Biomedical subjects

E T Rolls

Publications and source records attributed to E T Rolls.

At least 19 recordsLinked to original sources

PubMed1999

Backward projections in the cerebral cortex: implications for memory storage.

Cortical areas are characterized by forward and backward connections between adjacent cortical areas in a processing stream. Within each area there are recurrent collateral connections between the pyramidal cells. We analyze the properties of this architecture for memory storage and processing. Hebb-like synaptic modifiability in the connections and attractor states are incorporated. We show the following: (1) The number of memories that can be stored in the connected modules is of the same order of magnitude as the number that can be stored in any one module using the recurrent collateral connections, and is proportional to the number of effective connections per neuron. (2) Cooperation between modules leads to a small increase in memory capacity. (3) Cooperation can also help retrieval in a module that is cued with a noisy or incomplete pattern. (4) If the connection strength between modules is strong, then global memory states that reflect the pairs of patterns on which the modules were trained together are found. (5) If the intermodule connection strengths are weaker, then separate, local memory states can exist in each module. (6) The boundaries between the global and local retrieval states, and the nonretrieval state, are delimited. All of these properties are analyzed quantitatively with the techniques of statistical physics.

Cerebral Cortex↗

PubMed1999

Firing rate distributions and efficiency of information transmission of inferior temporal cortex neurons to natural visual stimuli.

The distribution of responses of sensory neurons to ecological stimulation has been proposed to be designed to maximize information transmission, which according to a simple model would imply an exponential distribution of spike counts in a given time window. We have used recordings from inferior temporal cortex neurons responding to quasi-natural visual stimulation (presented using a video of everyday lab scenes and a large number of static images of faces and natural scenes) to assess the validity of this exponential model and to develop an alternative simple model of spike count distributions. We find that the exponential model has to be rejected in 84% of cases (at the p < 0.01 level). A new model, which accounts for the firing rate distribution found in terms of slow and fast variability in the inputs that produce neuronal activation, is rejected statistically in only 16% of cases. Finally, we show that the neurons are moderately efficient at transmitting information but not optimally efficient.

Animals↗

PubMed1999

The representation of pleasant touch in the brain and its relationship with taste and olfactory areas.

Although there has been much investigation of brain pathways involved in pain, little is known about the brain mechanisms involved in processing somatosensory stimuli which feel pleasant. Employing fMRI it was shown that pleasant touch to the hand with velvet produced stronger activation of the orbitofrontal cortex than affectively neutral touch of the hand with wood. In contrast, the affectively neutral but more intense touch produced more activation of the primary somatosensory cortex than the pleasant stimulus. This indicates that part of the orbitofrontal cortex is concerned with representing the positively affective aspects of somatosensory stimuli, and in further experiments it was shown that this orbitofrontal area is different from that activated by taste and smell. The finding that three different primary or unlearned types of reinforcer (touch, taste, and smell) are represented in the orbitofrontal cortex helps to provide a firm foundation for understanding the neural basis of emotions, which can be understood in terms of states elicited by stimuli which are rewarding or punishing.

Brain↗

PubMed1999

Responses to the sensory properties of fat of neurons in the primate orbitofrontal cortex.

The primate orbitofrontal cortex is a site of convergence of information from primary taste, olfactory, and somatosensory cortical areas. We describe the responses of a population of single neurons in the orbitofrontal cortex that responds to fat in the mouth. The neurons respond, when fatty foods are being eaten, to pure fat such as glyceryl trioleate and also to substances with a similar texture but different chemical composition such as paraffin oil (hydrocarbon) and silicone oil [Si(CH3)2O)n]. This is evidence that the neurons respond to the oral texture of fat, sensed by the somatosensory system. Some of the population of neurons respond unimodally to the texture of fat. Other single neurons show convergence of taste inputs, and others of olfactory inputs, onto single neurons that respond to fat. For example, neurons were found that responded to the mouth feel of fat and the taste of monosodium glutamate (both found in milk), or to the mouth feel of fat and to odor. Feeding to satiety reduces the responses of these neurons to the fatty food eaten, but the neurons still respond to some other foods that have not been fed to satiety. Thus sensory-specific satiety for fat is represented in the responses of single neurons in the primate orbitofrontal cortex. Fat is an important constituent of food that affects its palatability and nutritional effects. The findings described provide evidence that the reward value (or pleasantness) of the mouth feel of fat is represented in the primate orbitofrontal cortex and that the representation is relevant to appetite.

