Magnocellular cells, also called M-cells, are neurons located within the magnocellular layer of the lateral geniculate nucleus of the thalamus. The cells are part of the visual system. They are termed "magnocellular" since they are characterized by their relatively large size compared to parvocellular cells.
Structure
thumb|Schematic diagram of the primate LGN.
The full details of the flow of signaling from the eye to the visual cortex of the brain that result in the experience of vision are incompletely understood. Many aspects are subject to active controversy and the disruption of new evidence.
In the visual system, signals mostly travel from the retina to the lateral geniculate nucleus (LGN) and then to the visual cortex. In humans the LGN is normally described as having six distinctive layers. The inner two layers, (1 and 2) are magnocellular cell (M cell) layers, while the outer four layers, (3,4,5 and 6), are parvocellular cell (P cell) layers. An additional set of neurons, known as the koniocellular cell (K cell) layers, are found ventral to each of the M cell and P cell layers. These layers were named this way because cells in the M layers of the LGN are larger than cells in the P layers.
M cells in the LGN receive input from parasol ganglion cells (which some neuroscientists call M cells),
none|thumb|400x400px|Visual representation of the parvocellular and magnocellular pathways
From the LGN, the M pathway continues by sending information to the interblob regions of the 4Cα layer of the V1 region of the visual cortex, also called the "striate cortex". The M pathway has high light/dark contrast detection, and is more sensitive at low spatial frequencies than high spatial frequencies. Due to this contrast information, M cells are essential for detecting changes in luminance, and performing visual search tasks and detecting edges.
The M pathway is also important for providing information about the location of objects. M cells can detect the orientation and position of objects in space, information that is sent through the dorsal stream. This information is also useful for detecting the difference in positions of objects on the retina of each eye, an important tool in binocular depth perception.
Cells in the M pathway have the ability to detect high temporal frequencies and can thus detect quick changes in the position of an object. The information sent to the intraparietal sulcus (IPS) of the posterior parietal cortex allows the M pathway to direct attention and guide saccadic eye movements to follow important moving objects in the visual field. In addition to following objects with the eyes, the IPS sends information to parts of the frontal lobe that allows the hands and arms to adjust their movements to correctly grasp objects based on their size, position, and location.
Some information has also been found to support the hypothesis that the M pathway is necessary for facial processing.
Clinical significance
Abnormal magnocellular pathways and magnocellular cells can be associated with various disorders and ocular impairments, including dyslexia, prosopagnosia and schizophrenia.
Another line of research suggests that defective eye movement caused by M cells is the cause of dyslexia. Since the magnocellular system is sensitive to image movement, and dyslexia is posited to be caused by abnormalities in M cells, dyslexics tend to focus on words longer, take shorter scans when reading, and stop more often per line. The study postulates that this is not caused by dyslexia but rather, low comprehension of the text causing abnormal eye movements in M cells. Therefore, it is difficult to conclude the importance of M cells in dyslexia from this study.