Visual Perception and Eye Function: A Comprehensive Guide
Visual Perception and Eye Function
Receiver Operating Characteristic (ROC)
A graphical plot of the observer’s hit rate as a function of the false alarm rate. Plotting the ROC curve allows prediction of the proportion of hits for a given proportion of false alarms. High beta indicates a low probability of a false alarm and a lower probability of a hit, and vice versa. Detecting a signal in noise involves altering the steady state’s criterion using a payoff matrix that selectively rewards specific criteria.
Light and the Eye
Light is a band of electromagnetic information, a wave or stream of photons, spanning 3/4 of an octave in frequency. It stimulates photoreceptors with energies ranging from 3eV (short waves) to 1.7eV (long waves). Light can be absorbed, scattered, reflected, transmitted, refracted (path altered), and dispersed (angle of refraction, creating a spectrum; rainbow). The visible spectrum contains seven colors and ranges from 400-700 nanometers.
Optical Power
Optical power is the capacity of a medium to bend light rays. One diopter is the optical power needed to bring parallel light rays incident upon a lens to a focal point 1 meter away. The cornea has an optical power of 45 diopters.
Photochromatic Interval
The difference in detection (rods) and color recognition (cones) threshold is significant below 550nm.
Eye Functions
Function 1: Eye Movements
Eyes move to align the optic axis of each retina on an object of interest or to follow a moving object. The superior colliculi process visual information and connect to cervical nerves of the neck at the tectospinal tract to control head and eye movements.
Function 2: Image Formation
The eye creates an image on the retina through the following order: tear layer, cornea, aqueous humor, pupil, iris, crystalline lens, vitreous humor, and retina. The tapetum lucidum is a reflective coating that allows some animals, like cows, to see at night. Pupil size is controlled by photoreceptors through the consensual pupillary reflex. Accommodation is the automatic adjustment of the eye’s focus for near vision by adding variable amounts of optical power. Accommodative power decreases with age (presbyopia), changing the near point from 40cm to 4m by age 70. Emmetropia refers to 20/20 vision. Myopia (nearsightedness) is the inability to see far away, while hyperopia (farsightedness) is the inability to see close up. Astigmatism is a visual defect caused by unequal curving of refractive surfaces, usually the cornea.
Function 3: Transduction
The eye transduces light from the retinal image into neural signals. From the vitreous chamber to the retina, the order is: axons of optic nerve, ganglion cells, inner synaptic layer, inner nuclear layer (amacrine, bipolar, horizontal cells), outer synaptic layer, outer nuclear layer (rods and cones), pigment epithelium, and choroid. Melanopsin retinal ganglion cells (mRGCs) are intrinsically photosensitive and contribute to non-image-forming functions like the pupillary reflex and circadian rhythms. They are driven by strong light levels, and the reciprocal relationship between intensity and duration holds for long durations, contributing to light adaptation.
Light and Dark Adaptation
Light Adaptation: The pupil constricts, and the number of photopigment molecules in the receptors decreases, reducing sensitivity despite increased light. Dark Adaptation: Rod and cone pigments regenerate in the dark with vitamin A, taking about 20 minutes for full adaptation. More light leads to fewer pigments, while less light leads to more pigments.
Retinal Ganglion Cell (RGC) Receptive Fields
RGC receptive fields are regions on the retina where light stimuli influence the ganglion cell’s firing rate. Sizes vary across the retina, with the smallest in the fovea (midget RGCs) and the largest in the periphery. The convergence ratio is the number of receptors contributing to an RGC’s action potential. RGCs act as filters, sensitive to light intensity differences between cells.
Retinal Information Processing
Cones process color and concentrate near the fovea, resulting in poor spatial resolution and color vision in the periphery. The central fovea is rod-free, causing blindness at the center of gaze in dim light (due to the image falling on the optic nerve). Rod density is high around the fovea, while cone density is low.
Visual Angle
Visual stimulus size is specified in degrees of visual angle (visual angle = arctan(object height/viewing distance)).