Animal Nutritional Physiological Phenomena↗

PubMed1999

Head direction cells in the primate pre-subiculum.

The function of the primate hippocampus and related structures was analysed by making recordings from the hippocampus, subiculum, presubiculum, and parahippocampal gyrus in monkeys actively walking in the laboratory. Head direction cells were found in the presubiculum. The firing rate of these cells was a function of the head direction of the monkey, with a response that was typically 10-100 times larger to the best as compared to the opposite direction. The mean half-amplitude width of the tuning of the cells was 76 degrees. The response of head direction cells in the presubiculum was not influenced by the place where the monkey was, there being the same tuning to head direction at different places in a room, and even outside the room. The response of these cells was also independent of the "spatial view" observed by the monkey, and also the position of the eyes in the head. The average information about head direction was 0.64 bits, about place was 0.10 bits, about spatial view was 0.27 bits, and about eye position was 0.04 bits. The cells maintained their tuning for periods of at least several minutes when the view details were obscured or the room was darkened. This representation of head direction could be useful together with the hippocampal spatial view cells and whole body motion cells found in primates in such spatial and memory functions as path integration.

Action Potentials↗

PubMed1999

The neurophysiology of backward visual masking: information analysis.

Backward masking can potentially provide evidence of the time needed for visual processing, a fundamental constraint that must be incorporated into computational models of vision. Although backward masking has been extensively used psychophysically, there is little direct evidence for the effects of visual masking on neuronal responses. To investigate the effects of a backward masking paradigm on the responses of neurons in the temporal visual cortex, we have shown that the response of the neurons is interrupted by the mask. Under conditions when humans can just identify the stimulus, with stimulus onset asynchronies (SOA) of 20 msec, neurons in macaques respond to their best stimulus for approximately 30 msec. We now quantify the information that is available from the responses of single neurons under backward masking conditions when two to six faces were shown. We show that the information available is greatly decreased as the mask is brought closer to the stimulus. The decrease is more marked than the decrease in firing rate because it is the selective part of the firing that is especially attenuated by the mask, not the spontaneous firing, and also because the neuronal response is more variable at short SOAs. However, even at the shortest SOA of 20 msec, the information available is on average 0.1 bits. This compares to 0.3 bits with only the 16-msec target stimulus shown and a typical value for such neurons of 0.4 to 0.5 bits with a 500-msec stimulus. The results thus show that considerable information is available from neuronal responses even under backward masking conditions that allow the neurons to have their main response in 30 msec. This provides evidence for how rapid the processing of visual information is in a cortical area and provides a fundamental constraint for understanding how cortical information processing operates.

Animals↗

PubMed1998

The neurophysiology of taste and olfaction in primates, and umami flavor.

To investigate the neural encoding of glutamate (umami) taste in the primate, recordings were made from taste responsive neurons in the cortical taste areas in macaques. Most of the neurons were in the orbitofrontal cortex (secondary) taste area. First, it was shown that there is a representation of the taste of glutamate which is separate from the representation of the other prototypical tastants sweet (glucose), salt (NaCl), bitter (quinine) and sour (HCl). Second, it was shown that single neurons that had their best responses to sodium glutamate also had good responses to glutamic acid. Third, it was shown that the responses of these neurons to the nucleotide umami tastant inosine 5'-monophosphate were more correlated with their responses to monosodium glutamate than to any prototypical tastant. Fourth, concentration response curves showed that concentrations of monosodium glutamate as low as 0.001 M were just above threshold for some of these neurons. Fifth, some neurons in the orbitofrontal region, which responded to monosodium glutamate and other food tastes, decreased their responses after feeding with monosodium glutamate to behavioral satiety, revealing a mechanism of satiety. In some cases this reduction was sensory-specific. Sixth, it was shown in psychophysical experiments in humans that the flavor of umami is strongest with a combination of corresponding taste and olfactory stimuli (e.g., monosodium glutamate and garlic odor). The hypothesis is proposed that part of the way in which glutamate works as a flavor enhancer is by acting in combination with corresponding food odors. The appropriate associations between the odor and the glutamate taste may be learned at least in part by olfactory to taste association learning in the primate orbitofrontal cortex.

Animals↗

PubMed1998

View-invariant representations of familiar objects by neurons in the inferior temporal visual cortex.

