In the ventrolateral frontal lobe of the human brain there is a distinct entity, cytoarchitectonic area 44 (Broca's area), which is crucial in speech production. There has been controversy over whether monkeys possess an area comparable to human area 44. We have addressed this question in the macaque monkey by combining quantitative architectonic analysis of the cortical areas within the ventrolateral frontal region with electrophysiological recording of neuron activity and electrical intracortical microstimulation. Here we show that, immediately in front of the ventral part of the agranular premotor cortical area 6, there is a distinct cortical area that is architectonically comparable to human area 44 and that this monkey area 44 is involved with the orofacial musculature. We suggest that area 44 might have evolved originally as an area exercising high-level control over orofacial actions, including those related to communicative acts, and that, in the human brain, area 44 eventually also came to control certain aspects of the speech act.

1. Three men and seven women, 25-40 yr of age, were asked to use the thumb and index fingers to grasp, lift, and hold the armature of a linear motor generating a 2.0-N opposing force (simulating an object weighing approximately 200 g) for 2 s. The surface in contact with the fingers was composed of smooth or polyamide plastic etched with 1.0-mm high Braille beads separated at 2.0- or 3.0-mm intervals measured from apex to apex. The surfaces were left either untreated or coated with talc, water, or sucrose films designed to change the coefficient of friction with the skin. Talc reduced the coefficient of friction, whereas water and sucrose both increased the friction against the skin. In all, 12 surface conditions were used to evaluate the effects of texture and friction on the grip force during lifting and holding. 2. For all subjects the inverse coefficient of friction was associated with proportionately scaled increases in grip force, regardless of surface texture. The peak lifting force as well as the static force used to hold the object stationary were significantly correlated with the inverse of the coefficient of friction. When coatings were applied to dissimilar surface textures to produce similar coefficients of friction, the grip force profiles were nearly identical. When strong adhesives increased the friction of the smooth surface compared with textured surfaces, grip forces decreased as friction increased. That is, although the untreated smooth surface had less friction than either of the two textured surfaces, the addition of sucrose increased the smooth surface friction to a higher level than either of the similarly treated textured surfaces. As a result, the effect of surface friction could be dissociated from the effect of either surface texture or coating. Friction appears to be a more important factor in determining the grip force than either texture or surface films at least for the range of textures and coatings examined in this study.

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The present study examined the role of the prefrontal cortex in retrieval processing using functional magnetic resonance imaging in human subjects. Ten healthy subjects were scanned while they performed a task that required retrieval of specific aspects of visual information. In order to examine brain activity specifically associated with retrieval, we designed a task that had retrieval and control conditions that were perfectly matched in terms of depth of encoding, decision making and postretrieval monitoring and differed only in terms of whether retrieval was required. In the retrieval condition, based on an instructional cue, the subjects had to retrieve either the particular stimulus that was previously presented or its location. In the control condition, the cue did not instruct retrieval but shared with the instructional cues the function of alerting the subjects of the impending test phase. The comparison of activity between the retrieval and control conditions demonstrated a significant and selective increase in activity related to retrieval processes within the ventrolateral prefrontal cortical region, more specifically within area 47/12. These activity increases were bilateral but stronger in the right hemisphere. The present study by strictly controlling the level of encoding, postretrieval monitoring, and decision making has demonstrated a specific increase in the ventrolateral prefrontal region that could be clearly related to active retrieval processing, i.e. the active selection of particular stored visual representations.

