MCAT Behavioral Sciences Review - Kaplan Test Prep 2021–2022

2.2 Vision
Sensation and Perception

LEARNING OBJECTIVES

After Chapter 2.2, you will be able to:

· List the functions of the parts of the eye, including the cornea, pupil, iris, ciliary body, canal of Schlemm, lens, retina, and sclera

· Describe parallel processing

· Identify the cell types responsible for color, shape, and motion detection

· Recall the structures in the visual pathway:

Vision is a highly adapted sense in human beings. With the ability to sense brightness, color, shape, and movement, and then to integrate this information to create a cohesive three-dimensional model of the world, the visual pathways are extremely important to everyday life. In fact, vision is the only sense to which an entire lobe of the brain is devoted: the occipital lobe.

STRUCTURE AND FUNCTION OF THE EYE

The anatomy of the eye is shown in Figure 2.2.

Figure 2.2. Anatomy of the Eye

The eye is a specialized organ used to detect light in the form of photons. Most of the exposed portion of the eye is covered by a thick structural layer known as the sclera, or the white of the eye. The sclera does not cover the frontmost portion of the eye, the cornea. The eye is supplied with nutrients by two sets of blood vessels: the choroidal vessels, a complex intermingling of blood vessels between the sclera and the retina, and the retinal vessels. The innermost layer of the eye is the retina, which contains the actual photoreceptors that transduce light into electrical information the brain can process.

When entering the eye, light passes first through the cornea, a clear, domelike window in the front of the eye, which gathers and focuses the incoming light. The front of the eye is divided into the anterior chamber, which lies in front of the iris, and the posterior chamber between the iris and the lens. The iris, which is the colored part of the eye, is composed of two muscles: the dilator pupillae, which opens the pupil under sympathetic stimulation; and the constrictor pupillae, which constricts the pupil under parasympathetic stimulation. The iris is continuous with the choroid, which is a vascular layer of connective tissue that surrounds and provides nourishment to the retina. The iris is also continuous with the the ciliary body, which produces the aqueous humor that bathes the front part of the eye before draining into the canal of Schlemm. The lens lies right behind the iris and helps control the refraction of the incoming light. Contraction of the ciliary muscle, a component of the ciliary body, is under parasympathetic control. As the muscle contracts, it pulls on the suspensory ligaments and changes the shape of the lens to focus on an image as the distance varies, a phenomenon known as accommodation. Behind the lens lies the vitreous humor, a transparent gel that supports the retina.

The retina is in the back of the eye and is like a screen consisting of neural elements and blood vessels. Its function is to convert incoming photons of light to electrical signals. It is actually considered part of the central nervous system and develops as an outgrowth of brain tissue. The duplexity or duplicity theory of vision states that the retina contains two kinds of photoreceptors: those specialized for light-and-dark detection and those specialized for color detection.

The retina is made up of approximately 6 million cones and 120 million rods. Cones are used for color vision and to sense fine details. Cones are most effective in bright light and come in three forms, which are named for the wavelengths of light they best absorb, as shown in Figure 2.3.

Figure 2.3. Relative Absorption of the Three Types of Cones at Different WavelengthsThe cones are named for the wavelengths at which they have highest light absorption: short (S, also called blue), medium (M, green), and long (L, red).

In reduced illumination, rods are more functional than cones because each rod cell is highly sensitive to photons and is somewhat easier to stimulate than a cone cell. In part, the sensitivity of rods has to do with the fact that all rods contain only a single pigment type called rhodopsin. In general, color vision requires far more light because each cone responds only to certain wavelengths of light. By contrast, a rod can be stimulated by light of any color. However, while rods permit vision in reduced light, the tradeoff is that rods only allow sensation of light and dark. Also, even though individual rods are highly sensitive to light, as a whole they are less useful for detecting fine details because rods are spread over a much larger area of the retina.

Mnemonic

Cones are for color vision. Rods function best in “roduced” light.

While there are many more rods than cones in the human eye, the central section of the retina, called the macula, has a high concentration of cones; in fact, the centermost region of the macula, called the fovea, contains only cones. As one moves further away from the fovea, the concentration of rods increases while the concentration of cones decreases. Therefore, visual acuity is best at the fovea, and the fovea is most sensitive in normal daylight vision. Some distance away from the center of the retina, the optic nerve leaves the eye. This region of the retina, which is devoid of photoreceptors, is called the optic disk, and gives rise to a blind spot, as shown in Figure 2.4.

Figure 2.4. Specialized Regions of the Retina

Rods and cones are specialized neurons and, like most neurons, connect with other neurons through synapses. However, rods and cones do not connect directly to the optic nerve. Rather, there are several layers of neurons in between, as shown in Figure 2.5. Rods and cones synapse directly with bipolar cells, which highlight gradients between adjacent rods or cones. Bipolar cells then synapse with ganglion cells, the axons of which group together to form the optic nerve. These bipolar and ganglion cells not only fall “in between” the rods and cones and the optic nerve, but also the bipolar and ganglion cells are actually located in front of the rods and cones, closer to the front of the eye. This arrangement means that a photon must actually navigate past several layers of cells to reach the rods and cones at the “back” of the retina; the information is then transmitted “forward” (in the form of action potentials) from the rod and cone cells until the signal reaches the ganglion cells. Observe in Figure 2.5 that there are significantly more photoreceptor cells than ganglion cells, so the output from each ganglion cell represents the combined activity of many rods and cones. The result is a pruning of details as information from the photoreceptors is combined. As the number of receptors that converge through the bipolar neurons onto one ganglion cell increases, the resolution decreases. On average, the number of cones converging onto an individual ganglion cell is smaller than for rods. This arrangement helps explain why color vision has a greater sensitivity to fine detail than black-and-white vision does.