A view-invariant representation of objects in the brain would have many computational advantages. Here we describe a population of single neurons in the temporal visual cortex (IT) that have view-invariant representations of familiar objects. Ten real plastic objects were placed in the monkeys' home cages for a period of time before neurophysiological experiments in which neuronal responses were measured to four views of each object. The macaques performed a visual fixation task, and had never been trained in object discrimination. The majority of the visual neurons recorded were responsive to some views of some objects and/or to the control stimuli, as would be expected from previous studies. However, a small subset of these neurons were responsive to all views of one or more of the objects, providing evidence that these neurons were coding for objects, rather than simply for individual views or visual features within the image. This result was confirmed by information theoretic analyses, which showed that the neurons provided information about which object was being seen, independently of the view. The coding scheme was shown to be sparse distributed, with relatively independent information being provided by the different neurons. Hypotheses about how these view-invariant cells are formed are described.

Action Potentials↗

PubMed1998

Spatial view cells in the primate hippocampus: effects of removal of view details.

Hippocampal function was analyzed by making recordings from hippocampal formation neurons in macaques actively walking in the laboratory. "Spatial view" cells, which respond when the monkey looks at a part of the environment were analyzed. It is shown that many of these cells retain their spatial characteristics when the view details are obscured totally by curtains and by darkness. It is shown that many of these cells respond more when the monkey is gazing toward one location in the room than toward other locations, even though none of the view details can be seen. Such cells were found in the CA1 region, the parahippocampal gyrus, and the presubiculum. Other cells stopped responding when the monkey looked toward the normally effective location in the environment if the view details were obscured. These cells were in the CA3 region of the hippocampus. The results indicate that for CA3 cells, the visual input is necessary for the normal spatial response of the neurons, and for other cells in the primate hippocampal formation, the response still depends on the monkey gazing toward that location in space when the view details are obscured. These latter cells therefore could reflect the operation of a memory system, in which the neuronal activity can be triggered by factors that probably include not only eye position command/feedback signals, but also probably vestibular and/or proprioceptive inputs. This representation of space "out there" would be an appropriate part of a primate memory system involved in memories of where in an environment an object was seen and more generally in the memory of particular events or episodes for which a spatial component normally provides part of the context.

Action Potentials↗

PubMed1998

Information about spatial view in an ensemble of primate hippocampal cells.

Hippocampal function was analyzed by making recordings from hippocampal neurons in monkeys actively walking in the laboratory. "Spatial view" cells, which respond when the monkey looks at a part of the environment, were analyzed. To assess quantitatively the information about the spatial environment represented by these cells, we applied information theoretic techniques to their responses. The average information provided by these cells about which location the monkey was looking at was 0.32 bits, and the mean across cells of the maximum information conveyed about which location was being looked at was 1.19 bits, measured in a period of 0.5 s. There were 16 locations for this analysis, each being one-quarter of one of the walls of the room. It also was shown that the mean spontaneous rate of firing of the neurons was 0.1 spikes/s, that the mean firing rate in the center of the spatial field of the neurons was 13.2 spikes/s, and that the mean sparseness of the representation measured in a 25-ms period was 0.04 and in a 500-ms time period was 0.19. (The sparseness is approximately equivalent to the proportion of the 25- or 500-ms periods in which the neurons showed one or more spikes.) Next it was shown that the mean size of the view fields of the neurons was 0.9 of a wall. In an approach to the issue of how an ensemble of neurons might together provide more precise information about spatial location than a single neuron, it was shown that in general the neurons had different centers for their view fields. It then was shown that the information from an ensemble of these cells about where in space is being looked at increases approximately linearly with the number of cells in the ensemble. This indicates that the number of places that can be represented increases approximately exponentially with the number of cells in the population. It is concluded that there is an accurate representation of space "out there" in the primate hippocampus. This representation of space out there would be an appropriate part of a primate memory system involved in memories of where in an environment an object was seen, and more generally in the memory of particular events or episodes, for which a spatial component normally provides part of the context.

Algorithms↗

PubMed1997

Responses of neurons in primary and inferior temporal visual cortices to natural scenes.

