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Biomedical subjects

Edmund T Rolls

Publications and source records attributed to Edmund T Rolls.

11 recordsLinked to original sources
PubMed2003

Human cortical responses to water in the mouth, and the effects of thirst.

In an event-related functional magnetic resonance imaging (fMRI) study in humans it was shown, first, that water produces activations in cortical taste areas (in particular the frontal operculum/anterior insula which is the primate primary taste cortex, and the caudal orbitofrontal/secondary taste cortex) comparable to those produced by the prototypical tastants salt and glucose. Second, the activations in the frontal operculum/anterior insula produced by water when thirsty were still as large after the subjects had consumed water to satiety. Third, in contrast, the responses to water in the caudal orbitofrontal cortex were modulated by the physiological state of the body, in that responses to the oral delivery of water in this region were not found after the subjects had drunk water to satiety. Fourth, further evidence that the reward value or pleasantness of water is represented in the orbitofrontal cortex was that a positive correlation with the subjective ratings of the pleasantness of the water was found with activations in the caudal and anterior orbitofrontal cortex, and also in the anterior cingulate cortex. Fifth, it was found that a region of the middle part of the insula was also activated by water in the mouth, and further, that this activation only occurred when thirsty. Sixth, analyses comparing pre- and postsatiety periods (i.e., when thirsty and when not thirsty) independently of stimulus delivery revealed higher activity levels in the rostral anterior cingulate cortex. The activity of the rostral anterior cingulate cortex thus appears to reflect the thirst level or motivational state of the subjects.

Cerebral Cortex↗
PubMed2003

Neurons in the primate orbitofrontal cortex respond to fat texture independently of viscosity.

The primate orbitofrontal cortex (OFC) is a site of convergence from primary taste, olfactory, and somatosensory cortical areas. We describe the responses of a population of single neurons in the OFC that respond to orally applied fat (e.g., safflower oil) and to substances with a similar texture but different chemical composition, such as mineral oil (hydrocarbon) and silicone oil [(Si(CH3)2O)n]. These findings provide evidence that the neurons respond to the oral texture of fat, sensed by the somatosensory system. Use of an oral viscosity stimulus consisting of carboxymethyl-cellulose in the range 1-10,000 centipoise (cP) showed that the responses of these fat-sensitive neurons are not related to stimulus viscosity. Thus a textural component independent of viscosity and related to the slick or oily property is being used to activate these oral fat-sensitive neurons. Moreover, a separate population of neurons responds to viscosity (produced, e.g., by the carboxymethyl-cellulose series), but not to fat with the same viscosity. Thus there is a dissociation between texture channels used to sense fat viscosity and non-fat-produced viscosity. Further, free fatty acids such as linoleic acid do not activate these neurons, providing further evidence that the oral fat-sensing mechanism through which these OFC neurons are activated is not gustatory but textural. Most of this population of fat-sensitive neurons receive convergent taste inputs. These results provide evidence about how oral fat is sensed and are relevant to understanding the physiological and pathophysiological processes related to fat intake.

Action Potentials↗
PubMed2003

An information theoretic approach to the contributions of the firing rates and the correlations between the firing of neurons.

To analyze the extent to which populations of neurons encode information in the numbers of spikes each neuron emits or in the relative time of firing of the different neurons that might reflect synchronization, we developed and analyzed the performance of an information theoretic approach. The formula quantifies the corrections to the instantaneous information rate that result from correlations in spike emission between pairs of neurons. We showed how these cross-cell terms can be separated from the correlations that occur between the spikes emitted by each neuron, the auto-cell terms in the information rate expansion. We also described a method to test whether the estimate of the amount of information contributed by stimulus-dependent synchronization is significant. With simulated data, we show that the approach can separate information arising from the number of spikes emitted by each neuron from the redundancy that can arise if neurons have common inputs and from the synergy that can arise if cells have stimulus-dependent synchronization. The usefulness of the approach is also demonstrated by showing how it helps to interpret the encoding shown by neurons in the primate inferior temporal visual cortex. When applied to a sample dataset of simultaneously recorded inferior temporal cortex neurons, the algorithm showed that most of the information is available in the number of spikes emitted by each cell; that there is typically just a small degree (approximately 12%) of redundancy between simultaneously recorded inferior temporal cortex (IT) neurons; and that there is very little gain of information that arises from stimulus-dependent synchronization effects in these neurons.

Algorithms↗
PubMed2003

The receptive fields of inferior temporal cortex neurons in natural scenes.

Inferior temporal cortex neurons have generally been found to have large visual receptive fields that typically include the fovea and extend throughout much of the visual field. However, a problem of such a large receptive field is that it does not easily support object selection by subsequent processing areas, in that all objects within such a large receptive field might activate inferior temporal cortex cells. To clarify this, we recorded from inferior temporal cortex neurons while macaques searched for objects in complex natural scenes or in plain backgrounds, as normally used. Inferior temporal cortex neuron receptive fields were much smaller in natural scenes (mean radius, 11 degrees) than in plain backgrounds (39 degrees). With two objects in a scene, one of which was a target for action (a touch), the firing rates were equally high during foveation of the effective stimulus when it was the target and when it was the distractor in both the plain and the complex scenes. With a plain background and two objects present, the receptive fields were much larger (24 degrees ) for the stimulus when it was the target than when it was the distractor (9 degrees ). This effect of object-based attention was much less evident in the complex scene, when the receptive fields were small both when the stimulus was a distractor and when it was a target. The results show that the temporal visual cortex provides an unambiguous representation in natural scenes by responding to the object shown at or close to the fixation point.

