Chapter 6 The Visual System From Your Eyes to Your Cortex This multimedia product and its contents are protected under copyright law. The following are prohibited by law: any public performance or display, including transmission of any image over a network; preparation of any derivative work, including the extraction, in whole or in part, of any images; any rental, lease, or lending of the program.

What do we see? Somehow a distorted and upside-down 2D retinal image is transformed into the 3D world we perceive 2 types of research needed to study vision Research probing the components of the visual system Research assessing what we see

Light Enters the Eye and Reaches the Retina No species can see in the dark, but some are capable of seeing when there is little light Light Photons of energy Waves of electromagnetic radiation Humans see light between 380-750 nanometers

The Conversion of Light to Neural Signals Transduction –conversion of one form of energy to another Visual transduction – light energy to neural signals by visual receptors Pigments absorb light Absorption spectrum determines spectral sensitivity

The retina-geniculate-striate pathway ~90% of axons of retinal ganglion cells Information from the left visual field of each eye projects to the right lateral geniculate nucleus (LGN) and vice versa Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV

Retinotopic organization Information received at adjacent portions of the retina remains adjacent More cortex is devoted to areas of high acuity – like the disproportionate representation of sensitive body parts in somatosensory cortex About 25% of primary visual cortex is dedicated to input from the fovea

Lateral Inhibition and Contrast Enhancement Visual system detects change Mach bands – nonexistent stripes that visual system creates to enhance the contrast and make edges easier to see – an example of contrast enhancement A consequence of lateral inhibition

Receptive Fields of Visual Neurons The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex

Receptive Fields of Visual Neurons Similarities seen at all 3 levels: Receptive fields of foveal areas smaller than those in the periphery Circular receptive fields Monocular Many had a center-surround organization

Receptive Fields in Striate Cortex Neurons of lower layer IV are an exception – circular receptive fields (as in retinal ganglion cells and LGN) Most neurons in V1 are either Simple – receptive fields are rectangular with “on” and “off” regions Complex – also rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field

Receptive Fields in Striate Cortex SIMPLE Rectangular “on” and “off” regions, like cells in layer IV Orientation and location sensitive All are monocular COMPLEX Rectangular Larger receptive fields Do not have static “on” and “off” regions Not location sensitive Motion sensitive Many are binocular

Columnar Organization of V1 Cells with simpler receptive fields send information on to cells with more complex receptive fields Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance Ocular dominance columns – as you move horizontally, the dominance of the columns changes Retinotopic organization is maintained

Seeing Color – 2 Theories Trichromatic theory (component theory) Proposed by Young, refined by Helmholtz 3 types of receptors, each with a different spectral sensitivity

Seeing Color – 2 Theories Opponent-process theory Hering 2 different classes of cells encoding color, and another class encoding brightness Each encodes two complementary color perceptions

Seeing Color – 2 Theories Both are correct – coding of color by cones seems to operate on a purely component basis, opponent processing of color is seen at all subsequent levels

Color Constancy and the Retinex Theory Color constancy – color perception is not altered by varying reflected wavelenths Retinex theory – color is determined by the proportion of light of different wavelengths that a surface reflects Relative wavelengths are constant, so perception is constant