A bias-free test of human temporal bisection: Evidence against bisection at the arithmetic mean

The temporal bisection procedure has been used to assess theories of time perception. A problem with the procedure for measuring the perceived midpoint of two durations is that the spacing of probe durations affects the length of the bisection point. Linear spacing results in longer bisection points closer to the arithmetic mean of the durations than logarithmic spacing. In three experiments, the influence of probe duration distribution was avoided by presenting a single probe duration of either the arithmetic or geometric mean of the trained durations. It was found that the number of participants that categorised the arithmetic mean as long was significantly larger than those that categorised it as short. The number of participants that categorised the geometric mean as either short or long did not significantly differ. This was true for trained durations of 0.4 s vs. 1.6 s (Experiments 1-3), 0.2 s vs. 3.2 s (Experiment 2) and 0.4 s vs. 6.4 s (Experiment 3). In Experiment 4, the probe trial distribution effect was replicated with logarithmic and linearly distributed probe durations, demonstrating that bisection occurs close to the arithmetic mean with linearly spaced probe durations. The results provide evidence against bisection at the arithmetic mean when probe spacing bias is avoided and, instead, the results are consistent with logarithmic encoding of time, or a comparison rule based on relative rather than absolute differences.

Knockout of NMDARs in CA1 and dentate gyrus fails to impair temporal control of conditioned behavior in mice

The hippocampus has been implicated in temporal learning. Plasticity within the hippocampus requires NMDA receptor-dependent glutamatergic neurotransmission. We tested the prediction that hippocampal NMDA receptors are required for learning about time by testing mice that lack postembryonal NMDARs in the CA1 and dentate gyrus (DG) hippocampal subfields on three different appetitive temporal learning procedures. The conditional knockout mice (Grin1ΔDCA1 ) showed normal sensitivity to cue duration, responding at a higher level to a short duration cue than compared to a long duration cue. Knockout mice also showed normal precision and accuracy of response timing in the peak procedure in which reinforcement occurred after 10 s delay within a 30 s cue presentation. Mice were tested on the matching of response rates to reinforcement rates on instrumental conditioning with two levers reinforced on a concurrent variable interval schedule. Pressing on one lever was reinforced at a higher rate than the other lever. Grin1ΔDGCA1 mice showed normal sensitivity to the relative reinforcement rates of the levers. In contrast to the lack of effect of hippocampal NMDAR deletion on measures of temporal sensitivity, Grin1ΔDGCA1 mice showed increased baseline measures of magazine activity and lever pressing. Furthermore, reversal learning was enhanced when the reward contingencies were switched in the lever pressing task, but this was true only for mice trained with a large difference between relative reinforcement rates between the levers. The results failed to demonstrate a role for NMDARs in excitatory CA1 and DG neurons in learning about temporal information.

Reinforcement rate and the balance between excitatory and inhibitory learning: Insights from deletion of the GluA1 AMPA receptor subunit

Conditioned responding is sensitive to reinforcement rate. This rate-sensitivity is impaired in genetically modified mice that lack the GluA1 subunit of the AMPA receptor. A time-dependent application of the Rescorla-Wagner learning rule can be used to derive an account of rate-sensitivity by reflecting the balance of excitatory and inhibitory associative strength over time. By applying this analysis, the impairment in GluA1 knockout mice may be explained by reduced sensitivity to negative prediction error and thus, impaired inhibitory learning, such that excitatory associative strength is not reduced during the nonreinforced periods of a conditioned stimulus. The article describes a test of the role of GluA1 in inhibitory learning that requires summing of the associative strengths of cues presented in compound. Mice were trained on a feature negative discrimination of the form A+/AX-. GluA1 knockout mice acquired the discrimination to a similar extent as controls. The inhibitory properties of cue X were verified in a summation test that included a control for nonassociative, external inhibition. The performance of GluA1 knockout mice was similar to that of controls. However, in line with previous findings, GluA1 deletion impaired the precision of timing of conditioned responding. These results provide further evidence that impaired sensitivity to reinforcement rate is not a consequence of impaired inhibitory learning. The results may more readily fit with accounts of rate sensitivity that propose that it reflects encoding of temporal and numeric information rather than being a consequence of changes in associative strength over time. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Uncertainty and predictiveness modulate attention in human predictive learning

[Correction Notice: An Erratum for this article was reported online in Journal of Experimental Psychology: General on Jan 14 2021 (see record 2021-07705-001). In the article, formatting for UK Research Councils funding was omitted. The author note and copyright line now reflect the standard acknowledgment of and formatting for the funding received for this article. All versions of this article have been corrected.] Attention determines which cues receive processing and are learned about. Learning, however, leads to attentional biases. In the study of animal learning, in some circumstances, cues that have been previously predictive of their consequences are subsequently learned about more than are nonpredictive cues, suggesting that they receive more attention. In other circumstances, cues that have previously led to uncertain consequences are learned about more than are predictive cues. In human learning, there is a clear role for predictiveness, but a role for uncertainty has been less clear. Here, in a human learning task, we show that cues that led to uncertain outcomes were subsequently learned about more than were cues that were previously predictive of their outcomes. This effect occurred when there were few uncertain cues. When the number of uncertain cues was increased, attention switched to predictive cues. This pattern of results was found for cues (1) that were uncertain because they led to 2 different outcomes equally often in a nonpredictable manner and (2) that were used in a nonlinear discrimination and were not predictive individually but were predictive in combination with other cues. This suggests that both the opposing predictiveness and uncertainty effects were determined by the relationship between individual cues and outcomes rather than the predictive strength of combined cues. These results demonstrate that learning affects attention; however, the precise nature of the effect on attention depends on the level of task complexity, which reflects a potential switch between exploration and exploitation of cues. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Dissociating Representations of Time and Number in Reinforcement-Rate Learning by Deletion of the GluA1 AMPA Receptor Subunit in Mice

Theories of learning differ in whether they assume that learning reflects the strength of an association between memories or symbolic encoding of the statistical properties of events. We provide novel evidence for symbolic encoding of informational variables by demonstrating that sensitivity to time and number in learning is dissociable. Whereas responding in normal mice was dependent on reinforcement rate, responding in mice that lacked the GluA1 AMPA receptor subunit was insensitive to reinforcement rate and, instead, dependent on the number of times a cue had been paired with reinforcement. This suggests that GluA1 is necessary for weighting numeric information by temporal information in order to calculate reinforcement rate. Sample sizes per genotype varied between seven and 23 across six experiments and consisted of both male and female mice. The results provide evidence for explicit encoding of variables by animals rather than implicit encoding via variations in associative strength.

The GluA1 AMPAR subunit is necessary for hedonic responding but not hedonic value in female mice

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GluA1 containing AMPA receptors are necessary for palatability responses to sucrose.

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Mice lacking GluA1 showed were sensitive to changes in the hedonic value of sucrose.

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This suggests that GluA1 is necessary for hedonic responding but not hedonic value.

