Kaspar Meyer

Convergence and divergence in a neural architecture for recognition and memory

How does the brain represent external reality so that it can be perceived in the form of mental images? How are the representations stored in memory so that an approximation of their original content can be re-experienced during recall? A framework introduced in the late 1980s proposed that mental images arise from neural activity in early sensory cortices both during perception and recall. Neurons in the association cortices, by contrast, would not code explicit mental content; rather, they would hold the records needed to reconstruct an approximation of the original perceptual maps in early cortices. Several neurophysiological and neuroimaging studies now lend growing support to this proposal.

Predicting visual stimuli on the basis of activity in auditory cortices

Using multivariate pattern analysis of functional magnetic resonance imaging data, we found that the subjective experience of sound, in the absence of auditory stimulation, was associated with content-specific activity in early auditory cortices in humans. As subjects viewed sound-implying, but silent, visual stimuli, activity in auditory cortex differentiated among sounds related to various animals, musical instruments and objects. These results support the idea that early sensory cortex activity reflects perceptual experience, rather than sensory stimulation alone.

Seeing Touch Is Correlated with Content-Specific Activity in Primary Somatosensory Cortex

There is increasing evidence to suggest that primary sensory cortices can become active in the absence of external stimulation in their respective modalities. This occurs, for example, when stimuli processed via one sensory modality imply features characteristic of a different modality; for instance, visual stimuli that imply touch have been observed to activate the primary somatosensory cortex (SI). In the present study, we addressed the question of whether such cross-modal activations are content specific. To this end, we investigated neural activity in the primary somatosensory cortex of subjects who observed human hands engaged in the haptic exploration of different everyday objects. Using multivariate pattern analysis of functional magnetic resonance imaging data, we were able to predict, based exclusively on the activity pattern in SI, which of several objects a subject saw being explored. Along with previous studies that found similar evidence for other modalities, our results suggest that primary sensory cortices represent information relevant for their modality even when this information enters the brain via a different sensory system.

Behind the looking-glass.

To understand how mirror neurons help to interpret actions, we must delve into the networks in which these cells sit, say Antonio Damasio and Kaspar Meyer.

Primary sensory cortices, top-down projections and conscious experience

Beginning with a prominent article by Crick and Koch in 1995 (Nature 375, 121-123), cognitive neuroscience has witnessed an intensive debate about whether or not neural activity in primary visual cortex correlates with conscious visual experience. While some studies--especially those employing functional magnetic resonance imaging--imply that this is the case, others--particularly those recording from single neurons--suggest that it is not. In the light of this ongoing controversy, it is surprising that the analogous question in other sensory modalities has received far less attention. The first part of the present article reviews studies relevant to the role of primary auditory and primary somatosensory cortices in conscious auditory and tactile experience. As will become evident, the results of these studies, at least at first sight, appear no less contradictory than those obtained in the visual modality--in fact, they evidence discrepancies that resemble those found in the visual system to an impressive degree. The second part of the article attempts to reconcile the seemingly contradictory data by suggesting that only activity induced in the primary sensory cortices through cortico-cortical top-down signals can become consciously accessible, whereas activity induced by bottom-up signals from the thalamus cannot. This conclusion is in line with the earlier proposals of several prominent neuroscientists that portrayed conscious perception as the result of an active interpretative process by the brain, rather than a passive reflection of the environment.

Sight and Sound Converge to Form Modality-Invariant Representations in Temporoparietal Cortex

People can identify objects in the environment with remarkable accuracy, regardless of the sensory modality they use to perceive them. This suggests that information from different sensory channels converges somewhere in the brain to form modality-invariant representations, i.e., representations that reflect an object independently of the modality through which it has been apprehended. In this functional magnetic resonance imaging study of human subjects, we first identified brain areas that responded to both visual and auditory stimuli and then used crossmodal multivariate pattern analysis to evaluate the neural representations in these regions for content specificity (i.e., do different objects evoke different representations?) and modality invariance (i.e., do the sight and the sound of the same object evoke a similar representation?). While several areas became activated in response to both auditory and visual stimulation, only the neural patterns recorded in a region around the posterior part of the superior temporal sulcus displayed both content specificity and modality invariance. This region thus appears to play an important role in our ability to recognize objects in our surroundings through multiple sensory channels and to process them at a supramodal (i.e., conceptual) level.

