Saccade preparation signals in the human frontal and parietal cortices

Our ability to prepare an action in advance allows us to respond to our environment quickly, accurately, and flexibly. Here, we used event-related functional MRI to measure human brain activity while subjects maintained an active state of preparedness. At the beginning of each trial, subjects were instructed to prepare a pro- or antisaccade to a visual cue that was continually present during a long and variable preparation interval, but to defer the saccade's execution until a go signal. The deferred saccade task eliminated the mnemonic component inherent in memory-guided saccade tasks and placed the emphasis entirely on advance motor preparation. During the delay while subjects were in an active state of motor preparedness, the blood oxygen level-dependent signal in the frontal cortex showed 1) a sustained elevation throughout the preparation interval; 2) a linear increase with increasing delay length; 3) a bias for contra- rather than ipsiversive movements; 4) greater activity when the specific metrics of the planned saccade were known compared with when they were not; and 5) increased activity when the saccade was directed toward an internal versus an external representation (i.e., anticue location). These findings support the hypothesis that both the human frontal and parietal cortices are involved in the spatial selection and preparation of saccades.     

Acetylcholinesterase histochemistry in the macaque thalamus reveals territories selectively connected to frontal, parietal and temporal association cortices

The patterns of histochemical staining for acetylcholinesterase (AChE) activity in the macaque thalamus were analyzed and compared with the distribution of cells and terminals labeled from injections of axonal tracers in the dorsolateral and orbital prefrontal cortex, in area 7a of the posterior parietal cortex and in the polysensory cortex of the superior temporal sulcus. AChE histochemistry is very useful in delineating the thalamic nuclei connected with the association cortex and in uncovering thalamic subdivisions that are barely evident on cytoarchitectonic grounds. Moreover, AChE activity reveals previously unrecognized heterogeneities within several thalamic nuclei, like the ventral anterior (VA), where a new ventromedial subdivision (VAvm) is described, the medial pulvinar (PuIM) or the mediodorsal nucleus (MD). In this nucleus three distinct chemical domains are present: the medial, ventral and lateral sectors characterized by low, moderate and high AChE activities, respectively. The staining pattern of the lateral sector is markedly heterogeneous with patches of intense AChE activity surrounded by a moderately stained matrix. The MD medial sector is connected with the orbitofrontal cortex, whereas the AChE-rich patches in the lateral sector are selectively connected with the dorsolateral prefrontal, parietal and temporal association cortices. In the PulM, a dorsomedial AChE-rich patch is selectively connected with the orbitofrontal cortex, whereas the surrounding territory, which shows moderate AChE activity, is preferentially connected with the parietal and temporal cortices. Chemically specific domains in the anterior, ventral anterior, midline, and intralaminar thalamic nuclei are also connected with the examined association cortices. These findings indicate that the topographic patterns of the thalamo-cortical connections of primate association areas conform to the chemical architecture of the thalamus. This implies that because each cortical area is connected to a particular set of thalamic regions, the influence of the thalamus on cortical function is exclusive for each area, highly diverse among the various association areas, and subject to a wide range of modulation at the thalamic level.     

Contributions of human parietal and frontal cortices to attentional control during conflict resolution: a 1-Hz offline rTMS study

Various brain regions contribute to aspects of attentional control in conflict resolution. Here, we used transcranial magnetic stimulation (TMS) to examine the functions of posterior parietal cortex (PPC) and dorsal medial frontal cortex (dMFC) in a visual flanker task. Participants responded to a central target that was flanked by congruent, neutral or incongruent stimuli on the left or right. Offline low-frequency repetitive TMS (1 Hz, 110% motor threshold, 20 min) was applied to right PPC or dMFC. Performance, as measured by reaction times and accuracy, was established at baseline, after rTMS, and sham stimulation before or after active rTMS. After rTMS to right PPC, the interference of flankers presented in the left visual hemispace diminished selectively. By contrast, after rTMS over the right dMFC, flanker effects in both visual fields remained. Our results suggest that right PPC specifically contributes to the assignment of spatial attention during stimulus encoding.     

Regional cerebral blood flow in the frontal, parietal and occipital cortices increases independently of systemic arterial pressure during slow walking in conscious rats