The GluA1 subunit of the AMPA receptor has been implicated in anhedonia. Mice that lack GluA1 (Gria1 knockout mice) show reduced lick cluster size, a measure of palatability in feeding behaviour. This deficit may reflect a role for GluA1 in encoding the hedonic value of palatable substances or instead a role for GluA1 in the behavioural expression of hedonic value. We tested the role of GluA1 in hedonic value by assessing sensitivity to changes in the rewarding property of sucrose as a consequence of negative/positive contrast effects in female mice. During training, on half of the days consumption of a flavour (CS+) mixed with 4% sucrose was preceded by consumption of 1% sucrose (positive contrast). On the other half of days consumption of a different flavour (CS–) mixed with 4% sucrose was preceded by consumption of 16% sucrose (negative contrast). In the test session both wild-type, controls and Gria1 knockout mice consumed more of the CS+ flavour than the CS– flavour. While Gria1 knockout mice showed reduced lick cluster sizes, both genotypes made larger lick clusters for the CS+ flavour than the CS– flavour suggesting that the CS+ was more palatable than the CS–. A follow up experiment in normal mice demonstrated that the negative contrast procedure resulted in a conditioned reduction of palatability of the CS– in comparison to an associatively neutral, novel flavour. The results failed to demonstrate a role for GluA1 in hedonic value suggesting that, instead, GluA1 is necessary for hedonic responding.

Spontaneous object-location memory based on environmental geometry is impaired by both hippocampal and dorsolateral striatal lesions

We examined the role of the hippocampus and the dorsolateral striatum in the representation of environmental geometry using a spontaneous object recognition procedure. Rats were placed in a kite-shaped arena and allowed to explore two distinctive objects in each of the right-angled corners. In a different room, rats were then placed into a rectangular arena with two identical copies of one of the two objects from the exploration phase, one in each of the two adjacent right-angled corners that were separated by a long wall. Time spent exploring these two objects was recorded as a measure of recognition memory. Since both objects were in different locations with respect to the room (different between exploration and test phases) and the global geometry (also different between exploration and test phases), differential exploration of the objects must be a result of initial habituation to the object relative to its local geometric context. The results indicated an impairment in processing the local geometric features of the environment for both hippocampus and dorsolateral striatum lesioned rats compared with sham-operated controls, though a control experiment showed these rats were unimpaired in a standard object recognition task. The dorsolateral striatum has previously been implicated in egocentric route-learning, but the results indicate an unexpected role for the dorsolateral striatum in processing the spatial layout of the environment. The results provide the first evidence that lesions to the hippocampus and dorsolateral striatum impair spontaneous encoding of local environmental geometric features.

Cue duration determines response rate but not rate of acquisition of Pavlovian conditioning in mice

The duration of a conditioned stimulus (CS) is a key determinant of Pavlovian conditioning. Rate estimation theory (RET) proposes that reinforcement rate is calculated over cumulative exposure to a cue and the reinforcement rate of a cue, relative to the background reinforcement rate, determines the speed of acquisition of conditioned responding. Consequently, RET predicts that shorter-duration cues require fewer trials to acquisition than longer-duration cues due to the difference in reinforcement rates. We tested this prediction by reanalysing the results of a previously published experiment. Mice received appetitive Pavlovian conditioning of magazine approach behaviour with a 10-s CS and a 40-s CS. Cue duration did not affect the rate at which responding emerged or the rate at which it peaked. The 10-s CS did elicit higher levels of responding than the 40-s CS. These results are not consistent with rate estimation theory. Instead, they are consistent with an associative analysis that assumes that asymptotic levels of responding reflect the balance between increments and decrements in associative strength across cumulative exposure to a cue.

Delay of reinforcement versus rate of reinforcement in Pavlovian conditioning

Conditioned stimulus (CS) duration is a determinant of conditioned responding, with increases in duration leading to reductions in response rates. The CS duration effect has been proposed to reflect sensitivity to the reinforcement rate across cumulative exposure to the CS, suggesting that the delay of reinforcement from the onset of the cue is not crucial. Here, we compared the effects of delay and rate of reinforcement on Pavlovian appetitive conditioning in mice. In Experiment 1, the influence of reinforcement delay on the timing of responding was removed by making the duration of cues variable across trials. Mice trained with variable duration cues were sensitive to differences in the rate of reinforcement to a similar extent as mice trained with fixed duration cues. Experiments 2 and 3 tested the independent effects of delay and reinforcement rate. In Experiment 2, food was presented at either the termination of the CS or during the CS. In Experiment 3, food occurred during the CS for all cues. The latter experiment demonstrated an effect of delay, but not reinforcement rate. Experiment 4 ruled out the possibility that the lack of effect of reinforcement rate in Experiment 3 was due to mice failing to learn about the nonreinforced CS exposure after the presentation of food within a trial. These results demonstrate that although the CS duration effect is not simply a consequence of timing of conditioned responses, it is dependent on the delay of reinforcement. The results provide a challenge to current associative and nonassociative, time-accumulation models of learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

The NMDA receptor antagonist MK-801 fails to impair long-term recognition memory in mice when the state-dependency of memory is controlled

NMDA receptor-dependent synaptic plasticity has been proposed to be important for encoding of memories. Consistent with this hypothesis, the non-competitive NMDA receptor antagonist, MK-801, has been found to impair performance on tests of memory. Interpretation of some of these findings has, however, been complicated by the fact that the drug-state of animals has differed during encoding and tests of memory. Therefore, it is possible that MK-801 may result in state-dependent retrieval or expression of memory rather than actually impairing encoding itself. We tested this hypothesis in mice using tests of object recognition memory with a 24-hour delay between the encoding and test phase. Mice received injections of either vehicle or MK-801 prior to the encoding phase and the test phase. In Experiment 1, a low dose of MK-801 (0.01 mg/kg) impaired performance when the drug-state (vehicle or MK-801) of mice changed between encoding and test, but there was no significant effect of MK-801 on encoding. In Experiment 2, a higher dose of MK-801 (0.1 mg/kg) failed to impair object recognition memory when mice received the drug prior to both encoding and test compared to mice that received vehicle. MK-801 did not affect object exploration, but it did induce locomotor hyperactivity at the higher dose. These results suggest that some previous demonstrations of MK-801 effects may reflect a failure to express or retrieve memory due to the state-dependency of memory rather than impaired encoding of memory.

Continual Trials Spontaneous Recognition Tasks in Mice: Reducing Animal Numbers and Improving Our Understanding of the Mechanisms Underlying Memory

Spontaneous recognition tasks are widely used as a laboratory measure of memory in animals but give rise to high levels of behavioral noise leading to a lack of reliability. Previous work has shown that a modification of the procedure to allow continual trials testing (in which many trials are run concurrently in a single session) decreases behavioral noise and thus significantly reduces the numbers of rats required to retain statistical power. Here, we demonstrate for the first time that this improved method of testing extends to mice, increasing the overall power of the approach. Moreover, our results show that the new continual trials approach provides the additional benefits of heightened sensitivity and thus provides greater insight into the mechanisms at play. Standard (c57) and transgenic Alzheimer model (TASTPM) mice were tested both at 7 and 10 months of age in both object recognition (OR) and object-location (OL) spontaneous recognition tasks using the continual trials methodology. Both c57 and TASTPM mice showed age-dependent changes in performance in OR. While c57 mice also showed age-related changes in performance of OL, TASTPM mice were unable to perform OL at either age. Significantly, we demonstrate that differences in OL performance in c57s and TASTPM animals is a result of proactive interference rather than an absolute inability to recognize OL combinations. We argue that these continual trials approaches provide overall improved reliability and better interpretation of the memory ability of mice, as well as providing a significant reduction in overall animal use.