The Role of Dendritic Signaling in the Anesthetic Suppression of Consciousness

Despite considerable progress in the identification of the molecular targets of general anesthetics, it remains unclear how these drugs affect the brain at the systems level to suppress consciousness. According to recent proposals, anesthetics may achieve this feat by interfering with corticocortical top–down processes, that is, by interrupting information flow from association to early sensory cortices. Such a view entails two immediate questions. First, at which anatomical site, and by virtue of which physiological mechanism, do anesthetics interfere with top–down signals? Second, why does a breakdown of top–down signaling cause unconsciousness? While an answer to the first question can be gleaned from emerging neurophysiological evidence on dendritic signaling in cortical pyramidal neurons, a response to the second is offered by increasingly popular theoretical frameworks that place the element of prediction at the heart of conscious perception.

Another Remembered Present

Could conscious perception reflect a memory process? Intuition tells us that perceptual experience—the seamless flow of conscious images of vision, sound, touch, and so forth—reflects the external world. Accordingly, information flow along the brains sensory pathways has been thought to follow a caudo-rostral direction, away from the ports of entry, toward integrative cortices in the anterior parts of the frontal and temporal lobes. However, this view of a unidirectional, “bottom-up” processing cascade is challenged by findings which suggest that there is also information transfer in the opposite, “top-down” direction, from association areas toward early sensory cortices. A particularly intriguing observation is that while the initial bottom-up activation sweep along the sensory pathways can accomplish stimulus processing of considerable complexity and yield certain automated behaviors, conscious awareness of a sensory object appears to depend on top-down signals (1–3), as observed in the visual (4), auditory (5), and somatosensory (6) systems. Why is this the case?

Convergent and invariant object representations for sight, sound, and touch.

We continuously perceive objects in the world through multiple sensory channels. In this study, we investigated the convergence of information from different sensory streams within the cerebral cortex. We presented volunteers with three common objects via three different modalities—sight, sound, and touch—and used multivariate pattern analysis of functional magnetic resonance imaging data to map the cortical regions containing information about the identity of the objects. We could reliably predict which of the three stimuli a subject had seen, heard, or touched from the pattern of neural activity in the corresponding early sensory cortices. Intramodal classification was also successful in large portions of the cerebral cortex beyond the primary areas, with multiple regions showing convergence of information from two or all three modalities. Using crossmodal classification, we also searched for brain regions that would represent objects in a similar fashion across different modalities of presentation. We trained a classifier to distinguish objects presented in one modality and then tested it on the same objects presented in a different modality. We detected audiovisual invariance in the right temporo‐occipital junction, audiotactile invariance in the left postcentral gyrus and parietal operculum, and visuotactile invariance in the right postcentral and supramarginal gyri. Our maps of multisensory convergence and crossmodal generalization reveal the underlying organization of the association cortices, and may be related to the neural basis for mental concepts. Hum Brain Mapp 36:3629–3640, 2015.

Direct comparison between properties of adaptation of the auditory nerve and the ventral cochlear nucleus in response to repetitive clicks

The present study was designed to complete two previous reports [Loquet, G., Rouiller, E.M., 2002. Neural adaptation to pulsatile acoustical stimulation in the cochlear nucleus of the rat. Hear. Res. 171, 72-81; Loquet, G., Meyer, K., Rouiller, E.M., 2003. Effects of intensity of repetitive acoustic stimuli on neural adaptation in the ventral cochlear nucleus of the rat. Exp. Brain Res. 153, 436-442] on neural adaptation properties in the auditory system of the rat. Again, auditory near-field evoked potentials (ANEPs) were recorded in response to 250-ms trains of clicks from an electrode chronically implanted in the ventral cochlear nucleus (VCN). Up to now, our interest had focused on the adaptive behavior of the first one (N1) of the two negative ANEP components. A re-examination of our data for the second negative component (N2) was now undertaken. Results show that the adaptation time course observed for N2 displayed the same three-stage pattern previously reported for N1. Similarly, adaptation became more pronounced and occurred faster as stimulus intensity and/or repetition rate were increased. Based on latency data which suggest N1 and N2 to be mainly due to the activity of auditory-nerve (AN) fibers and cochlear nucleus neurons, respectively, it was concluded that neural adaptation assessed by gross-potentials was similar in the AN and VCN. This finding is meaningful in the context of our search to restore normal adaptation phenomena via electro-auditory hearing with an auditory brainstem implant on the same lines as our work in cochlear implants.