receiving area for LGN inputs in the cerebral cortex lies below the inion.  This area’s also named: primary visual cortex (V1), area 17, or striate cortex: area of the cerebral cortex that receives direct inputs from the LGN, as well as feedback from other brain areas, and is responsible for processing visual information.  Striate cortex consists of 6 major layers, some of which have sublayers.  Fibers from LGN project mainly to layer 4, with magnocellular axons coming in to sublayer 4C alpha and parvocellular axons projecting to sublayer 4c beta.  Striate cortex contains on the order of 200 million cells(more than 100x as LGN).  Figure 3.14 illustrates 2 important features of the visual cortex: topography and magnification. o The fact that the image of the woman’s right eye brow is mapped onto regions corresponding to the numbers 3 and 4 in the striate cortex, tells the visual system that the eyebrow must be in positions 3 and 4 of the visual field. This is topographical mapping. o Objects imaged on or near the fovea are processed by neurons in a large part of the striate cortex, but objects imaged in the far right or left periphery are allocated only a tiny portion of the striate cortex. o This distortion of the visual-field map on the cortex is known as cortical magnification because the cortical representation of the fovea is greatly magnified compared to the cortical representation of the peripheral vision. TOPOGRAPHY OF THE HUMAN CORTEX  Much of what we know about cortical topography and magnification comes from anatomical and physiological studiesin animals.  Earliest studies in humans were based on correlating visual field defects with cortical lesions.  MRI is useful for anatomical imaging of soft tissues, including the brain.  MRI lets us see the structure of the brain.  fMRI is a noninvasive technique for measuring and localizing brain activity  fMRI doesn’t measure neural activity directly, it measures changes in blood oxygen level that reflect neural activity.  Blood oxygen level-dependent (BOLD) signals reflect a range of metabolically demanding neural signals.  By comparing blood flow when visual stimuli are presented in one portion of the visual field to blood flow when the field is blank, you can find portions of the brain that respond specifically to that stimulation of that portion of the field.  This makes it possible to map the topography of VI in the living human brain. SOME PERCEPTUAL CONSEQUENCES OF CORTICAL MAGNIFICATION  One consequence of cortical magnification is that visual acuity declines in an orderly fashion with eccentricity. This was demonstrated by Hermann Rudolf Aubert.  High resolution requires a great number of resources: a dense array of photoreceptors, one-to- one lines from photoreceptors to retinal ganglion cells,and a large chunk of striate cortex.  To see the entire visual field with such high resolution, we’d need eyes and brains too large to fit in our heads. Thus we evolved to have a visual system that provides high resolution in the center and lower resolution in the periphery.  Although visual acuity falls off rapidly in peripheral vision, it’s not the major obstacle to reading or object recognition in the visual periphery, the real problem is visual crowding: the deleterious effect of clutter on peripheral object recognition. Objects that can be easily identified in isolation seem indistinct and jumbled in clutter.  Crowding impairs not only the discrimination of object features and contours but the ability to recognize and respond appropriately to objects in clutter.  Crowding simplifies the appearance of the peripheral array by promoting consistent appearance among adjacent objects at the expense of an ability to pick out individual objects. RECEPTIVE FIELDS IN STRIATE CORTEX  David Hubel and Torsten Wiesel tried to map the recept
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