A biphasic reduction in a measure of palatability following sucrose consumption in mice

Consumption of foods results in a transient reduction in hedonic value that influences the extent and nature of feeding behavior. The time course of this effect, however, is poorly specified. In an initial experiment, using an analysis of the microstructure of licking in mice we found that consumption of sucrose led to a rapid reduction in lick cluster size, a measure of palatability, which recovered after 10 min, but reemerged 60 min after initial consumption. We then replicated the finding that lick cluster size is reduced after 60 min, but not 10 min, under conditions in which a number of potential behavioural confounds were removed. In Experiment 2 the effect was replicated using a between-subjects design that ruled out the possibility that the effect was a specific consequence of the within-subjects procedures used in the first experiment, in which mice may have come to expect sucrose at different time points within the feeding session. While Experiments 1 and 2 confounded the time between periods of access to sucrose with time since the start of the feeding session, this confound was removed in Experiment 3, and, similar to the previous experiments, it was found that a second reduction in palatability occurred after 60 min. Therefore, the effect was dependent only on the time since the previous exposure to sucrose, demonstrating that sucrose consumption initiates a biphasic reduction in palatability. The reduction in lick cluster size after 60 min was not typically accompanied by a reduction in consumption suggesting that the more slowly developing reduction in the palatability measure was not simply a consequence of post-ingestive satiety. The cause of the biphasic change is not yet clear, and may reflect independent processes or the consequence of a single process that initiates multiple changes in palatability over time.

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Recent consumption of a palatable food results in a reduction in palatability.

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The time course of recovery of palatability after sucrose consumption was tested.

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Palatability was measured using the mean lick cluster size.

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Recent consumption of sucrose resulted in a biphasic change in lick cluster size.

The group II metabotropic glutamate receptor agonist LY354740 and the D2 receptor antagonist haloperidol reduce locomotor hyperactivity but fail to rescue spatial working memory in GluA1 knockout mice

Group II metabotropic glutamate receptor agonists have been suggested as potential anti‐psychotics, at least in part, based on the observation that the agonist LY354740 appeared to rescue the cognitive deficits caused by non‐competitive N‐methyl‐d‐aspartate receptor (NMDAR) antagonists, including spatial working memory deficits in rodents. Here, we tested the ability of LY354740 to rescue spatial working memory performance in mice that lack the GluA1 subunit of the AMPA glutamate receptor, encoded by Gria1, a gene recently implicated in schizophrenia by genome‐wide association studies. We found that LY354740 failed to rescue the spatial working memory deficit in Gria1^−/− mice during rewarded alternation performance in the T‐maze. In contrast, LY354740 did reduce the locomotor hyperactivity in these animals to a level that was similar to controls. A similar pattern was found with the dopamine receptor antagonist haloperidol, with no amelioration of the spatial working memory deficit in Gria1^−/− mice, even though the same dose of haloperidol reduced their locomotor hyperactivity. These results with LY354740 contrast with the rescue of spatial working memory in models of glutamatergic hypofunction using non‐competitive NMDAR antagonists. Future studies should determine whether group II mGluR agonists can rescue spatial working memory deficits with other NMDAR manipulations, including genetic models and other pharmacological manipulations of NMDAR function.

Memory-dependent effects on palatability in mice

While palatability depends on the properties of particular foods, it is also determined by prior experience, suggesting that memory affects the hedonic value of a substance. Here, we report two procedures that affect palatability in mice: negative contrast and flavour habituation. A microstructure analysis of licking behaviour was employed, with the lick cluster size (the number of licks made in quick succession before a pause) used as a measure of palatability. It was first confirmed that lick cluster size increased monotonically as a function of sucrose concentration, whereas consumption followed an inverted U-shaped function. In a successive negative contrast procedure it was found that when shifted from a high sucrose concentration (32%) to a low sucrose concentration (4%), mice made smaller lick clusters than a group that only received the low concentration. Mice exposed to flavours (cherry or grape Kool Aid) mixed with sucrose (16%) made larger lick clusters for familiar flavours compared to novel flavours. This habituation effect was evident after short (5 min) and long (24 h) test intervals. Both successive negative contrast and flavour habituation failed to affect levels of consumption. Collectively, the results show that prior experience can have effects on lick cluster size that are equivalent to increasing or decreasing the sweetness of a solution. Thus, palatability is not a fixed property of a substance but is dependent on expectation or familiarity that occurs as a result of memory.

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Consumption in mice is maximal with intermediate concentrations of sucrose.

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Lick cluster size increases monotonically as a function of sucrose concentration.

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A successive negative contrast procedure reduced lick cluster size.

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Flavour habituation led to an increase in lick cluster size.

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Memory has effects on palatability similar to altering the sweetness of a solution.

Contexts control negative contrast and restrict the expression of flavor preference conditioning

Consumption of a high concentration of sucrose can have either a detrimental, negative contrast effect or a facilitatory, preference conditioning effect on subsequent consumption of a low concentration of sucrose, depending on the cues that are present during consumption. The role of context and flavor cues in determining these effects were studied using analysis of the microstructure of licking in mice. Exposure to a high concentration followed by exposure to a low concentration resulted in a transient reduction in mean lick cluster size, which was context dependent (Experiment 1). However, there was no change in the total number of licks or overall consumption. When a flavor that had previously been paired with a high concentration was paired with a low concentration, there was an increase in the total number of licks, and overall consumption, but no change in the mean lick cluster size (Experiment 2). Pairing a high concentration with a flavor in a particular context before pairing the context and flavor compound with a low concentration resulted in abolishing the expression of the flavor preference conditioning effect on the total number of licks and consumption (Experiment 3). These results demonstrate that although context and flavor cues have dissociable effects on licking behavior, their interaction has an antagonistic effect on the behavioral expression of memory.

The effect of US signalling and the US-CS interval on backward conditioning in mice

The effect of US signalling and the US-CS interval in backward conditioning was assessed in mice. For one group of mice the presentation of food was signalled by a tone and for another group, food was unsignalled. For half of the mice, within each group, the presentation of food preceded a visual cue by 10 s. For the other half, food was presented at the start of the visual cue (0-s US-CS interval), resulting in simultaneous pairings of these events. A summation test and a subsequent retardation test were used to assess the inhibitory effects of backward conditioning in comparison to training with a non-reinforced visual cue that controlled for the possible effects of latent inhibition and conditioned inhibition caused as a consequence of differential conditioning. In the summation test unsignalled presentations of the US resulted in inhibition when the US-CS interval was 10 s, but not 0 s. Signalled presentations of the US resulted in inhibition, independent of the US-CS interval. In the retardation test, independent of US signalling, a US-CS interval of 10 s failed to result in inhibition, but an interval of 0 s resulted in greater conditioned responding to the backward CS than the control CS. A generalisation decrement account of the effect of signalling the US with a 0-s US-CS interval, which resulted in reduced responding in the summation test and faster acquisition in the retardation test, is discussed.

