Showing papers in "Behavioral Neuroscience in 2006"

A mechanistic account of striatal dopamine function in human cognition: psychopharmacological studies with cabergoline and haloperidol.

Abstract: The authors test a neurocomputational model of dopamine function in cognition by administering to healthy participants low doses of D2 agents cabergoline and haloperidol. The model suggests that DA dynamically modulates the balance of Go and No-Go basal ganglia pathways during cognitive learning and performance. Cabergoline impaired, while haloperidol enhanced, Go learning from positive reinforcement, consistent with presynaptic drug effects. Cabergoline also caused an overall bias toward Go responding, consistent with postsynaptic action. These same effects extended to working memory and attentional domains, supporting the idea that the basal ganglia/dopamine system modulates the updating of prefrontal representations. Drug effects interacted with baseline working memory span in all tasks. Taken together, the results support a unified account of the role of dopamine in modulating cognitive processes that depend on the basal ganglia.

Gap detection deficits in rats with tinnitus: A potential novel screening tool.

Abstract: The study describes a novel method for tinnitus screening in rats by use of gap detection reflex procedures. The authors hypothesized that if a background acoustic signal was qualitatively similar to the rat's tinnitus, poorer detection of a silent gap in the background would be expected. Rats with prior evidence of tinnitus at 10 kHz (n = 14) exhibited significantly worse gap detection than controls (n = 13) when the gap was embedded in a background similar to their tinnitus. No differences between tinnitus and control rats were found with 16 kHz or broadband noise backgrounds, which helped to rule out explanations related to hearing loss or general performance deficits. The results suggest that gap detection reflex procedures might be effective for rapid tinnitus screening in rats.

Arousal and cortisol interact in modulating memory consolidation in healthy young men.

Abstract: Animal studies indicate that adrenal glucocorticoids enhance memory consolidation while impairing memory retrieval. In humans, beneficial effects on consolidation have been observed infrequently. In the current double-blind study, subjects (N = 29) received placebo or cortisol (30 mg) 10 min before viewing emotionally arousing or neutral pictures. Cortisol treatment had no effects on immediate recall. In the 24-hr delayed recall condition, cortisol led to an enhanced emotional memory facilitation because of decreased neutral and increased emotional memory recall. No effects of cortisol treatment were observed for recognition memory or mood. Results support the notion that glucocorticoids specifically enhance the consolidation of emotional material.

Fear conditioning and extinction: influence of sex and menstrual cycle in healthy humans.

Abstract: Although sex differences have been demonstrated in behavioral paradigms of fear conditioning, the findings have been inconsistent, and fear extinction has been little studied. The present study investigated the influence of sex and menstrual cycle phase on the recall of fear extinction. Three groups of healthy adult participants were studied: women at 2 different phases of the menstrual cycle (early follicular [early cycle] and late follicular [midcycle]) and men. Participants underwent a 2-day fear conditioning and extinction protocol. The paradigm entailed habituation, fear conditioning, and extinction learning on Day 1 and extinction recall and fear renewal on Day 2. Skin conductance served as the dependent variable. During fear acquisition on Day 1, men showed significantly larger conditioned responses relative to women; early cycle and midcycle women did not differ. No significant group differences were found during extinction learning. On Day 2, men and early cycle women expressed greater extinction memory than midcycle women. These data confirm sex differences in conditioned fear acquisition and suggest that midcycle hormones attenuate extinction recall.

Effects of bed nucleus of the stria terminalis lesions on conditioned anxiety: aversive conditioning with long-duration conditional stimuli and reinstatement of extinguished fear.

Abstract: Four experiments investigated the effects of lesions of the bed nucleus of the stria terminalis (BNST) on conditioned fear and anxiety. Though BNST lesions did not disrupt fear conditioning with a short-duration conditional stimulus (CS; Experiments 1 and 3), the lesion attenuated conditioning with a longer duration CS (Experiments 1 and 2). Experiment 3 found that lesions attenuated reinstatement of extinguished fear, which relies on contextual conditioning. Experiment 4 confirmed that the lesion reduced unconditioned anxiety in an elevated zero maze. The authors suggest that long-duration CSs, whether explicit cues or contexts, evoke anxiety conditioned responses, which are dissociable from fear responses to shorter CSs. Results are consistent with behavioral and anatomical distinctions between fear and anxiety and with a behavior-systems view of defensive conditioning.

