The argument against the distinct area-ness of 3a is in its transition type behavior - it looks and acts like an area that is halfway between sensory and motor, which it literally is, located in the depth of the central sulcus. There are several arguments in the primate against this assertion, which are highlighted below as we get to them, but for a central sulcus area this region has been slow to yield to clear definition. For a great review of this history of the attempt to define this region, see Jones and Porter (1980) "What is Area 3a?"
The suprathreshold responses of this region are deep and proprioceptive, as Powell and Mountcastle (1959) were among the first to observe.
About 10% of neurons give cutaneous responses and 90% proprioceptive/deep responses (e.g., Taoka et al., 1998; Iwamura (many papers)), which a preponderance of Type I muscle afferent responses (Oscarsson and Rosen, 1966). These inputs (muscle spindles) are key in the perception of arm position, which is why vibration of the tendon in the elbow can create the illusion of arm movement (e.g., see McCloskey 1982ish for a review).
Antidromic stimulation from M1 shows that while only 10% of 3a neurons show suprathreshold responses to cutaneous and deep inputs, about 60% of MI-projecting 3a neurons receive convergent "multi-modal" inputs (reviewed in Zarzecki, 1986). This could be important for plasticity.
Ramp and hold stimuli (Wise and Tanji, 1981) are both effective in activating area 3a, and there tends to be directionality in the response to the ramp in area 3a. Other groups have emphasized the importance of the ramp (Hore et al., 1976).
One argument for its unique-ness as an area is that 3a is an island between the tactile sensory representations of area 3b and area 4 posterior (which has suprathreshold action potential receptive fields) - this suggests that the reception fields here are not simply spill over from either "sensory" or "motor" sides of the strip but are a distinct representation in their own right.
Vestibular Reponses
In the neck muscle regions, about 30-50% of neurons receive vestibular input (in Squirrel Monkey) (see Guldin and Grusser, 1998 and Fredrickson and Rubin, 19-- for reviews). This area has been named 3aV and is one of 3 cortical areas that project to the vestibular nuclei.
Pain/Thermal Stimuli
Tommerdhal et al (1995; 1998) have observed that thermal/painful stimuli uniquely activate area 3a (as opposed to 3b and 1). If true, this could be a major finding, as 3b neurons are curiously devoid of pain input (e.g., Kenshalo's work) - it's surprising that no one found this type of results prior to 1996.
Spike-triggered averaging of area 3a activity was only very rarely (1 cell) associated with EMG activity.
Receives input from the distinct anterior and dorsal "shell" region of the VPL thalamic nucleus, an area with deep and proprioceptive receptive fields. The anterior portion of this shell projects exclusively to 3a, and the posterior to 3a and 2 (Friedman and Jones, 1981; Jones, 1983; see Jones, 1986 for a review). In contrast, 3b and 1 receive input from the cutaneous "core" nucleus. 3a also received input from the anterior pulvinar, and CM nuclei.
Receives input from: 3b, 1, 2, 4 (Note, this is NOT reciprocal), SII, insular cortex, SMA, cingulate motor cortex, and 5 (Darian-Smith et al., 1993b; Jones, 1986 for a review), contralateral input from 1 and 2 (Shanks, et al., 1985)
Sends output to: 3b, 1, 2, SII, SMA
Vestibular nuclei, dorsal column nuclei, and dorsal horn of the spinal cord, although these projections are less dense than those from 3b, 1 and 2 (eg only 2% of contralateral projections in Galea and Darian-Smith, 1994, as compared with ~10% in 3b/1 and 5 and 50% in area 4). Darian-Smith et al. (1993a, b) have mapped many of the connections described above, and rightly point out that area 3a receives sensory input from the thalamus, but its non-local cortico-cortical input is from primarily motor-like areas.
Extension training on a temporal discrimination (Recanzone et al., 1992) leads to the expansion/emergence of cutaneous receptive fields in area 3a for the relevant training surfaces. Lesions of area 3b (Xerri et al., 1998) in monkeys trained to perform a pellet-grasping task also lead to expansion into 3a, most dramatically of trained finger surfaces.
"Lesion, electrophysiological and neuroanatomical studies of the human central sulcus region have provided inconclusive evidence for the existence of a distinct cortical area 3a, and/or of a distinct region encoding proprioceptive information. Head (1920) concluded from the psychophysical evaluation of patients with cerebral lesion that a cortical proprioceptive area existed in the central sulcus region, independent of the representation of tactile perception. Penfield and Rasmussesn (1950) and Corkin et al., (1970) also reported deficits in position sense following excisions of the PoCG hand area. Penfield and Rasmussen (1950) further observed that kinesthetic sensations were frequently reported following electrical stimulation of the PoCG. These authors, however, did not observe the segregation of distinct somatosensory representations in the central sulcus region for proprioceptive or for tactile processing. The cytoarchitectonic definition of area 3a in the human (and monkey) has varied across researchers (see Jones and Porter, 1980, for a review), and has less rigid cytoarchitectonic borders than other regions within the central sulcus, (Jones and Porter, 1980; White et al., 1997; but see Geyer et al., 1997). Passive movement of the arm has been reported to activate the Rolandic cortex in the human (Weiller et al., 1996), but a recent PET study of the vibration-induced motion illusion of the arm failed to activate area 3a (Naito et al., 1999)."
Our work shows a clear delineation of an area in the depth of the central sulcus that is not responsive to punctate tactile input but is responsive to volitional movement of the hand (Moore et al., 2000). We have since replicated this result with vibrotactile input to the fingertip and palm, which never activates "3a" in the awake behaving human preparation...