Chapter 3: Spatial Vision (Part the Second) Flashcards
Simple cells
responds primarily to oriented edges and grating
With even DIRECTION sensitivity:
only respond to motion in one direction!
Complex cells
respond primarily to oriented edges and gratings, however they have a degree of spatial invariance.
Simple cells in the primary visual cortex can be formed by the linking of outputs from concentric lateral geniculate nucleus (LGN) cells with adjacent receptive fields.
In addition to signaling the presence of an edge, simple cells are selective for orientation.
Each LGN cell responds to one eye or the other….
…but never to both
Ocular dominance in V1
Each striate cortex cell can respond to input from both eyes with preference for one eye’s input.
COMPLEX cells
sensitive to motion.
Respond to bar regardless of location, as long as within RF.
two flavors of simple cells
an edge detector and a stripe detector
End Stopping
Process by which cells in the cortex first increase their firing rate as the bar length increases to fill up its receptive field, and then decrease their firing rate as the bar is lengthened further.
HYPERCOMPLEX cells: IMPORTANT for LUMINANCE BOUNDARIES and discontinuities.
LAST: cells can respond differently due to feedback from other visual areas.
Column
A vertical arrangement of neurons
Columns:
Hubel & Wiesel’s Discoveries
Found systematic, progressive change in preferred orientation;
All orientations were encountered in a distance of about 0.5 mm;
Same orientation preference in columns perpendicular to the surface of cortex.
Column Arrangement
Each of the 200 million cells in the Striate Cortex responds to a stripes or edges, gratings, oriented at a particular angle, with a particular width, perhaps moving in a specific direction.
But they are not arranged randomly…
Once they had the orientation of a neuron, Hubel & Wiesel pocked a bit harder and the neurons below would be sensitive to the same orientation.
If pocked tangentially, all orientations in succeeding manner…
Hubel and Wiesel’s Columns:
Summary
Found systematic, progressive change in preferred orientation; all orientations were encountered in a distance of about 0.5 mm
Hypercolumn
A 1x1-mm block of striate cortex containing “all the machinery necessary to look after everything the striate cortex is responsible for, in a certain small part of the visual world” (Hubel, 1982)
COLOR PROCESSING
Regular array of “CO blobs” in systematic columnar arrangement (discovered by using cytochrome oxidase staining technique)
cytochrome oxidase blobs
Method of Adaptation
The diminishing response of a sense organ to a sustained stimulus.
TILT AFTEREFFECT
Look for a long time (about one minute) to the fixation bar, keeping your eyes on the line between left gratings, maybe moving , but always on the bar.
Then, quickly move your eyes to the point between right gratings. Keeping your eyes on the dot, try to judge the orientation of the bars.
demonstrates selective adaptation.
Demonstration of Adaptation that is specific to Spatial Frequency
ADAPT to top panel. 1 minute
Then switch to panel immediately below.
Same spatial frequency, but lower contrast: you won’t see it.
Repeat testing with panel on right or left and the dim panel will not disappear.
After adaptation: contrast sensitivity is reduced, so that you need a STRONGER (higher contrast) stimulus to be detected.
Neurons responding to different enough spatial frequencies are not affected by the fatigue.
Tilt aftereffect
Perceptual illusion of tilt, provided by adapting to a pattern of a given orientation.
Supports the idea that the human visual system contains individual neurons selective for different orientations.
Selective Adaptation
Evidence that human visual system contains neurons selective for specific stimulus properties (e.g. orientation, frequency, motion, color)
Other aftereffects: Motion 1, Motion 2
If adaptation to a stimulus occurs, then it must be that a group of neurons was coding that stimulus and became fatigued.
Easy non-invasive method of measuring what the human visual system is sensitive to.
Spatial frequency channel
Pattern analyzers, implemented by a group of neurons with each set of neurons tuned to a limited range of spatial frequencies.
Why would the visual system use spatial frequency filters to analyze images?
Different spatial frequencies emphasize different types of information
IOW, sets of groups extract from image different “aspects” of the image:
how much left tilt at a high frequency,
how much horizontal at a lower freq…
Why sine waves?
Many stimuli can be broken down into a series of sine wave components using Fourier analysis
Any sound, including music & speech
Any complex image, including photographs, movies, objects, and scenes
Any movement, including head & limb movements
Also, our brains seem to analyze stimuli in terms of their sine wave components!
- Vision
- Audition
Period or wavelength
The time or space required for one cycle of a repeating waveform.
Phase
1) In vision, the relative position of a grating
2) In hearing, the relative timing of a sine wave
Amplitude
The height of a sine wave, from peak to trough, indicating the amount of energy in the signal
Even something as complicated and artificial as a square wave can be reproduced by adding the correct sine waves together.
….
Joseph Fourier
(1768–1830) developed another useful tool for analyzing signals
Fourier analysis
A mathematical procedure by which any signal can be separated into component sine waves at different frequencies.
Combining these component sine waves will reproduce the original signal.
Sine wave:
- In hearing, a waveform for which variation as a function of time is a sine function.
Also called a “pure tone” - In vision, a pattern for which variation in a property, like brightness or color as a function of space, is a sine function.
Show high spatial-frequency mask: Guess who?
lincoln
