Visual Sensation & Perception

How do we see?

Structure of the eye

The Retina

Visual Receptors• Rods

– Slowly adapting– Black & White vision– 120 million; None in fovea

• Cones– Rapidly adapting– Color vision– 5 million; 50,000 in fovea

Retinal Ganglion cells• Gather information from many rods and cones

across an area of the retina. – How many rods and cones depends on the size of the

ganglion cell’s receptive field

– The closer to the fovea, the smaller the receptive field.

• Project out of the eye through the optic nerve, creating a blind spot.

• 1 million retinal ganglion cells (receiving signals from 125 million receptors).

Receptive fields of retinal ganglion cells

• Center-surround– Most are excitatory

center, inhibitory surround.

– Some are the opposite

Out of the retina

• Signals from the two eyes cross over to the opposite brain hemisphere at the optic chiasm.– Not all signals from an eye go to contra-lateral

hemisphere.– Which hemisphere the signal goes to is based

on which visual hemifield the ganglion cell receives information from.

Into the brain• Ganglion cells synapse in the lateral geniculate

nucleus (LGN) of the thalamus.• For comparison, the auditory nerve synapses in the medial

geniculate nucleus.

• The LGN divides the signals into layers depending on which eye they come from, and whether they come from the fovea or not.

• 1, 4, & 6 from the contralateral eye; 2, 3, and 5 from the ipsilateral eye.

• 1 and 2 from the fovea.

• 400,000 cells leave the LGN

V1• From the LGN, the signals are sent to area

V1 in the very back of the occipital lobe.

• Signals are organized into a retinotopic map based on where on the retina they come from, and which eye they come from.

Feature detectors in V1• The retinotopic map is not simply a

light/dark detector. Signals are beginning to be combined into simple feature detectors that can detect lines at various orientations.

• All of the feature detectors for a particular area of the retina are anatomically organized into a column.

• A hypercolumn is two columns from corresponding parts of both retinas.

Beyond V1• From V1, signals go to area V2 where the

combine into more complex features (corners and simple shapes).

• After V2, the signal splits into two streams of information.– The what stream passes through V3 (which

does color detection) into the temporal lobe.– The where stream passes through V4 (which

aids with motion detection) into the parietal lobe.