Subcortical Processing Research Articles

Letter processing interferes with inhibition of return: Evidence for cortical involvement

Inhibition of return (IOR) refers to the finding that, when the time lag between a cue and a target is prolonged, the reaction to the target, when it eventually appears, is actually slower than with no cue. This phenomenon is thought to make visual search more efficient, and it is subserved by the left inferior parietal cortex and the supramarginal gyrus bilaterally. Interestingly, the very same brain structures are also involved in letter processing. Accordingly, we asked whether the two mental processes interfere with each other when simultaneously probed. The first experiment used a typical IOR procedure, but the cue/target placeholders were either simple geometric shapes or English letters. The results show that, although IOR is approximately the same across visual fields when shape placeholders are used, it is significantly lessened in the right visual field when letters are used as cue and target placeholders. To examine if this finding was due to potential spatial frequency differences between the placeholders, a second experiment using shapes and Japanese letters was conducted, and no differences in IOR were found. The supramarginal gyrus appears to be the most likely locus for the letter-IOR interference effect because it is active bilaterally in IOR, but only in the left hemisphere during letter processing. These findings provide support for the notion that IOR is not simply due to subcortical processes but also involves processing from cortical structures.

Cross-Orientation Suppression: Monoptic and Dichoptic Mechanisms Are Different

The response of a cell in the primary visual cortex to an optimally oriented grating is suppressed by a superimposed orthogonal grating. This cross-orientation suppression (COS) is exhibited when the orthogonal and optimal stimuli are presented to the same eye (monoptically) or to different eyes (dichoptically). A recent study suggested that monoptic COS arises from subcortical processes; however, the mechanisms underlying dichoptic COS were not addressed. We have compared the temporal frequency tuning and stimulus adaptation properties of monoptic and dichoptic COS. We found that dichoptic COS is best elicited with lower temporal frequencies and is substantially reduced after prolonged adaptation to a mask grating. In contrast, monoptic COS is more pronounced with mask gratings at much higher temporal frequencies and is less prone to stimulus adaptation. These results suggest that monoptic COS is mediated by subcortical mechanisms, whereas intracortical inhibition is the mechanism for dichoptic COS.

Rapid visuo-motor processes drive the leg regardless of balance constraints

Abstract When reaching for an object that unexpectedly moves, the hand is ‘magnetically' drawn toward the object. This occurs even before the subject perceives object motion [1–3], possibly through sub-cortical visuo-motor processes [4]. Whether a similar process exists for the lower limb is open to question. The evolution of a rapid visuo-motor process has obvious advantages for the hand, for example for catching prey, but less obvious advantage for the foot. Rapid visual driving of the foot may even be hazardous because of the need to maintain balance: this is normally maintained during a step by a pre-step predictive throw of the body which is tightly coupled to intended future foot placement [5,6]. Visually driven, mid-step deviations of the foot would upset this coupling and threaten balance. We ask whether the foot, like the hand, is automatically and rapidly driven by vision, and if so whether the process is suppressed by the balance constraint of stepping.

Modulation of involuntary and voluntary behavior following emotional stimuli in healthy subjects

Introduction Alterations of behavior control are core symptoms of various psychiatric disorders. Patients present with changes in complex behavior patterns and basic motor functions. Little is known about emotional modulation of voluntary behavior. Therefore, a paradigm was developed to study simultaneously influence of emotions on voluntary and involuntary movements. Method Healthy volunteers ( n=30) documented aspects of their mood and personality including depression, anxiety, and impulsivity. Individuals were instructed to view color slides of different emotional content and switch off the slide after by pressing a button, which was placed in a standardized distance from the resting point. The startle reflex was elicited while looking at the slides and quantified by EMG. Kinematic measures of hand movements by infrared detection were used to analyze the patterns of voluntary movements. Results This study confirmed previous findings about the modulation of the startle reflex by emotional stimuli which may reflect activity of the amygdala and subcortical stimulus processing. Voluntary movements, which may result from cortical processing of stimuli, were not influenced by the emotional context. In individuals with higher impulsivity scores, the startle reflex amplitudes were lower and relative time to peak velocity of the movement smaller. Conclusion Voluntary movements were not modulated by emotional stimuli, but time to peak velocity was shorter in individuals with greater impulsivity. The ability to generate adequate behavior as a key function of the brain is relevant for social functioning and activities of daily living. The studied paradigm could be useful to assess impulsivity and behavior control in psychiatric disorders.

