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PSYB51H3 Chapter Notes -Retinal Ganglion Cell, Visual Acuity, Receptive Field

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Matthias Niemeier

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Visual Acuity: Oh Say, Can You See?
-Contrast: diff. in luminance b/w and object and the background, or b/w
lighter and darker parts of the same object
-Acuity: smallest spatial detail that can be resolved
-Cycle: for a grating, a pair consisting of one dark bar and one bright bar
-Visual angle: angle subtended by an object at the retina (formed by lines
going from topbottom, through the centre of the lens, and on to the retina
oResolution acuity rep. one of the fundamental limits of spatial vision
finest high-contrast detail that can be resolved
oLimit is determined primarily by spacing of photoreceptors in the retina
-Sine Wave Gratings: a grating with a sinusoidal luminance profile
-Aliasing: misperception of a grating due to undersampling
Acuity for Low-Contrast Stripes:
-Spatial Frequency: number of cycles of a grating per unit of visual angle
(specified in degrees)
-Cycles per degree: number of dark and bright bars per degree of visual angle
-Contrast sensitivity function(CSF): a function describing how the sensitivity
to contrast depend on the spatial frequency (size) of the stimulus
-Contrast Threshold: smallest amount of contrast required to detect a pattern
Why Sine Wave Gratings?
- Although “pure” SWG maybe rare in real world, patterns of stripes w/ more or
less fuzzy boundaries are quite common (trees in forests, books on bookshelves)
- Visual system appears to break down real-world images into vast number of
components essentially a SWG with a particular spatial frequency
Retinal Ganglion Cells and Stripes:
- spatial freq. of grating is too low G.cell responds weakly b/c part of the fat,
brighter bar of grating lands in inhibitor surround, damping the cell’s response
- spatial freq. of grating is too high-> G.cell responds weakly b/c both dark and
bright stripes fall within the receptive-field centre, washing out the response
- spatial freq.is just right with a bright bar filling the centre and dark bars in the
surround, the cell responds vigorously
oTherefore, retinal ganglion cell responds best to a specific spatial freq. that
matches its receptive field size
-Phase: relative position of a grating
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Cortical Topography and Cortical Magnification:
- Important features of the visual cortex:
oTopographical mapping
oDramatic scaling of info. From diff parts of the visual field.
-Coritcal Magnification: amount of cortical area (usually in mm) devoted to
specific region (eg: 1 degree) in the visual field
oimp. Consequence visual acuity declines in an orderly fashion with
Receptive Fields in Striate Cortex:
-Hubel + Wiesel: cat experiement
oMost fundamental discovery: neurons in striate cortex respond to stripes
not stars
Receptive fields of striate cortex neurons are not circular, they are
elongated, responding much more vigorously to bars, lines, edges,
and gratings than to round circles of light
Orientation Selectivity:
- Other imp. Properties of receptive fields of neurons in S. cortex discovered by
Hubel and Wiesel:
oIndiv. Neuron will not respond equivalently to just any old stripe in its
receptive field. Responds best when line/edge is at just the right
orientation and hardly at all when line is tilted more than 30 degrees away
from optimal orientation
Orientation Tuning: Tendency of neurons in S. cortex to
respond optimally to certain orientations and less to others.
Other Receptive-field Properties:
-Filter: an acoustic, electrical, electronic, or optical device, instrument,
computer, program, or neuron that allows the passage of some frequencies
or digital elements and blocks others
- Another discovery: many cortical cells respond especially well to moving lines,
bars, edges, and gratings
oMany respond strongly when a line moves in one direction
-Ocular dominance: property of the receptive fields of S. cortex neurons by
which they demonstrate a preference, responding somewhat more rapidly
when a stimulus is presented in one eye than when it is presented in the other
Simple and Complex Cells:
- cortical neurons come in variety of types:
oSimple cells: clearly defined excitatory and inhibitory regions
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