What is Spatial Frequency?
Spatial frequency describes the rate that a stimulus changes across space. It is usually measured with black and white line gratings, similar to a zebra’s stripes. Spatial frequency measures the number of black-white gratings that the retina can see in a given distance.
- Spatial frequency is a measure of how rapidly an image changes across a given distance.
- Low spatial frequencies allow us to see large details. High spatial frequencies allow us to see small details, like letters.
- Spatial frequency plays a role in motion perception and estimating how fast something is moving.
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Understanding Spatial Frequency
Spatial frequency is the frequency of change per unit distance across an image. Some images have high spatial frequencies, whereas other ones have low spatial frequency. An image’s spatial frequency depends on how much detail it contains. Low spatial frequencies comprise changes that happen over wider distances and allow us to see large shapes. High spatial frequencies allow us to see small, fine details. In general, the human visual system is most sensitive to spatial frequencies between 2 to 6 cycles per degree.
Visual acuity is a different measurement than spatial frequency. Visual acuity is measured with an eye chart containing letters. Visual acuity measures the smallest detail that can be interpreted by the visual system. On the other hand, spatial frequency measures the ability to tell whether objects are spaced close together or far apart. The spatial frequency can be accurately measured using patterns of black and white bars, known as ‘gratings’.
Low and High Spatial Frequency: Visual Perception
According to theories on visual perception, scenes are processed in terms of spatial frequencies. High Spatial Frequencies transmit precise details of the scene. Low Spatial Frequencies carry coarse information, like shapes and patterns. However, it is still unclear how and where spatial frequencies are processed within the brain. Spatial frequency helps us perceive motion. If something has a lot of detail within a small distance, our brain thinks that it’s moving faster. Spatial frequency is used in drawings and art to trick our eyes into thinking that a still object is moving.
The maximum spatial frequency that humans can resolve is 60 cycles per degree. At this spatial frequency, humans can see very fine details. Above this spatial frequency, the object appears blurry and individual lines cannot be distinguished from the background. You can compare high spatial frequency to a metro train. At high speeds, the advertisements written on the train are so blurry that the words cannot be read. The visual system is best at interpreting options of low spatial frequency, which have large details.
Visual Cortex and Spatial Frequency
It is proven that the visual cortex is mapped for spatial frequency processing. The occipital lobe, the part of the brain that processes vision, is activated by high spatial frequency. Neurons within the visual cortex are tuned to process spatial frequency. A small change in spatial frequency can elicit a change in the way a neuron responds. In addition to spatial frequency, neurons in the visual cortex also respond to an image’s contrast, orientation, and luminance.
Spatial Frequency and Contrast Sensitivity Function
When the eye doctor checks the vision, she is usually using an eye chart that is high contrast, meaning that there are dark black letters against a white background. High contrast letters are easier to see. When letters are low contrast, such as gray letters against a gray background, they become harder to see. Low contrast means that a letter is hard to distinguish from its background.
The visual system uses spatial frequency clues to make out letters and shapes. It also uses contrast to detect an object’s details. The colors that form an image may be dark (high contrast) or light (low contrast). Humans are able to detect contrast easier when viewing low spatial frequency images (such as shapes and patterns). The contrast sensitivity is reduced at high spatial frequencies. Practically, this means that humans will benefit from better lighting when reading, because bright lighting creates better contrast. People with eye diseases such as macular degeneration, cataracts, or glaucoma usually have reduced contrast sensitivity. People with impaired vision may rely more heavily on high contrast and large print size when reading. Reading speed and other visual tasks depend on both spatial frequency and contrast.