The primary visual cortex (V1) is the first cortical area to receive visual input, and inferior temporal (IT) areas are among the last along the ventral visual pathway. We recorded, in area V1 of anaesthetized cats and area IT of awake macaque monkeys, responses of neurons to videos of natural scenes. Responses were analysed to test various hypotheses concerning the nature of neural coding in these two regions. A variety of spike-train statistics were measured including spike-count distributions, interspike interval distributions, coefficients of variation, power spectra, Fano factors and different sparseness measures. All statistics showed non-Poisson characteristics and several revealed self-similarity of the spike trains. Spike-count distributions were approximately exponential in both visual areas for eight different videos and for counting windows ranging from 50 ms to 5 seconds. The results suggest that the neurons maximize their information carrying capacity while maintaining a fixed long-term-average firing rate, or equivalently, minimize their average firing rate for a fixed information carrying capacity.

Animals↗

PubMed1997

The representational capacity of the distributed encoding of information provided by populations of neurons in primate temporal visual cortex.

It has been shown that it is possible to read, from the firing rates of just a small population of neurons, the code that is used in the macaque temporal lobe visual cortex to distinguish between different faces being looked at. To analyse the information provided by populations of single neurons in the primate temporal cortical visual areas, the responses of a population of 14 neurons to 20 visual stimuli were analysed in a macaque performing a visual fixation task. The population of neurons analysed responded primarily to faces, and the stimuli utilised were all human and monkey faces. Each neuron had its own response profile to the different members of the stimulus set. The mean response of each neuron to each stimulus in the set was calculated from a fraction of the ten trials of data available for every stimulus. From the remaining data, it was possible to calculate, for any population response vector, the relative likelihoods that it had been elicited by each of the stimuli in the set. By comparison with the stimuli actually shown, the mean percentage correct identification was computed and also the mean information about the stimuli, in bits, that the population of neurons carried on a single trial. When the decoding algorithm used for this calculation approximated an optimal, Bayesian estimate of the relative likelihoods, the percentage correct increased from 14% correct (chance was 5% correct) with one neuron to 67% with 14 neurons. The information conveyed by the population of neurons increased approximately linearly from 0.33 bits with one neuron to 2.77 bits with 14 neurons. This leads to the important conclusion that the number of stimuli that can be encoded by a population of neurons in this part of the visual system increases approximately exponentially as the number of cells in the sample increases (in that the log of the number of stimuli increases almost linearly). This is in contrast to a local encoding scheme (of "grandmother" cells), in which the number of stimuli encoded increases linearly with the number of cells in the sample. Thus one of the potentially important properties of distributed representations, an exponential increase in the number of stimuli that can be represented, has been demonstrated in the brain with this population of neurons. When the algorithm used for estimating stimulus likelihood was as simple as could be easily implemented by neurons receiving the population's output (based on just the dot product between the population response vector and each mean response vector), it was still found that the 14-neuron population produced 66% correct guesses and conveyed 2.30 bits of information, or 83% of the information that could be extracted with the nearly optimal procedure. It was also shown that, although there was some redundancy in the representation (with each neuron contributing to the information carried by the whole population 60% of the information it carried alone, rather than 100%), this is due to the fact that the number of stimuli in the set was limited (it was 20). The data are consistent with minimal redundancy for sufficiently large and diverse sets of stimuli. The implication for brain connectivity of the distributed encoding scheme, which was demonstrated here in the case of faces, is that a neuron can receive a great deal of information about what is encoded by a large population of neurons if it is able to receive its inputs from a random subset of these neurons, even of limited numbers (e.g. hundreds).

Algorithms↗

PubMed1997

Olfactory sensory-specific satiety in humans.

It is shown that olfactory sensory-specific satiety, measured by ratings of the pleasantness of the odour of a food eaten relative to the change in pleasantness of other foods, can be produced by eating a food to satiety. It is also shown that olfactory and taste sensory-specific satiety can be produced by chewing samples of a food for approximately as long as the food would normally be eaten in a meal. It is further shown that partial olfactory sensory-specific satiety can be produced by smelling the food for approximately as long as it would be in the mouth during a meal. These sensory-specific changes in the pleasantness of a food do not appear to reflect changes in the intensity of the foods, which were small and not highly correlated with the changes in pleasantness. The results show that at least partial olfactory, as well as taste, sensory-specific satiety does not require food to enter the gastrointestinal system, and does not depend on the ingestion of calories. The implications for the control of food intake, and the way in which the brain computes sensory-specific satiety, are considered.

Adult↗

PubMed1997

Invariant face and object recognition in the visual system.

