Question

Please answer these for me! Thank you :)

1. The parvocellular system is dedicated to fine-grained, conscious processing of color and other details. The magnocellular system contributes to conscious processing of motion and spatial information but also to lower-level, non-conscious function of motion. Describe 3 differences in the activity of these two systems and how they support these two different functions.
2. What information from the retinas crosses at the optic chiasm, and what doesn’t cross but stays on the same side of the axis of symmetry?
3. Describe why the somatosensory homunculus looks like he does.

Answer 1

Parvocellular cells, also called P-cells, are neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus.Magnocellular cells, also called M-cells, are neurons located within the Adina magnocellular layer of the lateral geniculate nucleus of the thalamus.

The parvocellular neurons are sensitive to colour, and are more capable of discriminating fine details than their magnocellular counterparts. Parvocellular cells have greater spatial resolution, but lower temporal resolution, than the magnocellular cells.

At optic chiasm, the fibers from the nasal half cross while the fibers from the temporal half remain uncrossed. From the optic chiasm, fibers pass to the optic tract. The left optic tract contains uncrossed temporal fibers from the left eyeball and crossed nasal fibers from the right eyeball.

This crossing over of optic fibers is important in such a way that it allows the same hemispheric visual field to be perceived by visual cortex from both eyes. This can be understood from the example that the left visual cortex receives the temporal visual field from the right eye (by crossed right nasal fibers) and the nasal visual field from the left eye (via uncrossed left temporal fibers).

This allows the left visual cortex to form a binocular image of right hemispheric field. The net result of crossing at the optic chiasm is that the right visual cortex senses and processes left-hemispheric vision while the left visual cortex senses and processes right-hemispheric field of vision.

The reason for the distorted appearance of the homunculus is that the amount of cerebral tissue or cortex devoted to a given body region is proportional to how richly innervated that region is, not to its size. The homunculus is like an upside-down sensory or motor map of the contralateral side of the body.