Biomechanical characteristics, patient preference and activity level with different prosthetic feet: a randomized double blind trial with laboratory and community testing

Providing appropriate prosthetic feet to those with limb loss is a complex and subjective process influenced by professional judgment and payer guidelines. This study used a small load cell (Europa™) at the base of the socket to measure the sagittal moments during walking with three objective categories of prosthetic feet in eleven individuals with transtibial limb loss with MFCL K2, K3 and K4 functional levels. Forefoot stiffness and hysteresis characteristics defined the three foot categories: Stiff, Intermediate, and Compliant. Prosthetic feet were randomly assigned and blinded from participants and investigators. After laboratory testing, participants completed one week community wear tests followed by a modified prosthetics evaluation questionnaire to determine if a specific category of prosthetic feet was preferred. The Compliant category of prosthetic feet was preferred by the participants (P=0.025) over the Stiff and Intermediate prosthetic feet, and the Compliant and Intermediate feet had 15% lower maximum sagittal moments during walking in the laboratory (P=0.0011) compared to the Stiff feet. The activity level of the participants did not change significantly with any of the wear tests in the community, suggesting that each foot was evaluated over a similar number of steps, but did not inherently increase activity. This is the first randomized double blind study in which prosthetic users have expressed a preference for a specific biomechanical characteristic of prosthetic feet: those with lower peak sagittal moments were preferred, and specifically preferred on slopes, stairs, uneven terrain, and during turns and maneuvering during real world use.

What causes aberrant salience in schizophrenia? A role for impaired short-term habituation and the GRIA1 (GluA1) AMPA receptor subunit

The GRIA1 locus, encoding the GluA1 (also known as GluRA or GluR1) AMPA glutamate receptor subunit, shows genome-wide association to schizophrenia. As well as extending the evidence that glutamatergic abnormalities have a key role in the disorder, this finding draws attention to the behavioural phenotype of Gria1 knockout mice. These mice show deficits in short-term habituation. Importantly, under some conditions the attention being paid to a recently presented neutral stimulus can actually increase rather than decrease (sensitization). We propose that this mouse phenotype represents a cause of aberrant salience and, in turn, that aberrant salience (and the resulting positive symptoms) in schizophrenia may arise, at least in part, from a glutamatergic genetic predisposition and a deficit in short-term habituation. This proposal links an established risk gene with a psychological process central to psychosis and is supported by findings of comparable deficits in short-term habituation in mice lacking the NMDAR receptor subunit Grin2a (which also shows association to schizophrenia). As aberrant salience is primarily a dopaminergic phenomenon, the model supports the view that the dopaminergic abnormalities can be downstream of a glutamatergic aetiology. Finally, we suggest that, as illustrated here, the real value of genetically modified mice is not as 'models of schizophrenia' but as experimental tools that can link genomic discoveries with psychological processes and help elucidate the underlying neural mechanisms.

Hippocampal synaptic plasticity, spatial memory and anxiety

Recent studies using transgenic mice lacking NMDA receptors in the hippocampus challenge the long-standing hypothesis that hippocampal long-term potentiation-like mechanisms underlie the encoding and storage of associative long-term spatial memories. However, it may not be the synaptic plasticity-dependent memory hypothesis that is wrong; instead, it may be the role of the hippocampus that needs to be re-examined. We present an account of hippocampal function that explains its role in both memory and anxiety.

Dissecting spatial knowledge from spatial choice by hippocampal NMDA receptor deletion

Hippocampal NMDA receptors (NMDARs) and NMDAR-dependent synaptic plasticity are widely considered crucial substrates of long-term spatial memory, although their precise role remains uncertain. Here we show that Grin1(ΔDGCA1) mice, lacking GluN1 and hence NMDARs in all dentate gyrus and dorsal CA1 principal cells, acquired the spatial reference memory water maze task as well as controls, despite impairments on the spatial reference memory radial maze task. When we ran a spatial discrimination water maze task using two visually identical beacons, Grin1(ΔDGCA1) mice were impaired at using spatial information to inhibit selecting the decoy beacon, despite knowing the platform's actual spatial location. This failure could suffice to impair radial maze performance despite spatial memory itself being normal. Thus, these hippocampal NMDARs are not essential for encoding or storing long-term, associative spatial memories. Instead, we demonstrate an important function of the hippocampus in using spatial knowledge to select between alternative responses that arise from competing or overlapping memories.

Hippocampal lesions can enhance discrimination learning despite normal sensitivity to interference from incidental information

Spatial properties of stimuli are sometimes encoded even when incidental to the demands of a particular learning task. Incidental encoding of spatial information may interfere with learning by (i) causing a failure to generalize learning between trials in which a cue is presented in different spatial locations and (ii) adding common spatial features to stimuli that predict different outcomes. Hippocampal lesions have been found to facilitate acquisition of certain tasks. This facilitation may occur because hippocampal lesions impair incidental encoding of spatial information that interferes with learning. To test this prediction mice with lesions of the hippocampus were trained on appetitive simple simultaneous discrimination tasks using inserts in the goal arms of a T-maze. It was found that hippocampal lesioned mice were facilitated at learning the discriminations, but they were sensitive to changes in spatial information in a manner that was similar to control mice. In a second experiment it was found that both control and hippocampal lesioned mice showed equivalent incidental encoding of egocentric spatial properties of the inserts, but both groups did not encode the allocentric information. These results demonstrate that mice show incidental encoding of egocentric spatial information that decreases the ability to solve simultaneous discrimination tasks. The normal egocentric spatial encoding in hippocampal lesioned mice contradicts theories of hippocampal function that suggest that the hippocampus is necessary for incidental learning per se, or is required for modulating stimulus representations based on the relevancy of information. The facilitated learning suggests that the hippocampal lesions can enhance learning of the same qualitative information as acquired by control mice. © 2011 Wiley Periodicals, Inc.

Dissociations within short-term memory in GluA1 AMPA receptor subunit knockout mice

GluA1 AMPA receptor subunit knockout mice display a selective impairment on short-term recognition memory tasks. In this study we tested whether GluA1 is important for short-term memory that is necessary for bridging the discontiguity between cues in trace conditioning. GluA1 knockout mice were not impaired at using short-term memory traces of T-maze floor inserts, made of different materials, to bridge the temporal gap between conditioned stimuli and reinforcement during appetitive discrimination tasks. Thus, different aspects of short-term memory are differentially sensitive to GluA1 deletion. This dissociation may reflect processing of qualitatively different short-term memory traces. Memory that results in performance of short-term recognition (e.g. for objects or places) may be different from the memory required for associative learning in trace conditioning.

Competitive short-term and long-term memory processes in spatial habituation

Exposure to a spatial location leads to habituation of exploration such that, in a novelty preference test, rodents subsequently prefer exploring a novel location to the familiar location. According to Wagner's (1981) theory of memory, short-term and long-term habituation are caused by separate and sometimes opponent processes. In the present study, this dual-process account of memory was tested. Mice received a series of exposure training trials to a location before receiving a novelty preference test. The novelty preference was greater when tested after a short, rather than a long, interval. In contrast, the novelty preference was weaker when exposure training trials were separated by a short, rather than a long interval. Furthermore, it was found that long-term habituation was determined by the independent effects of the amount of exposure training and the number of exposure training trials when factors such as the intertrial interval and the cumulative intertrial interval were controlled. A final experiment demonstrated that a long-term reduction of exploration could be caused by a negative priming effect due to associations formed during exploration. These results provide evidence against a single-process account of habituation and suggest that spatial habituation is determined by both short-term, recency-based memory and long-term, incrementally strengthened memory.

