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Queen's University
PSYC 215
Niko Troje

Page 1 of10 Chapter 8: Spatial Vision Fundamental Functions Sensors as Filters • Our sensors act as filters of environmental information, and must balance sensitivity to low levels of stimulus with specificity for a certain particular stimulus • Light photoreceptors in chaos of environment can use a screen for direction tuning. However, don’t want to be too selective like pinhole cameras, which only respond to light from one single direction. • Other tuning functions in vision for direction, wavelength, spatial frequency, and orientation • Adaptation happens on many different time scales: o Transient <1 is good vision -- you see letters at 20 feet which a normal person can only see at 10 feet; 20/30 a ratio Page 4 of 10 <1 is poor vision – you need 20 feet to see what others can read at 30 feet. Normal acuity is 2 arcmin (1 arcmin = 1/60 of degree). • Garbor Functions: Gratings of different spatial frequencies are seen through a Gaussian window. • Contrast sensitivity depends on spatial frequency, function of Garbor spectrum has inverted U shape. Temporal Contrast Sensitivity • Temporal features of an image describe changes in image intensity over time, as natural visual images are never entirely stationary. This is measured with a small flickering light, to find the smallest amount of flicker that the observer can detect. • A light that alternates repeatedly over time, between bright and dark intensities, has the following properties: • 1) Contrast: The difference between the maximum and minimum alternating intensities. • 2) Temporal frequency: The number of flicker cycles that repeat in one second (Hz). The sequence for one flicker cycle is one bright light, one dark light. Temporal Contrast Sensitivity Function • When temporal contrast sensitivity is measured at a range of flicker rates, the results can be plotted in a temporal contrast sensitivity function. • The temporal CSF shows that the contrast required for detection of a flickering light depends on its temporal frequency. In photopic vision, temporal contrast sensitivity peaks at 8 Hz, and declines more rapidly at higher than at lower temporal frequencies. Frequencies higher than 50 Hz are undetectable even at maximum contrast. • The shape of the temporal contrast sensitivity function is determined by neural factors. The different temporal responses of rods and cones feed retinal ganglion cells, which begin complex temporal interactions between the LGN and cortex. Spatiotemporal Sensitivity • We measured spatial sensitivity by a stimulus periodic over space (grating), and temporal sensitivity by a stimulus periodic over time (flickering light). Spatiotemporal features of an image describe changes in image intensity across space and over time. Stimuli • Spatiotemporal contrast sensitivity is measured with a flickering grating: alternates repeatedly in luminous intensity, so that bright bars become dark as dark bars become bright, and cycles the other way again. • This can be shown in a space–time plot: rate at which each bar completes one cycle of contrast reversal is temporal frequency, and grating at any one time shows spatial frequency. Spatiotemporal Sensitivity Function • The spatiotemporal contrast sensitivity function shows that the contrast required for detection of a flickering grating depends on the interaction between its spatial frequency and its temporal frequency. • At low temporal frequencies, spatial contrast sensitivity is band-pass (most sensitive to a band of mid- range spatial frequencies). At high temporal frequencies, spatial contrast sensitivity is low-pass (most sensitive to low spatial frequencies). Origins of Variation in Spatiotemporal Sensitivity Page 5 of10 •There are two alternative neural explanations for the shape of the spatiotemporal contrast sensitivity function: •1) Receptive field organization: The balance between the excitatory and inhibitory influences of center–surround receptive fields changes with temporal frequency. •The centre and surround each have a low-pass frequency response, but the centre responds to higher frequencies than the surround. •At low temporal frequencies, center and surround have equal influence. The net response of the receptive field is the difference between the low-pass center and the low-pass surround  band-pass spatial frequency response. •At high temporal frequencies, the center has more influence than the surround. The difference between the dominant low-pass center and the weak low-pass surround  low-pass spatial frequency response. •2) Parallel pathways: Two separate cell populations make parallel visual pathways. •A sustained channel (parvo), most sensitive to high spatial frequencies and low temporal frequencies – information about pattern and shape. •A transient channel (magno), most sensitive to low spatial frequencies and high temporal frequencies – information about movement. •Similar to certain clinical conditions in humans, selective damage to macaque monkey parvo cells reduces sensitivity to gratings with a combination of high spatial and low temporal frequencies. •The dominant view is that parallel pathways provide the best account of spatiotemporal sensitivity. It is likely that the visual system uses both cell divisions together, rather than switching between them, and varying between relative contributions of the channels depending on stimulus conditions. Representation at Multiple Spatial Scales Spatial Scale • Natural images contain a diverse range of: o Coarse-scale information: Low spatial frequency gratings representing the general shape of objects, such as identifying a human face and locating eyes, nose, and mouth o Fine-scale information: High spatial frequency gratings representing surface texture and sharp edges, such as estimating the age of a person from their skin texture Spatial Scale & Spatial Frequency Fourier Theory • Low frequency gratings contain coarse-scale information, while high-frequency gratings contain fine-scale information. • Any natural image can be decomposed into a collection of sine wave gratings at various spatial frequencies, contrasts, orientations, and phases by spatial Fourier analysis. Spatial Filtering • Each point in a Fourier spectrum plot represents one of the sin
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