Reduced variability of postural strategy prevents normalization of motor changes induced by back pain: a risk factor for chronic trouble?

Abstract: Variability is fundamental to biological systems and is important in posturomotor learning and control. Pain induces a protective postural strategy, although variability is normally preserved. If variability is lost, does the normal postural strategy return when pain stops? Sixteen subjects performed arm movements during control trials, when the movement evoked back pain and then when it did not. Variability in the postural strategy of the abdominal muscles and pain-related cognitions were evaluated. Only those subjects for whom pain induced a reduction in variability of the postural strategy failed to return to a normal strategy when pain stopped. They were also characterized by their pain-related cognitions. Ongoing perception of threat to the back may exert tighter evaluative control over variability of the postural strategy.

Facial affect recognition training in autism: can we animate the fusiform gyrus?

Abstract: One of the most consistent findings in the neuroscience of autism is hypoactivation of the fusiform gyrus (FG) during face processing. In this study the authors examined whether successful facial affect recognition training is associated with an increased activation of the FG in autism. The effect of a computer-based program to teach facial affect identification was examined in 10 individuals with high-functioning autism. Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) changes in the FG and other regions of interest, as well as behavioral facial affect recognition measures, were assessed pre- and posttraining. No significant activation changes in the FG were observed. Trained participants showed behavioral improvements, which were accompanied by higher BOLD fMRI signals in the superior parietal lobule and maintained activation in the right medial occipital gyrus.

The impact of selective amygdala, orbital frontal cortex, or hippocampal formation lesions on established social relationships in rhesus monkeys (Macaca mulatta).

Abstract: Social dominance, personality ratings, and frequency, duration, and timing of social behaviors were measured pre- and postsurgically in 6 groups of rhesus monkeys (Macaca mulatta), each consisting of 1 sham-operated control and 1 monkey each with a selective amygdala, hippocampal, or orbital frontal cortex lesion. Unlike previous reports, none of the operated groups showed changes in social dominance postsurgery, although changes in other measures varied by lesion site. Although sham-operated monkeys displayed heightened avoidant, anxious, and aggressive behaviors, those with hippocampal lesions also showed increased exploration and excitability, along with reduced responses to affiliative signals. Amygdala lesions yielded several personality changes that precluded positive social interactions (increased exploration and excitability, decreased affiliation and popularity) and altered responses to threatening social signals. By contrast, monkeys with orbital frontal lesions were involved in more aggressive interactions and responded differently to both affiliative and threatening signals. Although several findings differ from earlier nonhuman primate studies, they are largely in agreement with human data and emphasize the context-specific nature of social behavior studies. Interpretation of results in relation to cognitive processes mediated by each structure is discussed.

Chronic Stress Impairs Spatial Memory and Motivation for Reward Without Disrupting Motor Ability and Motivation to Explore

Abstract: This study uses an operant, behavioral model to assess the daily changes in the decay rate of short-term memory, motivation, and motor ability in rats exposed to chronic restraint. Restraint decreased reward-related motivation by 50% without altering memory decay rate or motor ability. Moreover, chronic restraint impaired hippocampal-dependent spatial memory on the Y maze (4-hr delay) and produced CA3 dendritic retraction without altering hippocampal-independent maze navigation (1-min delay) or locomotion. Thus, mechanisms underlying motivation for food reward differ from those underlying Y maze exploration, and neurobiological substrates of spatial memory, such as the hippocampus, differ from those that underlie short-term memory. Chronic restraint produces functional, neuromorphological, and physiological alterations that parallel symptoms of depression in humans.

D-cycloserine facilitates extinction but does not eliminate renewal of the conditioned emotional response.

Abstract: The effect of D-cycloserine (DCS), an NMDA partial agonist, on extinction of fear was investigated in rats using the conditioned emotional response preparation. Fear extinction was facilitated when the first 4 trials occurred with a 30-mg/kg dose of DCS. However, extinguished fear was "renewed" regardless of the drug treatment when the rats were returned to the context in which fear had been conditioned. Additional results suggest that DCS's facilitation of extinction is a small but meaningful effect in the current method. The results suggest caution regarding the use of DCS as an adjunct to extinction: Although the drug may modestly facilitate extinction learning, it does not necessarily destroy the potential for relapse. Behavioral mechanisms are discussed.

Human amygdala activity during the expression of fear responses.