Regional cerebral blood flow (rCBF) in the frontal, parietal and occipital cortices was measured using laser Doppler flowmetry during walking in conscious rats at moderate speed on a treadmill (4 cm/s) for a 30 s period. During walking rCBF increased in all these three cortices. The rCBF in the parietal cortex started to increase within a few seconds after the start of walking, and continued to increase 42 ± 16% (mean ± S.D.) until the end of walking. Within 90 s after walking had ceased, the increased rCBF returned to pre-walking basal levels. The rCBF responses in the frontal and occipital cortices were identical to that in the parietal cortex. Mean arterial pressure (MAP) in a caudal artery of the tail during walking was increased by about 10%. Injection of atropine (5 mg/kg, i.p.), a muscarinic cholinergic receptor antagonist that permeates the blood brain barrier (BBB), reduced the walking-induced increase in cortical rCBF, as determined by measurement of parietal rCBF, from 42 ± 12% to 28 ± 15%. However, injection of methylatropine (5 mg/kg, i.p.), a muscarinic cholinergic receptor antagonist that does not permeate the BBB, did not affect the response of rCBF. Neither drug affected the walking-induced response of MAP. Injection of mecamylamine (20 mg/kg, s.c.), a nicotinic cholinergic receptor antagonist that permeates the BBB, reduced the walking-induced increase in cortical rCBF from 47 ± 12% to 30 ± 12%. Injection of hexamethonium (20 mg/kg, s.c.), a nicotinic cholinergic receptor antagonist that does not permeate the BBB, did not affect the responses of rCBF. Both mecamylamine and hexamethonium decreased the resting basal MAP by about 50%, and abolished the walking-induced increase in MAP. It is suggested that the increase in cortical rCBF during walking, which is independent of MAP, is attributable to the walking-associated activation of cortical cholinergic nerves. Possible contribution of cholinergic neurons in the nucleus basalis of Meynert (NBM) to the walking-induced increase in cortical rCBF is discussed.     

Evidence for context sensitivity of grasp representations in human parietal and premotor cortices

Grasp-related responses in neurons of the macaque rostral inferior parietal lobule [PF/PFG and the anterior intraparietal area (AIP)] are modulated by task context. Event-related functional MRI was used to determine whether this is true in putative homologs of the human cortex, the rostral inferior parietal lobule (rIPL) and the anterior intraparietal sulcus (aIPS). Fifteen healthy, right-handed adults were required to select prospectively the most comfortable way to grasp a horizontally oriented handle using the cued hand (left or right). In the "no-rotation" condition, the task was simply to grasp the handle, whereas in the "rotation" condition, the goal was to plan to grasp and rotate it into a vertical orientation with the cued end (medial or lateral) pointing downward. In both conditions, participants remained still and indicated their grip preferences by pressing foot pedals. As in overt grasping, participants' grip preferences were significantly influenced by anticipation of the demands associated with handle rotation. Activity within the aIPS and rIPL increased bilaterally in both the rotation and no-rotation conditions. Importantly, these responses were significantly greater in the rotation vs. no-rotation condition. Similar context effects were detected in the presupplementary motor area, caudal intraparietal sulcus/superior parietal lobule, and bilateral dorsal and left ventral premotor cortices. Grasp representations within the rIPL and aIPS are sensitive to predicted task demands and play a role in context-sensitive grip selection. Moreover, the findings provide additional evidence that areas involved in the sensorimotor control of grasp also contribute to feedforward planning.     

Modulation of activity in medial frontal and motor cortices during error observation

We used measures of the human event-related brain potential (ERP) to investigate the neural mechanisms underlying error processing during action observation. Participants took part in two conditions, a task execution condition and a task observation condition. We found that activity in both the medial frontal cortex and the motor cortices, as measured via the error-related negativity and the lateralized readiness potential, respectively, was modulated by the correctness of observed behavior. These data suggest that similar neural mechanisms are involved in monitoring one's own actions and the actions of others.     

Evidence for entorhinal and parietal cortices involvement in path integration in the rat

Rats with lesions of the entorhinal or parietal cortex were tested in a homing task on a circular platform containing food cups and surrounded by curtains. The animals had to leave a refuge, explore the platform to find a hidden piece of food and carry it back to the refuge. Once the rats were proficient at performing the procedural aspects of the task, they were tested in two successive types of trials in which the food pellet was either always located in the central cup (food at center, FAC trials) or placed in a randomly chosen cup (food at random, FAR trials). Except in the first FAC trials, all groups displayed similar outward paths in FAC and FAR trials, showing that both types of trials involved equivalent path integration demand. Analysis of the homing accuracy showed that rats with entorhinal cortex or parietal cortex lesions exhibited inaccurate returns to the starting hole, suggesting that these two cortical areas are part of a neural network mediating path integration.     

Contributions of the retrosplenial and posterior parietal cortices to cue-specific and contextual fear conditioning

The retrosplenial cortex (RSP) and the posterior parietal cortex (PPC) are the primary sources of cortical sensory input to the postrhinal cortex (POR) in rodents. Together, these areas compose a major corticohippocampal circuit that is involved in processing visuospatial information. The POR has been implicated in contextual learning and memory, consistent with the type of information presumably being processed by this region. By comparison, little is known about the role of the RSP or the PPC in contextual learning. In the present study, rats were trained either before or after surgery in a standard signaled fear conditioning task in which an auditory cue was paired with foot shock. Contextual fear and tone-specific fear were assessed in subsequent test sessions. In Experiment 1, electrolytic damage to the RSP either before or immediately after training impaired the expression of contextual fear but not tone-specific fear. In contrast, electrolytic damage to the PPC had no effect on conditional fear to the context or the tone in Experiment 2. These findings indicate that the RSP, but not the PPC, contributes to the processing of contextual information by the POR corticohippocampal processing stream.     

