<p>Visual acuity is highest in the foveal region, namely around <sup>1</sup>/<sub>60</sub> of a degree, <strong>or 1 minute of arc (1 MAR)</strong>. At about <strong>30 degrees</strong> of eccentricity, visual acuity is reduced to anywhere between <strong>1.5 and 10 MAR</strong> when determined by a shape recognition task, as shown in Fig. 1 taken from <a href="http://www.ncbi.nlm.nih.gov/pubmed/7210524" rel="nofollow noreferrer">Lie, 1980</a>:</p>

<p><img src="https://i.sstatic.net/TfwW4.jpg" alt="Lie, 1980"></p>

<p>A later study (<a href="http://jp.physoc.org/content/442/1/47.short" rel="nofollow noreferrer">Anderson et al., 1991</a>) shows the following picture:</p>

<p><img src="https://i.sstatic.net/vRGny.jpg" alt="Anderson, 1991"></p>

<p>This figure shows that chromatic (color) visual acuity is about 60 cycles/degree in the fovea (i.e., 1 MAR) and approximately 0.4 cycles/degree at 30 degrees eccentricity temporally, which means a <strong>reduction in acuity of 150x</strong>.  At 30 degrees eccentricity to the nasal side acuity drops to 1 cycle per degree, which means a <strong>60x reduction</strong>. Under achromatic (black-and-white) conditions it drops about <strong>60x temporally</strong> and <strong>30x nasally</strong> at 50 degrees eccentricity. These tests were done using colored (red/green) and black-white grating-visual acuity tasks. </p>

<p>The <a href="http://www.ncbi.nlm.nih.gov/pubmed/7210524" rel="nofollow noreferrer">Lie, 1980</a> study mentions several references and the <a href="https://i.sstatic.net/vRGny.jpg" rel="nofollow noreferrer">Anderson, 1991</a> study is definitely worth checking out as well.</p>

<p>To get back to your scaling question: it is difficult to directly translate these findings into scaling factors for your stimuli as you make no mention of their approximate size, or their eccentricities. What the figures do show, however, is that people may not be able to see much detail in their periphery and that <strong>acuity may drop as much as 150x</strong> at an eccentricity of 30 degrees. It also shows that <strong>black-and-white stimuli may be better visible</strong> in the far periphery and also, quite interestingly, that <strong>acuities deteriorate at a faster rate</strong> with eccentricity <strong>in the temporal retina</strong>.</p>

CC BY-SA 3.0
2015-01-06T14:50:13.927Z
2015-03-20T20:27:22.753Z
2015-03-20T20:27:22.753Z
AliceD
<p>I'm currently setting up and experiment that utilises a visual search task that contains a circular array of target letters and a distractor that falls outside the circle.</p>

<p>Obviously the further away from the fovea the distractor moves the more difficult it is to see.</p>

<p>Can anyone point me to some literature as to how one would go about scaling the size of the distractor relative to the target letters to maintain comparable levels of acuity.</p>

<p>Most of the literature I've read doing similar experiments make the distractor bigger for this very reason. |But I'm having a hard time figuring how how one goes about deciding how much bigger the distractor should be. </p>

<neuroscience><experimental-psychology><vision>
Visual acuity and offset stimuli
Rooirokbokkie
# Visual acuity and offset stimuli

(Asked by: username_0 on 2014-12-27)

I'm currently setting up and experiment that utilises a visual search task that contains a circular array of target letters and a distractor that falls outside the circle.

Obviously the further away from the fovea the distractor moves the more difficult it is to see.

Can anyone point me to some literature as to how one would go about scaling the size of the distractor relative to the target letters to maintain comparable levels of acuity.

Most of the literature I've read doing similar experiments make the distractor bigger for this very reason. |But I'm having a hard time figuring how how one goes about deciding how much bigger the distractor should be.

**Tags:** neuroscience, experimental-psychology, vision


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## 1 answers

### Answer by username_1 on 2015-01-06. Score: 6 | Accepted answer

Visual acuity is highest in the foveal region, namely around 1/60 of a degree, **or 1 minute of arc (1 MAR)**. At about **30 degrees** of eccentricity, visual acuity is reduced to anywhere between **1.5 and 10 MAR** when determined by a shape recognition task, as shown in Fig. 1 taken from [Lie, 1980](http://www.ncbi.nlm.nih.gov/pubmed/7210524):

![Lie, 1980](https://i.sstatic.net/TfwW4.jpg)

A later study ([Anderson et al., 1991](http://jp.physoc.org/content/442/1/47.short)) shows the following picture:

![Anderson, 1991](https://i.sstatic.net/vRGny.jpg)

This figure shows that chromatic (color) visual acuity is about 60 cycles/degree in the fovea (i.e., 1 MAR) and approximately 0.4 cycles/degree at 30 degrees eccentricity temporally, which means a **reduction in acuity of 150x**. At 30 degrees eccentricity to the nasal side acuity drops to 1 cycle per degree, which means a **60x reduction**. Under achromatic (black-and-white) conditions it drops about **60x temporally** and **30x nasally** at 50 degrees eccentricity. These tests were done using colored (red/green) and black-white grating-visual acuity tasks.

The [Lie, 1980](http://www.ncbi.nlm.nih.gov/pubmed/7210524) study mentions several references and the [Anderson, 1991](https://i.sstatic.net/vRGny.jpg) study is definitely worth checking out as well.

To get back to your scaling question: it is difficult to directly translate these findings into scaling factors for your stimuli as you make no mention of their approximate size, or their eccentricities. What the figures do show, however, is that people may not be able to see much detail in their periphery and that **acuity may drop as much as 150x** at an eccentricity of 30 degrees. It also shows that **black-and-white stimuli may be better visible** in the far periphery and also, quite interestingly, that **acuities deteriorate at a faster rate** with eccentricity **in the temporal retina**.


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