The First Steps in Vision: From Light to Neural Signals

Chapter 2: The First Steps in Vision: From Light to Neural Signals

2.1 A Little Light Physics

• Light is a narrow band of electromagnetic radiation that can be conceptualized as a wave or a stream of photons.

• A photon is a quantum of visible light (or other form of electromagnetic radiation) demonstrating both particle and wave properties.

• Light can be:

  • Absorbed: Energy (e.g., light) that is taken up and not transmitted.

  • Scattered: Energy dispersed in an irregular fashion (e.g., when light enters the atmosphere).

  • Reflected: Energy redirected when it strikes a surface, usually back to its point of origin.

  • Transmitted: Energy passed on through a surface (when it is neither reflected nor absorbed).

  • Refracted: Energy that is altered as it passes into another medium.

2.2 Eyes That Capture Light

• The human eye consists of various parts:

  • Cornea: The transparent "window" into the eyeball.

  • Aqueous humor: The watery fluid in the anterior chamber.

  • Crystalline lens: The lens inside the eye, which focuses light onto the back of the eye.

  • Pupil: The dark circular opening at the center of the iris where light enters the eye.

  • Iris: The colored part of the eye, a muscular diaphragm that regulates light entering the eye by expanding and contracting the pupil.

  • Vitreous humor: The transparent fluid that fills the large chamber in the posterior part of the eye.

  • Retina: A light-sensitive membrane in the back of the eye that contains rods and cones. The lens focuses an image on the retina, which then sends signals to the brain through the optic nerve.

• Refraction, accomplished by the lens, is necessary to focus light rays onto the retina.

  • Accommodation: The process in which the lens changes its shape, thus altering its refractive power.

  • Presbyopia: Age-related loss of accommodation, making it difficult to focus on near objects.

• Problems of Refraction:

  • The lens may focus the image either in front of or behind the retina, requiring corrective lenses for normal vision.

  • Emmetropia: The condition of no refractive error.

  • Myopia: Nearsightedness; light is focused in front of the retina, so distant objects cannot be seen sharply.

  • Hyperopia: Farsightedness; light is focused behind the retina, so near objects cannot be seen sharply.

  • Astigmatism: Unequal curving of one or more refractive surfaces of the eye, usually the cornea.

• Camera Analogy for the Eye:

  • F-stop: Iris/pupil—regulates the amount of light coming into the eye.

  • Focus: Lens—changes shape to change focus.

  • Film: Retina—records the image.

• Ophthalmoscope allows doctors to view the fundus (back surface) of patients’ eyes.

• Photoreceptors: Cells in the retina that transduce light energy into neural energy.

  • Rods: Photoreceptors specialized for night vision; respond well in low luminance conditions and do not process color.

  • Cones: Photoreceptors specialized for daytime vision, fine visual acuity, and color; respond best in high luminance conditions.

• Light passes through several layers of cells before reaching rods and cones.

  • Light activates a photoreceptor, which signals the horizontal and bipolar cells that synapse with it.

  • Bipolar cells are connected to amacrine cells and ganglion cells.

  • Ganglion cells have axons that leave the retina through the optic disc (blind spot).

• The distribution of rods and cones is not constant over the retina, leading to poor color vision in the periphery.

• Visual angle is a measure of the size of visual stimuli based on how large an image appears on the retina.

  • The standard way to measure retinal size is in terms of “degrees of visual angle.”

  • Rule of thumb: the width of your thumbnail at arm's length is about 2 degrees of visual angle.

2.3 Dark and Light Adaptation

• The human visual system can adjust to a wide range of luminance levels.

• Two mechanisms for dark and light adaptation:

  • Pupil dilation.

  • Photoreceptors and their replacement.

• Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels.

• The amount of photopigment available in photoreceptors changes over time:

  • More light entering the retina leads to faster photopigment usage, reducing the amount available to process more light.

  • Less light entering the retina leads to slower photopigment usage, increasing the amount available to process what little light is there.

• In bright light, the pupil constricts, letting in less light, and the number of photopigments in the photoreceptors decreases.

• Being light-adapted means that even though there are more photons coming into the eye, there are fewer photopigments available to process them, so some of the light is “thrown away.”

• Age-related macular degeneration (AMD) is a disease associated with aging that affects the macula (central part of the retina containing the fovea) and gradually destroys sharp central vision, causing a scotoma (blind spot) in the visual field.

• Retinitis pigmentosa (RP) is a family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium, primarily affecting peripheral vision.

• New technologies for visual field loss:

  • Prosthetic retinas may replace damaged photoreceptors with an implanted device.

  • Gene therapies can improve the functioning of surviving photoreceptors.

  • Chemical therapies convert retinal ganglion cells into photoreceptors.

2.4 Retinal Information Processing

• Capturing a photon initiates photoactivation when light hits a photoreceptor.

• Photoreceptors:

  • Contain an outer segment (adjacent to the pigment epithelium), an inner segment, and a synaptic terminal.

  • Visual pigments are manufactured in the inner segment and then stored in the outer segment.

  • Contain a chromophore that captures photons and a protein called an opsin, whose structure determines the wavelength of light to which the photoreceptor responds.

    • Rods have rhodopsin.

    • Cones have three different opsins, which respond to long, medium, or short wavelengths.

  • Some photoreceptors contain melanopsin and can monitor ambient light levels and influence our sleep/wake cycle.

• Once photoactivation starts, photoreceptors become hyperpolarized (negatively charged).

• Changes in photoreceptor activation are communicated to the bipolar cells in the form of graded potentials.

  • Graded potentials vary continuously in their amplitudes.

• Bipolar cells synapse with retinal ganglion cells, which fire in an all-or-none fashion rather than in graded potentials.

• Cones work best in photopic (high-illumination) situations.

• Rods work best in scotopic (low-illumination) situations.

• The retina’s horizontal pathway includes horizontal and amacrine cells.

  • Horizontal cells: Specialized retinal cells that run perpendicular to the photoreceptors and contact both photoreceptors and bipolar cells; they are responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells.

  • Amacrine cells: These cells synapse horizontally between bipolar cells and retinal ganglion cells; they are implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time).

• The retina’s vertical pathway includes photoreceptors, bipolar cells, and ganglion cells.

  • Bipolar cell: Synapses with one or more rods or cones and with horizontal cells, then passes the signals to ganglion cells.

    • Diffuse bipolar cell: Receives input from multiple photoreceptors.

    • Midget bipolar cell: Receives input from a single cone.

  • P ganglion cells connect to the parvocellular pathway; they receive input from midget bipolar cells, involved in fine visual acuity, color, and shape processing; poor temporal resolution but good spatial resolution.

  • M ganglion cells connect to the magnocellular pathway; they receive input from diffuse bipolar cells, involved in motion processing; excellent temporal resolution but poor spatial resolution.

• Receptive field: The region on the retina in which stimuli influence a neuron’s firing rate.

• ON-center ganglion cells: Excited by light that falls on their center and inhibited by light that falls in their surround.

• OFF-center ganglion cells: Inhibited when light falls in their center and excited when light falls in their surround.

• Center-surround receptive fields allow each ganglion cell to respond best to spots of a particular size and act like a filter for information coming to the brain.

• Retinal ganglion cells are most sensitive to differences in intensity of light between center and surround and are relatively unaffected by average intensity.

  • Luminance variations tend to be smooth within objects and sharp between objects, thus center-surround receptive fields help to emphasize object boundaries.

• P ganglion cells: Small receptive fields, high acuity, work best in high luminance situations, sustained firing; provide information mainly about the contrast in the retinal image.

• M ganglion cells: Large receptive fields, low acuity, work best in low luminance situations, burst firing; provide information about how an image changes over time.

• Intrinsically photosensitive retinal ganglion cells (ipRGCs) in the developing retina:

  • Respond to light, but receive no input from rods or cones.

  • Are the first photoreceptors to mature in the retina.

  • Send light signals to the developing brain, as early as in the second trimester.

  • Babies in the womb can detect light long before they can see images.

• People can detect a single photon.