Sensing Light and the Human Eye

Sensing Light

• Visible light constitutes a small portion of the electromagnetic spectrum.
• Light can be described as waves of energy, varying in height (amplitude) and distance between peaks (wavelengths).
• The physical dimensions of light waves lead to changes in perception, such as brightness, colour and saturation.
• The length of a light wave determines the perception of colour/hue.
• The amplitude of a light wave determines the perception of brightness.
• Purity is the degree to which a light source emits a single wavelength, or a mixture of wavelengths, influencing colour perception.
• High saturation means that the light sources are perceived as having very rich colours by humans.

The Eye and Light Detection

• The cornea is a clear, smooth outer tissue that bends light and sends it through the pupil.
• The iris is a translucent, doughnut-shaped muscle that controls the size of the pupil, regulating the amount of light entering the eye.
• The lens bends the light again and focuses it onto the retina.
• Accommodation is the process where muscles change the shape of the lens to focus on objects at varying distances.
• When the eyeball is too long, images focus in front of the retina, causing nearsightedness (myopia).
• When the eyeball is too short, images focus behind the retina, causing farsightedness (hyperopia).
• Corrective measures include eyeglasses, contact lenses, and surgical procedures like LASIK to reshape the lens.

Light Conversion in the Retina

• The retina is the interface between light and the central nervous system.
• Photoreceptor cells (cones and rods) contain light-sensitive proteins that transduce light into electrical signals.
• Cones detect colour and fine details under normal daylight conditions.
• Rods operate under low-light conditions for night vision, sensing shades of grey.
• Rods are more sensitive than cones but do not provide colour information.
• There are about 120 million rods distributed evenly around each retina, except in the fovea.
• The fovea is the area of the retina where vision is clearest, containing no rods.
• There are about 6 million cones in each retina, densely packed in the fovea.
• The high density of cones in the fovea allows objects to be seen in great detail.
• Bipolar cells collect electrical signals from rods and cones and transmit them.
• Retinal ganglion cells (RGCs) organize signals and send them to the brain.
• Bundled RGC axons form the optic nerve, which leaves the eye through a hole in the retina, creating a blind spot.
• The perceptual system automatically "fills in" the blind spot using surrounding information.

The Optic Nerve and Brain Pathways

• Streams of action potentials travel to the brain along the optic nerve.
• Information from the right visual field is relayed to the left hemisphere, and vice versa.
• Information goes to the lateral geniculate nucleus (LGN) in the thalamus.
• Visual signals travel to area V1 in the occipital lobe, containing the primary visual cortex.

Colour Perception

• Colour perception is created by the brain.
• The visible spectrum includes colours from deep purple (shortest wavelengths) to red (longest wavelengths).
• White light contains about the same amounts of many different wavelengths across the visible spectrum.
• Rods signal brightness, not colour, due to having only one type of photopigment.
• Cones come in three types: L-cones (long wavelengths), M-cones (medium wavelengths), and S-cones (short wavelengths).
• The relative activity between pairs of cone types signals colour.
• Genetic disorders causing missing cone types can lead to colour vision deficiency.
• Sensory adaptation by staring at one colour can cause colour afterimages.
• The colour-opponent system involves pairs of cone types working in opposition (L-cone vs. M-cone, S-cone vs. M-cone).
• Colour aftereffects occur due to fatigued cones responding weakly compared to their opposing cones.

The Visual Brain

• Complex aspects of vision require brain processing.
• Area V1 is sensitive to edge orientation.
• Neurons in V1 selectively respond to bars and edges in specific orientations.
• The visual receptive field is small for neurons in V1.
• V1 contains populations of neurons tuned to respond to edges oriented in specific ways.
• Visual information is processed through 32 or more distinct brain areas from V1.
• Receptive fields become larger, and the features to which neurons respond become more complex in areas farther from V1.

Visual Streams

• The ventral stream travels to the temporal lobes and represents an object's shape and identity ("what" pathway).
• The dorsal stream travels to the parietal lobes and identifies where an object is and how it is moving
• Damage to the lateral occipital cortex can result in visual form agnosia, the inability to recognize objects by sight.
• Damage to the parietal lobe can cause difficulty using vision to guide reaching and grasping.
• The two streams work together during visual perception to integrate perception for identification (ventral) and perception for action (dorsal).
• The binding problem involves how the brain links features together to see unified objects.
• "Binding neurons" in the ventral visual stream receive coincident input from neurons representing different features of an object.