"PO" - Defined on the basis of physiology and architectonics by Gross, Colby, etc. ~ '85
"V6" - Defined on the basis of physiology and connectivity (area surrounded by a ring of callosal terminals)
"V6/V6A" - Area V6 was subdivided on the basis of numerous physiological, anatomical, and connectivity differences in Galletti et al. 1996, which is a damn good paper. Physiologists (including the Gross group) now use this terminology, although anatomists (eg Lewis and Van Essen 2000) still refer to PO.
V6 and V6A are in the medial component of Brodmann's area 19.
PO/(V6 + V6A) occupies the posterior face of the superior parietal gyrus (/lobule?)(SPG). This is a vertical bank of cortex adjacent to the midline medially, with V6 and V6A occupying the upper (dorsal) and lower (ventral) portions of the bank, respectively. Laterally to V6/V6A, the cortex curves anteriorly to become the medial bank of the intraparietal sulcus, or MIP. Since this part of the lobule is in the parietal-occipital sulcus (POS), it can also be called the anterior face of the POS. A good figure for seeing the location of PO/V6/V6A is figure 1 of Galletti et al. 1999b:
Unfortunately, nobody seems to record from V6/V6A when the animal is behaving, so all physiological data is from passive viewing. Receptive fields show a bias towards the contralateral inferior quadrant and a moderate size-eccentricity correlation (tighter correlation for V6). Receptive field sizes range from 10-50 degrees for V6A and 5-40 degrees for V6. (Galletti et al. 1999a, 1999c).
Glossary of response properties (not meant to refer to any other definition of simple/complex, by the way):
Simple: Cell activated by oriented bars, light/dark edges, spots.
Complex: Cell not activated by simple stimuli, requires moving corners, expanding/contracting light/dark bars, etc.
Eye position effect: Firing rate varies as a function of eye position in the dark.
Gain field: Cell has a retinotopic receptive field (either simple or complex), but the amplitude of response varies with the position of the eye in the orbit.
Real-position: Cell's receptive field (simple or complex) remains stationary with respect to the head as the animal changes the position of the eye in the orbit.
The story:
V6:
All cells in V6 are simple visual cells. (Galletti et al. 1996)
Receptive fields are topographically organized (Galletti et al. 1999c).
V6A:
In V6A, 60-70% of cells are visual.
Of visual cells in V6A, 48% show eye position effects in the dark. (Galletti et al. 1995).
Of visual cells in V6A, 61% show eye position effects on the visual response to stimuli (gain fields). (Galletti et al. 1995).
Of visual cells in V6A, 10% are real-position cells. (Galletti et al. 1993, 1999b).
Of non-visual cells in V6A, 32% show eye position effects in the dark (Galletti et al 1995).
Receptive field topography seems rougher than in V6. Galletti et al. (1999a) claim that V6A is nontopographic, but they may have missed topography due to oblique penetration angles.
Galletti et al. 1996 briefly report attentional effects in V6A (reduced firing for novel stimuli/shifts of attention).
Note: "Nonvisual" cells in V6A can often be excited by complex, moving patterns of light and dark (Galletti et al. 1996). This obviously implies that these are actually visual cells, and that the battery of visual stimuli being used to probe the cells isn't rich enough. But even if all V6A cells are in fact visual, they clearly have more complex response properties than the cells in V6.