Abstract: The initial learning and subsequent behavioral expression of fear are often viewed as independent processes with potentially unique neural substrates. Laboratory animal studies of Pavlovian fear conditioning suggest that the amygdala is important for both forming stimulus associations and for subsequently expressing learned behavioral responses. In the present article, human amygdala activity was studied during the autonomic expression of conditional fear in two differential conditioning experiments with event-related functional magnetic resonance imaging and concurrent recording of skin conductance responses (SCRs). Trials were classified on the basis of individual participants' SCRs. Significant amygdala responding was detected only during trials on which a signal both predicted shock and elicited significant conditional SCR. Conditional stimulus presentation or autonomic activity alone was not sufficient. These results indicate that amygdala activity may specifically reflect the expression of learned fear responses and support the position that this region plays a central role in the expression of emotional reactions.

Anxiogenic-like effect of chronic corticosterone in the light-dark emergence task in mice.

Abstract: Chronic hypercortisolemia is a hallmark of neuroendocrine and psychiatric disorders, such as Cushing's disease and depression. Whether cortisol directly contributes to the altered mood and anxiety symptoms seen in these diseases remains unclear. To address this, the authors have modeled hypercortisolemia by administering corticosterone in the drinking water of female Swiss Webster mice for 17 or 18 days (13 mg/kg). Light-dark emergence, startle habituation, and startle reactivity were measured. Chronic but not acute treatment with corticosterone increased the latency to emerge into the light compartment, an anxiogenic-like effect. Chronic corticosterone treatment did not affect startle habituation, but did reduce startle reactivity. This study suggests that chronic hypercortisolemia may contribute to anxiety-related behavior in patients with Cushing's disease and depression.

Early maternal rejection affects the development of monoaminergic systems and adult abusive parenting in rhesus macaques (Macaca mulatta)

Abstract: This study investigated the effects of early exposure to variable parenting style and infant abuse on cerebrospinal fluid (CSF) concentrations of monoamine metabolites and examined the role of monoaminergic function in the intergenerational transmission of infant abuse in rhesus monkeys (Macaca mulatta). Forty-three infants reared by their biological mothers and 15 infants that were cross-fostered at birth and reared by unrelated mothers were followed longitudinally through their first 3 years of life or longer. Approximately half of the infants were reared by abusive mothers and half by nonabusive controls. Abused infants did not differ from controls in CSF concentrations of 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), or 3-methoxy-4-hydroxyphenylgycol (MHPG). Abused infants, however, were exposed to higher rates of maternal rejection, and highly rejected infants had lower CSF 5-HIAA and HVA than low-rejection infants. The abused females who became abusive mothers in adulthood had lower CSF 5-HIAA than the abused females who did not. A similar trend was also observed among the cross-fostered females, suggesting that low serotonergic function resulting from early exposure to high rates of maternal rejection plays a role in the intergenerational transmission of infant abuse.

Genetic predisposition to anxiety-related behavior determines coping style, neuroendocrine responses, and neuronal activation during social defeat

Abstract: Genetic background may influence an individual's susceptibility to, and subsequent coping strategy for, an acute stressor. When exposed to social defeat (SD), rats bred for high (HAB) or low (LAB) trait anxiety, which also differ in depression-like behavior, showed highly divergent passive and active coping behaviors, respectively. HABs spent more time freezing and emitted more ultrasound vocalization calls during SD than LABs, which spent more time rearing and grooming. Although the behavioral data confirmed the prediction that heightened trait anxiety would make rats more prone to experience stress, adrenocorticotropin and corticosterone were secreted to a higher extent in LABs than in HABs. In the latter, Fos expression upon SD was enhanced in the amygdala and hypothalamic areas compared with LABs, whereas it was diminished in prefrontal and brainstem areas.

Stress facilitates consolidation of verbal memory for a film but does not affect retrieval.

Abstract: The effect of psychosocial stress on distinct memory processes was investigated in 157 college students using a brief film, which enabled comparison of verbal and visual memory by using a single complex stimulus. Participants were stressed either following stimuli presentation (consolidation) or before testing 48 hr later (retrieval) and were compared with no-stress controls. Salivary cortisol was measured before and 20 min after stress. The consolidation group significantly outperformed controls on total and verbal film scores. Stress did not impair retrieval relative to controls. Exploratory analyses revealed a significant correlation between cortisol and verbal scores across all groups (r = .18). Results provide the first evidence of a facilitative effect of a stressor on verbal memory, but failed to replicate retrieval findings.