Stimulus-dominance effects and lateral asymmetries for language in normal subjects and in patients with a single functional hemisphere

The assessment of language laterality by the dichotic fused-words test may be impaired by interference effects revealed by the dominant report of one member of the stimuli-pair. Stimulus-dominance and ear asymmetry were evaluated in normal population (48 subjects of both sex and handedness) and in 2 patients with a single functional hemisphere. Results show that in normals the number of stimulus-dominated responses is five times higher than in patients, and is negatively correlated to the index of laterality. It is suggested that dichotic stimuli may interfere one with another during the subcortical acoustic processing and at cortical level, when competing for verbal output. Subcortical interference accounts for stimulus-dominance in the single-hemisphere patients. In normal subjects, the dichotic discrimination is disturbed mainly during the hemispheric cross-talk needed for the oral–verbal processing of dichotic inputs. The frequency of ‘interhemispheric’ interference, as well as the extents of ear asymmetry, may both depend on differences in the processing stage of the competing inputs, and then they may be influenced by differential verbal skills of LH and RH. Very unequal levels of verbal analysis of the dichotic stimuli may hinder reciprocal interference during the hemispheric cross-talk thus yielding large ear asymmetries associated to small dominance effects.

Decision-making deficits and overeating: a risk model for obesity.

To demonstrate that human overeating is not just a passive response to salient environmental triggers and powerful physiological drives; it is also about making choices. The ventromedial prefrontal cortex has been strongly implicated in the neural circuitry necessary for making advantageous decisions when various options for action are available. Decision-making deficits have been found in patients with ventromedial prefrontal cortex lesions and in those with substance dependence--impairments that reflect an inability to advantageously assess future consequences. That is, they choose immediate rewards in the face of future long-term negative consequences. We extended this research to the study of overeating and overweight, testing a regression model that predicted that poor decision making (as assessed by a validated computerized gambling task) and a tendency to overeat under stress would correlate with higher BMI in a group of healthy adult women (N = 41) representing a broad range of body weights. We found statistically significant main effects for both independent variables in the predicted direction (p < 0.05; R2 = 0.35). Indeed, the decision-making impairments across the 100 trials of the computer task were greater in those with high BMI than in previous studies with drug addicts. Findings suggested that cortical and subcortical processes, which regulate one's ability to inhibit short-term rewards when the long-term consequences are deleterious, may also influence eating behaviors in a culture dominated by so many, and such varied, sources of palatable and calorically dense sources of energy.

Corticofugal modulation of amplitude domain processing in the midbrain of the big brown bat, Eptesicus fuscus

Recent studies have shown that the corticofugal system systematically modulates and improves subcortical signal processing in the frequency, time and spatial domains. The present study examined corticofugal modulation of amplitude sensitivity of 113 corticofugally inhibited neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus. Cortical electrical stimulation decreased the number of impulses and increased the response latency of these neurons. They had an average of 5.9±4.4 kHz best frequency (BF) differences between collicular and electrically stimulated cortical neurons. Cortical electrical stimulation synchronized with sound stimulation for 30 min compressed the rate–amplitude functions of half (56, 49.6%) of these collicular neurons and shifted their minimum thresholds (MT) and dynamic ranges (DR) toward that of electrically stimulated cortical neurons for as long as 40 min. These collicular neurons had an average of 1.6±1.4 kHz BF differences. The shift in collicular MT and DR significantly increased with differences in MT and DR between collicular and cortical neurons. Cortical electrical stimulation also shifted the BF and best amplitude (BA) of collicular neurons toward that of cortical neurons. The BF shift increased with BF differences and the BA shift increased with BA differences. These data suggest that the corticofugal system modulates collicular responses on the basis of topographic projections between the IC and auditory cortex. However, corticofugal modulation of collicular amplitude sensitivity is primarily dependent upon the difference but not the absolute amplitude sensitivity between collicular and cortical neurons.

Temporal specificity of perceptual learning in an auditory discrimination task.

Although temporal processing is used in a wide range of sensory and motor tasks, there is little evidence as to whether a single centralized clock or a distributed system underlies timing in the range of tens to hundreds of milliseconds. We investigated this question by studying whether learning on an auditory interval discrimination task generalizes across stimulus types, intervals, and frequencies. The degree to which improvements in timing carry over to different stimulus features constrains the neural mechanisms underlying timing. Human subjects trained on a 100- or 200-msec interval discrimination task showed an improvement in temporal resolution. This learning generalized to a perceptually distinct duration stimulus, as well as to the trained interval presented with tones at untrained spectral frequencies. The improvement in performance did not generalize to untrained intervals. To determine if spectral generalization was dependent on the importance of frequency information in the task, subjects were simultaneously trained on two different intervals identified by frequency. As a whole, our results indicate that the brain uses circuits that are dedicated to specific time spans, and that each circuit processes stimuli across nontemporal stimulus features. The patterns of generalization additionally indicate that temporal learning does not rely on changes in early, subcortical processing, because the nontemporal features are encoded by different channels at early stages.

Neuropsychology for movement disorders neurosurgery.