Deletion of the GluA1 AMPA receptor subunit impairs recency-dependent object recognition memory

Deletion of the GluA1 AMPA receptor subunit impairs short-term spatial recognition memory. It has been suggested that short-term recognition depends upon memory caused by the recent presentation of a stimulus that is independent of contextual–retrieval processes. The aim of the present set of experiments was to test whether the role of GluA1 extends to nonspatial recognition memory. Wild-type and GluA1 knockout mice were tested on the standard object recognition task and a context-independent recognition task that required recency-dependent memory. In a first set of experiments it was found that GluA1 deletion failed to impair performance on either of the object recognition or recency-dependent tasks. However, GluA1 knockout mice displayed increased levels of exploration of the objects in both the sample and test phases compared to controls. In contrast, when the time that GluA1 knockout mice spent exploring the objects was yoked to control mice during the sample phase, it was found that GluA1 deletion now impaired performance on both the object recognition and the recency-dependent tasks. GluA1 deletion failed to impair performance on a context-dependent recognition task regardless of whether object exposure in knockout mice was yoked to controls or not. These results demonstrate that GluA1 is necessary for nonspatial as well as spatial recognition memory and plays an important role in recency-dependent memory processes.

Deletion of the GluA1 AMPA receptor subunit alters the expression of short-term memory

Deletion of the GluA1 AMPA receptor subunit selectively impairs short-term memory for spatial locations. We further investigated this deficit by examining memory for discrete nonspatial visual stimuli in an operant chamber. Unconditioned suppression of magazine responding to visual stimuli was measured in wild-type and GluA1 knockout mice. Wild-type mice showed less suppression to a stimulus that had been presented recently than to a stimulus that had not. GluA1 knockout mice, however, showed greater suppression to a recent stimulus than to a nonrecent stimulus. Thus, GluA1 is not necessary for encoding, but affects the way that short-term memory is expressed.

The role of habituation in hippocampus-dependent spatial working memory tasks: evidence from GluA1 AMPA receptor subunit knockout mice

Spatial alternation, win-shift behavior has been claimed to be a test of working memory in rodents that requires active maintenance of relevant, trial-specific information. In this review, we describe work with GluA1 AMPA receptor subunit knockout mice that show impaired spatial alternation, but normal spatial reference memory. Due to their selective impairment on spatial alternation, GluA1 knockout mice provide a means by which the psychological processes underlying alternation can be examined. We now argue that the spatial alternation deficit in GluA1 knockout mice is due to an inability to show stimulus-specific, short-term habituation to recently experienced stimuli. Short-term habituation involves a temporary reduction in attention paid to recently presented stimuli, and is thus a distinct process from those that are involved in working memory in humans. We have recently demonstrated that GluA1 knockout mice show impaired short-term habituation, but, surprisingly, show enhanced long-term spatial habituation. Thus, GluA1 deletion reveals that there is competition between short-term and long-term processes in memory. © 2010 Wiley Periodicals, Inc.

Spatial working memory deficits in GluA1 AMPA receptor subunit knockout mice reflect impaired short-term habituation: evidence for Wagner's dual-process memory model

Genetically modified mice, lacking the GluA1 AMPA receptor subunit, are impaired on spatial working memory tasks, but display normal acquisition of spatial reference memory tasks. One explanation for this dissociation is that working memory, win-shift performance engages a GluA1-dependent, non-associative, short-term memory process through which animals choose relatively novel arms in preference to relatively familiar options. In contrast, spatial reference memory, as exemplified by the Morris water maze task, reflects a GluA1-independent, associative, long-term memory mechanism. These results can be accommodated by Wagner's dual-process model of memory in which short and long-term memory mechanisms exist in parallel and, under certain circumstances, compete with each other. According to our analysis, GluA1^−/− mice lack short-term memory for recently experienced spatial stimuli. One consequence of this impairment is that these stimuli should remain surprising and thus be better able to form long-term associative representations. Consistent with this hypothesis, we have recently shown that long-term spatial memory for recently visited locations is enhanced in GluA1^−/− mice, despite impairments in hippocampal synaptic plasticity. Taken together, these results support a role for GluA1-containing AMPA receptors in short-term habituation, and in modulating the intensity or perceived salience of stimuli.

Suppression to visual, auditory, and gustatory stimuli habituates normally in rats with excitotoxic lesions of the perirhinal cortex

In 3 habituation experiments, rats with excitotoxic lesions of the perirhinal cortex were found to be indistinguishable from control rats. Two of the habituation experiments examined the habituation of suppression of responding on an appetitive, instrumental baseline. One of those experiments used stimuli selected from the visual modality (lights), the other used auditory stimuli. The third experiment examined habituation of suppression of novel-flavored water consumption. In contrast to the null results on the habituation experiments, the perirhinal lesions disrupted transfer performance on a configural, visual discrimination, indicating the behavioral effectiveness of the lesions. Implications for comparator theories of habituation are considered, and it is concluded that others' demonstrations of the sensitivity of object recognition to perirhinal cortex damage is not the result of standard habituation.

Enhanced long-term and impaired short-term spatial memory in GluA1 AMPA receptor subunit knockout mice: evidence for a dual-process memory model

The GluA1 AMPA receptor subunit is a key mediator of hippocampal synaptic plasticity and is especially important for a rapidly-induced, short-lasting form of potentiation. GluA1 gene deletion impairs hippocampus-dependent, spatial working memory, but spares hippocampus-dependent spatial reference memory. These findings may reflect the necessity of GluA1-dependent synaptic plasticity for short-term memory of recently visited places, but not for the ability to form long-term associations between a particular spatial location and an outcome. This hypothesis is in concordance with the theory that short-term and long-term memory depend on dissociable psychological processes. In this study we tested GluA1-/- mice on both short-term and long-term spatial memory using a simple novelty preference task. Mice were given a series of repeated exposures to a particular spatial location (the arm of a Y-maze) before their preference for a novel spatial location (the unvisited arm of the maze) over the familiar spatial location was assessed. GluA1-/- mice were impaired if the interval between the trials was short (1 min), but showed enhanced spatial memory if the interval between the trials was long (24 h). This enhancement was caused by the interval between the exposure trials rather than the interval prior to the test, thus demonstrating enhanced learning and not simply enhanced performance or expression of memory. This seemingly paradoxical enhancement of hippocampus-dependent spatial learning may be caused by GluA1 gene deletion reducing the detrimental effects of short-term memory on subsequent long-term learning. Thus, these results support a dual-process model of memory in which short-term and long-term memory are separate and sometimes competitive processes.

To what extent can increasing the magnification of visual feedback of the centre of pressure position change the control of quiet standing balance?

Previous research has shown that standing sway can be reduced when real-time visual feedback of the centre of pressure (COP) position is provided and that it can be further reduced when the visual feedback is magnified. The objective of this study was to determine the magnification beyond which there was no further change in the control of standing sway, as indicated by measures of the root mean square (RMS) and mean power frequency (MPF) of the COP and COM positions. Participants stood with as little movement as possible on a force platform for 2 min while being provided with visual feedback of their COP position at seven different magnifications (1 x, 4 x, 8 x, 16 x, 32 x, 48 x and 64 x) in two support surface conditions: standing on a foam surface and on a non-compliant surface. The RMS of the COM position decreased while the MPF of the COP position increased with increasing magnification. In the non-compliant surface conditions, these changes reached a plateau when visual feedback was magnified 8 x. When balance was made more difficult by standing on a foam surface, plateaus occurred at larger magnifications and, in some measures, did not reach a plateau at all. These data suggest that: (1) with increasing magnification of visual feedback, small movements of the COP were more easily detected, allowing corrective postural adjustments to be made before accelerations of the COM could lead to large deviations in postural sway and (2) the visual feedback was relied upon to a greater extent when standing on a foam surface.