The role of inferior parietal and inferior frontal cortex in working memory

Verbal working memory involves two major components: a phonological store that holds auditory-verbal information very briefly and an articulatory rehearsal process that allows that information to be refreshed and thus held longer in short-term memory (A. Baddeley, 1996, 2000; A. Baddeley & G. Hitch, 1974). In the current study, the authors tested two groups of patients who were chosen on the basis of their relatively focal lesions in the inferior parietal (IP) cortex or inferior frontal (IF) cortex. Patients were tested on a series of tasks that have been previously shown to tap phonological storage (span, auditory rhyming, and repetition) and articulatory rehearsal (visual rhyming and a 2-back task). As predicted, IP patients were disproportionately impaired on the span, rhyming, and repetition tasks and thus demonstrated a phonological storage deficit. IF patients, however, did not show impairment on these storage tasks but did exhibit impairment on the visual rhyming task, which requires articulatory rehearsal. These findings lend further support to the working memory model and provide evidence of the roles of IP and IF cortex in separable working memory processes.     

Top-down and bottom-up attentional guidance: investigating the role of the dorsal and ventral parietal cortices

Recent neuroimaging studies suggest that the superior parietal lobule (SPL) of the human cortex mediates goal-directed attentional orienting, while the temporo-parietal junction (TPJ) mediates stimulus-driven attentional orienting. Here, we investigated these brain-behavior correspondences by examining the performance of patients with an attentional deficit following a right hemisphere lesion. Patients completed two tasks, one sensitive to stimulus-driven attentional orienting and the other to goal-directed attentional orienting. Based on the behavioral profiles obtained on each task, patients were assigned to different groups and their lesion overlap explored. Patients who exhibited difficulties with goal-directed attentional orienting and showed concurrent “hyper-capture” presented with lesion overlap centered over superior portions of the parietal lobule with spared inferior portions of the parietal lobule. Patients who performed normally on the goal-directed orienting task, while remaining abnormally immune to attentional capture, presented with lesion overlap centered over the inferior portions of the parietal lobule but spared superior parietal lobule. The findings from this study clearly suggest that (a) SPL and TPJ are anatomical regions that are recruited for the purposes of top-down and bottom-up orienting, respectively, and that damage to SPL and TPJ leads to disorders of top-down and bottom-up orienting, and (b) albeit dissociable, top-down and bottom-up orienting (and, by extension, SPL and TPJ) are not entirely independent mechanisms.     

A frontal but not parietal neural correlate of auditory consciousness

Hemodynamic correlates of consciousness were investigated in humans during the presentation of a dichotic sequence inducing illusory auditory percepts with features analogous to visual multistability. The sequence consisted of a variation of the original stimulation eliciting the Deutsch’s octave illusion, created to maintain a stable illusory percept long enough to allow the detection of the underlying hemodynamic activity using functional magnetic resonance imaging (fMRI). Two specular 500 ms dichotic stimuli (400 and 800 Hz) presented in alternation by means of earphones cause an illusory segregation of pitch and ear of origin which can yield up to four different auditory percepts per dichotic stimulus. Such percepts are maintained stable when one of the two dichotic stimuli is presented repeatedly for 6 s, immediately after the alternation. We observed hemodynamic activity specifically accompanying conscious experience of pitch in a bilateral network including the superior frontal gyrus (SFG, BA9 and BA10), medial frontal gyrus (BA6 and BA9), insula (BA13), and posterior lateral nucleus of the thalamus. Conscious experience of side (ear of origin) was instead specifically accompanied by bilateral activity in the MFG (BA6), STG (BA41), parahippocampal gyrus (BA28), and insula (BA13). These results suggest that the neural substrate of auditory consciousness, differently from that of visual consciousness, may rest upon a fronto-temporal rather than upon a fronto-parietal network. Moreover, they indicate that the neural correlates of consciousness depend on the specific features of the stimulus and suggest the SFG-MFG and the insula as important cortical nodes for auditory conscious experience.     

Modality-specific frontal and parietal areas for auditory and visual spatial localization in humans.