The neuropsychologist plays a crucial role in three phases of the neurosurgical treatment of movement disorder patients, namely screening, outcome evaluation and research. In screening patients, the differential diagnosis of dementia, impact of depression or other psychiatric conditions, and the influence of disease and medication-induced symptoms on cognitive performance must be determined. Postoperatively, systematic evaluations elucidate the cognitive costs or benefits of the procedure. The neuropsychologist is then able to provide feedback and counselling to the professional staff, patient and family to inform management strategies. Neuropsychologists also study alteration of cognitive processing due to lesions or stimulation, which, in tandem with functional imaging, shed light on plasticity in cortical and subcortical processing.

Signal Processing Systems for Echolocation in the Bat's Brain

The neural base of the bat's biosonar system is discussed here. The bat emits complex biosonar sounds (pulses) and listens to echoes for orientation and hunting flying insects. Different types of biosonar information are carried by different parameters characterizing pulse-echo pairs. For example, distance information is conveyed by echo delay, while velocity information is carried by Doppler shift. In the auditory cortex of the bat, not only frequency but also other information baring parameters such as echo delay and Doppler shift are systematically mapped as subareas. These computational maps greatly depend on subcortical signal processing. The subcortical auditory nuclei create delay lines and multipliers (or AND gates) for processing distance (echo delay) information, and also create level-tolerant frequency tuning and multipliers (or AND gates) for processing velocity (Doppler shift) information. These multipliers are called FM-FM and CF/CF combination sensitive neurons, respectively. The neurophysiological investigations of the bat's biosonar system provide an excellent database for neural computational models and sonar systems. Application of an agonist of inhibitory neurotransmitter to DSCF or FM-FM area behaviorally revealed the functions of these auditory subareas. Findings made by an on-board telemetry microphone from flying bats confirmed Doppler-shift compensation.

In vivo imaging of anaesthetic action in humans: approaches with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)

In vivo imaging of anaesthetic action in humans: approaches with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)

What is special about the brains of musicians?

What is special about the brains of musicians?

The callosal dilemma: Explaining diaschisis in the context of hemispheric rivalry via a neural network model

It is often suggested that a major factor in diaschisis is the loss of transcallosal excitation to the intact hemisphere from the lesioned one. However, there is long-standing disagreement in the broader experimental literature about whether transcallosal interhemispheric influences in the human brain are primarily excitatory or inhibitory. Some experimental data are apparently better explained by assuming inhibitory callosal influences. Past neural network models attempting to explore this issue have encountered the same dilemma: in intact models, inhibitory callosal influences best explain strong cerebral lateralization like that occurring with language, but in lesioned models, excitatory callosal influences best explain experimentally observed hemispheric activation patterns following brain damage. We have now developed a single neural network model that can account for both types of data, i.e., both diaschisis and strong hemisphere specialization in the normal brain, by combining excitatory callosal influences with subcortical cross-midline inhibitory interactions. The results suggest that subcortical competitive processes may be a more important factor in cerebral specialization than is generally recognized. [Neurol Res 2001; 23: 465-471]

Cortical map asymmetries in the context of transcallosal excitatory influences.

There is long-standing disagreement among experimentalists about whether transcallosal interhemispheric influences are primarily excitatory or inhibitory. Past computational models exploring this issue have encountered a similar dilemma: inhibitory callosal influences best explain hemispheric functional asymmetries, but excitatory callosal influences best explain transcallosal diaschisis. We recently hypothesized that this dilemma might be resolved by assuming excitatory callosal influences and a subcortical mechanism for cross-midline inhibition. Here we explore the feasibility of this hypothesis by examining a model of map formation in corresponding left and right cortical regions. The results show for the first time that both map asymmetries and diaschisis-like changes can be produced in a single model, suggesting that subcortical inhibitory processes may contribute more to asymmetric cortical functionality than is generally recognized.

Corticofugal inhibition compresses all types of rate-intensity functions of inferior collicular neurons in the big brown bat

Recent studies have shown that the auditory corticofugal system modulates and improves signal processing in the frequency, time and spatial domains. In this study, we examine corticofugal modulation of rate-intensity functions of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, by electrical stimulation in the primary auditory cortex (AC). Cortical electrical stimulation compressed all types of rate-intensity functions so as to increase the slope but decrease the dynamic range of IC neurons. Cortical electrical stimulation also shifts the responsive intensity of IC neurons to higher levels. These data indicate that corticofugal modulation also improves subcortical signal processing in intensity domain. The implication of these findings to bat echolocation is discussed.

Neuroimaging of chronic pain: phantom limb and musculoskeletal pain.