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The aim of this study was to determine whether relatively long-term changes in skin friction induced by a pharmacological blockade of sweat excretion would alter the grip forces applied to objects of a variety of different surface textures and frictions. Five men and three women were asked to lift the vertically mounted armature of a linear motor between the thumb and index finger and to hold it against an opposing force for 2 s. A 1.0-kHz tone indicated to the subject that the manipulandum had been correctly positioned between the upper and lower position limits. The linear motor generated a 2.5-N force tangential to the skin surface simulating an object weighing approximately 250 g. Three different polyamide plastic surfaces (either smooth or etched with 1.0 mm high Braille beads evenly spaced at 2- or 3-mm intervals) contacted the fingers in these experiments. Subjects lifted and held in a precision grip one of the three surfaces for blocks of ten consecutive trials, but the order of presentation of the three different textures was varied to offset the effect of expectancy. On a second block of ten trials the subjects were requested to release the object slowly to measure the ratio of the grip force normal to the grasped surface to the tangential load force at the moment of slip. This ratio or its inverse provided the coefficient of friction or the slip ratio for a particular subject and surface condition. Twelve hours prior to a second recording session all subjects placed transdermal patches of 1.5 mg scopolamine behind each ear to reduce palmar sweating by blocking the muscarinic receptors of exocrine sweat glands. The subjects were re-tested following procedures that were identical to the first session. Scopolamine significantly reduced the friction of the skin on the smooth and 2-mm beaded surfaces, but the friction of the 3-mm beaded texture was unaffected. Scopolamine also caused subjects to increase both the peak and static grip forces for all the textures including the 3-mm beaded surface, suggesting that for two of the three surfaces they were responding to the increased slipperiness of the skin due to reduced sweat production.

Two monkeys were trained to use the thumb and forefinger to lift and hold an instrumented apparatus within a narrow position window for 1 s. The device was equipped to measure the position and the grip and lifting forces exerted by the animal. On blocks of trials the weight and surface texture could be varied or a force-pulse perturbation could be systematically delivered 750 ms after the object entered the window. If unopposed, the perturbation would displace the hand from the position window, and in preparation for this perturbation the monkeys either increased their grip force before the perturbation or raised the object higher within the position window. Two clearly separated clusters of cells in the medial wall of the frontal lobe were found to be active in relation to the task. One group of cells (n = 115) was located in the caudal and medial part of area 6, in the supplementary motor area (SMA), and the other (n = 92) was located in the ventral bank of the cingulate sulcus (CMAv), in area 23c. In each area, neurons were characterized by their sensorimotor features clearly related to the hand in addition to their modulated activity in the task. In the SMA, 71% (42 of 59) of the neurons tested for receptive fields responded to peripheral and mainly proprioceptive stimulation, and 71% of them (30 of 42) received inputs from the hand. In the CMAv, 77% (48 of 62) of the neurons responded to peripheral proprioceptive stimulation, and 77% (37 of 48) exhibited receptive fields originating from the hand. Intracortical microstimulation applied to 43 sites in the SMA evoked discrete hand movements at 12 loci, whereas in the CMAv hand movements were observed at 8 of 27 sites tested with an average threshold of > 15 microA. A strong similarity was observed between the SMA and CMAv neurons in their sensorimotor features as well as the modulation of their activity in relation to the prehension task. In both areas the activity was poorly related to grip force and significant correlation with peak grip force was observed for only 9 and 7% of the CMAv and SMA neurons, respectively. In the SMA only five cells exhibited increased activity before the perturbation and in the CMAv no changes in activity were found despite the presence of clear preparatory increases in grip force in anticipation of the perturbation. The perturbation evoked reflexlike excitation of 38% (25 of 65) of the neurons in the CMAv and 28% (20 of 71) of the cells in the SMA; these cells were similar in magnitude and latency (approximately 50 ms) in both areas. In both the SMA and CMAv, most of the neurons increased their firing rate < 200 ms before the grip force onset and the overlap in the distribution of neuronal response times suggests a parallel activation of the SMA and CMAv neurons during the prehension task.

It is controversial whether monkeys, like human subjects, can recall, upon instruction, specific information about an event in memory. We therefore tested macaque monkeys on a task that was originally developed to study such active controlled memory retrieval in human subjects and we were able to demonstrate that monkeys, like human subjects, can retrieve, upon command, specific components of previously encoded events. Furthermore, following earlier functional neuroimaging work with human subjects showing the mid-ventrolateral prefrontal cortex to be involved in such active controlled retrieval, we recorded single-neuron activity within this region of the monkey brain while the monkeys performed the active retrieval task. Neuronal responses were related to the retrieval and the decision whether the retrieved information was the instructed one. These findings demonstrate, for the first time, an impressive capacity by macaque monkeys for controlled memory retrieval and, in addition, provide neurophysiological evidence about the role of the mid-ventrolateral prefrontal cortex in such controlled retrieval.