Also shown in Figure 2.5 are amacrine and horizontal cells, which receive input from multiple retinal cells in the same area before the information is passed on to ganglion cells. Amacrine and horizontal cells can thereby accentuate slight differences between the visual information in each bipolar cell. For example, these cells are important for edge detection, as they increase our perception of contrasts.

Figure 2.5. Cells of the Retina

VISUAL PATHWAYS

Visual pathways refer to both the anatomical connections between the eyes and the brain and to the flow of visual information along these connections. As demonstrated in Figure 2.6, each eye’s right visual field (blue in the figure) projects onto the left half of each eye’s retina, and each eye’s left visual field (black in the figure) projects onto the right half of each eye’s retina. As the signal travels through the optic nerves toward the brain, the first significant event occurs at the optic chiasm. Here, the fibers from the nasal half of each retina cross paths. Because the temporal fibers do not cross in the chiasm, this reorganization means that all fibers corresponding to the left visual field from both eyes project into the right side of the brain, and all fibers corresponding to the right visual field from both eyes project into the left side of the brain. These reorganized pathways are called optic tracts once they leave the optic chiasm.

Figure 2.6. Visual Pathways

From the optic chiasm, the information goes to several different places in the brain: some nerve fibers pass to the lateral geniculate nucleus (LGN) of the thalamus where they synapse with nerves that then pass through radiations in the temporal and parietal lobes to the visual cortex in the occipital lobe. Other nerve fibers pass to the superior colliculus, which controls some reflexive responses to visual stimuli and reflexive eye movements.

Real World

When there is a loud, sudden sound, the superior colliculus aligns the eyes with the likely stimulus. In other words, it’s the superior colliculus (as well as the sympathetic nervous system) that gives us the “deer in the headlights” appearance during the startle response.

PROCESSING

The ability to sense light information in the environment around us is useful in its own right. But, to effectively interact with the environment, we must also be able to make sense of visual stimuli. The connections between optic tract, LGN, and visual cortex help create a cohesive image of the world through a phenomenon known as parallel processing. Visual parallel processing is the brain’s ability to analyze information regarding color, form, motion, and depth simultaneously, i.e. “in parallel”, using independent pathways in the brain. For example, most people can quickly and easily recognize a moving car from a distance. The speed of recognition is facilitated, in part, by the fact that the form of the car (i.e. its shape) and the motion of the car are processed simultaneously in separate, parallel pathways in the brain.

Now let’s explore where each of these four aspects of vision is processed and the specialized cells that contribute to their detection. As described previously, cones are responsible for our perception of color. Form refers not only to the shape of an object, but also our ability to discriminate an object of interest from the background by detecting its boundaries. Neurons carrying information from the fovea and surrounding central portion of the retina synapse with parvocellular cells in the lateral geniculate nucleus. These cells have very high color spatial resolution; that is, these cells permit us to detect very fine detail when thoroughly examining an object. However, parvocellular cells can only work with stationary or slow-moving objects because these cells have very low temporal resolution.

Conversely, magnocellular cells are well-suited for detecting motion because these cells have very high temporal resolution. Reflecting the fact that form and motion are processed in parallel, magnocellular cells and parvocellular cells are located in distinct layers of the lateral geniculate nucleus. Also, magnocellular cells predominantly receive inputs from the periphery of our vision, allowing more rapid detection of objects approaching us from the sides. However, magnocellular cells have low spatial resolution, so much of the rich detail of an object can no longer be seen once the object is motion. Magnocellular cells therefore provide a blurry but moving image of an object.

Depth perception, our ability to discriminate the three-dimensional shape of our environment and judge the distance of objects within it, is largely based on discrepancies between the inputs the brain receives from our two eyes (more on this to follow in MCAT Behavioral Sciences Review, Section 2.5, Object Recognition). Specialized cells in the visual cortex known as binocular neurons are responsible for comparing the inputs to each hemisphere and detecting these differences.

Finally, our brains wouldn’t be very good at processing visual information if they didn’t learn to associate certain patterns of stimuli with expected behaviors or outcomes. To assist in this, a whole slew of even more specialized cells called feature detectors exist in the visual cortex. Each feature detector cell type detects a very particular, individual feature of an object in the visual field. For example, if we were to look at a stop sign we would activate: a feature detector for the color red, while another feature detector would respond to the white border and letters. Yet another type of feature detector would recognize the horizontal lines, while still others would be activated by the angled lines of the octagon. Rather than needing to individually process each of these features every time, the overall combination of feature detectors become activated in parallel. Finally, our response to the stop sign, i.e. to STOP, also is stored for future retrieval.

Mnemonic

Magnocellular cells specialize in motion detection.

MCAT Concept Check 2.2:

Before you move on, assess your understanding of the material with these questions.

1. List the functions of the various parts of the eye:

o Cornea:

o Pupil:

o Iris:

o Ciliary body:

o Canal of Schlemm:

o Lens:

o Retina:

o Sclera:

2. List the structures in the visual pathway, from where light enters the cornea to the visual projection areas in the brain.

3. What is parallel processing?

4. In feature detection, what type of cells are responsible for color? Form? Motion? Depth?

o Color:

o Form:

o Motion:

o Depth: