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The parvocellular pathway from lateral geniculate nucleus to visual cortex is most sensitive for the stimulus of:

A Color contrast
B Luminance contrast

C Temporal frequency
D Saccadic eye movements

Ans. A Color contrast

Pathways to the cortex (Visual pathways)

a. The axons of retinal ganglion cells project a detailed spatial representation of the retina on the lateral geniculate body Q

b. Each geniculate body contains six well defined layers .

a. Layers - 1 and 2 : have large cells, called Magnocellular (provides rapidly' conducting pathway to visual cortex and transmit only white and black information)

b. Layers 3 - 6 : have small cells called Parvocellular (transmit colour and convey accurate point to point spatial information) Q

c. On each side (Lateral geniculate body)

a. Layers 1, 4 and 6 : Receive input from contralateral eye i.e. from Nasal retina because only nasal (medial) side of optic nerve crosses to opposite side.

b. Layers 2, 3 and 5 : Receive input from Ipsilateral eye i.e. from Temproal retina (lateral half of retina) because optic nerves that originates from Temporal side does not crosses to opposite side.

d. Retina have two kinds of ganglion cells :

a. Y-Retinal ganglion or Large ganglion cells or Magno or M-cells : which add responses from different kinds of cones and are concerned with movement, flicker and stereopsis; M-cells project to Magnocellular portion of LGB.

b. X-retinal ganglion or small ganglion cells or Parvo or P-cells : which substract input from one type of cone from input from another and are concerned with color, texture and shape; P-cells project to parvocellular portion of LGB.

e. 5. From Lateral geniculate body (nucleus) ; pathways project to visual cortex (area 17): Q

a. Magnocellular pathway: arises from Layers 1 and 2 and carries signals for detection of movement, depth and flicker.

b. Parvocellular pathway: arises from lavers 3, 4, 5 and 6 and carries signals for Color vision. texture, shape and fine details.

Cell in the interlaminar region of LGB (nucleus) à receive input from P-ganglion cells à they project via a separate component of the P pathway to blobs in the visual cortex (necessary for detection of color) (end in layer 2 and 3 of visual cortex)

Visual area

Primary visual cortex: area 17 à visual pathways (Magnocellular and parvocellular) end in the layer 4C of visual cortex. It is also known as VI. Q V8-àcolor vision Q

Fig. : Ganglion cells projection from the right hemiretina of each eye to the right LGB and from this nucleus to the right primary visual cortex Q. Note that six layers of the geniculate; P-ganglion cells (also known as X-ganglion), project to layers 3-6 and M ganglion cells (also known as Y ganglion) project to layers 1 and 2. The ipsilateral (I) and contralateral (C) eyes project to alternate layers.

Functions of LGB

a. Relay function - is very accurate, exact point to point transmission with high degree of spatial fidelity all the way from retina to the visual cortex.

b. Gate the transmission of signals to visual cortex.

Vision Flashcard List

11 flashcards