Ch10-Vision

The Visual Stimulus

• Visible light is a small part of the electromagnetic spectrum (380-760 nm).
• Humans are not sensitive to electromagnetic radiation outside this range, while some animals, like honeybees, can detect ultraviolet radiation.
• Light acts as both a particle and a wave, with wavelength determining color.
• Longer wavelengths correspond to lower frequency and appear as red, shorter to higher frequency, and are perceived as violet.
• The intensity of the light wave corresponds to brightness, a constant speed of 186,000 miles/second.

The Visual Stimulus: Color, Purity, and Brightness

• A mixture of all wavelengths creates white light.
• Light with only one wavelength is called saturated light.
• Variations in saturation and wavelengths lead to different hues.
• Brightness is determined by the magnitude of the light wave.

The Human Eye: Anatomy

• Light travels in straight lines; the iris controls pupil size (dilation = sympathetic, contraction = parasympathetic).
• The lens focuses light on the retina.
• The image on the retina is inverted.
• The fovea is optimized for high acuity and color vision.
• The rods, located outside the fovea, are responsible for scotopic and mesopic vision (low light).
• Cones, located mostly in the fovea, are responsible for mesopic and photopic vision (bright light).
• Photoreceptors at the back of the retina consist of 120 million rods and 6 million cones.
• Vergence movements involve both eyes focusing on a single target.
• Saccadic movements are quick eye shifts from one point to another.
• Pursuit movements smoothly track moving objects.

The Retinal Ganglion Cells

• Bipolar cells and photoreceptors are not action potential generating cells.
• Retinal Ganglion Cells generate action potentials. -There are 4 types of retinal ganglion cells: parvocellular, magnocellular, bistratified, and photosensitive.
• Parvocellular - high sensitivity to color and detail.
• Magnocellular - high sensitivity to movement and brightness, also involved in depth perception.
• Bistratified - inputs from rods, cones, and amacrine cells.
• Photosensitive - contribute to synchronizing daily rhythms and pupillary constriction.

The Visual Pathway

• Information from the left visual field travels to the right side of the brain, and vice versa.
• The optic nerves join at the optic chiasm.
• Signals are relayed to the dorsal lateral geniculate nucleus (LGN) in the thalamus.
• Finally, information is processed by the primary visual cortex (V1).

Coding of Light & Dark

• Visual information processing involves reception, transduction, and coding.
• Receptors contain photopigments (opsin and retinal) that change their structure upon receiving light.
• Transduction involves the conversion of light to an electrical signal. In the presence of light the photoreceptors hyperpolarize.
• Coding involves the communication of that signal to the brain, producing graded potentials in photoreceptors and bipolar cells, and action potentials in retinal ganglion cells. Lateral inhibition from horizontal and amacrine cells contributes to contrast enhancement.

Coding of Color

• Humans have three types of cones, each sensitive to different wavelengths of light (short, medium, and long).
• The Trichromatic theory says cone responses combine to perceive all colors.
• The Opponent-process theory explains afterimages and color vision by explaining how specific neurons respond to opposing pairs of colors (red/green, blue/yellow).

The Primary Visual Cortex

• The Primary visual cortex processes information about location, color, ocular dominance, and orientation.
• Magno- and parvocellular pathways provide parallel processing.
• Ocular dominance columns show a dominance for input from one eye.
• Orientation columns in the cortex respond to different orientations of lines.

Perception of Visual Information

• Spatial frequency relates to contrast differences in the visual field.
• Visual processing involves two major pathways, the dorsal "where" and the ventral "what" pathways.
• Depth perception uses monocular and binocular cues.
• The Dorsal stream processes information about spatial location and movement.
• The Ventral stream processes information about object recognition and color.