Your question about the human eye is an interesting one.
Well vertical lines, horizontal lines, and large dots are all able to be detected by the human eye. Many experiments have been conducted in trying to understand vision and what people have learned is that is not so easy to understand since there is processing of information in nerves of the eye before the signals even go into the brain (through the optic nerve). Then there is a lot of processing of the neural signals in the brain. So when you ask if someone sees an object better than another person it might not be just a difference in the eye detecting the signal (visual object) and processing in the eye but the processing in the brain.
I am not sure there is an easy answer to your question. I searched a data base of published studies (www.pubmed.gov) using key words such as "eye vertical horizontal line large dots sensitivity" and obtained zero number of primary articles. When I switched to "eye vertical horizontal line dot sensitivity" I obtained 2 hits. Only one article was of any interest and the research does not directly answer your question.
Vision Res. 1994 Nov;34(21):2859-77 Spatial integration in position acuity. Wang H, Levi DM. College of Optometry, University of Houston, TX 77204-6052.
"To determine whether and how spatial integration takes place in position acuity, bisection and Vernier thresholds were measured in the fovea of four normal observers with spatially "undersampled" dark lines (i.e. lines comprised of discrete samples). The size, contrast, and density of samples, and the separation of the lines were varied. For a given sampling density, sample size (0.17-2.72 min) has negligible effect on position threshold. For all sample sizes, position threshold decreases as sampling density increases, indicating that spatial integration takes place. The form of spatial integration depends on line separation. At the optimal line separation (2 min for bisection and 0 min for Vernier), position threshold decreases as sampling density increases with a slope of about -0.8 on log axes, steeper than a slope of -0.5 as would be expected from statistical position averaging. This effect of sampling density can be completely explained by spatial contrast summation for visibility. At the 16 min line separation, position threshold also decreases as sampling density increases but with a slope shallower than -0.5. However, this effect of sampling density can not be explained by contrast summation. Position thresholds decrease even after discounting the effect of contrast summation on visibility, suggesting a genuine position averaging. These findings are independent of line orientation (horizontal or vertical), and hold for both random and uniform dot distributions, and for both bisection and Vernier. Thus, two separate mechanisms of position acuity are suggested. A spatial filter mechanism operates at the optimal (or narrow) line separation where position threshold is critically dependent on stimulus visibility. A local sign mechanism operates at the relatively wider line separation where position acuity benefits from local sign position averaging. For both mechanisms, spatial integration is not perfect"
When I did a different search for " human eye line dot sensitivity", I obtained this article which is partly answering your question.
"Vision Res. 1990;30(11):1913-21 Simultaneous orientation contrast for lines in the human fovea. Westheimer G. Department of Molecular and Cell Biology, University of California, Berkeley 94720.
When it is surrounded by lines of a differing orientation, a test line changes its apparent orientation in a direction away from that of the surround lines. Using a nulling technique to arrive at numerical values, the properties of this simultaneous orientation contrast have been analyzed: it diminishes with distance of the surround lines; rises and then falls off as a function of surround line orientation; decreases with exposure duration; is sharply tuned (+/- 100 msec) for synchrony of test and surround line presentation; is robust to differences between test and surround line disparity but not intensity; and is reduced with dichoptic presentation of test and surround lines. Orientation contrast can be induced in a variety of oriented features, including illusory contours, an ellipse, a moving dot and a row of dots or lines, but two dots alone don't suffice. The results are taken as evidence that orientation is a domain sui generis, in which simultaneous contrast is exhibited in the same manner as in the domains of color, brightness and disparity."
The facts known to date are that the primary sensory cells can detect lines and dots. Also dots in a line then can be detected as a line when close together (kind of the same idea as the text you are reading now is a bunch of dots close together. The human eye has different layers of neurons that take the signals and process the signal into electrical events. These electrical events then get sent to another level of neurons that process the signal. These later neurons (layer 3 or ganglion cells) still in the eye then are sensitive to many primary neurons that detected the light signal. Some of these ganglion cells respond best when light is a big dot with dark around the dot. These are called "on-center" cells. While other ganglion cells respond to dark dots and light around the dark circle. These are called "off-center". So as you can see now that large dots are both stimulatory in different ways-if it is a light dot or a dark dot. A line is a composite of the dots so it is the same process but many more "on- or off-center cells will be activated". Now in the brain these signals can be processed as bars of light in particular orientations. This is a higher order of processing the signals. So to answer your question is not so straight forward as far as I can tell.
To help understand the many processes of the eye and more insight into your question please see http://webvision.med.utah.edu/
All the best,
Robin Cooper
Try the links in the MadSci Library for more information on Neuroscience.