VISION AND READING

 

There are important pre-attentive processes involving the magnocellular (in particular) & parvocellular subdivisions of the visual system

Start by reviewing some interesting symptoms noted by disabled readers. These related to the illusory text movement phenomenon noted as a symptoms of ocular motor dysfunction.

 

blurriness
blanching

channels
swirling

low contrast / glare

 

Now look at the anatomy of Magno and parvo pathways

From this summary of the anatomical pathways of the two systems, the P pathway terminates in inferior temporal cortex, the area of the brain that is responsible for identification processes; whereas, the M pathway terminates in the superior temporal cortex and the posterior parietal lobes. These areas are essential for motion detection and object localization.

 

Therefore, one can conceptualize the P pathway as the "what" system and the M pathway as the "where" system

The chart illustrates the distribution of M and P ganglion cells in the retina. Note that the M pathway is gnerally more predominant ine periphery; P cells in the foveal area

Physiological properties of m and p cells

Note on the left that M cells have larger receptive fields than P cells and accordingly on the right have a lower cutoff spatial frequency and the maximum contrast sensitivity is found at a lower spatial frequncy than P cells.

One can conclude that P cells "see" the higher spatial frequencies of a visual stimulus and the M cells are more sensitive to lower.

This illustrates the differences between M and P cells when they "look" at a picture of you know who.

M&P Physiological functions

Magnocellular

Parvocellular

transient responses

sustained responses

higher contrast sensitivity - low spatial frequencies

faster

achromatic

color vision channel

motion detection

visual acuity - high spatial frequencies

inhibits the parvo system

m&p reading2