Biomedical subjects

Alexander Easton

Publications and source records attributed to Alexander Easton.

10 recordsLinked to original sources

PubMed2006

Frontal-temporal disconnection abolishes object discrimination learning set in macaque monkeys.

Two previous studies have shown that frontal-temporal disconnection in monkeys, produced by unilateral ablation of frontal cortex in one hemisphere and of visual inferior temporal cortex in the opposite hemisphere is entirely without effect on visual object-reward association learning in concurrent discrimination tasks. This is a surprising finding in light of the severe impairments that follow frontal-temporal disconnection in many other tests of visual learning and memory, including delayed matching-to-sample and several conditional learning tasks. To explore the limits of this preserved object-reward association learning, we trained monkeys on visual object discrimination learning set (DLS) prior to frontal-temporal disconnection. As a result of training with single object-reward associations, the monkeys acquired a proficient learning set, evidenced by the rapid learning of new single object-reward association problems. This rapid learning was not affected by unilateral ablations of either inferior temporal cortex alone or frontal cortex alone but was severely impaired after final surgery to complete the disconnection. Moreover, each individual monkey now learned single object-reward association problems at the slow rate at which that individual had learned such problems before the formation of learning set. This result shows that frontal-temporal disconnection abolishes visual learning set.

Animals↗

PubMed2006

48 year old male with sudden onset of right sided weakness.

This 48 year-old male presented with sudden right sided weakness and hypertension, and on CT was found to have a deep left hemispheric intracerebral hematoma. There were accompanying bilateral hypointensities in the occipital lobes, unchanged 1 day later, when expansion of the hematoma and uncal herniation resulted in death. The past medical history included liver transplantation followed by long term cyclosporin, features of thrombotic thrombocytopenic purpura (TTP) attributed to cyclosporin, type 1 diabetes mellitus and acute renal failure attributed to diabetic nephropathy. There was a 2-month history of hypertension poorly responsive to therapy. The occipital lobes at autopsy showed focal cortical hemorrhages with bilateral discoloration of white matter sparing subcortical zones. Microscopy showed white matter pallor with parenchymal cysts, enlarged perivascular spaces and focal acute hemorrhages consistent with edema secondary to acute hypertension. Cortical hemorrhages were associated with intravascular thrombi and fibrin exudates, as well as an accompanying microinfarct, attributed to TTP. The occipital pathology reflects the posterior leukoencephalopathy syndrome (PLS) combined with cortical lesions of TTP. PLS is usually diagnosed radiologically by occipital hypointensities, and is reversible, so that autopsy pathology is rarely examined. TTP may predispose to the development of the PLS in certain cases.

Brain Edema↗

PubMed2005

Recollection in an episodic-like memory task in the rat.

Episodic memory in humans is the conscious recollection of a past event. Animal models of episodic-like memory assess the memory for "what"happened, "where" it happened, and either "when" it happened, or in "which" context it happened, although recollection on such tasks is often difficult to measure. Here we present the first evidence of successful recollection of a past event in a rat in a task which is easily performed, requires little training, and is easily adaptable for other commonly used laboratory species.

Animals↗

PubMed2005

The role of prefrontal cortex in object-in-place learning in monkeys.

Previous ablation studies in monkeys suggest that prefrontal cortex is involved in a wide range of learning and memory tasks. However, monkeys with crossed unilateral lesions of frontal and temporal cortex are unimpaired at concurrent object-reward association learning but are impaired at conditional learning and the implementation of memory-based performance rules. We trained seven monkeys preoperatively on an associative learning task that required them to associate objects embedded in unique complex scenes with reward. Three monkeys then had crossed unilateral lesions of frontal and inferior temporal cortex and the remaining monkeys had bilateral prefrontal cortex ablation. Both groups were severely impaired postoperatively. These results show that both bilateral prefrontal cortex ablation and frontal-temporal disconnection impair associative learning for objects embedded in scenes. The results provide evidence that the function of frontal-temporal interactions in memory is not limited to conditional learning tasks and memory-dependent performance rules. We propose that rapid object-in-place learning requires the interaction of frontal cortex with inferotemporal cortex because visual object and contextual information which is captured over multiple saccades must be processed as a unique complex event that is extended in time. The present results suggest a role for frontal-temporal interaction in the integration of visual information over time.

