Perception Lecture 6

Photoreceptor

Specialized cells in the retina (rods and cones) that convert light into electrical signals.

Inner Segment

The part of a photoreceptor that contains the cell nucleus and synaptic terminals.

Outer Segment

The part of a photoreceptor containing photopigments.

Photopigment

A light-sensitive molecule in photoreceptors composed of opsin and a chromophore.

Opsin

A protein component of photopigments that determines the wavelength sensitivity of the photoreceptor.

Chromophore

The light-absorbing molecule within photopigments; in mammals, this is 11-cis retinal.

Phototransduction

The process by which photoreceptor cells convert light into electrical signals.

Photoisomerization

The process by which 11-cis retinal changes to all-trans retinal upon exposure to light.

Photopigment Regeneration

The process of converting all-trans retinal back to 11-cis retinal.

Rods

Photoreceptors specialized for low-light (scotopic) vision; highly sensitive to light, but do not convey colour information.

Cones

Photoreceptors specialized for bright light (photopic) vision, responsible for colour vision.

Rhodopsin

The photopigment found in rods.

Photopsin

The photopigment found in cones.

S-Cones

Cones sensitive to short wavelengths, associated with blue perception.

M-Cones

Cones sensitive to medium wavelengths, associated with green perception.

L-Cones

Cones sensitive to long wavelengths, associated with red perception.

Melanopsin

Photopigment found in some photoreceptors that monitor ambient light and influence circadian rhythm.

Duplex Retina

Refers to the dual functionality of the retina with both rods (for scotopic vision) and cones (for photopic vision).

Photopic Vision

Vision under well-lit conditions, primarily mediated by cones.

Scotopic Vision

Vision in low-light conditions, primarily mediated by rods.

Dark Adaptation

The process by which the eyes adjust to low-light conditions.

Lateral Inhibition

A neural mechanism where active neurons suppress neighbouring neurons.

Receptive Field

The region on the retina in which stimuli influence a neuron’s firing rate.

ON-center/OFF-surround Cells

Ganglion cells that are activated by light in the centre and inhibited by light in the surround.

OFF-center/ON-surround Cells

Ganglion cells that are activated by light in the surround and inhibited by light in the centre.

Bipolar Cells

Retinal interneurons that connect photoreceptors to ganglion cells.

Graded Potentials

Continuous variations in the amplitude of a postsynaptic potential.

Ganglion Cells

Retinal neurons that send signals from the retina to the brain via the optic nerve.

Visual Acuity

The smallest spatial detail that can be resolved.

Visual Angle

The size of a visual stimulus on the retina.

Grating

A pattern of alternating black and white stripes used to measure visual acuity.

Spatial Frequency

The number of cycles of a pattern within one degree of visual angle.

Convergence

The number of photoreceptors that synapse onto a single bipolar cell.

Fovea

The central region of the retina, specialised for high acuity.

Optic Chiasm

The site where half of the optic nerve fibers from each eye decussate to the contralateral brain hemisphere.

Temporal Retina

The part of the retina that receives light from the contralateral visual field, and does not decussate in the optic chiasm.

Nasal Retina

The part of the retina that receives light from the ipsilateral visual field, and decussates in the optic chiasm.

Optic Tract

Retinal projections beyond the optic chiasm.

Lateral Geniculate Nucleus (LGN)

A nucleus in the thalamus where most optic tract fibers synapse.

Magnocellular Layers

Layers 1 and 2 of the LGN that process peripheral vision and are input from parasol retinal ganglion cells.

Parvocellular Layers

Layers 3-6 of the LGN that process central vision and are input from midget retinal ganglion cells.

Koniocellular Layers

Located in intralaminar regions of the LGN, specialized for colour perception.

Superior Colliculus (SC)

A midbrain structure that receives direct input from the retina, involved in visual reflexes and unconscious visual processing.

Blindsight

Unconscious visual perception in patients with damage to the primary visual cortex.

Optic Radiations

Axons from the LGN that project to the primary visual cortex (V1).

Primary Visual Cortex (V1)

The first area of the cortex to process visual information.

Retinotopic Mapping

The mapping of the visual field onto the retina and then onto visual areas of the brain.

Magnocellular Pathway

A visual pathway that projects to the parietal cortex and is specialized for processing motion.

Parvocellular Pathway

A visual pathway that projects to the temporal cortex and is specialized for object recognition.