New developments in brain imaging lead to a better understanding of cortical and subcortical processes involved in pain perception and the establishment of chronic pain. For different forms of chronic pain long-term changes in cortical structures have been described. In patients with phantom limb pain and back pain alterations in the somatotopic organization of the primary somatosensory (SI) could be observed. The amount of this reorganization is correlated with the subjective pain rating. These changes, which are based on processes of neuronal plasticity, can partially be reversed by analgesic interventions. For the investigation of cortical processes concerning reorganization, EEG and MEG methods are most suitable because of their high temporal and spatial resolution. In conclusion, these findings open a new way for therapeutic interventions to prevent the development of chronic pain.

Pavlovian aversive and appetitive odor conditioning in humans: subjective, peripheral, and electrocortical changes.

Differential Pavlovian conditioning of aversive and appetitive odors was examined in 30 male healthy subjects. The appetitive conditioning group (n=15) received a pleasant odor (vanilla), the aversive conditioning group (n=15) an unpleasant odor (fermented yeast) as unconditioned stimulus. Slides of two different neutral faces that were easy to discriminate served as conditioned stimuli (CS). An EEG was recorded from nine electrodes. Electromyographic activity was measured bilaterally from the m. corrugator supercilii and m. zygomaticus. The startle response was obtained from the m. orbicularis oculi. Finally, heart rate and skin conductance response were assessed. The subjective data and the skin conductance response revealed successful differential aversive conditioning. By contrast, the pleasant odor failed to produce appetitive odor conditioning. The conditioned and unconditioned response of the corrugator muscles confirm previous reports on the m. corrugator being strongly involved in the expression of negative affect. Contrary to previous findings, magnitude of the startle reflex was not found to be modulated depending on the valence of the CS. Central psychophysiological parameters showed little change during differential conditioning. The presence of subjective-evaluative conditioning and contingency awareness without significant changes in cortical and cardiovascular correlates might be due to extremely localized cortical processing of conditioned olfactory cues or primarily subcortical processing. The latter interpretation is strengthened by the presence of differential conditioning in non-voluntary responses such as the corrugator muscles.

Brain Structures in Verbal Communication: A Focus on Prosody

Human communication is made up of three domains: speech, language, and pragmatics, which can be fully specified as a set of rules, units, and practices. Prosodic function spans these domains. Neurolinguistic studies utilizing an array of disparate techniques reveal that many parts of the brain are required for normal communicative function. Speech and language are largely lateralized to the left cerebral hemisphere, while pragmatic competence appears to require an intact right hemisphere. Familiarity agnosias have been associated with right hemisphere dysfunction, leading to an explanation for preserved familiar proper noun recognition in severe aphasia. This model pertains to the adult brain; due to the poorly understood phenomenon of cerebral plasticity, laterality of function in children is established weakly and slowly. The basal ganglia may subserve well-known phrasal structures such as that seen in “automatic speech,” and some kinds of prosodic competence, both in production and perception. A thorough model of language and the brain takes into account two horizontal and one vertical dimension: the left-right dimension of hemispheric model differences; the anterior-posterior dimension reflecting sensory and motor processes; and the vertical interaction of cortical and subcortical processes. All of these participate crucially in the different domains of communication.

Gender differences in regional cerebral activity during sadness.

Functional magnetic resonance imaging and echo-planar-imaging were used to investigate affect related gender differences in regional cerebral activity. The experiment was conducted using a standardized mood induction procedure. Blood-oxygen-level-dependent effect was measured in 13 male and 13 female healthy subjects, during both moods of happiness and sadness, respectively. Parallel to earlier neuroimaging findings, our results show brain activity in the amygdala of males during negative affect. Females failed to demonstrate a similar activation pattern despite matched subjective ratings of negative affect to males. Results point to differential regional cerebral correlates of emotional experience in males and females, which is suggestive of a more focal and subcortical processing of sadness in men.

Absence of neurodegeneration in the thalamus and caudate of elderly patients with schizophrenia

The cognitive and functional deterioration observed in many ‘poor-outcome’ patients with schizophrenia suggests an ongoing neurodegenerative process. Diagnostic neuropathologic studies have excluded known neurodegenerative diseases as the cause of this dementia, and in a previous quantitative investigation of neurodegeneration and neural injury in this population we found no abnormalities in the cerebral cortex. However, it is possible that the deterioration observed in these patients could be due to subcortical neurodegenerative processes. Neurodegeneration and neural injury in the caudate nucleus and mediodorsal nucleus of the thalamus were investigated in a postmortem study of 11 prospectively accrued, clinically well-characterized elderly people with schizophrenia, 11 elderly control subjects with no neuropsychiatric illness, and 12 subjects with Alzheimer's disease. Traditional and immunohistochemical staining and unbiased computerized counting methods were used to quantify common markers of neurodegeneration and neural injury (neuron loss, neurofibrillary tangles, astrocytosis, microgliosis). No statistically significant differences were found between schizophrenia and control subjects for the densities of any markers. There is no evidence that abnormal neurodegeneration occurs in these two important subcortical structures.