Animals↗

PubMed2004

Differential reward outcome learning in adult humans.

Differential reward outcome learning provides a unique reward outcome for each condition of a conditional discrimination task, and this increases the rate at which these tasks can be learned. The present experiment aims to show that irrespective of any difference in learning rate, conditional discrimination tasks and differential reward outcome tasks are solved using different strategies. Two groups of 30 adult subjects were taught a series of conditional visual discriminations. One group received different reward outcomes for each condition, whilst the other group received the same reward outcome for each condition. Both groups learned the visual discriminations at the same rate. Subjects were then taught a new conditional cue with a subset of previously learnt discrimination problems and then required to transfer the newly learnt instruction cue to the remaining discrimination problems that had been previously learnt. Although both groups appeared to transfer the newly learnt cue at the same rate, the subjects performing the differential reward outcome task learnt to a criterion level of performance with fewer errors. The results are discussed in relation to evidence from monkeys indicating different neural mechanisms underlying the learning of both tasks.

Adolescent↗

PubMed2002

Insights into the nature of fronto-temporal interactions from a biconditional discrimination task in the monkey.

Previous work in monkeys has shown that both frontal and inferior temporal cortices are required to solve visual learning tasks. When communication between these cortical areas is prevented within the same hemisphere by crossed lesions of the frontal cortex in one hemisphere and the inferior temporal cortex in the opposite hemisphere, most learning tasks are impaired, but learning of object-reward associations is unimpaired. The current experiment aims to understand further the role of the interaction between the frontal and inferior temporal cortices in learning tasks. We trained monkeys on a biconditional discrimination task, in which different visual cues guided behaviour towards choice objects. One visual cue predicted immediate delivery of reward to a correct response, the other visual cue predicted a delayed delivery of reward to a correct response. Pre-operative behavioural data clearly shows that the monkeys form expectations of the reward outcome for the individual cues and choice objects. Crossed lesions of frontal and inferior temporal cortices, however, produce no impairment on this task. The result suggests (in combination with previous experiments) that task difficulty does not determine the reliance of a task on interactions between the frontal cortex and the inferior temporal cortex within the same hemisphere. Instead, we propose that tasks that can be solved by using expectation of the reward outcome do not require interaction of frontal and inferior temporal cortices within the same hemisphere. The results are discussed in the context of other data on frontal interactions with inferior temporal cortex in learning tasks.

Animals↗

PubMed2002

The uses of colour vision: behavioural and physiological distinctiveness of colour stimuli.

Colour and greyscale (black and white) pictures look different to us, but it is not clear whether the difference in appearance is a consequence of the way our visual system uses colour signals or a by-product of our experience. In principle, colour images are qualitatively different from greyscale images because they make it possible to use different processing strategies. Colour signals provide important cues for segmenting the image into areas that represent different objects and for linking together areas that represent the same object. If this property of colour signals is exploited in visual processing we would expect colour stimuli to look different, as a class, from greyscale stimuli. We would also expect that adding colour signals to greyscale signals should change the way that those signals are processed. We have investigated these questions in behavioural and in physiological experiments. We find that male marmosets (all of which are dichromats) rapidly learn to distinguish between colour and greyscale copies of the same images. The discrimination transfers to new image pairs, to new colours and to image pairs in which the colour and greyscale images are spatially different. We find that, in a proportion of neurons recorded in the marmoset visual cortex, colour-shifts in opposite directions produce similar enhancements of the response to a luminance stimulus. We conclude that colour is, both behaviourally and physiologically, a distinctive property of images.