Although the importance of the posterior parietal and prefrontal regions in spatial localization of visual stimuli is well established, their role in auditory space perception is less clear. Using positron emission tomography (PET) during auditory and visual spatial localization in the same subjects, modality-specific areas were identified in the superior parietal lobule, middle temporal and lateral prefrontal cortices. These findings suggest that, similar to the visual system, the hierarchical organization of the auditory system extends beyond the temporal lobe to include areas in the posterior parietal and prefrontal regions specialized in auditory spatial processing. Our results may explain the dissociation of visual and auditory spatial localization deficits following lesions involving these regions.     

Damage to the frontal and parietal lobes of the brain and conditioned reflexes in unconstrained subjects

Summary Higher nervous activity and motor disturbances were studied in three unconstrained dogs before and after in jury to the frontal or parietal lobes. There was no difference in the recovery time of conditioned reflex activity as between dogs with frontal or parietal lobe injury. However, damage to the frontal lobes led to motor disturbances and prolonged motor excitation, and the animals moved about aimlessly. These effects were observed both in the laboratory and in the animal house.(Presented by Academician V. N. Chernigovskii) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 55, No. 4, pp. 22–25, April, 1963     

The role of inferior frontal gyrus and inferior parietal lobule in semantic processing of Chinese characters

Functional magnetic resonance imaging was used to explore the neural correlates of semantic judgments to Chinese characters. Adult participants were asked to indicate if character pairs were related in meaning that were arranged in a continuous variable according to association strength. This parametric manipulation allowed for a more precise determination of the role of the left inferior parietal lobule in processing meaning, which has not been reported in previous Chinese studies. Consistent with previous findings in English, participants showed activation in left inferior frontal gyrus (BA 47, 45) and left posterior middle temporal gyrus (BA 21). Characters with stronger semantic association elicited greater activation in left inferior parietal lobule (BA 39), suggesting stronger integration of highly related semantic features. By contrast, characters with weaker semantic association elicited greater activation in both an anterior ventral region (BA 47) and a mid-ventral region of left inferior frontal gyrus (BA 45), suggesting a controlled retrieval process and a selection process. Our findings of association strength are discussed in a proposed neuro-anatomical model of semantic processing.     

Dissociation of visual, motor and predictive signals in parietal cortex during visual guidance

The role of the posterior parietal cortex (PPC) in the visual guidance of movements was studied in monkeys trained to use a joystick to guide a spot to a target. Visual and motor influences were dissociated by transiently occluding the spot and by varying the relationship between the direction of joystick and spot movements. We found a strong segregation of function in PPC during visual guidance. Neurons in area MST were selectively modulated by the direction of visible moving stimuli, whereas neurons in area MIP were selectively modulated by the direction of hand movement. In contrast, the selectivity of cells in the lateral intraparietal area (LIP) did not directly depend on either visual input or motor output, but rather seemed to encode a predictive representation of stimulus movement. These predictive signals may be an important link in visuomotor transformations.     

Reduced volume of parietal and frontal association areas in patients with schizophrenia characterized by passivity delusions

BACKGROUND: In patients with schizophrenia, passivity delusions are characterized by a difficulty in determining the agency of purposive actions. Neuropsychological and functional neuroimaging data suggest that passivity delusions are associated with dysfunction of the parietal lobe association cortex. METHOD: Cortical volume calculated from magnetic resonance imaging data in a group of 12 patients with schizophrenia characterized by motor passivity delusions was compared statistically with the cortical volume of 11 patients without passivity delusions. RESULTS: Reduced cortical volume was observed in parietal and frontal association cortices in the passivity group. CONCLUSIONS: These data provide direct evidence for the involvement of the parietal lobe in the pathophysiology of passivity delusions in schizophrenia.     

Scopolamine impairs memory performance and reduces frontal but not parietal visual P3 amplitude

It has been suggested that the P3 event-related potential (ERP) may mark the operation of certain working or long-term memory processes. It has also been reported that cholinergic blockade by scopolamine induces significant memory impairment and is associated with an increased latency, as well as amplitude reduction or abolition of the auditory P3, thus supporting hypothesised links between P3 and long-term memory function. An intriguing anomaly is that, while visual P3 latency is also increased by scopolamine, amplitude is not changed. The aim of this study was to make a more detailed assessment of the effects of scopolamine on the visual P3 at a drug dose known to induce memory impairment. After drug administration, memory performance was significantly impaired and visual P3 latency was significantly increased. There was little evidence of parietal P3 amplitude reduction, but frontal P3 amplitude was significantly reduced in both target and non-target conditions. These findings, when considered in the light of a more recent study of the effects of scopolamine on auditory P3, suggest that cholinergic blockade produces a common effect in both visual and auditory modalities of significant frontal P3 amplitude reduction, but no significant parietal P3 amplitude reduction. These results are consistent with the view that there are modality-independent generators of the parietal and frontal P3. The finding of drug-induced memory impairment and modulations of frontal ERP deflections is also consistent with recent evidence of a significant role for regions of the frontal lobe in encoding and retrieval of long-term memories.