Action Potentials↗
PubMed2003

Activity of primate subgenual cingulate cortex neurons is related to sleep.

The most frequent type of neuronal response found in the subgenual cingulate cortex (area 25) of the rhesus macaque was a highly significant increase of firing rate when the monkey fell asleep (median rate = 1.6 spikes/s) compared with the awake state (median rate = 0.1 spikes/s). On average, the firing rate of the neurons when awake was 23% of that when the monkeys were asleep. Neurons were not found in this region with responses related to taste, olfactory, and visual stimuli including faces or related to movement. These results are relevant to understanding the function of this region in humans, in which it has been suggested that activation may be related to disengagement from tasks and to induced sadness, both of which we note lead to a more passive or resting behavior. A decrease in the activation of this area in humans has been observed during the recovery from depression, which we note leads to a more active state of behavior.

Action Potentials↗
PubMed2002

A unified model of spatial and episodic memory.

Medial temporal lobe structures including the hippocampus are implicated by separate investigations in both episodic memory and spatial function. We show that a single recurrent attractor network can store both the discrete memories that characterize episodic memory and the continuous representations that characterize physical space. Combining both types of representation in a single network is actually necessary if objects and where they are located in space must be stored. We thus show that episodic memory and spatial theories of medial temporal lobe function can be combined in a unified model.

Animals↗
PubMed2002

Object-based visual neglect: a computational hypothesis.

Some patients with damage to the right parietal cortex show neglect for the left half of each of a series of objects shown in a horizontal row in the visual field. The neglect is thus not based on failure to see objects in any part of left visual space, but is object-based. We show that in a model of attention with separate V1, object (inferior temporal cortex, IT) and spatial (posterior parietal cortex, PP) modules the effect can arise after graded damage increasing towards the right of the PP module when the lateral inhibition between neurons in the PP and V1 modules is short-range. The local lateral inhibition produces high contrast effects at the edges of each object, and it is when this interacts with gradually increasing damage through the left visual field that the visibility of the left half of each object is especially impaired. This result was found in a formal model completely specified by mean field equations to quantify the dynamical interactions between the modules. This is the first quantitative account of object-based neglect found in humans with right parietal cortex damage, and provides evidence that the model of attention we describe can account for even detailed and extraordinary phenomena that can occur in visual perception.

Attention↗
PubMed2002

Perirhinal cortex neuronal activity is actively related to working memory in the macaque.

Lesion studies suggest that the perirhinal cortex plays a role in object recognition memory. To analyze its role, we recorded the activity of single neurons in the perirhinal cortex in a rhesus monkey (Macaca mulatta) performing a delayed matching-to-sample task with up to four intervening stimuli. Certain neurons (40 of 90 analyzed) showed a smaller response to an image when it was shown the second time within a trial (as a match image) than when it had been shown (as a sample image) the first time. A new finding was that the perirhinal cortex neurons were actively reset between trials: when a particular image was shown as a sample on a succeeding trial, the response was much larger than when it had been shown as a match image a short time previously on the previous trial. This resetting between trials appears to reflect the operation of an active working memory process rather than a passive temporal decay in a neuronal response. The results thus provide evidence that the perirhinal cortex plays an active role in visual working memory, perhaps in association with other brain areas such as the prefrontal cortex.

Animals↗
PubMed2002

Invariant object recognition in the visual system with novel views of 3D objects.

To form view-invariant representations of objects, neurons in the inferior temporal cortex may associate together different views of an object, which tend to occur close together in time under natural viewing conditions. This can be achieved in neuronal network models of this process by using an associative learning rule with a short-term temporal memory trace. It is postulated that within a view, neurons learn representations that enable them to generalize within variations of that view. When three-dimensional (3D) objects are rotated within small angles (up to, e.g., 30 degrees), their surface features undergo geometric distortion due to the change of perspective. In this article, we show how trace learning could solve the problem of in-depth rotation-invariant object recognition by developing representations of the transforms that features undergo when they are on the surfaces of 3D objects. Moreover, we show that having learned how features on 3D objects transform geometrically as the object is rotated in depth, the network can correctly recognize novel 3D variations within a generic view of an object composed of a new combination of previously learned features. These results are demonstrated in simulations of a hierarchical network model (VisNet) of the visual system that show that it can develop representations useful for the recognition of 3D objects by forming perspective-invariant representations to allow generalization within a generic view.

Neural Networks, Computer↗
PubMed2002

Fast, fully automated global and local magnetic field optimization for fMRI of the human brain.

The aim of this novel technique is to allow researchers, particularly those operating at high static magnetic field strengths on fMRI applications, to tailor the static magnetic field within the brain. The optimum solution for their experimental needs is reached, utilizing the full potential of the active shims at their disposal. The method for shimming human brain, which incorporates automatic brain segmentation to remove nonbrain tissue from the optimization routine, is presented and validated. The technique is fast, robust, and accurate, achieving the global minimum to a static field homogeneity function of the in vivo brain. Both global and specified local regions of the brain can be selected on which to optimize the shims without requiring skilled intervention. The effectiveness of the automated local shim is demonstrated in an olfactory fMRI study where significant activations in the orbitofrontal cortex were very clear when the above method was employed.

Adult↗