Animals↗

PubMed2002

Interaction of inferior temporal cortex with frontal cortex and basal forebrain: double dissociation in strategy implementation and associative learning.

Macaque monkeys learned a strategy task in which two groups of visual objects needed to be treated differently, one with persistent and one with sporadic object choices, to obtain food rewards. After preoperative training, they were divided into two surgical groups of three monkeys each. One group received crossed unilateral removals of frontal cortex and inferior temporal cortex (IT x FC) and were severely impaired in performing the strategy task. The other group received bilateral transection of anterior temporal stem, amygdala, and fornix (TS+AM+FX) and were unimpaired in performing the strategy task. Subsequently the same animals were tested in visual object-reward association learning. Here, confirming previous results, group IT x FC was unimpaired, whereas group TS+AM+FX was severely impaired. The results show that the amnesic effects of TS+AM+FX cannot be generally attributed to the partial temporal-frontal disconnection that this lesion creates, and therefore support the hypothesis that the amnesic effects of this lesion are caused primarily by the disconnection of temporal cortex from ascending inputs from the basal forebrain. The results also show that temporal-frontal interaction in strategy implementation does not require those routes of temporal-frontal interaction that are interrupted in TS+AM+FX, and therefore support the hypothesis that projections to other posterior cortical areas allow temporal and frontal cortex to interact with each other by multisynaptic corticocortical routes in strategy implementation.

Amygdala↗

PubMedDate not supplied

A cholinergic explanation of dense amnesia.

Traditionally, dense amnesia in humans has been thought to be caused by damage to memory structures within the medial temporal lobe, either the hippocampus, the perirhinal cortex or a combination of the two. Recent work in animals has indicated that damage to the hippocampus results in memory impairments that are specific to memories for events. In contrast, damage to the perirhinal cortex impairs object memories in primates, which are likely to be analogous to human semantic memory. Discrete damage to either of these structures does not appear to result in severe and global memory impairments. We propose an alternative to this medial temporal lobe explanation of dense amnesia, based on our recent work. We have shown that the cholinergic cells of the basal forebrain are essential for new learning of both object and event-like memories in the monkey. Our model of learning in the primate brain rests on the proposal that there is a route of communication between the frontal cortex and inferior temporal cortex and hippocampus via the basal forebrain, which is critical for memory encoding. Interruption to this circuit at any stage will result in a severe and global anterograde amnesia.

Amnesia↗

PubMedDate not supplied

Feedback from V1 and inhibition from beyond the classical receptive field modulates the responses of neurons in the primate lateral geniculate nucleus.

It is well established that the responses of neurons in the lateral geniculate nucleus (LGN) can be modulated by feedback from visual cortex, but it is still unclear how cortico-geniculate afferents regulate the flow of visual information to the cortex in the primate. Here we report the effects, on the gain of LGN neurons, of differentially stimulating the extraclassical receptive field, with feedback from the striate cortex intact or inactivated in the marmoset monkey, Callithrix jacchus. A horizontally oriented grating of optimal size, spatial frequency, and temporal frequency was presented to the classical receptive field. The grating varied in contrast (range: 0-1) from trial to trial, and was presented alone, or surrounded by a grating of the same or orthogonal orientation, contained within either a larger annular field, or flanks oriented either horizontally or vertically. V1 was ablated to inactivate cortico-geniculate feedback. The maximum firing rate of LGN neurons was greater with V1 intact, but was reduced by visually stimulating beyond the classical receptive field. Large horizontal or vertical annular gratings were most effective in reducing the maximum firing rate of LGN neurons. Magnocellular neurons were most susceptible to this inhibition from beyond the classical receptive field. Extraclassical inhibition was less effective with V1 ablated. We conclude that inhibition from beyond the classical receptive field reduces the excitatory influence of V1 in the LGN. The net balance between cortico-geniculate excitation and inhibition from beyond the classical receptive field is one mechanism by which signals relayed from the retina to V1 are controlled.

Animals↗