MIP was initially considered to be part of area 5. Now it is considered a separate area and Lewis and Van Essen have added to the mess by "delineating" 5v and 5d on anatomical criteria in addition to the more medial MIP. It does not appear to have any other former nomenclature.
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Area 5 is located in the superior parietal lobule. In the monkey it is immediately posterior to area 2 and anterior to the intraparietal sulcus (IPS). 5v is defined by Lewis and Van Essen as the more lateral extent of the anterior/medial wall of the IPS, the area which used to be known as MIP, which used to just be plain 'ole 5.
Function:
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Most representations are on the arm/shoulder as compared with the trunk (Sakata et al., 1973; Taoka et al., 1998; Pearson and Powell, 1985). Hand/wrist is also relatively well represented (e.g., ~30% of classified neurons in Taoka et al., 1998). Many receptive fields are bilateral and respond best to movement of the limbs bilaterally. Repetition of passive sensory stimulation can lead to adaptation of responses (see Seal, 1989 for a review) and these neurons, while able to be driven by sensory input, respond best when activated by movement (Mountcastle et al., 1975). Lesions of area 5 have only mild sensory deficits in roughness thresholds (Murray and Mishkin, 1984) suggesting that this is not a traditional "higher sensory" cortex.
Integration
Mountcastle and Sakata (old school) found very few visually driven neurons, and those they found tended to have overlapping directional sensitivity in vision and touch (like are VIP). Mackay and Crammand (1987) found that visual observation of approach to an area 5 receptive field on the hand or shoulder caused increased "anticipatory" firing.
Graziano et al.(2000) showed real arms in a certain position, and realistic fake arms in the same position and got modulation of cell firing, with vision giving a small but significant interaction with proprioception. 30% of neurons showed this effect, but when all neurons were considered the effect was still significant. In this study, for at least some cells where there was no independent effect, correlated visual and tactile stimulation on the hand caused a shift and an increase in firing in response to the presence of the fake arm (the Botvinik and Cohen illusion). This suggests that updating the proprioceptive field with visual and tactile input is one of the key goals of this area from a "sensory" point of view.
"Early" cells (~25%) show delay period premotor activity and do not have sensory fields (Mountcastle et al., 1975; Bioulac et al., 1999; Seal et al., 1982; but see Crammand and Kalaska, who report 60% of cells have early/delay period activity related to direction of movement). Low-threshold electrical stimulation of area 5 was not associated with muscle contraction (Seal 1989): This area is probably not an output layer in the opinion of many (see Wise et al., 1997, for a review). Of motor cells there are two distinct activity types, 15% = RS and 85% = Bursting.
Area 5 motor-related cells are tuned for the direction of movement (e.g., Crammand and Kalaska, 1989), and are not effected much by placing a load on the arm (Kalaska et al., 1990) suggesting an extrinsic / kinematic coding scheme, as opposed to M1 which is strongly modulated by load. Cells with directional sensitivities tend to be more posterior (in area MIP), though many studies have not made a distinction and it is therefore hard to be sure (see Wise et al., 1997, for a review).
... "Body-centered encoding of reaching movements has been recorded in area 5 cells (Lacquaniti et al., 1995; Caminiti et al., 1995). Regression analyses of neuronal responses during reaching to visual targets demonstrated that the responses were consistent with the separate encoding of azimuth, elevation and distance of the target in relation to a body-centered spherical coordinate system. Because both the body and the head of their monkeys were fixed, Lacquaniti et al. (1995) were unable to determine which of the regression models for co-ordinate systems centered on the eyes or on the shoulder fitted their data better. A shoulder-centered representation seems the more likely given the connections of area 5 with PMd and M1 where the preferred directions of reach-related cells are modulated by shoulder position (Caminiti et al. 1990, 1991) and the observation of area 5 cells that fire in relation to movements at more than one joint (Sakata et al. 1973)."
--- From Rushworth et al., 1997.
Scott et al. (1997) showed that most (95%) area 5 cells show a change in activity when reaching movements are performed using the same hand path (extrinsic variable) and different initial arm orientation ("intrinsic variable"). Almost all their cells showed modulation based on initial arm position, and about half of directionally tuned area 5 cells showed differences in direction tuning based on the arm orientation (Scott et al., 1997; see also Seal, 1989 for a review). So, intrinsic variables do seem to matter, even if extrinsically applied load does not.
Rushworth et al. (1997) lesioned either 7a, 7ab & LIP or 5, 7b, and MIP. With the first lesion, monkeys were impaired in a visually guided reaching task, in the second group monkeys were impaired in a proprioceptively guided reaching task (reaching to a remembered arm location established in the dark). Neither lesion impaired a non-spatial (blue light mean pull/ red light means twist a joystick) task.
Bioulac et al taught monkeys a sensory triggered arm movement, then deafferented by rhizotomy (C2-T7). They found that the animal makes many errors post surgery and has a slower reaction times for 5 months but then gets back to normal behavior by 9 months. During this time, the burst cells fire less, and have a broader interspike interval (ISI) range (slower ISI).
Projects to: SMA, PMd, 4 (these are primarily from "5d"), 1, 2, 3a, projections to 3b are "extremely rare" (Fabri and Burton, 1995a) (see also Jones, 1986 for a review, Pons and Kaas 1986), 7b, retro-insular cortex (Friedman et al., 1980), principal and arcuate sulci, cingulate, superior temporal gyrus, basal ganglia (caudate and putamen), claustrum, (see Hyvarinen, 1982 for a review).
Receives from: ipsilateral: 4, 3a, 1, 2, 3b (again this was rare), 7b: contralateral 2, 5, and 7b. The ipsilateral PoCG projections are topographic in organization (e.g., Jones et al., 1978). No input from or to area 7a (Neal et al., 1986), but does receive input from area 19, AIP, VIP, and LIP...
An important fact here is that areas 3a and 3b don't project forward to motor areas such that areas 1, 2, and 5 have to integrate/pass this information along.
Projects to: Dorsal column nuclei (Weisburg and Rustioni, 1977), corticospinal (~9% of CST projections from the cortex originate here, as compared to 50% from area 4, Toyoshima and Sakai, 1982), red nucleus, thalamic lateral posterior nucleus, superior colliculus, pons, tegmentum and substantia nigra.
Recieves from: Thalamic lateral posterior nucleus, central lateral nucleus and pulvinar. Dentate nucleus lesions effect the timing of cell firing here in a way that is correlated with recovery of reaction time in an arm movement task. Stimulation of that is correlated with recovery of reaction times in an arm movement task. Stimulation of the fastigial nucleus causes activation here that appears to be of thalamocortical origin (Yamamoto et al., 1983) though I did not find a reference detailing a direct thalamic input to this region.
More posterior representations in the PoCG project to more anterior parts of area 5 and vice versa, suggesting a mirror map (Pearson and Powell, 1985), one of the defining criteria for being a unique sensory area (and at least a weak argument against the subdivision into 5v and 5d by Lewis and van Essen, 2000).