Neurophysiological evidence is described, showing that some neurons in the macaque temporal cortical visual areas have responses that are invariant with respect to the position, size and view of faces and objects, and that these neurons show rapid processing and rapid learning. A theory is then described of how such invariant representations may be produced in a hierarchically organized set of visual cortical areas with convergent connectivity. The theory proposes that neurons in these visual areas use a modified Hebb synaptic modification rule with a short-term memory trace to capture whatever can be captured at each stage that is invariant about objects as the object changes in retinal position, size, rotation and view. Simulations are then described which explore the operation of the architecture. The simulations show that such a processing system can build invariant representations of objects.

Animals↗

PubMed1997

Information in the neuronal representation of individual stimuli in the primate temporal visual cortex.

To analyze the information provided about individual visual stimuli in the responses of single neurons in the primate temporal lobe visual cortex, neuronal responses to a set of 65 visual stimuli were recorded in macaques performing a visual fixation task and analyzed using information theoretical measures. The population of neurons analyzed responded primarily to faces. The stimuli included 23 faces and 42 nonface images of real-world scenes, so that the function of this brain region could be analyzed when it was processing relatively natural scenes. It was found that for the majority of the neurons significant amounts of information were reflected about which of several of the 23 faces had been seen. Thus the representation was not local, for in a local representation almost all the information available can be obtained when the single stimulus to which the neuron responds best is shown. It is shown that the information available about any one stimulus depended on how different (for example, how many standard deviations) the response to that stimulus was from the average response to all stimuli. This was the case for responses below the average response as well as above. It is shown that the fraction of information carried by the low firing rates of a cell was large--much larger than that carried by the high firing rates. Part of the reason for this is that the probability distribution of different firing rates is biased toward low values (though with fewer very low values than would be predicted by an exponential distribution). Another factor is that the variability of the response is large at intermediate and high firing rates. Another finding is that at short sampling intervals (such as 20 ms) the neurons code information efficiently, by effectively acting as binary variables and behaving less noisily than would be expected of a Poisson process.

Action Potentials↗

PubMed1997

Spatial view cells in the primate hippocampus.

Hippocampal function was analysed by making recordings in rhesus monkeys actively walking in the laboratory. In a sample of 352 cells recorded in the hippocampus and parahippocampal cortex, a population of 'spatial view' cells was found to respond when the monkey looked at a part of the environment. The responses of these hippocampal neurons (i) occur to a view of space 'out there', not to the place where the monkey is, (ii) depend on where the monkey is looking, as shown by measuring eye position, (iii) do not encode head direction, and (iv) provide a spatial representation that is allocentric, i.e. in world coordinates. This representation of space 'out there' would be an appropriate part of a primate memory system involved in memories of where in an environment an object was seen, and more generally in the memory of particular events or episodes for which a spatial component normally provides part of the context.

Animals↗

PubMed1997

Taste and olfactory processing in the brain and its relation to the control of eating.

Taste processing in primates through the nucleus of the solitary tract and the primary taste cortex is shown to represent the identity and intensity of taste inputs. In contrast, in the secondary taste cortex in the orbitofrontal area, single neurons respond to the taste of a food only if hunger is present. The neurons here reflect the reward value of taste. They show sensory-specific satiety. In addition to neurons with best responses to sweet, salt, bitter, and sour, there are separate representations of the taste of protein ("umami") and of astringency (e.g., tannic acid). In the orbitofrontal cortex, olfactory inputs converge onto neurons with taste inputs, forming representations of flavor. Neurons in this region may also respond to the sight of food and to its texture. The olfactory representation for some neurons reflects the taste association of odors; and olfactory sensory-specific satiety is represented in this part of the brain. Rapid learning of visual-to-taste associations is also a feature of the neural processing that occurs in the orbitofrontal cortex.

Animals↗

PubMed1996

Rapid visual learning in neurones of the primate temporal visual cortex.

The human visual system can learn to recognize visual stimuli rapidly. For example, humans can accurately reconstruct meaningful objects out of fragmentary evidence, once they have seen the same object in its unambiguous form. The anterior temporal cortical areas of macaques contain some neurones with invariant visual responses which appear to provide a representation of complex patterns and objects, such as faces. Remarkably, these neurones show an enhancement of response after brief (e.g. 5 s) exposure to the unambiguous stimulus, an effect that appears to reflect the neural basis of the rapid perceptual learning seen in humans.

Animals↗