Visual Sensation Perception How do we see Structure

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

V 1 • From the LGN, the signals are sent to area V 1 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 V 1 • 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 V 1 • From V 1, signals go to area V 2 where the combine into more complex features (corners and simple shapes). • After V 2, the signal splits into two streams of information. – The what stream passes through V 3 (which does color detection) into the temporal lobe. – The where stream passes through V 4 (which aids with motion detection) into the parietal lobe.