Prolonged rewarding stimulation of the rat medial forebrain bundle: neurochemical and behavioral consequences.

Abstract: Extracellular dopamine levels were measured in the rat nucleus accumbens by means of in vivo microdialysis. Delivery of rewarding medial forebrain bundle stimulation at a low rate (5 trains/min) produced a sustained elevation of dopamine levels, regardless of whether train onset was predictable. When the rate of train delivery was increased to 40 trains/min, dopamine levels rose rapidly during the first 40 min but then declined toward the baseline range. The rewarding impact of the stimulation was reduced following prior delivery of stimulation at the high, but not the low, rate. These results support the idea that dopamine tone plays an enabling role in brain stimulation reward and is elevated similarly by predictable and unpredictable stimulation.

The effects of orbital frontal cortex damage on the modulation of defensive responses by rats in playful and nonplayful social contexts.

Abstract: In a series of 3 experiments on rats, 2 hypotheses were tested: (a) that damage to the orbital frontal cortex (OFC) would alter the socially relevant context for executing defensive responses but not their performance and (b) that damage done to the OFC in early infancy would produce more deficits in social behavior than similar damage occurring in adulthood. Bilateral or unilateral OFC damage in adult males did not impair their ability to defend themselves during play fighting and when protecting their food but did impair their ability to modify the pattern of defense in response to different partners. Rats that sustained bilateral damage at 3 days of age not only had deficits in partner-related modulation of defense but also exhibited hyperactivity in their play. The findings thus supported the proposed hypotheses.

Stress facilitates consolidation of verbal memory for a film but does not affect retrieval. Commentary

Abstract: The effect of psychosocial stress on distinct memory processes was investigated in 157 college students using a brief film, which enabled comparison of verbal and visual memory by using a single complex stimulus. Participants were stressed either following stimuli presentation (consolidation) or before testing 48 hr later (retrieval) and were compared with no-stress controls. Salivary cortisol was measured before and 20 min after stress. The consolidation group significantly outperformed controls on total and verbal film scores. Stress did not impair retrieval relative to controls. Exploratory analyses revealed a significant correlation between cortisol and verbal scores across all groups (r = .18). Results provide the first evidence of a facilitative effect of a stressor on verbal memory, but failed to replicate retrieval findings.

Dorsal hippocampus inhibition disrupts acquisition and expression, but not consolidation, of cocaine conditioned place preference.

Abstract: Cocaine abusers may experience drug craving upon exposure to environmental contexts where cocaine was experienced. The dorsal hippocampus (DHC) is important for contextual conditioning, therefore the authors examined the specific role of the DHC in cocaine conditioned place preference (CPP). Muscimol was used to temporarily inhibit the DHC and was infused before conditioning sessions or tests for CPP to investigate acquisition and expression of cocaine CPP, respectively. To investigate consolidation, rats received intra-DHC muscimol either immediately or 6 hr after conditioning sessions. Inhibition of DHC, but not the overlying cortex, disrupted acquisition and expression of cocaine CPP. It is interesting to note that there was no effect of post-conditioning DHC inhibition. The findings suggest that the DHC is important for both acquisition and recall, but not consolidation, of context-cocaine associations.

The Role of Voluntary Exercise in Enriched Rearing: A Behavioral Analysis

Abstract: The effects of postweaning enriched rearing and home cage voluntary wheel-running exercise in adulthood were contrasted on a comprehensive battery of tests designed to assess mnemonic, attentional, emotional, and motor functions. In a 2 x 2 factorial design, female C57BL/6 mice were housed in groups in either standard or enriched cages, which were equipped with either a running or a locked wheel. They were maintained in the corresponding housing conditions for 2 months postweaning prior to, and throughout, testing. Enriched rearing was associated with anxiogenesis, hypolocomotor activity, enhanced motor skills, retarded extinction of conditioned responding, and improved water maze performance. Exercise as such enhanced motor coordination and facilitated extinction of contextual conditioning. Evidence for an interaction between enrichment and exercise was apparent in the open field test, conditioned freezing to a tone stimulus, prepulse inhibition, and acquisition of water maze reference memory. Hence, care should be taken to control for the unique contribution of wheel-running exercise when it is included as an integral component of the enrichment procedure.

Executive functions in the heterozygous reeler mouse model of schizophrenia.

