• Sensory coding issues
• Anatomy of the retina
• Receptive Fields
• Organization of striate cortex
• Feature analysis
• Spatial frequency analysis
• Visual association cortex

• Afferent vs. efferent
• Stimulus: any energy capable of exciting a receptor
• Mechanical
• Chemical
• Thermal
• Photic
• Sensory energies are measurable (unlike ESP)

• Receptors: specialized nerve cells that transduce energy
• Act as dendrites that eventually induce A.P.’s
• Receptors are "mode" specific
• Only detect a small range of energy levels
• Eye: 400-700 nM
• Ear: 20-20,000 Hz
• Taste buds: specific chemicals
• "Law of Specific Nerve Energies"

Receptive Fields

• Receptive Field (RF): Those attributes of a stimulus that will alter the firing rate of sensory cell
• Can measure RF at each level of sensory system
• There are as many RF’s as there are cells in a sensory system

Sensory System Issues

• How many synapses (order of system)
• Degree of decussation (crossover) ?
• Projects to what area of thalamus?
• Projects to what area of cortex?
• Does cortex show "topical" organization?
• Does cortex show "columnar" organization?
• Modification of sensory coding?
• Experience, hormones?

• Function of visual systems is to detect EMR emitted by objects
• Nature of visible light (400-700 nM)
• Functions of vision
• Locate figure vs. ground
• Detect movement (predator/prey?)
• Detect color (adaptive value of color?)

• An eye consists of:
• Aperture (pin hole, pit, or pupil)
• Lens or not?
• Photoreceptive elements (retina)

• Light passes through several layers of retina to reach the photoreceptors

Rods and Cones

• Rods: 120 million
• Light sensitive (not color)
• Found in periphery of retina
• Consist of stacked protein disks
• Low activation threshold
• Cones: 6 million
• Are color sensitive
• Found mostly in fovea
• One continuous membrane

Retinal Circuitry

• Photorec.: 1st order
• Bipolar: 2nd order
• Ganglion cell: 3rd order
• Rods: more diffusely connected to bipolar cells

• Photopigments
• Consist of opsin and retinal
• In the dark, NA⁺ channels are open -> glutamate is released
• Light breaks opsin and retinal apart->
• Activates transducin (G protein)->
• Activates phosphodiesterase->
• Reduces cGMP -> closes NA⁺ channels
• Net effect: light hyperpolarizes the retinal receptor

Receptive Fields: Ganglion Cell

• Ganglion cells exhibit low baseline firing rates:
• Receptive fields: circular in shape with ring-shaped surround:
• "ON-Cell":
• Light placed on center increases firing
• Light placed on surround decreases firing

Visual Pathways

• Within retina:
• Photoreceptor -> bipolar cell - > ganglion cell
• Beyond retina:
• Ganglion cell -> through optic chiasm -> lateral geniculate -> primary visual cortex (striate)

• Magnocellular system
• Cells from retina terminate in LGN layers 1,2
• Carry info on contrast and movement (color insensitive)
• Carry input from "A" retinal ganglion (Y type) cells
• Parvocellular system
• Cells from retina terminate in LGN layers 3-6
• carry info on fine detail, and color
• Carry input from "B" retinal ganglion cells (X type)

LGN Receptive Fields

• LGN shows circular receptive fields
• Size: larger for magnocellular than parvocellular
• Color sensitivity: only for parvocellular
• Color receptive fields (circular: on-off)
• e.g. red-on, green-off

• Hubel and Wiesel were interested in the visual stimuli that would excite a nerve cell in area 17
• Anesthetized a cat, inserted microelectrode into area 17, recorded AP pattern during presentation on stimuli to retina
• Cells responded to features of visual field
• Shape
• Orientation
• Movement

• Columnar Organization (input from same part of retina)
• Ocular dominance: cells respond to only one eye
• Columns for left and right alternate
• Orientation columns:
• Cells respond to same orientation, adjacent cells are shifted by 10 degrees
• Are at right angle to ocular dominance column
• Do a 180 degrees in 1 mm
• Color "blobs"-stained for cytochrome oxidase
• -Show up every 0.5 mm (one blob for each eye)
• Removal of one eye - alternate rows of blobs disappear

Modules in Visual Cortex

• Visual neurons respond to a sine wave grating:
• Alternating patches of light and dark
• Low f: large areas of light and dark
• High f: fine details

• From striate cortex (V1): see 2 streams
• Dorsal: "where" an object is
• Projects to post. pariental association cortex
• Ventral: "what" an object is (analysis of form)
• Projects to extrastriate cortex (V2, V3, V4, V5)
• and to inferior temporal cortex (TEO, TE, STS)
• Dorsal: mostly magnocellular input
• Ventral: equal mix of magnocellular and parvocellular input

Visual Cortex

• V1: responds to color, orientation, eye dominance
• V4: responds to color (and form perception)
• Lesions impair color perception
• V5: responds to movement
• Inf. temporal cortex
• TEO: coding of object features (2-d patterns)
• TE: recognition of objects (a face or a hand)