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Visual / occipital:
V6 is more strongly connected with lower visual areas (V2, V3, V3A, V5) than is V6A, and is more strongly connected to MST. V6A gets no input from V2, weak input from V3 and V3A, and "ascending" input (see 'hierarchy') from V5. (Shipp et al. 1998) |
Parietal:
V6 and V6A have similar connection patterns with parietal areas. Both have strong connections to MIP, 7m/MDP, and PEc (dorsal/caudal pole of the SPL). Both V6 and V6A appear to project to LIP and VIP, although this is not as solid a results as with MIP, with lower overall label intensity and more variation between animals (Shipp et al. 1998). |
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Frontal:
Shipp et al. found that both V6 and V6A project to Brodmann's area 6, which is premotor cortex. These axons project primarily to PMdr, with weaker projections to PMv and the medial wall (SMA). This is interesting, especially given that most visual areas that project to frontal cortex (such as LIP) project mostly to FEF. "Suggests that V6/V6A is used for skeletomotor rather than oculomotor output." (Shipp et al. 1998). Retrograde tracer injection to PMdr labeled V6A, whereas injection into PMdc (Caminiti et al. 1999). |
Interesting issue: hierarchy
One of the more interesting things in Galletti et al. 1996 and Shipp et al. 1998 is an argument that V6 and V6A are actually at different levels of the cortical hierarchy. This is based on doing tracer injections confined to the dorsal part of posterior SPG (V6A) or the central part (V6) and looking at the pattern of terminal labelling across SPG. Laminar patterns of terminal labels can be designated "higher" (ascending in hierarchy), "intermediate," or "lower" (descending). These authors concluded that V6A is a "higher" area than V6. Although V6 and V6A have a similar sphere of target/input areas, they can be differentiated by hierarchical level: V6: Below V6A, equal to V4, V5 (MT). V6A: Above V6, equal to MST, LIP. See either Shipp et al. 1998 or Galetti et al. 1996 for a defense of inferring hierarchy from laminar patterns of terminal label, this seems to be something developed in the Shipp/Zeki lab. |
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Intrinsic:
V6 and V6A are strongly interconnected, and the laminar distribution of terminals suggests that V6A is above V6 in the hierarchy. |
Coming soon!
Caminiti, R.; Genovesio, A; Marconi, B; Mayer, AB; Onorati, P; Ferraina, S; Mitsuda, T; Giannetti, S; Squatrito, S; Maioli, MG; Molinari, M. Early coding of reaching: frontal and parietal association connections of parieto-occipital cortex. European Journal of Neuroscience, 1999 Sep, 11(9):3339-45. Retrograde tracer injections into V6A, PMdr, and PMdc.
Galletti, C; Battaglini, PP; Fattori, P. Parietal neurons encoding spatial locations in craniotopic coordinates. Experimental Brain Research, 1993, 96(2):221-9. First report of "real position cells" in PO. For some reason Galletti and friends don't ever show spike data from these cells in subsequent papers, but you can find lots of them here.
Galletti, C; Battaglini, PP; Fattori, P. Eye position influence on the parieto-occipital area PO (V6) of the macaque monkey. European Journal of Neuroscience, 1995 Dec 1, 7(12):2486-501.
Galletti, C; Fattori, P; Battaglini, PP; Shipp, S; Zeki, S. Functional demarcation of a border between areas V6 and V6A in the superior parietal gyrus of the macaque monkey. European Journal of Neuroscience, 1996 Jan, 8(1):30-52. Important paper, first differentiation of V6 from V6A based on physiological and anatomical criteria.
Galletti, C; Fattori, P; Kutz, DF; Gamberini, M. Brain location and visual topography of cortical area V6A in the macaque monkey. European Journal of Neuroscience, 1999a Feb, 11(2): 575-82.
Galletti, C; Fattori, P; Kutz, DF; Gamberini, M. Brain location and visual topography of cortical area V6A in the macaque monkey. European Journal of Neuroscience, 1999b Feb, 11(2): 575-82.
Galletti, C; Fattori, P; Kutz, DF; Gamberini, M. Brain location and visual topography of cortical area V6A in the macaque monkey. European Journal of Neuroscience, 1999c Nov, 11(11): 3922-36.
Lewis, JW; Van Essen, DC. Mapping of architectonic subdivisions in the macaque monkey, with emphasis on parieto-occipital cortex. Journal of Comparative Neurology, 2000 Dec 4, 428(1): 79-111.
Nakamura, K; Chung, HH; Graziano, MS; Gross, CG. Dynamic representation of eye position in the parieto-occipital sulcus. Journal of Neurophysiology, 1999 May, 81(5): 2374-85. Looks at temporal dynamics of eye position effects.
Shipp, S; Blanton, M; Zeki, S. A visuo-somatomotor pathway through superior parietal cortex in the macaque monkey: cortical connections of areas V6 and V6A. European Journal of Neuroscience, 1998 Oct, 10(10): 3171-93. The anatomy bible for V6/V6A. Table 2 is a qualitative chart of WGA-HRP (antero/retro tracer) found in ~20 frontal and parietal areas after injection into V6, V6A, or both.