Abstract: Deficits in working memory and executive functions are now considered among the most reliable endophenotypes for schizophrenia. To determine whether cognitive deficits exist in mouse models of the disease, the authors trained heterozygous reeler (+/rl) mice on a series of visual discriminations similar to those used to test executive abilities in primates. These mice resemble schizophrenia patients in that both have reduced levels of reelin protein and altered gamma aminobutyric acid neurotransmission in the prefrontal cortex. The +/rl mice showed a selective deficit in reversal learning, with a pattern of errors that suggested impaired visual attention rather than a deficiency in perseveration and inhibitory control. These results show that cognitive dysfunction may serve as a useful biomarker in mouse models of neuropsychiatric disease.

Attentional dysfunction, impulsivity, and resistance to change in a mouse model of fragile X syndrome.

Abstract: On a series of attention tasks, male mice with a mutation targeted to the fragile X mental retardation 1 (Fmrl) gene (Fmrl knockout [KO] mice) committed a higher rate of premature responses than wild-type littermates, with the largest differences seen when task contingencies changed. This finding indicates impaired inhibitory control, particularly during times of stress or arousal. The KO mice also committed a higher rate of inaccurate responses than controls, particularly during the final third of each daily test session, indicating impaired sustained attention. In the selective attention task, the unpredictable presentation of potent olfactory distractors produced a generalized disruption in the performance of the KO mice, whereas for controls, the disruption produced by the distractors was temporally limited. Finally, the attentional disruption seen following an error was more pronounced for the KO mice than for controls, further implicating impaired regulation of arousal and/or negative affect. The present study provides the first evidence that the Fmrl KO mouse is impaired in inhibitory control, attention, and arousal regulation, hallmark areas of dysfunction in fragile X syndrome. The resistance to change also seen in these mice provides a behavioral index for studying the autistic features of this disorder.

Enrichment to odors improves olfactory discrimination in adult rats.

Abstract: The authors tested how prior odor enrichment affects the spontaneous discrimination of both preexposed and novel odors. Experimental rats were exposed to single odors or to pairs of similar or dissimilar odors for 1-hr periods twice daily over 20 days. Spontaneous discriminations between pairs of similar odors were tested before and after the odor exposure period using an olfactory habituation task. The authors found that (a) experimental rats did not spontaneously discriminate similar odor pairs before the exposure period, whereas they spontaneously discriminated them after the enrichment period, and (b) the improvement of performance was not selective for the odors used during enrichment. These results show that odor experience changes perception in the manner predicted based on other groups' electrophysiological experiments.

Enduring effects of photoperiod on affective behaviors in Siberian hamsters (Phodopus sungorus).

Abstract: The effects of perinatal and postweaning photoperiods on subsequent affective behaviors were examined in adult Siberian hamsters (Phodopus sungorus). Hamsters exposed perinatally to short days (8 hr light/day) exhibited mixed results for adult anxiety-like behaviors and increased some depressive-like behaviors compared with hamsters exposed to long days (16 hr light/day). Postweaning exposure to short days increased depressive- and anxiety-like behaviors compared with long days. Sex differences in affective behaviors were observed. These results suggest that anxiety-like behaviors are organized early in life and endure throughout adulthood, and anxiety- and depressive-like behaviors are modified by postweaning photoperiod. The persistence of photoperiod-induced affective behaviors in rodents supports the hypothesis that symptoms of human affective disorders may reflect ancestral adaptations to seasonal environments.

Opioid receptors in the midbrain periaqueductal gray regulate prediction errors during Pavlovian fear conditioning.

Abstract: The authors used a within-subject blocking design to study the role of ventrolateral periaqueductal gray (v1PAG) opioid receptors in regulating prediction errors during Pavlovian fear conditioning. In Stage I, the authors trained rats to fear conditioned stimulus (CS) A by pairing it with shock. In Stage II, CSA and CSB were co-presented and followed with shock. Two novel stimuli, CSC and CSD, were also co-presented and followed with shock in Stage II. CSA blocked fear from accruing to CSB. Blocking was prevented by systemic pretreatment with naloxone. Blocking was also prevented in a dose-dependent and neuroanatomically specific fashion by vlPAG infusions of the micro-opioid receptor antagonist CTAP. These experiments show that v1PAG micro-opioid receptors contribute to Pavlovian fear learning by regulating predictive error.