Differential effects of startle on reaction time for finger and arm movements

Recent studies using a reaction time (RT) task have reported that a preprogrammed response could be triggered directly by a startling acoustic stimulus (115-124 dB) presented along with the usual "go" signal. It has been suggested that details of the upcoming response could be stored subcortically and are accessible by the startle volley, directly eliciting the correct movement. However, certain muscles (e.g., intrinsic hand) are heavily dependent on cortico-motoneuronal connections and thus would not be directly subject to the subcortical startle volley in a similar way to muscles whose innervations include extensive reticular connections. In this study, 14 participants performed 75 trials in each of two tasks within a RT paradigm: an arm extension task and an index finger abduction task. In 12 trials within each task, the regular go stimulus (82 dB) was replaced with a 115-dB startling stimulus. Results showed that, in the arm task, the presence of a startle reaction led to significantly shorter latency arm movements compared with the effect of the increased stimulus intensity alone. In contrast, for the finger task, no additional decrease in RT caused by startle was observed. Taken together, these results suggest that only movements that involve muscles more strongly innervated by subcortical pathways are susceptible to response advancement by startle.

Malaise in the water maze: untangling the effects of LPS and IL-1beta on learning and memory

It has been widely described that immune activation, such as that induced by bacterial endotoxin (lipopolysaccharide, LPS) or by interleukin-1beta (IL-1beta) causes deficits in learning and memory. These studies have been performed in a limited number of paradigms and have often failed in their experimental design to account for features of sickness behaviour and have thus introduced potential confounding factors. As such, this literature provides an oversimplified view of the issues. A detailed reading of the literature reveals that rodents treated with LPS or IL-1beta, whether systemically or centrally, do not reproducibly show clear impairments in spatial reference memory in the Morris Water Maze. Latency to find the platform is almost invariably increased, consistent with sickness and reduced locomotor speed in these animals. In studies where distance travelled or route to the platform have been examined there have been either very modest or no differences between treated groups, or stress-induced, thigmotaxic, strategies employed by the sick animals. This suggests that emotional and performance changes are more significant than cognitive impairments. There is better evidence for a deficit in contextual fear conditioning experiments induced by LPS and IL-1beta and these effects are clearly dose-dependent, with facilitation at low doses and impairment at higher doses. We propose that the field should be more cautious and more specific in its description of these cognitive effects and that new tasks be employed in these studies, that are not susceptible to confounding factors such as locomotor speed and elevated stress responses. Emerging data suggests that systemic insults induce more robust memory impairments in aged rodents or those with pre-exisiting neurodegenerative disease and these effects are consistent with the mild effects of infection on cognitive processes in young or healthy adults and the more severe effects, such as delirium, in the elderly and demented population.

Structural learning and the hippocampus

It is argued that while the hippocampus is not vital for all classes of configural learning, it is vital for a specific subclass of configural tasks called "structural learning." The defining feature of structural learning is that in addition to binding stimulus elements to make unique arrays (as in all configural learning), the relationship of these elements to each other, be it spatial or temporal, is specified. Direct evidence supporting the proposal that the hippocampus is required for structural learning comes from recent lesion studies with rats. While rats with hippocampal lesions were impaired at relearning a set of spatial structural problems, they showed no impairment when relearning two configural tasks (transverse patterning and the biconditional discrimination), neither of which required structural learning. Other support comes from surveys of spatial and temporal learning by amnesic patients with hippocampal damage, and from imaging studies of both humans and rats. While these studies offer consistent support for the structural hypothesis, very few provide a rigorous test as the tasks can often be solved by other strategies. For this reason, the present review details the design features of future stringent tests of the structural hypothesis.

Deletion of glutamate receptor-A (GluR-A) AMPA receptor subunits impairs one-trial spatial memory

Genetically modified mice lacking the glutamate receptor A (GluR-A) subunit of the AMPA receptor (GluR-A-/- mice) display normal spatial reference memory but impaired spatial working memory (SWM). This study tested whether the SWM impairment in these mice could be explained by a greater sensitivity to within-session proactive interference. The SWM performance of GluR-A-/- and wild-type mice was assessed during nonmatching-to-place testing under conditions in which potential proactive interference from previous trials was reduced or eliminated. SWM was impaired in GluR-A-/- mice, both during testing with pseudotrial-unique arm presentations on the radial maze and when conducting each trial on a different 3-arm maze, each in a novel testing room. Experimentally naive GluR-A-/- mice also exhibited chance performance during a single trial of spontaneous alternation. This 1-trial spatial memory deficit was present irrespective of the delay between the sample information and the response choice (0 or 45 min) and the length of the sample phase (0.5 or 5 min). These results imply that the SWM deficit in GluR-A-/- mice is not due to increased susceptibility to proactive interference.

Influence of cadence, power output and hypoxia on the joint moment distribution during cycling

The purpose of this study was to use a hypoxic stress as a mean to disrupt the normal coordinative pattern during cycling. Seven male cyclists pedalled at three cadence (60, 80, 100 rpm) and three power output (150, 250, 350 W) conditions in normoxia and hypoxia (15% O2). Simultaneous measurements of pedal force, joint kinematics, % oxyhaemoglobin saturation, and minute ventilation were made for each riding condition. A conventional inverse dynamics approach was used to compute the joint moments of force at the hip, knee, and ankle. The relative contribution of the joint moments of force with respect to the total moment was computed for each subject and trial condition. Overall, the ankle contributed on average 21%, the knee 29% and the hip 50% of the total moment. This was not affected by the relative inspired oxygen concentration. Results showed that the relative ankle moment of force remained at 21% regardless of manipulation. The relative hip moment was reduced on average by 4% with increased cadence and increased on average by 4% with increased power output whereas the knee moment responded in the opposite direction. These results suggest that the coordinative pattern in cycling is a dominant characteristic of cycling biomechanics and remains robust even in the face of arterial hypoxemia.

Softer, Higher-Friction Flooring Improves Gait of Cows With and Without Sole Ulcers

We studied dairy cows (n = 30) walking on concrete and on a soft, high-friction composite rubber surface to examine how flooring influenced gait and how this differed for cows with hoof lesions. Cows had hooves trimmed 9 wk after the trial and were classified as either with or without sole ulcers. Video recordings of the cows while walking were digitized using motion analysis software to calculate stride variables (length, height, overlap, duration, proportion of triple support, and speed). Gait was scored by a subjective scoring system (1 = sound to 5 = severely lame) and by a continuous visual analog scale for each of 7 gait attributes. Cows with sole ulcers walking on a composite rubber surface had longer strides (156.9 +/- 2.6 vs. 149.6 +/- 2.6 cm), higher stride heights (9.7 +/- 0.3 vs. 8.8 +/- 0.3 cm), more stride overlap (0.4 +/- 2.0 vs. -4.3 +/- 2.0 cm), shorter periods of triple support (3 legs in ground contact; 68.6 +/- 2.0 vs. 73.8 +/- 2.0%), walked faster (1.22 +/- 0.04 vs. 1.17 +/- 0.04 m/s) and had lower overall gait scores (2.9 +/- 0.1 vs. 3.1 +/- 0.1), better tracking-up (19 +/- 2 vs. 24 +/- 2), better joint flexion (29 +/- 2 vs. 33 +/- 2), more symmetric steps (31 +/- 3 vs. 36 +/- 3), and less reluctance to bear weight on their legs (12 +/- 2 vs. 16 +/- 2) compared with walking on concrete. Similar results were found for cows without sole ulcers. Most of the subjective gait measures could distinguish between cows with and without sole ulcers, but this was not the case for kinematic measures other than stride height. Cows with higher gait scores (more severe lameness) showed the greatest improvement in stride length (r = -0.51), triple support (r = 0.59), swing duration (r = -0.44), overall gait score (r = 0.46), and reluctance to bear weight (r = 0.66) when walking on the rubber surface compared with cows with lower gait scores. These results indicate that rubber flooring provides a more secure footing and is more comfortable to walk on, especially for lame cattle.

The importance of the rat hippocampus for learning the structure of visual arrays

It has been assumed that the integrity of the rodent hippocampus is required for learning the spatial distribution of visual elements in an array. Formally assessing this assumption is, however, far from straightforward as standard tests are amenable to alternative strategies. In order to provide a stringent test of this ability rats were trained on three concurrent visual discriminations in a water tank in which the stimuli in each pair of discriminations contained exactly the same elements but they differed in their spatial arrangement e.g. A|B vs. its mirror image B/A. Such 'structural' discriminations are a specific subtype of 'configural' or 'nonlinear' tasks. Following acquisition half of the rats received hippocampal lesions and all rats were retrained on the structural discriminations. Hippocampal lesions impaired the ability to relearn these 'structural' discriminations. In contrast, two other groups of rats with similar hippocampal lesions showed no impairment on relearning two non-structural, configural discriminations: transverse patterning and biconditional learning. All three tasks used the same apparatus, the same stimulus elements, and similar training regimes. Superior performance by the rats with hippocampal lesions during a generalization decrement probe showed that hippocampal lesions had diminished sensitivity to 'structural' features on the biconditional task. While the rat hippocampus need not be required for all configural learning, it is important for the special case when the spatial arrangements of the elements are critical. This ability may be a prerequisite for the creation of mental snapshots, which underlie episodic memory.

Supersmart mice: Surprising or surprised? Theoretical comment on Singer, Boison, Möhler, Feldon, and Yee (2007)

The glycine transporter (GlyT1) regulates levels of the neurotransmitter glycine, a coagonist of the N-methyl-D-aspartate receptor (NMDAR), and as such may represent a novel site for developing cognition-enhancing drugs. Genetically modified mice with reduced levels of GlyT1 have been generated to test this hypothesis. P. Singer, D. Boison, H. Möhler, J. Feldon, and B. K. Yee now show, through a spontaneous exploration task, that mice in which GlyT1 has been deleted, specifically in neurons in the forebrain, demonstrate enhanced object recognition memory. Whereas both control and mutant mice show a preference for a novel object over a familiar object 2 min after the initial presentation of 1 of the objects, only the mutant mice show a preference for the novel object when tested after a 2-hr delay. The longer-lasting habituation displayed by the GlyT1 mice is consistent with a role for glycine/NMDAR-dependent synaptic plasticity in supporting a nonassociative, short-term memory trace of a recently experienced stimulus. This short-term habituation process may be independent of associative learning mechanisms and may be best described by A. R. Wagner's (1981) sometimes opponent process model.

Gastrocnemius and soleus muscle length, velocity, and EMG responses to changes in pedalling cadence

Several authors have shown different excitation patterns for soleus and gastrocnemius muscles in response to cadence manipulation during cycling. The purpose of this study was to examine gastrocnemius and soleus length and velocity change as a function of pedalling cadence to consider mechanisms underlying these excitation differences. Ten male and two female cyclists rode at five randomly assigned cadences (50, 65, 80, 95, and 110 rpm) at a nominal 200 W power output while EMG of the gastrocnemius and soleus and sagittal plane video were recorded. Joint-coordinate data for the knee and ankle were used with equations of Grieve et al. [Grieve D, Pheasant S, Cavanagh PR. Prediction of gastrocnemius length from knee and ankle joint posture, in: E. Asmussen, K. Jorgensen, editors. International Series on Biomechanics, vol. 2A, Baltimore: University Park Press; 1978. p. 405-412] to compute gastrocnemius and soleus length and velocity. Consistent with previous publications, gastrocnemius displayed a significant (p<0.05) increase in integrated EMG with increased cadence, whereas cadence had no significant effect on integrated EMG of the soleus. The ankle became significantly (p<0.05) more plantar flexed and reflected a reduced range of motion with increased cadence while the knee became significantly (p<0.05) less extended. Soleus decreased its range of motion by 29%, whereas gastrocnemius decreased its range of motion by 9%. In contrast, soleus increased its velocity range by 32% and gastrocnemius increased by 45%. These data show that with increased cadence gastrocnemius operated over a narrower range of operating lengths but at a higher range of shortening velocity than soleus. The higher range of velocity may have resulted in the need for a relatively higher excitation, as indicated by the integrated EMG, as the muscle was working at a different range on its force-velocity curve. During the recovery portion of the pedalling cycle, the soleus was acting eccentrically while the gastrocnemius acted concentrically indicating the triceps surae complex did not always act in unison.

Neurotoxic lesions of the rat perirhinal and postrhinal cortices and their impact on biconditional visual discrimination tasks

It has been argued that damage to the perirhinal cortex should impair visual discriminations when the stimuli have overlapping features. In Experiment 1, rats with perirhinal cortex lesions were trained on a series of visual discriminations in a water tank, culminating in a biconditional discrimination. No evidence was found of a perirhinal lesion deficit, although the same rats showed an object recognition deficit. In Experiment 2 the lesions were extended to involve both the perirhinal and postrhinal cortices in a new group of rats. An impairment was now found on acquisition of the biconditional task, but this was not specific as impairments were also found on two elemental visual discriminations. Taken together, the study failed to find evidence that the rat perirhinal cortex is necessary for configural visual discriminations and so revealed that some ambiguous visual discriminations can be learnt when this area is removed. Furthermore, there was no evidence that the parahippocampal region is selectively dedicated to configural learning, even though the loss of this area can impair the acquisition of some configural tasks.

Adaptations to normal human gait on potentially slippery surfaces: the effects of awareness and prior slip experience

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. Here we quantify the effects of two aspects of prior knowledge - awareness of a possible slip and prior slip experience - on normal gait. Sixty-eight subjects (40F, 28M) each walked over 48 high-friction surfaces (control trials) and 12 low-friction surfaces. Within- and between-subject changes in lower limb muscle activation, gait kinematics and ground reaction forces were analyzed in three non-slip control trials: one before and one after the first unexpected slip exposure, and a third after repeated slip exposures. Subjects knew they might slip in the latter two trials but not the first trial. Twenty subjects slipped during their first low-friction exposure (early slip group), 32 in later low-friction exposures (late slip group), and 16 subjects did not slip at all. Simultaneous changes in awareness and experience between the first two analyzed trials of the early slip group altered the muscle activity in both limbs, reduced the foot and knee angles at heel strike in the slip limb and reduced the ground reaction forces, impulses and utilized friction after heel strike in the slip limb. A change in only awareness between the first two analyzed trials of the late slip group produced the same kinematic changes seen in the early slip group, but only small muscle activity change and no kinetic changes. Subsequent slip experience in the late slip group produced the muscle activation and kinetic changes observed in the early slip group, but no further kinematic changes. These results showed that awareness of a potential slip primarily alters how the slip-limb approaches the floor, whereas prior slip experience primarily alters the anticipatory muscle activation and how the foot interacts with the floor. These muscle, kinematic and kinetic changes were consistent with a more cautious "normal" gait, and can reduce the external validity of slip and fall experiments.

