On columns and pathways Flashcards
optic chiasm
visual signals from both eyes leave the back of the eye in the optic nerve and meet at the optic chiasm - an x-shaped bundle of fibers on the ventral surface of the brain, inferior to the hypothalamus. at the optic chiasm, some of the fibers cross to the contralateral brain hemisphere
double dissocation
in one person, damage to one areas of the brain causes function A to be absent while function B is present. In another person, damage to another area of the brain causes function B to be absent while function A is present. The conclusion is that the 2 functions can be disrupted and operate independently of one another
Visual agnosia
inability to recognize/describe common objects, faces or pictures, even though the ability to navigate through the world persists. Caused by damage to the occipitotemporal region.
Optic ataxia
inability to reach accurately towards visual targets or adjust their grasp to reflect the size of the target object, even though the ability to recognize them persists. Caused by damage to the posterior parietal region.
Visual prosopagnosia
inability to recognize faces. Different features of the faces can be perceived, but the person whose face this is cannot be indetified. Developmental prosopagnosia is something that is present from birth
inferotemporal (IT) cortex
receptive fields of neurons in the IT cortex are large enough to encompass whole objects in the visual field. evidence for faces and hands and face selectivity
medial temporal lobe (MTL)
some of the signals leaving the IT cortex reach the medial temporal lobe (MTL), more specifically the parahippocampal cortex, the entorhinal cortex and the hippocampus. also respond to memories of concepts.
Extrastriate cortex
areas outside the striate cortex to which the visual signals travels from V1. They are also separately known as V2, V3, V4, and V5.
What (ventral) pathway
The signal from the occipital lobe continues through the what pathway and the how pathway. A pathway leading from V1 to the temporal lobe. Important for determining an object’s identity. Sensitive to form, pattern and color.
How (dorsal) pathway
a pathway leading from V1 to the parietal lobe. Important for directing an action with regard to a visual stimulus (e.g., pick up an object). Is faster, but colorblind.
selective adaptation
constantly viewing a stimulus with a specific feature causes neurons that respond to that feature to adapt, decreasing their firing rate if the stimulus is presented again immediately
selective rearing experiments
animals are raised in an environment that contains only certain types of stimuli with the expectation that neurons that respond to these stimuli will become more prevalent
location columns
the striate cortex is organized into location columns, which are perpendicular to the cortical surface. All neurons within a location column have their receptive fields at approximately the same location in the retina.
orientation columns
the striate cortex is also organized into orientation columns. All neurons within an orientation column respond best to approximately the same orientation. Adjacent orientation columns have cells with slightly different preferred orientations.
Ocular dominance
V1 neurons have a preference for stimuli presented in one eye rather than the other eye.
ocular dominance columns
the striate cortex is also organized into ocular dominance columns, again perpendicular to the cortical surface. 1mm in the cortex contains oriention columns that represent the entire range of orientations. One location columns i large enough to contain orientation columns that cover all possible orientations
Hypercolumn
a location column with all of its orientation columns. There are 2 ocular dominance columns within each hypercolumn, one of the left eye and one for the right.
Simple cortical cells
neurons in the striate cortex, whose receptive fields have a side by side excitatory and inhibitory areas. These cells respond best to bars of lights, edges and lines of particular orientations
Orientation tuning curve
a graphical representation of the relationship between line orientation and firing of a neuron.
2 proposed mechanisms for how the circular receptive fields in the LGN are transformed to elongated receptive fields in the striate cortex
1) LGN cells that feed into a cortical cell are all in a row
2) neural interactions (e.g., lateral inhibition) between the cortex also play an important role for this
complex cells
neurons in the striate cortex, which do not have clearly defined excitatory & inhibitory regions. Some of them respond best to moving bar-like stimuli with specific orientations. Respond best to a particular direction of movement.
End-stopped cells
neurons in the striate cortex, which respond to moving lines of a specific length or to moving corners or angles
feature detectors
Simple, complex and end-stopped cells are called feature detectors, because they fire in response to specific features of a stimulus (e.g., orientation, direction of movement)
Primary visual (striate) cortex
from the LGN, the visual signal travels to the Primary visual cortex. Locatioons on the striate cortex correspond to location on the retina and the visual world
Retinotopic/topographical map
the orderly mapping of the world and the retina on the visual cortex and the LGN
Cortical magnification
spatial representation of the visual scene on the cortex is distorted, with more space being allotted to locations near the fovea than to locations in the peripheral retina.
Eccentricity
distance from the fovea (in degrees)
lateral geniculate nuclei (LGN)
After the optic chiasm, 90% of the tracts proceed to the LGN - thalamic nuclei in each cerebral hemisphere. Neurons in the LGN have center-surround receptive fields, just like the retinal ganglion cells. LGN output to the cortex is smaller than the input from the retina. LGN receives more feedback from the cortex than input from the retina. Each LGN has 6 layers and looks like a stack of pancakes that is bent in the middle. Layer 1,4 & 6 receive input from the contralateral eye. Layers 2,3, &5 receive input from the ipsilateral eye
magnocellular layers
the bottom layers are called magocellular layers. Receive input from M ganglion cells. Respond to large, fast-moving objects.
parvocellular layers
receive input from P ganglion cells. responsible for processing the details of stationary stimuli
Koniocellular cells
a neuron located between the magnocellular and parvocellular layers of the LGN. The layer is known as the koniocellular layer.
superior colliculus
plays a role in controlling eye movements
