PSYC 211 Midterm 2 Vision II

Vision Sensation

• Retina: light → photoreceptor cell → bipolar cell → RGC →

• → Forebrain: thalamus → cerebral cortex V1

• we understand vision in the retina because the activity there is solely determined by what the animal sees

• transition from sensation to perception occurs in V1

Visual Thalamus

• our understanding becomes more limited as soon as we get to the visual thalamus (4th cell in the circuit) because only 10% of the inputs to the visual thalamus come from the retina

• other inputs come from other areas of the brain, and the info encoded in those pathways is unknown

Lateral Geniculate Nucleus (LGN)

• a nucleus in the thalamus

• many RGCs project here and then LGN neurons project to the visual cortex

• left visual field goes to the right hemisphere

• right visual field goes to the left hemisphere

  • each hemisphere gets input from both eyes

Superior Colliculus

• involved in controlling fast reflexive movements in response to light

Hypothlamus

• regulates sleep/wake cycles

Receptive Field

• a description of the physical stimulus that activates a neuron

• area of visual space (relative to a fixation point) where light is capable of changing the activity of a neuron

• to identify receptive field, we train an animal to stare at the centre of a screen, and then systematically shine some pattern of light in different areas of their visual space

Horizontal Cell

• type of retinal cell

• accentuate contrast by boosting NT release from photoreceptor cells that are more dimly lit than their neighbours

• regulates adjacent photoreceptor and bipolar cells

• do not have action potentials

• release glutamate in a graded manner dependent on their membrane potential

Receptive Field of Photoreceptor Cell

• leaky sodium ions are open in the dark and are already depolarized

• release more glutamate in the dark than the light

• location of visual space where it detects light (relative to the fixation point)

• wavelength of light its most sensitive to

Bipolar Cells

• there are ON and OFF bipolar cells

• specialize in detecting small increases and small decreases in light intensity, respectively

• no action potentials

• graded glutamate release dependent on membrane potential

ON Bipolar Cells

• most sensitive to increases in brightness

• express inhibitory metabotropic glutamate receptors

  • activity patterns are the opposite of the photoreceptor cells that connect to them

  • inhibited by glutamate

• in the light:

  • depolarize and release glutamate

  • photoreceptor cells: sodium channels close → hyperpolarized, and release less glutamate

  • ON bipolar cells are less inhibited → so depolarizes

• In the dark:

  • hyperpolarize and stop releasing glutamate

  • photoreceptor cells: sodium channels open → depolarized, release more glutamate

OFF Bipolar Cells

• most sensitive to decreases in brightness

• express normal excitatory ionotropic glutamate receptors

  • activity patterns follow the photoreceptor cells connected to them

  • excited by glutamate

• in the light:

  • hyperpolarize and do not release glutamate

  • photoreceptor cells: sodium channels close → hyperpolarized, release less glutamate

• in the dark:

  • depolarize and continuously release glutamate

  • photoreceptor cells: sodium channels open → depolarized, release more glutamate

Bipolar Cell Receptive Fields

• horizontal cells create a “centre-surrounded” organization in the receptive fields of bipolar cells

• sum of the receptive fields of the photoreceptor cells that connect to it

• characterized by:

  • location in visual space (relative to fixation point)

  • wavelength of light it is most sensitive to (centre of its receptive field)

  • whether the light in the centre of its receptive field causes ON or OFF response

• in the fovea:

  • bipolar cells receive input from only one photoreceptor cell, so the centre of its receptive field is the same size as one photoreceptor cell (high resolution vision)

• outside the fovea:

  • bipolar cells receive input from many photoreceptor cells, so the centre of their receptive field is the size of many photoreceptor cells’ receptive fields summed together (low resolution vision)

Amacrine Cells

• detect motion and movement direction by integrating info from neighbouring bipolar cells

• regulate the excitability of adjacent bipolar and ganglion cells

• don’t report absolute light levels

  • only signal when there are changes in light across time

• release NT onto several cell types (RGCs, bipolar cell terminals, and other amacrine cells)

RGCs

• express excitatory glutamate receptors

  • activity mostly follows the bipolar cells that connect to them

• also have a “centre-surrounded” organization and are classified as ON and OFF for light and colour

ON RGCs

• increase their rate of spiking when light is in the centre of their receptive field

• decrease their rate of spiking when light is only in the surround area

OFF RGCs

• opposite of ON RGCs

• decrease their rate of spiking when the light is in the centre of the receptive field

• increase their rate of spiking when light is only in the surround area

RGCs in the Fovea

• process colour by integrating inputs from many bipolar cells

• A “Yellow On” center indicates the RGC is processing information from at least two cone cells in the center position

  • (one red and one green, via ON bipolar cells, that have overlapping receptive fields).

• A “Blue Off” surround indicates that it is also processing information from several blue cone cells (via OFF bipolar cells) that surround the red/green cone cells.

Receptive Field of Thalamic Neurons

• similar to that of RGCs

Receptive Field of Neurons in V1

• sum of many thalamic LGN neurons

• simple cells in V1 are sensitive to lines of light

  • their receptive fields are typically organized in centre-surrounded fashion

Primary Visual Cortex

• also known as V1 and striate cortex

• Neurons in V1 spike when there is a line of light in a particular orientation in their receptive field.

• some V1 neurons respond best to vertical lines, some to horizontal, and some to lines oriented somewhere in between

• V1 neurons have a larger receptive field and respond to various orientations of light

  • does this by all neurons in one cortical column analyzing the same pixel of visual space

  • this helps identify borders, edges, and corners

• neurons downstream of V1 put this info together to identify whole objects and their position in space

Extrasiate Cortex

• synonymous with visual association cortex

• areas V2, V3, V4, V5, etc.

Where Pathway (dorsal)

• dorsal stream to parietal lobe

• involved in identifying motion and spatial relationships

• encodes:

  • whether anything is moving in visual space (V5)

  • how can I move to interact or avoid the object

What Pathway (ventral)

• ventral stream to temporal lobe

• involved in identifying shapes and colours

• encodes:

  • what objects are there

  • what colour are the objects (V4)

• sensation:

  • retina - light detection

  • → V1 - edge detection (lines and orientation)

• perception:

  • → V2 - shape detection (two lines form an angle; two angles create a shape)

  • → early ventral visual stream - object identification (independent of viewing angle)

  • → late ventral visual stream - object identification (this is a banana)

• meaning:

  • → semantic interpretation - this is something i can eat

Monocular Vision

• some V1 neurons respond to visual input from just one eye

• depth perception

Binocular Vision

• most V1 neurons respond to visual input from both eyes

• each provide a different perspective to the same area of visual space

• depth perception

Depth Perception

• there are many monocular cues that can be used to estimate depth, such as relative size, amount of detail, relative movement as we move our eyes etc.

• we use these cues to perceive depth when looking at a 2D image, only one eye is required for monocular cues

Stereopsis

• perception of depth that emerges from the fusion of two slightly different projections of an image on the two retinas

• difference between the images from the two eyes is called retinal disparity

  • results from the horizontal separation of the two eyes

• improves the precision of depth perception, especially for moving objects, and is helpful when playing sports or when pouring a glass of water

Visual Agnosia

• V1 works, but part of their visual association cortex is damaged

• agnosia is a deficit (problem) in perception – a deficit in the ability to recognize or comprehend certain sensory information (like visual detection of objects, colors, and motion), even though sensation of the sensory information is not defective.

• people with an agnosia generally have intact memories and knowledge of the sensory features that they can no longer perceive (through a particular sensory modality)

Akinetopsia

• disability in movement perception

• damage in the dorsal stream (V5)

• people describe seeing the world as a series of snapshots

Cerebral Chromatopsia

• denying colour perception

• damage to ventral visual stream (V4)

• people describe things as looking dull or drab

  • shades of gray

• V4 is involved in both perception and memory of colour

Predictive Coding Theory

• every level of visual processing is affected by predictions of what’s interpreted at lower levels and a feedback error signal to correct future predictions