Neurological Examination, clinical cases and neuropsychological ...

23/07/54
V5/MT Visual area V5, V also known as visual area MT (middle
temporal), is a region of extrastriate visual cortex that is thought to play a major
role in the perception of motion, the integration of local motion signals into
global percepts and the guidance of some eye movements
MT is connected to a wide array of cortical and subcortical brain areas. Its inputs
include the visual cortical areas V1, V2, and dorsal V3 (dorsomedial area), the
koniocellular regions of the LGN, and the inferior pulvinar. The pattern of
projections to MT changes somewhat between the representations of the foveal
and peripheral visual fields, with the latter receiving inputs from areas located in
the midline ecortex and retrosplenial ospe region
A standard view is that V1 provides the "most important" input to MT.
Nonetheless, several studies have demonstrated that neurons in MT are capable
of responding to visual information, often in a direction‐selective manner, even
after V1 has been destroyed or inactivated. Moreover, research by Semir Zeki
and collaborators has suggested that certain types of visual information may
reach MT before it even reaches V1.
MT sends its major outputs to areas located in the cortex immediately
surrounding it, including areas FST, MST and V4t (middle temporal crescent).
Other projections of MT target the eye movement‐related areas of the frontal
and parietal lobes (frontal eye field and lateral intraparietal area).
Function of V5/MT
The first studies of the electrophysiological properties of neurons in MT showed that a
large portion of the cells were tuned to the speed and direction of moving visual stimuli
These results suggested that MT played a significant role in the processing of
visual motion.
Lesion studies have also supported the role of MT in motion perception and eye
movements and neuropsychological studies of a patient who could not see motion, seeing
the world in a series of static "frames" instead, suggested that MT in the primate is
homologous to V5 in the human.
However, since neurons in V1 are also tuned to the direction and speed of motion, these
early results left open the question of precisely what MT could do that V1 could not. Much
work has been carried out on this region as it appears to integrate local visual motion
signals into the global motion of complex objects ] For example, lesion to the V5 lead to
deficits in perceiving motion and processing of complex stimuli. It contains many neurons
selective for the motion of complex visual features (line ends, corners). Microstimulation of
a neuron located in the V5 affects the perception of motion. For example, if one finds a
neuron with preference for upward motion, and then we use an electrode to stimulate it,
the monkey becomes more likely to report 'upward' motion.
There is still much controversy over the exact form of the computations carried out in area
MT and some research suggests that feature motion is in fact already available at lower
levels of the visual system such as V1
MT was shown to be organized in direction columns.
NEUROPSYCHIATRY 190
Organization of V1 and V2.
A. Subregions in V1 (area 17)
and V2 (area 18). This section
from the occipital lobe of a
squirrel
monkey at the border of areas
17 and 18 was reacted with
cytochrome oxidase. The
cytochrome
oxidase stains the blobs in V1
and the thick and thin stripes
in V2. (Courtesy of M.
Livingstone.)
B. Connections between V1
and V2. The blobs in V1
connect primarily to the thin
stripes in V2, while
the interblobs in V1 connect to
interstripes in V2. Layer 4B
projects to the thick stripes in
V2 and to
the middle temporal area
(MT). Both thin and
interstripes project to V4.
Thick stripes in V2 also
project to MT.
Callosal Disconnection
Syndromes
NEUROPSYCHIATRY 192
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