Introduction
The image in this activity shows a Gabor patch—a sine wave grating seen through a Gaussian window. Gabor patches are popular stimuli in vision laboratories because they have characteristics that match the receptive field properties of neurons in primary visual cortex.
The Gabor patch appears as a series of vertical black and white stripes within a circular window. The visual pattern is analogous to what it feels like to touch corduroy fabric. The ridges of the Gabor patch become less distinct towards the edges of the circle, and finally become completely smooth at the border of the circle.
Description of Activity
The user can click on radio buttons to change the spatial frequency (highest, medium high, medium, medium low, lowest) or contrast (high, medium, low) of the displayed Gabor patch. We describe the various features of the Gabor patches below.
Sine Wave Grating
A sine wave grating is an image in which light intensity alternates between its brightest and darkest values according to a sine function. A tactile analogy of a sine wave grating would be the wavy ridges felt on a piece of corduroy, or a Ruffles potato chip. In the image, the intensity cycles from its brightest value (white in a high-contrast grating) through a series of intermediate grays, to its darkest value (black in a high-contrast grating), and then back to white again. In the tactile analogy of corduroy fabric, the ridges sticking up would be “white” in the image and the indentations between them would be “black.”
Gaussian Window
There is an image of a square window with vertical black and white stripes. Sharp edges are formed where the grating meets the square background, and in an experiment, these sharp edges could confuse a neuron. The edges can be eliminated by multiplying the grating by a circular Gaussian window, which causes the grating to blend in with the background by fading to gray near the edges. The Gaussian function is also known as the “normal curve” or the “bell curve.”
The next image shows the same vertical black and white stripes, but now they are in a circular window and the stripes fade to gray as they get near the edge of the circle. This would be like feeling a circular patch of corduroy where the ridges get smoother and smoother as you get near the edge of the circle, becoming completely smooth at the circle’s edge. The image’s intensity is highest in the middle and decreases towards the edges, just as the Gaussian function does.
Spatial Frequency
One important property of a Gabor patch is the spatial frequency of its sine wave grating. Spatial frequency is usually measured in cycles per degree: the number of times the sine wave repeats within one degree of visual angle. That is, the number of black and white stripes per unit area, which changes with the thickness of the stripes. Wide stripes have a low spatial frequency and narrow stripes have a high spatial frequency.
The concept of spatial frequency can be understood using the corduroy fabric analogy. A low spatial frequency would be corduroy with wide ridges and large spaces between them (i.e., wide wale corduroy). A high spatial frequency would be corduroy with very fine ridges spaced closely together (i.e., narrow wale corduroy).
Contrast
The contrast of a Gabor patch’s sine wave grating refers to the intensity difference between the lightest and darkest portions of the patch, as illustrated in the graphs below.
In a high contrast patch, the lightest regions of the image are white and the darkest regions are black. In a low contrast patch, the lightest regions are light gray and the darkest regions are dark gray.
Using the corduroy fabric analogy, a high contrast fabric would be one with tall ridges and deep valleys between them. A low contrast fabric would have small ridges with shallow valleys between them. Importantly, the number of fabric stripes per unit area would be the same, only the height of the fabric ridges would differ in this analogy.
Phase
The phase of a Gabor patch refers to the relative position, or shift, of the sine wave from left to right. A graph in this part of the activity illustrates the phases of a sine wave, from 0 degrees (negative 1 on the y-axis) through 90 (0 on the y-axis), 180 (positive 1 on the y-axis), 270 (0 on the y-axis), and back to 0 degrees (negative 1 on the y-axis).
Again using the analogy of corduroy fabric, imagine having two patches of the fabric placed next to each other such that the ridges of the corduroy in the two patches do not line up with each other. These two patches of fabric would be “out of phase” with each other. Shifting a sine wave grating to the left or right along the x-axis would change that grating’s phase.