Peripheral Visual System
Spatial sensitivity is equivalent to visual acuity. Central acuity, the ability to see fine spatial detail in the fovea, is tested with the Snellen chart viewed at 6 meters (optical infinity). 20/20 vision is achievable by 90% of the population.
Visual Perception
Visual perception involves converting light information into experience and supporting visually guided behavior.
Eye Movement Control
Six muscles control each eye’s movement for gaze stabilization (vestibulo-ocular and optokinetic reflexes) and gaze shifting (saccades and smooth pursuit).
Version
- Saccadic: Rapid jumps between fixation points, essential for reading.
- Smooth Pursuit: Keeps a moving object’s image under the fovea.
Visual motion is processed in the middle temporal and medial superior temporal cortex.
Vergence
Vergence adjusts the angle between the eyes’ lines of sight (converging or diverging) to align a visual stimulus with each fovea.
Binocular Coordination
Binocular coordination is the eyes’ ability to work together for clear vision. Convergence occurs when the eyes turn inwards for near viewing. This mechanism works with the accommodation system. Accommodation flexibility is the ability to change lens curvature for focusing. Accommodation endurance is the ability to maintain focus for extended periods. Latent hyperopia can cause headaches when uncorrected.
Peripheral Vision
Peripheral vision perceives objects outside the foveal region.
Depth Perception
Depth perception, the ability to judge relative distance, relies on monocular and binocular cues. Binocular depth perception (stereopsis) depends on proper eye alignment.
Color Vision
Color vision is the ability to discriminate between lights with varying spectral content. Anomalies are typically inherited.
Central Visual Pathways
Contralateral vision means light from the right visual field stimulates the left superior colliculus, left lateral geniculate nucleus (LGN), and left primary visual cortex (V1).
TectoPulvinar Pathway
This pathway detects spatial location and terminates in the prestriate cortex (V2). 10% of RGC axons inform the tectum about spatial location.
Geniculostriate Pathway
This pathway carries information from 90% of RGC axons to the LGN. Axons from LGN neurons form optic radiations projecting to V1. Damage to this pathway causes visual experience loss.
Visual Processing
Light-sensitive neurons exist throughout the brain, but not all regions receiving eye input support visual experience. The visual system analyzes retinal imagery peripherally and synthesizes information in cortical structures for higher visual centers.
Lateral Geniculate Nucleus (LGN)
Three cell types project from the LGN: midget axons to parvocellular layers, parasol axons to magnocellular layers, and bistratified axons to koniocellular layers.
Primary Visual Cortex (V1)
V1 organizes in ocular dominance columns (hypercolumns) containing three types of visual neurons: simple, complex, and hypercomplex. Layers 2, 3, and 4 receive LGN input. Ocular dominance columns are stripes of neurons preferring input from one eye. The cortical magnification factor relates V1 volume to retinal area.
V1 and Attention
V1 creates a saliency map to control saccades. Higher visual centers query V1 for information, mainly from the center of gaze.
Dorsal and Ventral Streams
- Dorsal Stream: Visually guided reaching, motion perception, spatial awareness (V5, V3a, V6 in parietal lobe).
- Ventral Stream: Object recognition, form, color (inferotemporal cortex).
Later Areas
- V2: Binocularity
- V3: Global motion
- V4: Color processing
- V5 (MT): Local motion
- Lateral Intraparietal Cortex: Saccades
- Frontal Eye Fields: Visual attention
- Lateral PFC: Planning
Color Vision
Color vision is crucial for object perception. Three physiological conditions are necessary: rods and cones, comparison of receptor signals, and neural coding.
Trichromacy
The retina has scotopic (rods) and photopic (cones) regions. Three cone types are sensitive to different parts of the spectrum due to different photopigments.
Univariance
Wavelength discrimination requires comparing multiple photoreceptor outputs. Cones produce a univariate response (graded change in membrane potential).
Photopigment Spectral Sensitivity
A photopigment’s spectral sensitivity curve shows the probability of light absorption as a function of wavelength. Multiple light spectra can elicit the same response from a single photopigment type, so color vision requires multiple cone types.