Impaired trace fear conditioning following neonatal ethanol: reversal by choline.

Abstract: Neonatal ethanol exposure in animals results in performance deficits on tests of hippocampus-dependent spatial memory, and recent studies have shown that extra dietary choline can ameliorate some of these impairments. In this experiment, rats were administered 5.25 g/kg ig ethanol per day or sham intubations on Postnatal Days (PD) 4-9 and choline (0.1 ml of an 18.8 mg/ml solution) or saline subcutaneously on PD 4-20. On PD 30, rats were given delay or trace fear conditioning trials and were tested for conditioned stimulus-elicited freezing 24 hr later. Neonatal ethanol produced a profound impairment in trace conditioning that was reversed by choline. Groups did not differ in delay conditioned responding, indicating that neonatal ethanol produces a relatively selective cognitive deficit that can be alleviated with supplemental choline.

Path integration and lesions within the head direction cell circuit: comparison between the roles of the anterodorsal thalamus and dorsal tegmental nucleus.

Abstract: Experiments were designed to determine whether 2 regions of the head direction cell circuit, the anterodorsal thalamic nucleus (ADN) and the dorsal tegmental nucleus (DTN), contribute to navigation. Rats were trained to perform a food-carrying task with and without blindfolds prior to receiving sham lesions or bilateral lesions of the ADN or DTN. ADN-lesioned rats were mildly impaired in both versions of the task. DTN-lesioned rats, however, were severely impaired and showed reduced heading accuracy in both task versions. These findings suggest that although both the DTN and ADN contribute to navigation based on path integration and landmarks, disruption of the head direction cell circuit at the level of the DTN has a significantly greater effect on spatial behavior than lesions of the ADN.

Immediate shock deficit in fear conditioning: effects of shock manipulations.

Abstract: Pavlovian contextual fear conditioning occurs when an aversive unconditional stimulus (US), such as a footshock, is presented to a rat shortly after it is placed in an experimental context. Contextual fear conditioning does not occur when the shock is presented immediately upon placement of the rat in the novel chamber. In the present study, the authors report that increasing either the number of immediate shock sessions (Experiment 1) or the immediate shock duration (Experiment 2) did not reverse this deficit. However, immediate shock seems to sensitize subsequent context conditioning (Experiment 3). These findings suggest that the associative deficit produced by immediate shock is not related to the rat's ability to process the footshock US.

The human mu-opioid receptor gene polymorphism 118A > G decreases cortical activation in response to specific nociceptive stimulation.

Abstract: The authors sought to investigate the role of a common single nucleotide polymorphism in the mu-opioid receptor gene (OPRM1) 118A > G for nociceptive sensory processing using event-related potentials (ERPs). Specific nociceptive (carbon dioxide [CO-sub-2]: 40% volume-to-volume [vol/vol] and 60% vol/vol) and nonnociceptive (hydrogen sulfide, 2 parts per million [ppm] and 4 ppm) stimuli were applied to the nasal mucosa of 45 volunteers. ERPs were recorded from a central lead. In this random sample, we found 37 noncarriers, 7 heterozygous carriers, and 1 homozygous carrier of the variant OPRM1 118G allele (allelic frequency, 10%). Amplitudes of nociceptive ERP in carriers of this allele were, on average, half as high as those of noncarriers. In discriminant analysis, ERP amplitude N1 response to the weaker nociceptive stimuli was the only ERP parameter that discriminated statistically significantly between carriers and noncarriers of the variant 118G allele. On the basis of N1-CO-sub-2 (40% vol/vol), the authors correctly backclassified 68.6% of the cases as carriers or noncarriers of the allele. The OPRM1 118A > G polymorphism specifically modulates nociceptive but not nonnociceptive cortical activation.

Primes and targets in rapid chases: Tracing sequential waves of motor activation.

Abstract: Single-cell recordings have indicated that visual stimuli elicit rapid waves of neuronal activation that propagate so fast that they might be free of intracortical feedback. Here, the time course of feedforward activation was traced by measuring pointing responses to color targets preceded by color primes initiating either the same or opposite response. The early time course of priming effects was strictly time locked to prime onset and depended only on properties of the primes, but was independent of the onset times of the actual targets as well as the perceptual effects of targets on primes. Results indicated that nonoverlapping feedforward signals by primes and targets traverse the visuomotor system in a rapid chase, controlling associated motor responses in strict sequence.