The Visual Pathway

Sensory Systems What is a Sensory System? –Window to the physical energies of the external environment. –Gives rise to sensory perceptions.

Sensory Systems Several different modalities –Vision –Audition –Somatosensory –Taste –Smell Laws of Specific Sense Energies –Johannes Muller (1826) –Labeled pathways

Sensory Systems Sensory Receptor A specialized neuron that detects a particular category of physical events. –Sensory transduction The process by which sensory stimuli are transduced into slow, graded receptor potentials. –Receptor potential A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.

Sensory Systems Sensory Receptor –Some transduce and encode (e.g. somatosensory, olfaction) –Some only transduce (e.g. vision, audition, taste) –The region of the sensory surface that modulates the activity of sensory neurons is called the Receptive Field of the neuron.

The Visual System Functions of the Visual System: –Answers two questions What is it? (recognition) Where is it? (location) –Transforms a distorted and upside-down 2-D retinal image into the 3-D world we perceive.

The Visual System Does the visual system create an exact copy of the external world? –No! The visual system creates a perception of reality. –Chaplin Movie

Visual Illusions

Visible Light Properties of Light –Wavelength – perception of color –Intensity(amplitude) – perception of brightness

Figure 6.2 The electromagnetic spectrum: colors and wavelengths visible to humans

Figure 6.4 The Eye Lens – focuses light on the retina Ciliary muscles alter the shape of the lens as needed Accommodation – the process of adjusting the lens to bring images into focus

Figure 6.4 The Blindspot

Eye Position and Binocular Disparity Convergence –eyes must turn slightly inward when objects are close Binocular disparity –difference between images on the two retinas Both are greater when objects are close –provides brain with 3-D image and distance information

The Retina The retina is in a sense “inside-out” –Light passes through several cell layers before reaching its receptors LIGHT-> retinal ganglion cells -> bipolar cells -> receptors cells –Lateral communication Horizontal cells Amacrine cells

Figure 6.5

Duplexity theory of vision –cones and rod mediate different kinds of vision –Cones – photopic (daytime) vision High-acuity Color information in good lighting –Rods – scotopic (nighttime) vision High-sensitivity, low-acuity vision in dim light Lacks detail and color information Cone and Rod Vision

Figure 6.8

Cone and Rod Vision More convergence in rod system, increasing sensitivity while decreasing acuity Only cones are found at the fovea Distribution of rods and cones Figure 6.9

Spectral Sensitivity Lights of the same intensity but different wavelengths may not all look as bright A spectral sensitivity curve shows the relationship between wavelength and brightness There are different spectral sensitivity curves for photopic (cone) vision and scotopic (rod) vision

Human photopic and scotopic spectral sensitivity curves Figure 6.10