The effect of subject awareness and prior slip experience on tribometer-based predictions of slip probability

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. We sought to quantify how the empirical relationship between slip probability and available floor friction was affected by subject awareness and prior slip experience. Sixty-eight subjects (40 females, 28 males) walked over three different low-friction surfaces inserted periodically between non-slip control trials. Three increasing levels of prior knowledge were used: deceived (unaware of the slippery surface), aware (20% chance of a slippery surface, but no prior slip experience) and experienced (aware with prior slip experience). Available friction was measured using a drag sled and a variable incidence tribometer. Of 620 low-friction trials, 124 generated slips greater than 27mm. The proportion of slips, the slip distance and the required friction (taken from the control trial immediately before a low-friction trial) generally decreased with increasing levels of prior knowledge. These adaptations were accommodated by logistically regressing slip outcome (yes/no) against the normalized friction (available friction minus required friction) rather than against available friction alone. The regressions showed that subject awareness biased the slip probability curve toward a lower slip risk for a given normalized friction, but that the subsequent addition of slip experience generated a slip risk curve that was not significantly different from that of deceived (and presumably unprepared) subjects. These findings suggest that data to validate a tribometer's ability to predict the risk of slipping (but not falling) can be acquired from subjects with prior slip experience.

Effects of Milking on Dairy Cow Gait

We studied cows with (n = 6) and without (n = 26) sole ulcers before and after milking to explore how milking influences dairy cattle gait and how this differs for cows with hoof injuries. Video recordings of cows were digitized using motion-analysis software to calculate stride variables for each hoof. Gait was scored using a 1-to-5 numerical rating system (1 = sound, 5 = severely lame) and a continuous 100-unit visual analog scale of gait attributes (back arch, head bob, tracking-up, and reluctance to bear weight). For cows with and without sole ulcers, differences in gait before and after milking were evident; after milking, all cows had significantly longer strides (123.3 vs. 133.5 +/- 2.0 cm, respectively), higher stride height (8.3 vs. 8.9 +/- 0.1 cm), shorter stride durations (1.49 vs. 1.41 +/- 0.03 s), walked faster (0.85 vs. 0.97 +/- 0.03 m/s), and had shorter periods of triple support (3 legs in ground contact; 80.0 vs. 71.7 +/- 2.0%). Tracking-up and reluctance to bear weight improved after milking (20 vs. 16 +/- 2; 20 vs. 15 +/- 1, respectively), but numerical rating scores and back arch did not. Cows with sole ulcers walked differently than cows without for all measures, except swing duration, both before and after milking. Interactions between hoof health and milking were found for swing duration and head bob but when tested separately, the only effect was that cows without sole ulcers had longer swing durations before milking (0.45 vs. 0.44 +/- 0.01 s, respectively). Gait differences were probably due to udder distention and motivation to return to the home pen. Our results suggest that the most suitable time to assess lameness is after milking when differences between cows with and without ulcers are most evident.

Hoof pathologies influence kinematic measures of dairy cow gait

To explore how hoof pathologies affect the gait of dairy cattle, we studied gait profiles of cows with no visible injuries (n = 17), sole lesions (n = 14), and sole ulcers (n = 7). Video recordings of dairy cows were digitized using motion analysis software to calculate 6 stride variables for each hoof. Compared with cows with sole ulcers, healthy cows walked faster (1.11 +/- 0.03 vs. 0.90 +/- 0.05 m/s, mean +/- SEM), had shorter stride durations (1.26 +/- 0.03 vs. 1.48 +/- 0.05 s), and longer strides (139.5 +/- 2.1 vs. 130.0 +/- 3.2 cm). Percentage of triple support in the gait cycle (time when cattle were supported by 3 legs) more than doubled for cows with sole ulcers compared with healthy cows (42 vs. 18%). Gait differences were likely due to cows reducing the load on an affected leg. Few differences were detected between healthy cows and those with sole lesions, perhaps because of variation in number, severity, and location of injuries. Kinematic gait analysis is a promising approach in understanding how hoof pathologies affect dairy cow gait.

Gradation of Neck Muscle Responses and Head/Neck Kinematics to Acceleration and Speed Change in Rear-end Collisions

Recent epidemiological evidence shows that the potential for whiplash injury varies with both the average acceleration and speed change of a rear-end collision. The goal of this study was to examine the gradation of neck muscle responses and the head and neck kinematics to rear-end collision pulses in which the acceleration and speed change were independently varied. Thirty subjects (15F, 15M) underwent 36 consecutive rear-end collisions consisting of three different average accelerations (ā = 0.5, 0.9 and 1.3 g) and three different speed changes (Deltav = 0.25, 0.50 and 0.75 m/s). Onset and amplitude of the sternocleidomastoid (SCM) and cervical paraspinal (PARA) muscle responses were measured using surface electromyography. Kinematic measures included linear and angular accelerations and displacements of the head and torso. The results showed that the amplitude of the muscle and kinematics responses was graded to both collision acceleration and speed change. The magnitude of early peaks in the head/neck kinematics correlated more strongly with collision acceleration (r(2) = 0.63 to 0.69), whereas the magnitude of later kinematic peaks correlated more strongly with collision speed change (r(2) = 0.59 to 0.95). Onset of the SCM muscle response correlated only weakly with collision acceleration and speed change (r(2) </= 0.07). A collision pulse descriptor obtained by multiplying average acceleration and speed change (āDeltav) yielded the strongest and most consistent correlations with neck muscle (r(2) = 0.48 to 0.58) and head/neck kinematic responses (r(2) = 0.78 to 0.94). This measure of collision severity is also consistent with the recent epidemiological evidence that whiplash symptom intensity and duration increases with both average acceleration and speed change.

Can prepared responses be stored subcortically?

Quick voluntary responses to environmental stimuli are required of people on a daily basis. These movements have long been thought to be controlled via cortical loops involving processing of the stimulus and generation of a suitable response. Recent experiments have shown that in simple reaction time (RT) tasks, the appropriate response can be elicited much earlier (facilitated) when the "go" signal is replaced by a startling (124 dB) auditory stimulus. In the present experiment we combined a startling acoustic stimulus with an established RT paradigm that involved simple and choice RT. In a simple RT condition the prepared voluntary response was elicited at very short latencies following the startle. However, when cortical processing was required prior to responding (choice RT task), the startle did not facilitate the voluntary response, and gave rise to more movement production errors. Since movements requiring ongoing cortical processing following the stimulus are not facilitated by startle, it is unlikely that the startle facilitation is due to increased neural activation. In contrast, it appears more likely that the startle acts as an early trigger for subcortically stored prepared movements since movements that are prepared in advance can be initiated at such short latencies (<60 ms).